UNIVERSITY OF CALIFORNIA,
IRVINE
Radiocarbon of Black Carbon
in Marine Dissolved Organic Carbon
DISSERTATION
submitted in partial satisfaction of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
in Earth System Science
by
Lori Anne Ziolkowski
Dissertation Committee:
Professor Ellen Druffel, Chair
Professor James Randerson
Professor Susan Trumbore
Professor Richard Chamberlin
Professor Caroline Masiello
2009
UMI Number: 3369198
Copyright 2009 by
Ziolkowski, Lori Anne
All rights reserved
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Chapter 3 c 2009 Lori Anne Ziolkowski
All other materials DEDICATION
For Oma,
whom I wish could be here
to see me become Dr. Z.
ii
TABLE OF CONTENTS
Page
LIST OF FIGURES
vi
LIST OF TABLES
viii
ACKNOWLEDGMENTS
ix
CURRICULUM VITAE
xi
ABSTRACT OF THE DISSERTATION
xv
1 Introduction
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Oxidation of PAHs using the BPCA method
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . .
2.3 Methods . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Sample treatment . . . . . . . . . . . . . .
2.4 Results and Discussion . . . . . . . . . . . . . . .
2.4.1 Nitration of BPCAs . . . . . . . . . . . . . .
2.4.2 Carbon yields . . . . . . . . . . . . . . . . .
2.4.3 BPCA products of PAHs . . . . . . . . . . .
2.4.4 Time course and mechanistic experiments
2.4.5 Analysis of black carbon ring trial materials
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Detection of fullerenes and carbon nanotubes using BPCAs
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.2 Introduction . . . . . . . . . .
3.3 Methods . . . . . . . . . . . .
3.4 Results . . . . . . . . . . . .
3.4.1 BPCA distributions . .
3.4.2 Carbon yield . . . . .
3.4.3 Mixtures in sediments
3.5 Conclusions . . . . . . . . . .
Bibliography . . . . . . . . . . . . .
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4 Quantification of extraneous carbon during CSRA of BC
4.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Chemical Oxidation . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Radiocarbon analysis of isolated samples . . . . . . . . . . .
4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 Carbon mass balance and corrections . . . . . . . . . . . . .
4.4.2 Extraneous carbon added during PCGC isolation (CPCGC ) . .
4.4.3 Extraneous carbon added during chemical oxidation and PCGC
isolation (Cchemistry+PCGC ) . . . . . . . . . . . . . . . . . . . . .
4.4.4 Correcting for extraneous carbon and associated uncertainties
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5 Black carbon in marine dissolved organic carbon
5.1 Abstract . . . . . . . . . . . . . . . . . . . . . . .
5.2 Introduction . . . . . . . . . . . . . . . . . . . . .
5.3 Approach . . . . . . . . . . . . . . . . . . . . . .
5.4 Results . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . .
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6 Conclusions and thoughts on future research
6.1 BPCA method and its applicability . . . . . . . . . . . . . . . . . . .
6.2 Evaluating extraneous material added during the preparation of CSRA
samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Black carbon in the marine DOC pool . . . . . . . . . . . . . . . . .
6.4 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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A Determination of Carbon Yields
105
A.1 Polycyclic Aromatic Hydrocarbons . . . . . . . . . . . . . . . . . . . 105
A.2 Calculating the percentage of black carbon in UDOM . . . . . . . . . 106
iv
B BPCA protocol
B.1 Chemical extraction of BPCAs . . . . . . . . . . . . . . . . . . . . .
B.1.1 Cleaning the bomb . . . . . . . . . . . . . . . . . . . . . . . .
B.1.2 Pre-treatment of samples (if required) . . . . . . . . . . . . .
B.1.3 Filter sample . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.1.4 Cation column (if required) . . . . . . . . . . . . . . . . . . .
B.1.5 Dry samples . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.1.6 Derivatize . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.1.7 Solvent change . . . . . . . . . . . . . . . . . . . . . . . . . .
B.2 PCGC settings and parameters . . . . . . . . . . . . . . . . . . . . .
B.2.1 Determination of sample concentration and retention times .
B.2.2 Program the preparative fraction collector (PFC) to collect at
selected RTs . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.2.3 Prime the PCGC for collection . . . . . . . . . . . . . . . . .
B.2.4 Collect sample(s) . . . . . . . . . . . . . . . . . . . . . . . . .
B.2.5 Check the concentration and purity of the isolate . . . . . . .
B.2.6 Prepare sample for combustion . . . . . . . . . . . . . . . . .
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LIST OF FIGURES
Page
1.1 The black carbon continuum, adapted from Masiello {2004}. . . . . .
2
2.1 Theoretical chemical structure of black carbon . . . . . . . . . . . .
2.2 The PAHs used in this study. . . . . . . . . . . . . . . . . . . . . . .
2.3 Structures of BPCAs used as markers of aromatic carbon in this
study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Schematic of the oxidation of phenanthrene into BPCAs . . . . . . .
2.5 Oxidation products and yields for anthracene as a function of time. .
2.6 Proposed reaction scheme for the oxidation of anthracene to B2CA.
2.7 Distribution of non-, mono- and di-nitrophthalic acid as a function of
reactants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Change in B3CA and B6CA oxidation products from perylene over
time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9 BPCA distribution of materials used in the black carbon ring trial. . .
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3.1
3.2
3.3
3.4
3.5
3.6
Oxidation schematic of the PAH perylene . . . . . . . . . . . .
Three of the six carbon nanoparticles studied. . . . . . . . . .
BPCA distribution of fullerenes, carbon lampblack and soot. . .
BPCA distribution of SWCNTs with and without cation column.
Standard addition of soot and SWCNTs to marine sediments. .
Theoretical and measured BPCA distributions in mixtures. . . .
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4.1 The magnitude of column bleed and oven temperature as a function
of retention time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Grass char and hexane soot before and after Cex correction. . . . . .
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5.1 Map illustrating sample locations. . . . . . . . . . . . . . . . . . . . .
5.2 BPCA distribution and 14 C of BC for the samples. . . . . . . . . . .
5.3 ∆14 C of black carbon and marine DOC as a function of depth. . . . .
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6.1 Chemical structure of asphaltene . . . . . . . . . . . . . . . . . . . . 101
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LIST OF TABLES
Page
2.1
2.2
2.3
2.4
PAH Carbon yield and BPCA distributions . . . . . . . . . . . .
Time course carbon yields . . . . . . . . . . . . . . . . . . . . .
Quantification of black carbon materials (g BC / kg dry weight)
Variations on converting BPCAs to BC . . . . . . . . . . . . . .
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3.1 Percent carbon yield for the carbon nanoparticles in this study . . . .
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4.1 Materials processed and associated solvents used for CSRA of black
carbon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Type and treatment of samples evaluated for mass and FM of extraneous carbon added during sample processing. . . . . . . . . . . . .
4.3 Radiocarbon values (fraction modern) and associated uncertainty of
black carbon reference materials before and after correction for Cex .
5.1 Sample information for UDOM samples in this study. . . . . . . . . .
5.2 Measurements of black carbon isolated from UDOM. . . . . . . . . .
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A.1 Carbon content of BPCAs. . . . . . . . . . . . . . . . . . . . . . . . 106
A.2 Example calculation of BPCAs in PAHs. . . . . . . . . . . . . . . . . 107
B.1 PCGC settings and parameters . . . . . . . . . . . . . . . . . . . . . 112
viii
ACKNOWLEDGMENTS
This work would not have possible without Ellen Druffel introducing me to the world
of black carbon, by not so naı̈vely assigning me a summer project isolating black
carbon from a dissolved organic carbon sample! Her quiet guidance and boundless
support allowed me to have the greatest freedom in carrying out this project. I am
eternally thankful to her for her patience, belief in my abilities, critical feedback and
willingness to discuss potentially frivolous matters. In the future whenever I cross
paths with a rubber chicken, I will think of you, with great fondness.
Sheila Griffin, the rock of the Druffel lab, was instrumental in my progress. She
provided support, structure and delicious baked goods even when I didn’t know I
needed it. I am forever indebted to her and her willingness to turn a blind eye to
my chaotic ways. I am grateful to my academic big brother, Steve Beaupré, who
was always willing to lend a helping and educational hand, and for teaching me the
value of returning to first principles.
I’d like to thank my thesis committee for grounding me, while still believing in my
ability to tackle this project. Sue Trumbore taught me the value in being challenged,
while Jim Randerson taught me the value of taking a step back, to evaluate the big
picture. Carrie Massiello provided the foundation for black carbon in the Druffel lab.
Thank you for introducing me to the wonderful world of biochar. Dick Chamberlin
was an indispensable organic chemistry resource, always willing to answer even
the simplest of my questions with great patience.
Without the guidance (and tough love) of the KECK Carbon Cycle AMS facility,
these radiocarbon measurements would be meaningless. Always willing to answer
my questions, from trivial to complex, John Southon was a valuable resource for
this work. I am honoured my CSRA blank evaluation work has received the Guaciara dos Santos seal of approval. For better or worse, never again will I be able
look at 14 C data without questioning the blank.
If it were not for Lihini Aluwihare and Matt McCarthy opening their freezers to me,
I would still be searching for marine UDOM samples. I would like to thank Claudia
Czimczik for first introducing me to the BPCA method and showing me that you
can have it all in science. I am most appreciative to Christopher Reddy for his accessibility, which has brought my understanding of (and confidence in) collegiality
to a new level. I am indebted to John Greaves at the UCI Mass Spec facility for his
patience and eye to detail with regards to my work.
Thank you to the UCI ESS graduate students, past and present, who have opened
my mind to different schools of thought and challenged my perceptions. With the
administrative support of Cynthia, Liz, Linda and Jeff, my paper work was always
in order, money was always deposited to my bank account and immigration always
let me back in the country. Thank you!
ix
Finally, I am indebted to my family for their support along this great journey. Thank
you to my parents, Werner and Shirley, and my sister Kathi, who always encouraged me to follow my heart. And thank you to Cam, who patiently listens to all my
ideas, humours me when I riddle off scientific nonsense to him, and continues to
challenge me.
x
CURRICULUM VITAE
Lori Anne Ziolkowski
EDUCATION
Doctor of Philosophy in Earth System Science
University of California, Irvine
2009
Irvine, California
Master of Science in Earth System Science
University of California, Irvine
2006
Irvine, California
Master of Science in Chemical Oceanography
Dalhousie University
2000
Halifax, Nova Scotia
Bachelor of Science in Environmental Chemistry
University of Waterloo
RESEARCH EXPERIENCE
Graduate Research Assistant
University of California, Irvine
Research Technician
Dalhousie University
Graduate Research Assistant
Dalhousie University
Guest Student Investigator
Woods Hole Oceanographic Institution
Research Technician
University of Notre Dame, Radiation Laboratory
Geochemical Assistant
National Water Research Institute
1998
Waterloo, Ontario
2004–2009
Irvine, California
2000–2004
Halifax, Nova Scotia
1998–2000
Halifax, Nova Scotia
Jan – Aug,1997
Woods Hole, Massachusetts
May – Aug,1996
South Bend, Indiana
Jan – Apr,1995
Burlington, Ontario
HONORS AND AWARDS
Origins Institute CREATE Astrobiology Postdoctoral Fellowship, 2009 - 2011
Isocompound Meeting, Young Investigator Award, June 2009
UC Irvine Earth System Science Outstanding Departmental Contributions, 2009
UC Irvine Graduate Dean’s Dissertation Quarter Fellowship, Summer 2009
UC Irvine School of Physical Sciences Faculty Endowed Fellowship, 2008-2009
Outstanding Presentation, UC Irvine Institute of Geophysics and Planetary Physics,
2008
Pedagogical Fellowship, University of California Irvine, 2006-2008
Jenkins Graduate Fellowship, University of California Irvine, 2004-2006
Graduate Student fellowship, Dalhousie University,1998-2000
xi
REFEREED JOURNAL PUBLICATIONS
Bourbonniere, R.A., S.L. Telford, L.A. Ziolkowski, J. Lee, M.S. Evans and P.A.
Meyers (1997) Biogeochemical marker profiles in cores of dated sediments from
larger North American lakes Molecular Markers in Environmental Geochemistry,
ACS Symposium Series, 671, 133-150.
Ziolkowski, L., K. Vinodgopal, and P.V. Kamat (1997), Photostabilization of organic dyes on poly(styrenesulfonate)-capped TiO2 nanoparticles Langmuir, 13(12):
3214-3128.
Xie, H., S.S. Andrews, W.R. Martin, J. Miller, L. Ziolkowski, C.D. Taylor, and O.C.
Zafiriou (2002), Validated methods for sampling and headspace analysis of carbon
monoxide in seawater, Marine Chemistry, 77(2-3), 93-108.
Clark, C.D., W.T. Hiscock, F.J. Millero, G. Hitchcock, L. Brand, W.L. Miller, L. Ziolkowski, R.F. Chen and R.G. Zika (2004) CDOM distribution and CO2 production
on the southwest Florida shelf, Marine Chemistry, 89(1-4): 145-167.
Bouillon, R.C., W.L. Miller, M. Levasseur, M. Scarratt, A. Michaud, and L. Ziolkowski (2006) The effects of mesoscale iron enrichment on the marine photochemistry of dimethylsulfide in the NE subarctic Pacific, Deep-Sea Research Part
II Topical studies in Oceanography, 53(20-22): 2384-2397,
doi:10.1016/j.dsr2.2006.05.024.
Ziolkowski, L. A. and W. L. Miller (2007), Marine photochemical production of
carbon monoxide. Marine Chemistry, doi:10.1016/j.marchem.2007.02.004.
Ziolkowski, L.A. and E.R.M. Druffel (2009), Feasibility of isolating and detecting
fullerenes and carbon nanotubes using the benzene polycarboxylic acid method.
Marine Pollution Bulletin, doi:10.1016/j.marpolbul.2009.04.018
PAPERS IN PREPARATION
Ziolkowski, L.A., E.R.M. Druffel, R.A. Chamberlin and J. Greaves, Evaluation of the
oxidation of polycyclic aromatic hydrocarbons using the benzene polycarboxylic
acid method.
Ziolkowski, L.A., E.R.M. Druffel and J. Southon. Microscale compound specific
radiocarbon analysis.
Ziolkowski, L.A. and E.R.M. Druffel. Radiocarbon of black carbon in marine dissolved organic carbon.
xii
ORAL PRESENTATIONS (presenter in bold)
Ziolkowski, L.A., W.L. Miller, D. J. Kieber, and K. Mopper, Rapid Shipboard Determination of the Efficiency Spectrum for Photochemical Carbon Monoxide Production, AGU Ocean Sciences, January 2000, Texas.
Ziolkowski, L. A., W.L. Miller, H. Xie and O.C. Zafiriou, Efficiency Spectra for the
Marine Photochemical Production of Carbon Monoxide Determined Using a Solar
Simulator, ASLO, February 2001, New Mexico.
Ziolkowski, L. A. and W.L. Miller, Spatial and Temporal Variation of CDOM Fading
Efficiency, AGU Ocean Sciences, February 2002, Hawaii.
Ziolkowski L.A. and C. Gardiner, The Professional Utility of Teaching Assistant
Training beyond the Classroom, Lilly West, March 2008, Pamona, CA.
Ziolkowski, L.A. and E. R. M. Druffel, Radiocarbon content of benzene polycarboxylic acids, GSA/SSA Joint Meeting, October 2008, Houston, TX (Invited presentation)
Ziolkowski, L.A., Black magic: can we make carbon disappear?, UCI ESS 1/2
Baked Seminar Series, February 2009 (Invited presentation)
Ziolkowski, L.A. and E.R.M. Druffel, Compound specific radiocarbon analysis
of black carbon in marine dissolved organic matter, Isocompound Meeting, June
2009, Potsdam, Germany.
POSTER PRESENTATIONS (presenter in bold)
Ziolkowski, L.A. and W.L. Miller, U.V. Optical Properties During the Evolution of a
Phytoplankton Bloom (SERIES), ASLO/TOS Ocean Sciences, 2004, Hawaii.
Ziolkowski, L.A., C.S. Law and W.L. Miller, Investigation of the Marine Photochemical Production of Carbon Monoxide in the Waters South-East of New Zealand,
IGAC Conference, September 2004, Christchurch, N.Z.
Ziolkowski, L.A., E.R.M. Druffel and S. Griffin, Progress Towards an Estimate of
the Radiocarbon Content of Black Carbon in Marine Organic Matter, AGU Ocean
Sciences, February 2006, Hawaii.
Ziolkowski, L.A. and E.R.M. Druffel. Black carbon measurements using a revised
BPCA method. EGU, April 2007, Vienna, Austria.
Ziolkowski, L.A. and E.R.M. Druffel, Radiocarbon Content of Soot and Charred
Black Carbon using the Benzene Polycarboxylic Acid Method, AGU Ocean Sciences, March 2008, Orlando, Fl.
Ziolkowski, L.A. and E.R.M. Druffel, Radiocarbon values of black carbon using
the Benzene Polycarboxylic Acid Method, AGU Fall Meting, December 2008, San
xiii
Francisco, CA.
FIELD EXPERIENCE
Cruise:
R/V Tangaroa, M. Harvey, ANZ-SOLAS, Mar 20-Apr 19, 2004, S. Ocean.
CCGS Martha Black, M. Gosselin, C-SOLAS, July 3-28, 2003, N. Atlantic.
R/V Pelican, R.T. Powell, SWISS-III, Aug 29-Sept 13, 2002, Gulf of Mexico.
R/V El Puma, M. Levasseur, C-SOLAS, July 4-30, 2002, N. Pacific.
R/V Pelican, W. Landing, SWISS-II, August 11-25, 2001, Gulf of Mexico.
R/V Pelican, R.T. Powell, SWISS-I, April 16-28, 2001, Gulf of Mexico.
R/V Endeavor, O. C. Zafiriou, March 13-April 2, 2000, Sargasso Sea.
R/V Endeavor, O. C. Zafiriou, August 2-20, 1999, Sargasso Sea.
R/V Endeavor, D. J. Keiber, July 8-28, 1999, Gulf of Maine.
R/V New Horizon, T. Hayward, CalCOFI, April 2-30, 1997, So. Cal. coast.
Other: Thompson, MB, Canada, BOREAS, 1993, 1994, 1995, collection of beaver
pond water.
OTHER ACTIVITIES
Reviewer
Publications: AGU Book Series, Environmental Science and Technology, Marine
Chemistry
Grants: National Science Foundation
Service & Other Activities
2008 GSA Short Course: Starting out in Undergraduate Research and Education:
A Professional Development Workshop for Young Faculty
2006-2008: Science Fair Judge, California State Science Fair
2007-present: AGU Education and Human Resources Student Advisory Board
2007-present: Earth System Science Journal Club Coordinator
2005-2006: Earth System Science Departmental Graduate Student Representative
TEACHING EXPERIENCE
UC Irvine
Introduction to the Earth System, ESS 25, Teaching Assistant (2005, 2007)
Organic Biogeochemistry, ESS 53, Teaching Assistant (2006)
Teaching Assistant Professional Development Program, Instructor (2006-08)
AFFILIATIONS
American Geophysical Union member
Earth Science Womens Network member
The Oceanography Society member
xiv
ABSTRACT OF THE DISSERTATION
Radiocarbon of Black Carbon
in Marine Dissolved Organic Carbon
By
Lori Anne Ziolkowski
Doctor of Philosophy in Earth System Science
University of California, Irvine, 2009
Professor Ellen Druffel, Chair
Black carbon (BC), a bi-product of combustion, is a major long-term carbon sink
in the Earth system. Known storage pools for BC are marine sediment and soil.
Previous studies found significant
14
C age differences between BC and organic
carbon in sediments, and projected that BC must reside in an intermediate pool,
such as dissolved organic carbon (DOC), before deposition to the sediment. This
research applied compound specific radiocarbon analysis (CSRA) of BC using the
benzene polycarboxylic acid (BPCA) method, to provide the first estimates of BC
cycling in marine DOC.
First, the BPCA method was adapted for CSRA of marine DOC. This method was
applied to nine polycyclic aromatic hydrocarbons (PAHs) to examine the oxidation
mechanism of the BPCA method. These experiments showed larger BPCAs are
preferentially formed for large (>4 ring) PAHs and an average C recovery of 26 ±
7 %. Quantification of nitrated BPCAs was found to be essential for accurate assessment of BC. Next, I evaluated the mass and radiocarbon of extraneous carbon
(Cex ) added in the processing and isolation of CSRA samples. The Cex originated
equally from column bleed and the processing steps prior to compound isolation.
xv
While constant over a few weeks, the mass and radiocarbon signature of Cex varied
over longer time periods and must be frequently re-evaluated.
Finally, the radiocarbon signatures of BC in marine high molecular weight (HMW)
DOC samples from a river and five locations in the Atlantic and Pacific Oceans
are presented. BC exported from the river was 14 C modern, while ocean samples
were uniformly old (average open ocean BC ∆14 C = -888 ± 25 h, n=6). The
concentration of BC in HMW DOC (also known as UDOM) ranged from 0.5 to 3.5 %
and suggests that a substantial portion of BC should be in the low molecular weight
DOC pool. The presence of 14 C-depleted BC in modern HMW DOC demonstrates
that there are widely different turnover times for these two pools in the marine
carbon cycle.
xvi
1
Introduction
Black carbon, (BC) is the term applied to materials that have undergone combustion and are characterized by a broad spectrum of properties. As detected in soil
and sediment reservoirs, BC is operationally defined and represents a range of
combustion residues (i.e.: char and charcoal) and combustion condensates (i.e.:
as soot, Figure 1.1). BC may represent a significant sink of the global carbon cycle
{Kuhlbusch, 1998}, affect the Earth’s radiative heat balance {Crutzen and Andreae,
1990; Ramanathan and Carmichael, 2008}, influence the albedo of snow {Flanner
et al., 2008}, serve as a paleo-tracer for Earth’s fire history {Bird and Cali, 1998}
and represent a significant portion of carbon buried in soil and sediment {Masiello
and Druffel, 1998}.
Loss processes for BC are not well understood. When Seiler and Crutzen {1980}
estimated the annual natural emissions of BC to be on the order of 120 Tg BC, they
pointed out that without a loss term, all carbon on the surface of the Earth would
be present as BC in less than 100,000 years. BC has been found in 400 million
year old sediments in various locations {Venkatesan, 1989; Killops and Killops,
1992} and terrestrial soil samples that range in age from thousands to millions of
years {Bird and Cali, 1998}. Herring {1985} found no obvious decay of BC upon
1
Figure 1.1: The black carbon continuum, adapted from Masiello {2004}.
2
visual inspection of Cenozoic sediments. Despite its inert nature, losses of BC
have been detected in some situations. For example, Bird et al. {1999} determined
that BC in African soils could be significantly degraded on centennial timescales.
Using stable carbon isotopes they found that coarse BC particles are degraded
faster than finer particles. In laboratory experiments, Winkler {1985} observed
that organic compounds within the BC undergo acidic breakdown in an anaerobic
bog and lake sediments. Measurements of BC decay in sediments were made by
Middelburg et al. {1999}, where they found a diminished amount of BC in Madeira
Abyssal Plain turbidites that were exposed to oxygen over long time periods (10-20
kyr).
There is no standardized analytical technique for measuring BC. Quantification
techniques generally fall into one of two categories, isolation techniques and identification techniques (Figure 1.1). Separation techniques isolate the BC from the
bulk sample using some form of oxidation to remove the non-BC carbon compounds. Quantification techniques analyze the bulk sample for chemical signatures
indicative of black carbon, such as condensed aromatic structures. Several review
papers {Schimdt and Noack , 2000; Schmidt et al., 2001; Preston and Schmidt,
2006} and intercomparison projects {Currie et al., 2002; Hammes et al., 2007} on
the quantification and importance of BC have recently been published. In the most
recent intercomparison project, termed the “BC ring trial” {Hammes et al., 2007},
seventeen different teams using seven different isolation methods measured the
BC content in 12 standard reference materials (SRMs), and recommended that all
future BC studies calibrate using this set of BC reference materials. The ”BC ring
trial” clearly demonstrated that different methods each have associated artifacts
and biases.
3
Outline of this thesis
The primary goal of this dissertation was to determine the role that BC plays in the
DOC cycle within the global ocean.
The method used to quantify BC is often chosen based on the type of BC of interest (Figure 1.1). Since we were interested in not only quantifying the BC but
also measuring its radiocarbon content, we selected a BC method that would allow
for unambiguous isotopic measurement of BC. The benzene polycarboxlyic acid
(BPCA) method seemed well suited for CSRA of BC, as it simultaneously oxidizes
non-BC and transforms BC into BPCAs, leaving the BC-signature in an aqueous
solution. This method can provide both qualitative and quantitative information
about the BC. Before CSRA of BPCAs could be employed, two methodological
issues needed to be resolved. First, the accurate quantification of BC as BPCAs
requires a reliable, repeatable and robust conversion of BPCAs to BC, which had
not been previously demonstrated in previous BPCA studies. Secondly, our interest in CSRA of BPCAs required minimizing the carbon added in derivatization
and a new derivatization method was essential for the most accurate 14 C measurements. These improvements, along with additional information a proposed reaction
scheme for the formation of BPCAs is discussed in Chapter 2.
The similarity of naturally produced BC to some manufactured BC-like products,
such as fullerenes and carbon nanotubes (CNTs), led to the work presented in
Chapter 3. Here we investigate the feasibility of using the BPCA method for isolating fullerenes and CNTs from BC and non-BC sedimentary material.
Marine DOC can be concentrated through either size ultratfiltration (UDOM) using
tangential flow filtration or solid phase extraction (SPE-DOM). Both methods are
costly especially for obtaining large quantities of DOC. Further, the DOC they iso4
late is only a fraction of the pool. UDOM concentrates the high molecular weight
DOC while SPE-DOM is composed of hydrophobic compounds within DOC. We
elected to isolated BC from UDOM in an effect to minimize potential artifacts that
may originate from the breakdown of the solid phase in SPE-DOM, which might
influence our 14 C measurements.
Because BC was postulated to be a small percentage of the UDOM, large quantities of UDOM are required to produce BPCAs for CSRA. Any sample handling
for radiocarbon analysis inadvertently adds extraneous carbon. Since the BPCAs
produced from the oxidation of UDOM would generate small CSRA samples (<
30 µg carbon), the extraneous carbon would contribute to the measured isotopic
composition of the CSRA samples. In Chapter 4 we discuss how we quantify the
magnitude and variability of extraneous carbon originating from the chemical oxidation and subsequent isolation by PCGC.
Based the work of Masiello and Druffel {1998}, it was postulated that BC was 4
to 22 % of deep DOC. Using a suite of samples, ranging from fresh river water to
the deep Pacific, we investigate the concentration and radiocarbon of BC in marine
DOC. These results are presented in Chapter 5.
The final chapter (Chapter 6) summarizes the work presented here and suggests
some potential avenues of future research that stem from this thesis.
Two appendixes follow the conclusion. Appendix A outlines carbon recovery calculations. Appendix B contains the method protocol used to generate the BPCA
data presented within this thesis.
5
Bibliography
Bird, M., and J. Cali, A million-year record of fire in sub-saharan africa, Nature,
394, 767 –769, 1998.
Bird, M., C. Moyo, E. Veenedaal, J. Lloyd, and P. Frost, Stability of elemental carbon
in savanna soil, Global Biogeochemical Cycles, 13, 923 – 932, 1999.
Crutzen, P., and M. Andreae, Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles, Science, 250(4988), 1669 –
1678, 1990.
Currie, L., et al., A critical evaluation of interlaboratory data on total, elemental, and
isotopic carbon in the carbonaceous particle reference material, nist srm 1649a,
Journal of Research of the National Institute of Standards and Technology, 107,
279 – 298, 2002.
Flanner, M. G., C. S. Zender, P. G. Hess, N. M. Mahowald, T. H. Painter, V. Ramanathan, and P. J. Rasch, Springtime warming and reduced snow cover from
carbonaceous particles, Atmospheric Chemistry and Physics Discussions, 8(6),
19,819–19,859, 2008.
Hammes, K., et al., Comparison of quantification methods to measure fire-derived
(black/elemental) carbon in soils and sediments using reference materials from
soil, water, sediment and the atmosphere, Global Biogeochem. Cycles, 21(3),
18, doi:10.1029/2006GB002914, 2007.
Herring, J., Charcoal fluxes into sediments of the north pacific ocean: the cenozoic
record of burning, in The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present., edited by E. Sunquist and W. Broecker, pp. 419 –
442, AGU, Washington, 1985.
6
Killops, S., and V. Killops, An Introduction to Organic Geochemistry, 228 pp., John
Wiley and Sons, 1992.
Kuhlbusch, T., Black carbon and the carbon cycle, Science, 280, 1903 –1904,
1998.
Masiello, C., New directions in black carbon organic geochemistry, Marine Chemistry, 92, 201–213, 2004.
Masiello, C., and E. Druffel, Black Carbon in Deep-Sea Sediments, Science,
280(5371), 1911–1913, doi:10.1126/science.280.5371.1911, 1998.
Middelburg, J., J. Nieuwenhuize, and P. van Breugel, Black carbon in marine sediments, Marine Chemistry, 65, 245 –252, 1999.
Preston, C., and M. Schmidt, Black (pyrogenic) carbon: a synthesis of current
knowledge and uncertainties with special consideration of boreal regions, Biogeoscience, 3, 397 – 420, 2006.
Ramanathan, V., and G. Carmichael, Global and regional climate changes due to
black carbon, Nature Geoscience, 1(4), 221, doi:doi:10.1038/ngeo156, 2008.
Schimdt, M., and A. Noack, Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges, Global Biogeochem. Cycles, 14(3),
777 – 793, 2000.
Schmidt, M., J. Skjemstad, C. Czimczik, B. Glaser, K. Prentice, Y. Gelinas, and
T. Kuhlbusch, Comparative analysis of black carbon in soils, Global Biogeochemical Cycles, 15, 163 – 167, 2001.
Seiler, W., and P. Crutzen, Estimates of gross and net fluxes of carbon between
the biosphere and the atmosphere from biomass burning, Climate Change, 2,
207 –247, 1980.
7
Venkatesan, M., Tetrahymanol, its widespread occurrence and geochemical significance, Geochim. Cosmochim. Acta, 53, 3095 – 3101, 1989.
Winkler, M., Charcoal analysis for paleoenvironmental interpretation, a chemical
assay, Quaternary Research, 23, 313 – 326, 1985.
8
2
Oxidation of polycyclic aromatic hydrocarbons
and natural materials using the benzene
polycarboxylic acid method
2.1
Abstract
Quantification of black carbon (BC), carbonaceous material of pyrogenic origin, has
typically required either chemical or thermal oxidation methods for isolation from
heterogeneous matrices, such as sediment or soil. The benzene polycarboxylic
acid (BPCA) method involves chemical oxidation of aromatic structures, such as
those in BC, into BPCAs. Using a revised BPCA method, we studied the oxidation of nine polycyclic aromatic hydrocarbons (PAHs). After 8 hours of oxidation
at 180 o C, the average carbon yield was 25.7 ± 6.8 % C and was not correlated
to the molecular weight of the PAH oxidized. The majority of the BPCAs observed
were nitrated, which has serious implications for the quantification of BC. Smaller
PAHs favor the formation of less substituted BPCAs, while larger PAHs, such as
coronene, favor the formation of more fully substituted BPCAs. Time course experiments revealed variations of BPCA distributions over time, favoring less substituted
BPCAs with longer oxidation times, while the carbon yield remained constant. No
9
decarboxylation of fully substituted mellitic acid (B6CA) was observed during the
time course experiments. Using the model compound anthracene, a potential internal standard, we proposed a mechanism for the oxidation reaction based on
time course experiment data. Quantification of BC in reference materials revealed
that this revision of the BPCA method is significantly more efficient than previous
versions and is effective for quantifying soot BC.
2.2
Introduction
Black carbon (BC) particles are by-products of combustion processes that can be
defined by a broad range of characteristics. Sizes range from nanometer soot particles to millimeter pieces of charcoal. Chemically, BC has a condensed, highly
aromatic structure (Figure 2.1). Environmental scientists are interested in isolating
BC from organic matrices, such as soils and sediments, to address questions regarding the time scales of carbon storage in the Earth system. The wide range
of physical and chemical characteristics of both the BC and the heterogenous
matrices in which it is found pose challenges when isolating and quantifying BC
{Masiello, 2004}. Various techniques are used to quantify BC isolated from environmental matrices including thermal oxidation {Gustafsson et al., 1997}, chemical
oxidation (nitric acid {Glaser et al., 1998; Brodowski et al., 2005} and acid dichromate {Wolbach and Anders, 1989}) and mild chemical or photo-oxidation (NaClO
{Simpson and Hatcher, 2004} and ultra-violet {Skemstad et al., 1996}) followed by
NMR.
The benzene polycarboxylic acid (BPCA) method {Glaser et al., 1998; Brodowski
et al., 2005} converts BC to benzene rings that are substituted with various numbers (2-6) of carboxylic acid groups. Assuming that BC is primarily aromatic carbon
10
O
C
OH
O
O
O
O
O
O
O
C=O
CH2
O
CH2
O
CH2
CH2
O
CH2
O
O
O
O
HO
O
HO-CH2
O
O
O
O
O
O
C
O
O
CH3
O
O
C=O
O
O
O
OH
H
H
C
C
O
C
C
(CH2)13
CH3
O
O
CH C
O
O
OH
O
Figure 2.1: Theoretical black carbon type of molecule Goldberg {1985}. Highlighted are a number of PAHs used in this study to mimic edge functionalities of the
theoretical BC structure.
and additional carbon is neither added nor exchanged, the method provides both
yield and structural information about the BC. Nitric acid oxidizes the BC structure to BPCAs in which a single aromatic ring from the BC is maintained and is
substituted with carboxylic acids derived from adjacent rings or side chains. This
method retains the carbon from the molecular structure, which is essential for subsequent isotopic abundance assays such as compound specific radiocarbon analysis {Eglinton et al., 1996}. While nitric acid oxidation of BC results in a significant
loss of carbon (roughly 25 % is retained), it provides both qualitative and quantitative information about the original BC with negligible methodological artifacts.
A recent intercomparison of BC analysis methods {Hammes et al., 2007} revealed
that the previous version of the BPCA method {Glaser et al., 1998; Brodowski
et al., 2005}, which silylated rather than methylated the BPCAs, was more efficient
for the analysis of char-like BC than soot-like BC. Char-like BC is less condensed,
contains more functional side chains and forms BPCAs that are less substituted.
11
Soot-like BC is highly condensed with few additional side chains and forms BPCAs
with a higher degree of substitution. Generally, the chemical structure of BC is unknown in environmental matrices, such as marine sediment. Therefore, a method
that provides both quantitative and qualitative data is needed.
To understand the distribution and yield of BPCA oxidation products, we studied
the oxidation of PAHs to model edge functionalities of BC (Figure 2.1). This is the
first study to systematically examine the oxidation products of the BPCA method
and suggest an oxidation mechanism. Polycyclic aromatic hydrocarbons (PAHs) of
various sizes and structures were digested and the resulting products were quantified. A previous version of this method {Glaser et al., 1998; Brodowski et al.,
2005}, using GC-FID analysis of silylated BPCAs, was not as efficient for quantifying all BPCAs and required the use of response factors. Also, previous methods
did not quantify nitrated BPCAs. Here we present results of experiments that quantified the BPCAs and nitrated-BPCAs as methyl esters. No response factors were
used, as all BPCAs were methylated with the same efficiency. We examined the
reaction kinetics by evaluating the reaction products as a function of time. Using
commercially available BPCAs, we also address the possibility of decarboxylation
of BPCAs during the oxidation and subsequent steps. Furthermore, we suggest a
mechanism of oxidation for the PAH anthracene. Finally, using this revised method
the BC in reference materials was quantified. Compared to previous versions of
the BPCA method, this method produced higher estimates of BC and was able to
quantify soot BC.
12
2.3
2.3.1
Methods
Sample treatment
All glassware and quartz filters that came in contact with the samples and standards were baked at 550 o C for 2 hours prior to use in order to minimized carbon
contamination. Individual PAHs (Figure 2.2), varying in amounts from 2 to 10 mg C
were weighed into 12 mL quartz digestion tubes. Two mL concentrated nitric acid
(grade ACS) were added to each tube, then were capped and heated to 180 o C in
a high pressure digestion apparatus {Schramel et al., 1980}. Briefly, the digestion
apparatus consisted of an aluminum block with holes to fit up to six teflon sleeves
and quartz digestion tubes and caps. The block was mechanically clamped closed
to secure the quartz tubes within the teflon sleeves and then placed into an oven
at 180 o C for 0.5 to 16 hours. Post digestion, the samples were filtered through
quartz fiber filters (27 mm diameter, 0.8 µm pore diameter) and rinsed with 15 mL
Milli-Q water that was generated immediately prior to use. The filtrate was then
freeze dried.
Dried samples were redissolved in 5 mL methanol and the internal standard,
biphenyl-2,2’-dicarboxylic acid (1 mg mL-1 in methanol) was added. Samples were
derivatized by titration with 2.0 M trimethylsilyl diazomethane in ethyl ether (Sigma
Aldrich). Derivatization was considered complete when the solution retained the
yellow color of the trimethylsil-diazomethane.
The derivatized oxidation products were blown dry under ultrahigh purity nitrogen
and re-dissolved in methylene chloride, and subsequently separated and quantified on a Hewlett Packard 6890 gas chromatograph outfitted with a Gerstel cooled
injection system and a DB-XLB capillary column (30 m x 0.53 mm I.D., 1.5 µm film
13
8
7
6
9
2
3
4
10
5
8
1
7
(a) Anthracene
8
1
6
10
9
10
9
2
4
5
7
3
6
1
2
4
5
(b) Phenanthrene
3
(c) Retene
NO2
9
8
1
12
11
4
10
7
6
9
2
12
7
5
11
1
12
9
9
8
7
6
4
5
(g) Perylene
8
7
8
7
6
11
5
4
12
1
2
3
10
3
4
9
4
5
(h) Benzo-ghi-perylene
Figure 2.2: The PAHs used in this study.
14
6
(f) 1-Nitropyrene
2
10
2
3
4
(e) Pyrene
2
3
10
6
1
10
9
8
5
1
2
3
3
(d) Chrysene
11
1
10
8
7
6
5
(i) Coronene
thickness) and a flame ionization detector (FID). After injection, the column temperature was maintained at 100 o C for 1 minute, then raised at 25 o C min-1 to 250
o
C followed by a 5 o C min-1 ramp to a final temperature of 280 o C. The column was
held at the final temperature for 10 minutes. The detector temperature was 300 o C.
The split-less injection volume was between 1 and 3 µL.
Compound verification was performed using a Finnigan Trace MS+ GC/MS system
operating in electron ion (EI) mode. The GC was equipped with a J&W Scientific
DB-5 capillary column (30 m x 0.32 mm I.D., 0.25 µm film thickness). Helium was
used as the carrier gas. The temperature program used was 50 o C ramping at 10
o
C min-1 to a final temperature of 290 o C. The injector temperature was 250 o C.
BPCAs were identified by comparison of their retention times with those obtained
for a commercially available mixture and were verified using the GC/MS. All methylated BPCAs were quantified relative to the biphyenl-2,2’-dicarboxylic acid internal
standard. Unlike previous BPCA studies {Glaser et al., 1998; Brodowski et al.,
2005}, response factors were not required to correct for incomplete derivatization.
All methylated BPCAs exhibited the same response factor and BPCA calibration
curves were calculated relative to the internal standard peak area. Except where
otherwise noted, samples were processed and analyzed in triplicate. Detection of
BPCAs was limited to 10 ng BPCA per injection.
15
COOH
COOH
COOH
COOH
HOOC
COOH
COOH
(a) phthalic acid
COOH
(b) hemimellitic acid
(c) trimellitic acid
COOH
HOOC
COOH
COOH
HOOC
COOH
COOH
COOH
COOH
COOH
COOH
(d) trimesic acid
(e) prehnitic acid
(f) mellophanic acid
COOH
COOH
HOOC
COOH
HOOC
COOH
HOOC
COOH
HOOC
COOH
(g) pyromellitic acid
(h) benzene pentacarboxylic
acid
HOOC
COOH
HOOC
COOH
COOH
(i) mellitic acid
Figure 2.3: Structures of benzene polycarboxylic acids used as markers of aromatic carbon in this study.
16
2.4
2.4.1
Results and Discussion
Nitration of BPCAs
Previous BPCA studies {Glaser et al., 1998; Brodowski et al., 2005} quantified only
the non-nitrated BPCAs produced during oxidation and did not consider whether
the oxidation products of the BC also included significant amounts of nitrated BPCAs. Early studies of the organic chemistry of electrophilic substitution found that
nitration of PAHs was important {Dewar and Mole, 1956; Watts, 1873}. We have
found that the majority of the BPCAs produced from all PAHs studied were substituted with at least one nitro-group (-NO2 ). Mono- and di-nitrated B2CAs were
observed. All other BPCAs (B3CA, B4CA and B5CA) were mono-nitrated. Both
3-nitrophthalic and 4-nitrophthalic acid are commercially available and were used
for calibration of the nitrated B2CAs. Larger nitrated BPCAs were not commercially available. However, since the calibration curves for 3-nitrophthalic and 4nitrophthalic acid were the same as that for phthalic acid, we applied the nonnitrated calibration curves to the larger BPCAs (e.g.: the calibration curve for B3CA
was applied to all nitrated B3CA isomers). The BPCA distribution and carbon yields
discussed below include nitrated BPCAs.
2.4.2
Carbon yields
For each oxidized and derivatized PAH, the BPCAs were quantified and a carbon
yield was calculated by comparing the sum of BPCA carbon to the initial carbon
(see Appendix A for an example). For all nine PAHs analyzed in this study, the
average carbon yield was 25.7 ± 6.8 % C, with values for individual PAHs ranging
17
18
Anthracene
Phenanthrene
Retene
Chrysene
Pyrene
1-Nitropyrene
Perylene
Benzo-ghi-perylene
Coronene
# of C
14
14
18
18
16
16
20
22
24
%C
94.4
94.4
92.3
94.7
95
77.4
95.2
95.5
96
% C recovered
24.2 ± 1.6
23.7 ± n.a.
29.3 ± 1.4
21.5 ± 2.3
37.0 ± 7.2
36.1 ± 9.3
22.5 ± 1.3
22.5 ± 3.9
19.5 ± 2.2
B2CA
100.0 ± 0.0
72.1 ± 2.1
1.3 ± 2.2
64.4 ± 2.5
4.0 ± 3.5
-
B3CA
62.4 ± 0.8
18.8 ± 3.5
17.9 ± 3.5
80.3 ± 1.7
6.9 ± 0.4
-
B4CA
27.9 ± 2.1
36.4 ± 1.7
35.6 ± 2.5
79.3 ± 5.2
78.1 ± 5.4
53.1 ± 1.2
67.4 ± 2.4
B5CA
-
B6CA
19.7 ± 1.7
40.0 ± 1.5
32.6 ± 2.4
Table 2.1: Carbon yield and percent BPCA distribution of the nine PAHs oxidized in this study. All samples were oxidized
for 8 hours. ± is the standard deviation of three replicates. - indicates no BPCAs were detected.
from 19.5 ± 2.2 to 37.0 ± 7.2 % C (Table 2.1). The two smallest PAHs studied,
anthracene and phenanthrene, exhibited carbon yields of 24.2 ± 1.6 % and 23.7 %
(n=1), respectively. Retene, a three ring PAH with two small side chains, exhibited
a significantly higher carbon yield (29.3 ± 1.4 %). Chrysene, a four ring PAH with
a structure similar to phenanthrene, had a carbon yield of 21.5 ± 2.3 %. Fourring pyrene and 1-nitropyrene had the highest measured carbon yields, 37.0 ± 7.2
% and 36.1 ± 9.3 % respectively. The five ring perylene and six ring benzo-ghiperylene had the same carbon yield (22.5 ± 1.3 % and 22.5 ± 3.9 % respectively).
The largest PAH studied, seven ring coronene, had the lowest carbon yield of 19.5
± 2.2 %. No significant correlations were found between the carbon yield and the
number of aromatic rings or the percentage of carbon in the PAH; thus we are not
able to draw any conclusions how the type or size of PAH oxidized is related to the
carbon yield.
The BPCA method requires the use of a conversion factor to convert the BPCAs
formed into an estimate of BC mass. Previously activated charcoal was used to
determine the conversion factor {Glaser et al., 1998; Brodowski et al., 2005}. Since
the composition and character of this material may vary between production lots of
activated charcoal, the distribution and yield of BPCAs from this material may vary.
We recommend using materials of known chemical formulas, such as PAHs, to
calibrate the BPCA method. Our results from the oxidation of PAHs suggest using
the average carbon yield of 25.7 ± 6.8 % C to calculate the BC mass in samples.
2.4.3
BPCA products of PAHs
The BPCA method oxidizes condensed aromatic structures to produce single benzene rings with carboxylic acid functional groups derived from adjacent aromatic
19
COOH
A’
A
B
HNO3
180oC
COOH
A
A’
or
COOH
COOH
HOOC
HOOC
B
or
HOOC
COOH
Figure 2.4: Schematic of the oxidation products of phenanthrene using the BPCA
method. One molecule of phenanthrene theoretically could produce either one
molecule B2CA (A or A’) or one molecule B4CA (B). Using the observed distribution of BPCAs produced and assuming no losses occurred during oxidation, the
theoretical carbon yield would be 57 %.
rings or side chains. For example, when phenanthrene is oxidized (Figure 2.4) only
two BPCAs are produced: phthalic acid (B2CA) and benzene-1,2,3,4-tetracarboxylic
acid (B4CA). If the method does not oxidize the aromatic structure preferentially
(regioselective), we would expect two molecules of B2CAs to form for every one
molecule of B4CA, based on the fact that one phenanthrene consists of two outer
rings suited to become B2CAs and one central ring suited to become B4CA. Indeed, oxidized phenanthrene preferentially formed B2CA (72.1 ± 2.1 % of the
carbon recovered) and the remainder was B4CAs (27.9 ± 2.1 %). Oxidized anthracene also is expected to yield 66.7 % B2CA and 33.3 % B4CA, however, it
produced exclusively B2CAs. The four-ring PAH chrysene is expected to yield
equal proportions of B2CA and B4CA, however it yielded 64.4 ± 2.5 % B2CA and
35.6 ± 2.5 % B4CA. Anthracene and chrysene also preferentially formed smaller
BPCAs than expected.
The oxidation of retene yielded the expected distribution of BPCAs (2 B3CAs:1
B4CA, Table 2.1). Carbon on each side chain was oxidized to a carboxylic acid.
This result suggests that aliphatic side chains of BC could be oxidized to carboxylic
acids. Theoretically, perylene would yield four B3CAs for every one B6CA and
indeed, we found 80.3 ± 1.7 % B3CAs and 19.7 ± 1.7 % B6CA formed. Previously,
20
perylene oxidation was reported {Glaser et al., 1998} to yield 75 % B3CAs and 25
% B6CAs, similar to ours. In contrast, Dittmar {2008} reported 19 % B3CA and 81
% B6CA from oxidized perylene, however his quantification methods (microwaveassisted oxidation and HPLC quantification) were different from our study. This
study did not use microwave-assisted oxidation and BPCAs were quantified as
methyl-esters by GC rather than carboxylic acids via HPLC.
Other PAHs used in this study formed more of the larger BPCAs than expected.
Pyrene formed 18.8 ± 3.5 % B3CA and 79.3 ± 5.2 % B4CA instead of the expected equal distribution. The oxidation products of nitrated pyrene, 1-nitropyrene,
was not significantly different from the non-nitrated compound, except that a small
percentage (4.0 ± 3.5 %) of B2CA was produced. The observed B2CA was a mixture of two isomers of dinitro-B2CA. Benzo-ghi-perylene was expected to form 33
% B3CA, 50 % B4CA and 17 % B6CA; instead it formed 6.9 ± 0.4 % B3CA, 53.1
± 1.2 % B4CA and 40.0 ± 1.5 % B6CA. The proportion of B4CAs formed was as
expected, however more B6CAs and less B3CAs were measured than expected.
The largest PAH studied, coronene, was expected to form 86 % B4CA and 14 %
B6CA. Instead, coronene produced 67.4 ± 2.4 % B4CA and 23.6 ± 2.4 % B6CA,
again more B6CAs than expected. These results show that larger PAHs generally
formed larger BPCAs than predicted.
These BPCA distribution data illustrates the complexity of the oxidation reaction.
Since there seems to be no systematic pattern of oxidation, we cannot accurately
model the oxidation products. Nor can we, without additional data, reconstruct the
original structure of the BC using the BPCA distribution. However, these distribution data are useful when drawing qualitative distinctions between different types
of BC {Ziolkowski and Druffel, 2009}. It is possible to distinguish between material
with aliphatic side-chains and fully condensed BC material.
21
Table 2.2: Carbon yields for time course experiments of two PAHs (anthracene
and perylene) and mellitic acid (B6CA). All samples were oxidized at 180 o C for
the time listed. ± is the standard deviation of three replicates. - indicates time
points that were not studied.
oxidation time
(hours)
0.5
1
2
4
8
16
2.4.4
% C recovered
Anthracene
Perylene
Mellitic acid
16.5 ± 8.5
11.7 ± 3.4
95.9 ± 7.7
18.6 ± 1.7
20.4 ± 0.1
97.4 ± 13
20.5 ± 0.7
24.0 ± 1.1
97.3 ± 5.7
23.6 ± 0.2
22.6 ± 1.3 104.7 ± 1.0
18.9 ± 3.1
19.9 ± 3.4 95.2 ± n.a.
Time course and mechanistic experiments
To evaluate the optimal time of the high pressure and high temperature oxidation of
PAHs to BPCAs, we conducted time course experiments. Anthracene was chosen
as a model compound for time course and mechanistic experiments because it was
being evaluated for use as an internal standard. First, we evaluated the evolution of
BPCAs from anthracene by conducting the high temperature nitric acid oxidation
from 0.5 to 16 hours. The carbon yield of BPCAs increased from 1 to 8 hours
(Table 2.2). The 16 hour oxidations did not yield significantly different amounts or
distributions of BPCAs than 8 hour oxidations, although the standard deviations at
16 hours was much larger than the previous three time points. Therefore oxidation
of at least 8 hours were optimal, as shorter oxidations gave lower carbon yields and
longer oxidations a greater variability of carbon yield. We also examined the degree
of nitration of the B2CAs formed from anthracene as a function of time (Figure
2.5). With oxidations of 0.5 and 1.0 hour the B2CA formed were exclusively dinitroB2CA. Between the one and two hour oxidations, the carbon yield increased and
the quantity of dinitro-B2CA produced decreased significantly and was replaced by
B2CA and mono-nitro-B2CA (Figure 2.5).
22
100%
% of total
80%
B2CA
mononitro-B2CA
dinitro-B2CA
C yield
60%
40%
20%
0%
0.5
1
2
4
8
duration of oxidation (hours)
16
Figure 2.5: Distribution of non-nitrated, mono-nitrated and di-nitrated dicarboxylic
acid formed and carbon yield from oxidation of anthracene as a function of oxidation time.
NO2 O
O
HNO3
180oC
w
slo
fas
t
O
O
NO2
O
NO2
COOH
COOH
+
NO2
O
COOH
COOH
+
NO2
O2N
COOH
COOH
NO2
Figure 2.6: Proposed reaction schematic for the high pressure, high temperature
oxidation of anthracene to B2CA. The initially formed products undergo thermodynamic equilibration to primarily mononitro-B2CA (see Figure 2.7).
23
4-nitrophthalic acid,
16hrs
3-nitrophthalic acid,
16hrs
B2CA
mononitro-B2CA
dinitro-B2CA
phthalic acid, 16 hrs
3,5-dinitrophthalic acid,
16 hours
anthracene, 45min
0%
10%
20%
30%
40%
50%
60%
70%
80%
90% 100%
% of total B2CA measured
Figure 2.7: Distribution of non-, mono- and di-nitrophthalic acid as a function of
reactants. Four forms of phthalic acid were oxidized for 16 hours: phthalic acid,
3-nitrophthalic acid, 4-nitrophthalic acid and 3,5-dinitrophthalic acid. Regardless
of the starting materials 3-nitrophthalic acid is the most abundant product after 16
hours. This experiment was conducted in duplicate and the difference between
duplicates was ≤ 5 %.
24
These results suggest that the oxidation mechanism of anthracene is a multistep
process with dinitrophthalic acid as the initial product (≤ 1 hr). With increased oxidation time (8 hr) the B2CA reaches an “equilibrium” state as nitro-phthalic acid.
When the shortest oxidations of anthracene were filtered, a solid remained, identified by GC/MS and NMR as 100 % anthraquinone. Oxidation of anthracene in nitric acid, under milder conditions, has been found to produce anthraquinone {Cho,
1995}. We hypothesize that the anthraquinone generated was then nitrated to dinitroanthraquinone before being oxidized to dinitrophthalic acid (Figure 2.6) which
was then denitrated to mononitro-B2CA, the thermodynamic sink under these conditions.
We tested this hypothesis by oxidizing four forms of B2CA: phthalic acid, 3-nitrophthalic acid, 4-nitrophthalic acid and 3,5-dinitrophthalic acid for 16 hours. With
the exception of 4-nitrophthalic acid, we found that regardless of the form of phthalic acid we oxidized, the primary product was 3-nitro-phthalic acid (Figure 2.7),
supporting our hypothesis. Furthermore, after 16 hours of oxidation 4-nitrophthalic
acid yielded 85 % mononitro-B2CA and 15 % dinitro-B2CA and more than half of
the mononitro-B2CA was 3-nitrophthalic acid. This demonstrated the conversion of
4-nitrophthalic acid to the more stable 3-nitrophthalic acid. These results confirm
the importance of quantifying nitrated BPCAs, because nitration occurs before the
formation of BPCAs and can comprise a significant portion of the products.
Perylene was also studied in a time course experiment. Although the carbon yield
was relatively constant over the course of the these experiments at 21.7 ± 1.9 %
(Table 2.2), the distribution of BPCAs changed significantly as a function of oxidation time (Figure 2.8). The shortest perylene oxidation (2 hours) yielded more
B6CA than the 16 hour oxidation (23.3 ± 0.5 % B6CA versus 14.4 ± 0.3 %). The
mononitro-B3CA was predominantly 4-nitro-1,2,3-benzenetricarboxylic acid and a
25
50
80
40
60
30
40
20
20
10
0
B3CA
B6CA
C yield
% carbon yield (triangles)
% of total BPCAs observed (circles)
100
0
0
5
10
oxidation duration (hours)
15
20
Figure 2.8: Change in B3CA (filled circles) and B6CA (open circles) oxidation
products (% of total BPCAs observed) and carbon yield (filled triangles) from perylene over time (% carbon yield). Error bars represent the standard deviation of
three replicates.
26
small proportion was 5-nitro-1,2,3-benzenetricarboyxlic acid (≤ 10 %). Relative distributions of these two acids did not vary with increased oxidation time (up to 16 hr).
Dewar and Mole {1956} reported that perylene nitrates at the #3 position, which is
consistent with our observation that 4-nitro-1,2,3-benzenetricarboyxlic acid is the
dominant B3CA formed. This provides further confirmation that nitration occurs
before the break-up of the PAHs. The decrease in B6CA with oxidation time may
indicate that decarboxylation may take place with longer oxidation times. It is important to note that, while the ratio of B3CA to B6CA doubled between 2 and 16
hours, the carbon yield did not significantly change. Thus if decarboyxlation is
occurring, in this case it did not change the carbon yield. At no point during the
perylene time course experiments did we observe equivalent amounts of oxidation
products, as reported when microwave assisted oxidation was employed {Dittmar,
2008}. In the future, it is important for users of the BPCA to quantify nitrated BPCAs and calibrate the oxidation performed in each lab. Comparing the ratio of
smaller to larger BPCAs (including nitrated ones) of a known compound, such as
a PAH, should be used to calibrate the method for inter-lab comparisons of BPCA
distributions.
To test for decarboxylation, we oxidized commercially available mellitic acid (B6CA)
at 180 o C for 1 to 16 hours (Table 2.2). The average recovery of mellitic acid for
all time points was 98.1 ± 3.8 %. For all time points except 8 hours, the amount
of mellitic acid remaining after oxidation was between 95 and 97 %. The 8 hour
oxidation yielded 104.7 ± 1.0 % of the initial carbon. These results demonstrate
that mellitic acid is not decarboxylated over the course of the oxidation.
The time course results demonstrate that oxidations conducted for 4 to 16 hours
show no change in the carbon yield, whereas the relative distribution of BPCAs
changes as a function of oxidation time. Since the anthracene carbon yield (Table
27
2.2) and nitration (Figure 2.5) continued to evolve from 4 to 8 hours of oxidation
time, we elected to conduct all further oxidations at 8 hours.
2.4.5
Analysis of black carbon ring trial materials
An analytical challenge in the analysis of black carbon is its wide variety of chemical
and physical characteristics. Many methods of BC quantification focus on particular components of BC (i.e. soot or char). Recently, an inter-comparison of BC
quantification methods for BC rich materials {Hammes et al., 2007} revealed that
the previous version of the BPCA method were more well suited for the analysis of
char than for soot BC. Additionally, the inter-comparison revealed that the conversion factor of BPCAs to BC was not easily reproducible.
Since many modifications of the BPCA method were made in this work, we analyzed a suite of BC rich materials to contextualize this version of the BPCA method
(Table 2.3, Figure 2.9). The PAH carbon yield data reported here were combined
with carbon yield data of soot-like BC materials in another study (Ziolkowski and
Druffel {2009}, Chapter 3) to generate a robust conversion factor, for converting
BPCAs to BC, of 4 ± 1 or the inverse of 25 ± 6 %. A wide range of BC materials
(e.g.: carbon nanotubes, soot, char, PAHs, etc) were used to generate this conversion factor. This is much higher than the 2.27 conversion factor reported in the
original BPCA study {Glaser et al., 1998}, but lower than the highest BPCA conversion factor measured (4.5) by Brodowski et al. {2005}. Activated charcoal, which
was used to generate previous BPCA conversion factors, was not used as in the
determination of the conversion factor reported here. Although, carbon yields of
oxidized activated charcoal (23.5 ± 1.0 % C yield, n=3) agree with the conversion
factor reported here.
28
29
Table 2.3: Quantification of black carbon materials (g BC / kg dry weight) by this and other methods. Data for CTO-375,
BPCA, C2 O7 and TOR/TOT are from Hammes, 2007 Hammes et al. {2007}. The quanty of BC in this work was converted
from all BPCAs (including those nitrated) to BC using a conversion factor of 4 (the inverse of 25 %). A dash indicates that
no data was reported. The uncertainty, s, is the propagated error of the BPCA to BC conversion factor (25 ± 6 %) or the
standard deviation between replicates, which ever is larger.
BPCA, this work
CTO-375
BPCA
C 2 O7
TOR/TOT
mean
s mean
s mean
s mean
s mean
s
aerosol
32.7
8.2
14.9 7.0
14.5
4.5
63.9
20.8
66.5
20.4
marine sediment
10.7
5.1 1.4
1.7
11.8
10.9
8.1
1.5
6.6
6.5
IHSS NOM
63.0
1.1 1.9
20.5
15.8
25.1
- 162.3 135.1
hexane soot
945.5
236.4 410.0 8.3 239.5 206.9 469.9
97.8 887.6
24.3
wood char
478.4
119.7
- 183.2
96.3 524.4 106.7 652.7
93.5
grass char
488.0
122.0
9.0 7.2 154.6
18.3 205.8
49.4 478.0
76.0
Figure 2.9: BPCA distribution of materials used in the black carbon ring trial.
Table 2.4: WoodPchar and hexane soot BC yield data (g BC / kg dry wt) using various
P scenarios. nBPCA is the summation of all BPCAs, including those nitrated.
BPCAs is the summation of all non-nitrated BPCAs. BPCA (last column) is the
data reported in the BC ring trial Hammes
P et al. {2007}
P summing the non-nitrated
BPCAs. For the work presented here ( nBPCA and BPCA), the uncertainty, s,
is the propagated error of the BPCA to BC conversion factor (25 ± 6 %).
BPCA summation
conversion factor
wood char
hexane soot
P
nBPCA
4.09
mean
s
478.4 119.6
945.5 236.5
P
P
BPCA
BPCA
4.09
2.27
mean
s mean
s
385.2
96.3 213.8 53.4
616.8 154.2 334.0 83.5
30
BPCA
2.27
mean
s
183.2
96.3
239.5 206.9
For all materials assessed, our BC yields were higher than both the CTO-375
and original BPCA method. Our results are similar to those generated using the
chromic acid oxidation method (C2 O7 ) for charred materials, while soot materials
are closer to the thermal optical method (TOR/TOT). The increased BC yield using
this version of the BPCA method is due to various factors, most like to the quantification of nitrated BPCAs and possibly in small part to the derivatization method.
For wood char, if the 2.27 conversion factor was applied to the sum of non-nitrated
BPCAs, the BC yield is lowered from 478 g/kg to 213 g/kg, much closer to the
previous BPCA methods of the amount of BC in wood char (Table 2.4). The BC
yield for hexane soot using only the non-nitrated BPCAs were converted to BC
with a 2.27 conversion factor is 334 g/kg and is still significantly higher than the
previous BPCA estimates for hexane soot. Thus, it appears that the BPCA method
presented here is not biased for char and can equally quantify char and soot BC.
After the conversion factor and quantification of nitrated peaks are accounted for,
the BC yield is still greater than previous versions of this method, likely due to the
increased oxidation temperature and derivatization method.
2.5
Conclusions
Using a revised BPCA method we have shown that oxidation of nine PAHs results
in nitrated BPCAs. On average, 25.7 % of the PAH carbon was recovered as BPCAs (including nitrated BPCAs). Although the number of acid groups is related to
the original structure, the distribution of oxidation products does not systematically
correlate with the structure of the original PAH. More highly substituted BPCAs are
preferentially formed from larger PAHs. Time course experiments revealed that the
ratio of oxidation products changed over time, favoring smaller BPCAs with longer
31
oxidation times, while the carbon yield did not change. We also found that quantifying the nitrated BPCAs is essential as the PAHs were nitrated before they were
oxidized. Future work with the BPCA method should assess the degree of nitrated
BPCAs when oxidizing BC in environmental samples, as it may provide further information about the oxidation process. Measurements of BC in reference materials
reveal that this version of the BPCA method is overall more efficient at quantifying
BC and no longer is biased against the quantification of soot BC. The increased
efficiency is a function of the oxidation conversion factor, quantification of nitrated
peaks, derivatization method and increased temperature of oxidation of BPCAs to
BC.
Acknowledgements
The authors would like to thank Sheila Griffin and Claudia Czimczik for their help
with this work. We acknowledge funding from the NSF Chemical Oceanography
Program.
Bibliography
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black carbon assessment using benzene polycarboxylic acids, Organic Geochemistry, 36(9), 1299–1310, 2005.
Cho, B., Recent progress in the synthesis of nitroarenes. A review., Organic Preparations and Procedures Int., 27(3), 243 – 272, 1995.
32
Dewar, M., and T. Mole, Electrophilic substitution. Part II. The nitration of napthalene and perylene., Journal of the Chemical Society, pp. 1441 – 1443, 1956.
Dittmar, T., The molecular level determination of black carbon in marine dissolved
organic matter, Organic Geochemistry, 39(4), 396–407, 2008.
Eglinton, T., L. Aluwihare, J. Bauer, E. Druffel, and A. McNichol, Gas chromatographic isolation of individual compounds from complex matrices for radiocarbon
dating, Analytical Chemistry, 68(5), 904–912, 1996.
Glaser, B., L. Haumaier, G. Guggenberger, and W. Zech, Black carbon in soils:
the use of benzenecarboxylic acids as specific markers, Organic Geochemistry,
29(4), 811–819, 1998.
Goldberg, E. D., Black carbon in the Environment, Wiley, 1985.
Gustafsson, Ö., F. Haghseta, C. Chan, J. Macfarlane, and P. Gschwend, Quantification of the dilute sedimentary soot phase: Implications for PAH speciation and
bioavailability, Environmental Science and Technology, (31), 203 – 209, 1997.
Hammes, K., et al., Comparison of quantification methods to measure fire-derived
(black/elemental) carbon in soils and sediments using reference materials from
soil, water, sediment and the atmosphere, Global Biogeochemical Cycles, 21(3),
18, 2007.
Masiello, C. A., New directions in black carbon organic geochemistry, Marine
Chemistry, pp. 201 – 213, 2004.
Schramel, P., A. Wolf, R. Seif, and B. Klose, Eine neue apparatur zur druckveraschung von biologischem material, Fresenius Zeitschrift fur Analytische
Chemie, (302), 62–64, 1980.
33
Simpson, M., and P. Hatcher, Determination of black carbon in natural organic
matter by chemical oxidation and solid-state
13
C nuclear magnetic resonance
spectroscopy, Organic Geochemistry, (358), 923 – 935, 2004.
Skemstad, J., P. Clarke, J. Taylor, J. Oades, and S. McClure, The chemistry and
nature of protected carbon in soil, Australian Journal of Soil Research, (34), 251
– 271, 1996.
Watts, H., A Dictionary of Chemistry and the Allied Branches of Other Sciences,
vol. IV, Longmans, Green and Co., 1873.
Wolbach, W., and E. Anders, Elemental carbon in sediments: Determination and
isotopic analysis in presence of kerogen, Geochimica Cosmochimica Acta, (53),
1637 – 1647, 1989.
Ziolkowski, L., and E. Druffel, The feasibility of isolation and detection of fullerenes
and carbon nanotubes using the benzene polycarboxylic acid method, Marine
Pollution Bulletin, 59, 213 – 218, 2009.
34
3
The feasibility of isolation and detection of
fullerenes and carbon nanotubes using the
benzene polycarboxylic acid method
Presented here with minor editing:
Ziolkowski and Druffel (2009), The feasibility of isolation and detection of fullerenes
and carbon nanotubes using the benzene polycarboxylic acid method, Marine Pollution Bulletin, 59 (4-7) 213-218, doi: 10.1016/j.marpolbul.2009.04.018.
3.1
Abstract
The incorporation of fullerenes and carbon nanotubes into electronic, optical and
consumer products will inevitably lead to the presence of these anthropogenic compounds in the environment. To date, there have been few studies isolating these
materials from environmental matrices. Here we report a method commonly used
to quantify black carbon (BC) in soils, the benzene polycarboxylic acid (BPCA)
method, for measurement of two types of single-walled carbon nanotubes (SWCNTs), two types of fullerenes and two forms of soot. The distribution of BC prod35
ucts (BPCAs) from the high pressure and high temperature oxidation illustrates
the condensed nature of these compounds because they form predominantly fully
substituted mellitic acid (B6CA). The conversion of carbon nanoparticles to BPCAs was highest for fullerenes (average of 23.2 ± 4.0 % C recovered for both C60
and C70 ) and lowest for non-functionalized SWCNTs (0.5 ± 0.1 % C). The recovery of SWCNTs was 10 times higher when processed through a cation-exchange
column, indicating the presence of metals in SWCNTs compromises the oxidation
chemistry. While mixtures of SWCNTs, soot and sediment revealed small losses
of black carbon during sample processing, the method is suitable for quantifying
total BC. The BPCA distribution of mixtures did not agree with theoretical mixtures
using model polyaromatic hydrocarbons, suggesting the presence of a matrix effect. Future work is required to quantify different types of black carbon within the
same sample.
3.2
Introduction
As the industrial application for carbon nanotubes (CNTs) and fullerene production
increases, their presence in the environment is an eventuality. While these compounds have much biomedical promise {Bianco et al., 2005}, there is conflicting
eco-toxic evidence {Oberdorster, 2004; Tong et al., 2007} about their impact on
organisms in nature. The analytical methods used to isolate these materials from
environmental matrices are limited, inhibiting our ability to directly quantify these
compounds.
Fullerenes, also known as buckyballs, consists of twelve pentagonal rings surrounded by an appropriate number of aromatic hexagonal rings. C60 , first reported
by Kroto et al. {1985} has 20 hexagon rings and is spherical, while C70 has 25
36
hexagons and an elongated shape. Used in industrial polymer products, such
as thin films, electro-optical devices {Prato, 1999} and drug delivery agents {Bosi
et al., 2003; Bianco et al., 2005}, fullerene production has increased annually.
Numerous techniques have been used to characterize fullerenes, such as mass
spectrometry and UV-Vis spectroscopy {Isaacson et al., 2007; Andrievsky et al.,
2002}, but few methods have successfully isolated fullerenes from environmental
matrices. Carbon nanotubes exhibit different chemical and physical properties depending on the method of production, removal of amorphous carbon and functionalization {Dai, 2002; Niyogi et al., 2002; Plata et al., 2008}. Commercially available
SWCNTs are typically produced on a metal catalyst and can be up to one-third
metal by weight {Plata et al., 2008}. To date, CNTs have been studied mostly by
size exclusion chromatography {Bauer et al., 2007}, electron microscopy {Rasheed
et al., 2007} and chemo-thermal oxidation {Sobek and Bucheli, 2009}.
Recently the benzene polycarboxylic acid (BPCA) method has been employed to
study BC in soils {Glaser et al., 1998; Brodowski et al., 2005}, marine sediment
{Sanchez-Garcia et al., 2007} and marine dissolved organic matter {Dittmar, 2008;
Ziolkowski and Druffel, 2008}. Using a high-temperature and high-pressure oxidation, the BC is chemically oxidized with concentrated nitric acid and converted
to BPCAs. The number of carboxylic acid groups on each BPCA is a function
of the number of aromatic carbons attached to it prior to oxidization (e.g.: Figure 3.1). Currently, the mechanism of this reaction is unknown. Fully substituted
BPCAs (B6CA) are formed from aromatic rings surrounded on all sides by other
aromatic rings, while less substituted BPCAs (i.e. B3CA) are formed from aromatic rings with only two adjacent aromatic rings. Thus by examining the relative
distribution of BPCAs, information about the source BC material may be obtained.
Post-oxidation, if a BPCA distribution is predominantly B6CA the original BC material was likely a condensed aromatic, while BC material that is less condensed and
37
Figure 3.1: Oxidation of the PAH perylene yields two BPCAs: 20 % mellitic acid
(B6CA) and 80 % trimellitic acid (B3CA), suggesting that the oxidation products
are reflective of the original structure of the condensed aromatic material. The
reported percentages are based on mg C recovered.
more oxidized will form fewer B6CAs with a greater proportion of smaller BPCAs.
Since BC and CNTs are similar in their condensed aromatic structure, it is likely
that similar extraction techniques could be used. Here we test the feasibility using the BPCA method to isolate and quantify carbon nanoparticles in the marine
environment. In this paper we present the distribution of BPCAs and the percent
carbon recovered for two fullerenes, two carbon nanotubes and two other carbon
nanoparticles, hexane soot and carbon lampblack. Using mixtures of these carbon
nanopoarticles in marine sediment, we quantify CNTs and evaluate possible matrix
effects.
3.3
Methods
We obtained polycyclic aromatic hydrocarbons fullerenes (C60 and C70 ) and two
single walled carbon nanotubes (SWCNT) from Sigma Aldrich (Figure 3.2). The
first SWCNT was 1 - 2 nm O.D. x 0.5 - 2 µm in length. The second was functionalized (SWCNT-F) with 3-6 % carboxylic acid groups and was 4 - 5 nm O.D. by 0.5
-1.5 µm in length. Hexane soot obtained from D.M. Smith (University of Denver)
was analyzed previously by Akhter et al. {1985} using spectroscopic techniques
and quantified for BC by Hammes et al. {2007}. Commercially available carbon
38
(a) Buckminsterfullerene, C60
(b) Buckminsterfullerene, C70 , after
Mckenzie et al. {1992}
(c) SWCNT, nonfunctionalized
Figure 3.2: Three of the six carbon nanoparticles studied.
lampblack (Fisher) was also analyzed. Perylene (Sigma Aldrich) was used as a
model compound to investigate the oxidation process (Figure 3.1). Marine sediment (NIST SRM 1941b) was used as an environmental matrix for mixed samples.
Two to seven mg carbon were digested in 2 mL 65 % HNO3 at 180o C for 8 hours
(unless otherwise noted) as reported by Glaser et al. {1998}; Brodowski et al.
{2005} and in Chapter 2. During the digestion BC is chemically oxidized to form
BPCAs. The solution was passed though a 0.8 µm pore size quartz fiber filter into
a filtration flask and washed with 30 mL of deionoized water. To remove polyvalent
cations from the filtrate that interfere with sample analysis, a number of samples
received additional treatment. This subset of samples was pretreated with 10 mL
39
of 4 M trifluoroacetic acid for 4 hours at 104 o C. Following nitric acid oxidation and
filtration, samples were passed through a cation column (H+ form, Dowex 50W-X8,
200-400 mesh, packed 18 cm ID x 10 cm high) that was subsequently rinsed with
an additional 30 mL of deionized water and combined with the filtrate. Samples
were then freeze-dried for 24 hours.
Five mL of methanol was added to the dried BPCAs along with 500 µL of a 1 mg
mL−1 solution of biphenyl-2,2-dicarboyxlic acid (Sigma Aldrich) in methanol that
was used as a derivatization standard. Samples were then methylated by titration with (trimethylsilyl)diazomethane in diethyl ether (Sigma Aldrich) until the sample solution remained yellow, indicating the presence of un-reacted diazomethane.
Samples were then dried under a stream of purified ultra-high purity nitrogen. A
fixed volume of dichloromethane was then added as a solvent. All samples were
separated on a Hewlett Packard 6890 GC outfitted with a Gerstel cooled injection system and a DB-XLB capillary column (30 m x 0.53 mm I.D., 1.5 µm film
thickness) and a flame ionization detector (FID). After injection, the column temperature was maintained at 100 o C for 1 minute, then raised 25 o C min−1 to 250 o C,
then raised 5 o C min−1 to a final temperature of 280 o C. The detector temperature
was 300 o C. The split-less injection volume was between 1 and 3 µL. Benzenepolycarboxylic acids were identified by comparison of their retention times with those
obtained by a commercially available mixture, verified by GC-mass spectrometry
and quantified by GC-FID. All methylated BPCAs were quantified relative to the
biphyenl-2,2-dicarboxylic acid internal standard. No additional response factors
were applied, as all methylated BPCAs exhibited an equal response to detection.
Initial work studying the oxidation products of PAHs with the BPCA method yielded
BPCAs substituted with -NO2 groups, due to the nitric acid oxidation (Chapter 2).
This study quantifies these nitrated BPCAs. Omitting these nitrated peaks from
40
quantification would lead to an underestimate of the BPCAs formed in oxidation.
Non-nitrated BPCAs are used as reference materials, as nitrated BPCAs are not
commercially available. Phthalic acid and 3-nitro and 4-nitrophthalic acid exhibited
nearly identical calibration curves, therefore we assumed the same relationship
would hold true for larger nitrated BPCAs. All measurements were performed in
triplicate, unless otherwise noted.
3.4
3.4.1
Results
BPCA distributions
The BPCA method forms BPCAs only from condensed aromatic materials, such
as char, soot or polycyclic aromatic hydrocarbons (PAHs) {Glaser et al., 1998;
Brodowski et al., 2005}. Although some PAHs and BC materials form BPCAs with
two carboxylic acids, only those compounds with three or more acids groups were
quantified in this work. This assumption is employed to avoid erroneously quantifying BC as BPCAs from non-BC material, such as lignin. Initially, the PAH perylene
was studied to understand the mechanism of high temperature and high pressure
nitric acid oxidation (Chapter 2). Upon oxidation of perylene we measured only two
BPCAs: the tri-substituted hemimelltic acid (B3CA) and the fully substituted mellitic
acid (B6CA) with the measured mole ratio of 4:1 (Figure 3.1). These results suggest that the quantitative distribution of BPCAs can provide structural information
about the material being oxidized.
Our results show that most of the oxidation products of the fullerenes (C60 and C70 )
and soots are the fully substituted mellitic acid (B6CA) with small portions of less
41
100
hexane soot
hexane soot + cation
carbon lampblack
C70
C60
C60 + cation
90
80
% of total BPCAs formed
70
60
50
40
30
20
10
0
B3CA
B4CA
B5CA
B6CA
Figure 3.3: Distribution of BPCAs formed upon high temperature and high pressure
acid oxidation relative to total BPCAs formed from fullerenes, carbon lampblack
and soot with and without cation column processing.
42
substituted BPCAs (Figure 3.3). Oxidation of both C60 and C70 , processed without cation removal, produces the greatest yield of B6CAs (94.4 ± 0.7 and 92.2 ±
2.8 % of total BPCAs, respectively). The BPCA distribution of C60 did not change
significantly when processed through the cation column (Table 3.1). Based on the
structure of C60 (Figure 3.2a), only B6CA should be formed, which is confirmed by
these results. Carbon lampblack, processed without the cation column, had almost
equal proportions of B3CA, B4CA and B5CA (about 10 % each) and 70.8 ± 10.0 %
B6CA, suggesting that the structure of carbon lampblack is predominantly aromatic
rings surrounded by other rings. Without the cation column, hexane soot produced
the smallest proportion of B6CA of the materials in this study (46.5 ± 4.6 %) with
10.3 ± 1.6 % of B5CA, 21.5 ± 0.6 % B4CA and 21.7 ± 5.3 % B3CA. Processing soot through the cation column (n=1) drastically shifted the BPCA distribution.
There was an increase in the proportion of B6CA and B5CA and a corresponding
decrease in the proportion of B4CA and B3CA. If one theorized about the structure
of hexane soot using the BPCA distribution using the cation processed samples,
a more condensed aromatic structure would be proposed than that using the noncation processed samples.
The distribution of oxidation products formed from SWCNTs varied markedly depending on sample treatment (Figure 3.4). The non-carboxylic acid functionalized
SWCNTs exhibited significantly different distributions when processed with and
without the cation column. Oxidation products of SWCNTs processed through the
cation column were all B6CA (98.4 ± 2.2 %) and a trace of B4CA, while SWCNT
samples not processed through the cation column contained a mixture of mostly
B3CA and B4CA (37 ± 13 % B3CA and 43 ± 8 % B4CA) with the remainder as
B6CA (20 ± 17). B5CA were not observed in either sample. The SWCNT not
processed through the cation exchange resin had the greatest uncertainty associated with the distribution of oxidation products, due to the low carbon yield (as
43
Table 3.1: Percent carbon yield for the carbon nanoparticles in this study. Unless
otherwise noted, all samples were treated for 8 hr.
No cation column processing
C70
C60
Carbon lamp black
Hexane soot
SWCNT
SWCNT-F
SWCNT 16 hr
SWCNT-F 16 hr
n
3
3
3
3
3
3
3
3
B3CA
0.9± 1.5
0.0± 0.0
11.3± 8.9
21.7± 5.3
37.1±13.3
18.4± 2.7
7.0± 6.5
27.1± 7.7
B4CA
3.0±1.2
2.6±0.7
9.1±1.8
21.5±0.6
43.3±8.1
9.2±1.5
5.3±2.2
11.0±1.9
B5CA
4.0±0.2
3.0±0.0
8.8±0.7
10.3±1.6
0.0±0.0
4.1±0.7
0.0±0.0
0.0±0.0
B6CA
92.2± 2.8
94.4± 0.7
70.8±10.0
46.5± 4.6
19.6±17.1
68.3± 3.5
87.7± 5.2
61.9± 9.6
% C Yield
26.0± 3.2
20.3± 2.1
20.0± 4.1
25.3± 4.0
0.8± 0.2
8.2± 1.5
7.8± 1.2
11.1± 2.7
B4CA
1.1± 5.0± 1.2±1.8
2.1±0.3
11.0±4.7
13.7±3.4
B5CA
8.8± 26.4± 0.0±0.0
10.9±3.7
5.9±8.4
5.2±3.8
B6CA
90.1±
68.6±
98.4± 2.2
87.0± 3.4
83.0±13.1
81.1± 7.2
% C Yield
17.0± 15.7± 7.2± 1.6
7.2± 1.5
6.0± 1.9
8.8± 0.3
Processed through cation
C60
Hexane soot
SWCNT
SWCNT-F
SWCNT 16 hr
SWCNT-F 16 hr
n
1
1
2
2
2
2
B3CA
-±
-±
-±
-±
-±
-±
-
discussed in the next section). Theoretically, SWCNT should form predominantly
B6CA because the structure consists of benzene rings surrounded on all sides by
other benzene rings. In contrast, oxidation of SWCNT-F produced mostly B6CA
regardless of cation processing (87.0 ± 3.4 % with and 68.3 ± 3.5 % without cation
processing). The other oxidation products (B4CA and B5CA) varied with cation
processing, such that fewer B5CA and more B4CAs and B3CAs were formed when
cation processing was not done. These results indicate a greater proportion of interfering cations, likely the metal catalyst, in SWCNTs that was not observed with
the SWCNT-F samples.
We also investigated the effect of oxidation duration on the SWCNT BPCA distribution. After 16 hours of oxidation (8 additional hours), the SWCNT-F oxidation
products were predominantly B6CA (81.1 ± 7.2 % with and 61.9 ± 9.6 % without
cation processing). The 16 hour oxidation of SWCNT did not yield a significantly
44
100
90
SWCNT , 8 hrs
SWCNT-F, 8 hrs
SWCNT, 16 hrs
SWCNT-F, 16 hrs
% of total BPCAs formed
80
70
60
50
40
30
20
10
0
100
% of total BPCAs formed
90
B3CA
SWCNT , 8 hrs
SWCNT, 16 hrs
B4CA
B5CA
B6CA
B5CA
B6CA
SWCNT-F, 8 hrs
SWCNT-F, 16 hrs
80
70
60
50
40
30
20
10
0
B3CA
B4CA
Figure 3.4: Distribution of BPCAs formed upon high temperature and high pressure
acid oxidation relative to total BPCAs formed for two types of single walled carbon
nanotubes for 8 hour oxidations and 16 hour oxidations. (a) samples processed
without cation column and (b) samples processed through cation column.
45
different distribution of oxidation products for samples processed with and without
cation column processing. These distributions are not different from those obtained
from shorter oxidations of SWCNT.
3.4.2
Carbon yield
Carbon recoveries were calculated as a percentage of mg BPCA C formed relative
to the mg C used in each experiment (Table 3.1). The BPCA carbon yields of the
compounds in this study ranged from 0.8 to 26 % and 6.0 to 17 % for samples
processed with and without the cation column. The range of the carbon yields
decreases with cation processing and the overall recoveries were lower. These
losses, due to additional sample handling, may be accounted for in the future by
incorporating an additional recovery standard that could be added before chemical
oxidation. For fullerenes not cation processed, the carbon yield was equal; C70
exhibited a carbon yield of 26.0 ± 3.2 % and C60 had a carbon yield of 20.3 ± 2.1
%. Processed through the cation, there was a small loss of C60 (17 % recovery).
A sample of C70 was not processed through the cation column. By definition, this
method cannot recover 100 % of the carbon from fullerenes because carbon is lost
due to the breakup of adjacent rings. The maximum number of B6CA molecules
that could form from one C70 is three (i.e.: 36 carbons), corresponding to only
51.4 % carbon yield. If the carbon yield is adjusted to account for only the carbon
available to form BPCAs, then the C yield for C70 would be 47.0 ± 6.4 % within this
study. Similarly, if only the carbon available to form BPCAs was used to calculate
the carbon yield for C60 then the maximum possible carbon yield would be 33.5
± 3.5 %. Adjusting the carbon yield to reflect only the available BPCAs formed is
not always practical or possible, because the correct structures of the compounds
studied are not always known.
46
Although carbon lampblack and hexane soot exhibited significantly different BPCA
distributions (Figure 3.3), the C yields of these materials (20.0 ± 4.1 % and 25.3
± 4.0 % respectively) are statistically equivalent and equal to the C60 yield. The C
yield for perylene was also similar to these values (22.6 ± 1.3 %, Chapter 2), suggesting an average C yield of 20.7 ± 4.2 % for the non-SWCNT materials in this
study. The conversion of BPCAs to BC was previously been made using activated
charcoal as a model BC material {Glaser et al., 1998; Brodowski et al., 2005}.
This average C yield is in agreement with the BPCA conversion factor reported by
Brodowski et al. {2005} for activated charcoal but about half of the original conversion factor reported by Glaser et al. {1998}.
After oxidation of SWCNTs, there was always black particulate material left in the
flask that was most likely undissolved SWCNTs. When processed without the
cation column, the nonfunctionalized SWCNT had the lowest carbon yield at 0.5
± 0.1 % and the functionalized SWCNT had a carbon yield of 6.8 ± 1.4 %. If the
oxidation duration was increased to 16 hours, the SWCNT carbon yield increased
to 7.3 ± 0.7 % with no change for the SWCNT-F. When processed with the cation
column both SWCNTs had the same carbon yield (7.2 ± 1.6 and 7.2 ± 1.5 %) after
8 hours and longer oxidations did not show a significant change in carbon yield
(6.0 ± 1.9 % SWCNT and 8.8 ± 0.3 % SWCNT-F) from the shorter oxidations.
Carboxylic acid functionalized SWCNTs are produced as a bi-product of metal catalyst removal using HNO3 alone or in combination with H2 SO4 {Liu et al., 1998}.
Since the carbon yield for the 16-hour functionalized SWCNT was not significantly
different from that of the non-functionalized SWCNT, it is plausible that the oxidation procedure that functionalized the SWCNT initially made it more susceptible to
BPCA formation. Future experiments should include longer oxidation times (i.e.:
32 hours) to assess the effect on the SWCNT C yield. Acid treatment has also
been shown to shorten the length of the CNTs {Chen et al., 2001; Liu et al., 1998}
47
and to form carbonaceous impurities {Hu et al., 2003}.
When pure compounds are assessed using the BPCA method (e.g.: perylene),
cation column processing is not required. However, environmental samples most
often contain a significant concentration of metals and other polyvalent cations that
requires removal by cation column. Internal standard, biphenyl-2,2-dicarboxylic
acid, recoveries are lower when polyvalent cations are present, suggesting that the
presence of polyvalent cations compromises the dervitization reaction. Since the
BPCA distributions of samples processed without the cation column were distinctly
different from those processed with the cation column, polyvalent cations apparently change the mechanism of the oxidation process. Hexane soot did not exhibit
a loss of the internal standard when processed without the cation column, yet the
BPCA distribution was significantly different under the two processing regimes.
Further study is required to determine under what conditions cation column processing affects BPCA distributions. Samples containing polyvalent cations not processed through the cation column form a smaller proportion of B6CA and a larger
proportion of smaller BPCAs. Therefore estimated structures of BC for samples
processed without the cation column would be a less condensed aromatic structure than for those samples processed through the cation column, underestimating
the aromaticity of the original structure.
3.4.3
Mixtures in sediments
A key challenge remains for applying this methodology to environmental matrices,
such as coastal sediments. In environmental samples it will be important to not only
quantify the amount of BC present, but also to determine the relative contributions
of different BC sources. Two methods were used to evaluate matrix effects that
48
may occur during oxidation. First, using standard addition of SWCNT we quantified BC to known mixtures of marine sediment, soot and SWCNT (Figure 3.5).
Marine sediment (NIST 1941b), without any additional BC, was found to contain
4.4 ± 0.4 g/kg BC, which is greater than previous BPCA estimates and not significantly different than chemo-thermal oxidation (Hammes et al. {2007}). While these
measurements were converted from BPCAs to BC using the 20.7 % conversion
factor determined in this study, other loss processes must be present. Since we
do not know the types of BC materials being quantified we must use this average
value. In some cases this may lead to an over estimate of BC abundance. The
slope of the data from the standard addition experiments indicates recovery of 95
± 20 % of the added BC when the samples were oxidized for 8 hours, when using
the 20.7 % conversion factor. Prolonged oxidation, of 16 hours, also fell on the
observed trend, indicating that the duration of oxidation did not affect the observed
matrix effect. SWCNTs oxidized in isolation were found to have a low carbon yield
(7.2 %) and in mixture samples a conversion factor of 20.7 % was applied, yet
the BC yield is lower than expected. The lower than expected BC yield in mixture
samples demonstrates that a small matrix effect is present. In other words, the
presence of sedimentary material, mainly clay, causes the nitric acid oxidation of
BC to proceed less efficiently than when no sedimentary material is present.
A second method used to evaluate matrix effects was to examine the BPCA distributions of the mixtures, comparing the theoretical and measured BPCA distributions. When oxidized together, soot and SWCNTs were not as fully substituted as
predicted by a theoretical mixture of these compounds (Figure 3.6a). This shift towards the production of less substituted BPCAs was more dramatic when soot and
SWCNTs were oxidized with marine sediments present (Figure 3.6b). Although the
matrix effect generates BPCA distributions similar to those not processed through
the cation column (Figure 3.4), these observed results are not due to residual
49
250
y = 0.95±0.20x - 3.4±28.4
r2=0.92
BC quantified (µg)
200
150
100
8 hour
16 hour
Linear (8
hour)
50
0
0
50
100
150
BC added (µg)
200
250
Figure 3.5: Standard addition of soot and SWCNTs to marine sediment (NIST
1941b) after 8 hours (filled squares) and 16 hours (grey circle). The trendline,
generated using only 8 hour oxidation samples, indicates the recovery of SWCNT.
The y-intercept corresponds to the BC content in the marine sediment and the
negative value indicates loss of BC. Error bars represent propagated errors.
50
cations. If cations were present, the internal standard recovery would have diminished, whereas this was not observed in the mixture samples. Thus, this shift could
be due to interactions with the cation column.
The presence of this matrix affect limits the applicability of identifying the types
of BC present in mixtures using the BPCA method alone. Compound specific
isotopic analysis of BPCAs, for the purpose of isolating the source of BC, is not
likely to be feasible when mixtures of BC are present. Stable carbon isotopes are
not suitable as SWCNTs show a wide range of δ13 C values (-53.2 to -23.5 h,
Plata et al. {2008}), which may be from the carbon source material or fractionation
during fabrication and post-production treatments. Since carbon source materials
for SWCNTs, fullerenes and most soots are mostly fossil in origin, radiocarbon
(∆14 C) analysis of BPCAs would not garner information about the source of BC.
BC oxidization techniques will not be able to parse the source of BC when mixtures
of soot and SWCNTs are present due to their structural similarity. Therefore, for
BC source appointment, additional analytical techniques must be employed.
3.5
Conclusions
This paper investigated the suitability of using the BPCA method to isolate two
SWCNTs, two fullerenes and two types of soot from natural samples. The materials
studied exhibit distinct BPCA distributions, favoring the production of larger BPCAs.
Mixtures of BC do not exhibit BPCA distributions predicted by oxidation of single
compounds. Although the BPCA method is suitable for isolating and quantifying
BC mixtures in environmental samples, matrix effects complicate the feasibility of
identifying the relative contributions of different types of BC using this method.
51
80
theoretical 1.3soot:SWCNT
measured 1.3soot:SWCNT
theoretical 2.2soot:SWCNT
measured 2.2soot:SWCNT
% of total BPCA distribution
70
60
50
40
30
20
10
0
B4CA
80
% of total BPCA distribution
70
60
B5CA
B6CA
theoretical sed, 1.3soot:SWCNT
measured sed, 1.3soot:SWCNT
theoretical sed, 3.3soot:SWCNT
measured sed, 3.3soot:SWCNT
50
40
30
20
10
0
B4CA
B5CA
B6CA
Figure 3.6: Theoretical and measured BPCA distributions in mixtures of (a) soot
and SWCNT and (b) marine sediment, soot and SWCNT. Error bars on theoretical
BPCA distributions are 5 % while measured BPCA distribution errors are propagated errors.
52
Acknowledgments
The authors thank Sheila Griffin, John Greaves, Richard Chamberlin and Dachun
Zhang for their technical expertise and advice. We acknowledge support of National Science Foundation EAR-04473232 and EAR-0502519 (to E.R.M.D.).
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4
Quantification of extraneous carbon during
compound specific radiocarbon analysis of black
carbon
4.1
Abstract
Radiocarbon (14 C) is a radioactive isotope that is useful for determining the age
and cycling of carbon-based materials in the Earth system. Compound specific radiocarbon analysis (CSRA) provides powerful insight into the cycling of individual
components that make up the carbon cycle. Extraneous, or non-specific background carbon (Cex ), is added during sample processing and subsequent isolation
of CSRA samples. Here, we evaluate the quantity and radiocarbon signature of Cex
added from two sources: during preparative capillary gas chromatography (PCGC,
CPCGC ) and sample processing of CSRA for black carbon samples (Cchemistry ). To
normalize our Cex estimates to samples with different isolation durations on the
GC, we report the amount of Cex as µg C per minute of GC collection over 50
injections. Using both a direct and indirect method of assessment, we determine
that the added CPCGC was 0.1 ± 0.05 and 0.5 ± 0.3 µg C min-1 50 injections-1 with
a fraction modern ranging from 0.15 to 0.2. We found that the direct and indirect
57
assessment of Cchemistry+PCGC agreed, both in magnitude and radiocarbon value
(1.1 ± 0.5 µg C, fraction modern = 0.2). Half of the Cex was introduced before
PCGC isolation, likely from solvents used in the extraction method. The magnitude of propagated uncertainties of CSRA samples were found to be a function of
sample size and collection duration. Small samples collected for a brief amount
of time have less propagated
14
C uncertainty than larger samples collected for a
longer period of time. CSRA users are cautioned to consider the magnitude of
uncertainty they require for their system of interest and to frequently evaluate the
magnitude of Cex added during sampling processing and isolation.
4.2
Introduction
Radiocarbon dating of bulk organic and inorganic carbon reservoirs has allowed
the average residence time of carbon in each of the respective pools to be calculated. However, these reservoirs comprise of complex heterogenous mixtures
whose components have different residence times that may not be well represented by bulk radiocarbon measurements. Initially, the heterogenous mixtures
were studied via compound class specific radiocarbon analysis (CCSRA). The subsequent introduction of compound specific radiocarbon analysis (CSRA) allowed
measurement of 14 C signature in a single compound {Eglinton et al., 1996}. CSRA
usually involves a multiple-step purification procedure that culminates in the collection of a single compound (or group of compounds) of high purity. The applications
of CCSRA and CSRA range from source apportionment of atmospheric particles
{Reddy et al., 2002; Sheesley et al., 2009}, biomarkers with paleoclimatic implications {Prahl and Wakeham, 1987; Sachs and Lehman, 1999; Mollenhauer et al.,
2005}, microbial incorporation of fossil material {Petsch et al., 2001; Slater et al.,
58
2005} and compound class studies in marine sediments {Hwang and Druffel, 2005}
and marine dissolved organic carbon {Aluwihare et al., 2002; Loh et al., 2004}.
New developments in accelerator mass spectrometry (AMS) have decreased the
sample size requirements for CSRA. Ultra-small samples{Santos et al., 2007} and
online 14 C measurements {von Reden et al., 2008} enable CSRA as small as 2 µg
C. Preparation of CSRA samples requires two-distinct and rigorous sets of laboratory protocols (sample isolation and
14
C analysis), each step inadvertently and
unavoidably introducing Cex . Thus a CSRA sample of 2 µg C may be ≥50% Cex . To
date few studies have quantified Cex {Shah and Pearson, 2007}. Accounting for Cex
has largely been avoided by processing samples large enough so as to overwhelm
the Cex . However, all environmental CSRA techniques allow for the preparation of
large sample sizes, because the compound of interest might be in low abundance.
Constraining the uncertainty of 14 C measurements is done by evaluating the mass
and variability of Cex added during sample preparation. Here we assess the mass
and radiocarbon signatures of Cex specific to the chemical oxidation of organic
matter for quantifying black carbon using PCGC. We employed the benzene polycarboxylic acid method that chemically oxidizes black carbon to benzene rings substitued with three to six carboxylic acid groups.
4.3
Methods
Natural and synthetic vanillin (4-hydroxy-3-methoxybenzaldehyde, Table 4.1) were
used as standards to assess the extraneous carbon added during PCGC isolation.
Black carbon (BC) reference materials were used as process standards to quantify
Cex added throughout the entire isolation procedure (Table 4.1){Hammes et al.,
59
2007, 2008}.
4.3.1
Chemical Oxidation
To minimize carbon contamination, all glassware and quartz filters that came in
contact with the samples and standards were baked at 550o C for 2 hours prior to
use. Samples were processed using a modification of the benzene polycarboxylic
acid (BPCA) method ({Ziolkowski and Druffel, 2009}, Chapter 2). Process materials, wood char and hexane soot (Table 4.1), were oxidized in 2 mL of concentrated
nitric acid (grade ACS) in quartz tubes inside a high pressure digestion apparatus
at 180o C for 8 hours. Post digestion, the samples were filtered through quartz fiber
filters (27 mm diameter, 0.8 µm pore diameter) and 15 mL Milli-Q water was used
to rinse any remaining BPCAs from the filter. The filtrate was collected and freeze
dried overnight.
Dried samples were redissolved in 5 mL methanol and the internal standard, biphenyl2,2’-dicarboxylic acid (1 mg mL-1 in methanol) was added. Samples were derivatized by titration with 2.0 M trimethylsilyl diazomethane in ethyl ether (Sigma Aldrich).
Derivatization was considered complete when the solution retained the yellow color
of the trimethylsil-diazomethane. Methanol was dried with a purified stream of UHP
nitrogen. A fixed volume of dichloromethane was added.
The derivatized oxidation products were separated and quantified on a Hewlett
Packard 6890N outfitted with a Gerstel cooled injection system, a DB-XLB capillary column (30 m x 0.53 mm I.D., 1.5 µm film thickness), and a flame ionization
detector (FID). After injection, the column temperature was maintained at 100o C
for 1 minute, then raised at 25o C min-1 to 250o C followed by a 5o C min-1 ramp to
280o C for 10 minutes and then raised to 320o C for 5 minutes of bake out (Figure
60
61
1
Assumed radiocarbon values.
Table 4.1: Materials processed and associated solvents used for CSRA of black carbon.
duplicate.
material
use
source
materials processed
modern vanillin
GC process standard
Sigma Aldrich
synthetic vanillin
GC process standard
Sigma Aldrich
grass char
method process standard Uni. of Zurich
hexane soot
method process standard Uni. of Denver
solvents and materials
methanol
solvent
dichloromethane
solvent
biphyenl-2,2’-dicarboxylic acid internal standard
Sigma Aldrich
TMS-diazomethane
derivatization agent
Sigma Aldrich
DB-XLB
GC column
Agilent
14
C was measured in
0.0001
0.0001
0.000 ± 0.001
0.0001
0.000 ± 0.001
1.052 ± 0.002
0.002 ± 0.001
1.056 ± 0.002
0.005 ± 0.001
bulk 14 C (FM)
The bulk
4.1). The FID temperature was 300o C. The splitless injection volume was 1 µL
for all samples in this study. Approximately 1 % of the flow eluting from the capillary column was diverted to the FID and 99 % was sent to the preparative fraction
collector (PFC), which consists of a zero-dead-volume valve in a heated interface
(320o C) and seven 200 µL glass U-tube traps (six sample traps and a waste trap).
The PFC transfer was kept constant at 320o C for all samples processed. U-tubes
were supported in isopropyl alcohol cooled units (-10o C). The auto-injector, CIS
and trapping device are programmable and computer controlled, and FID data was
acquired using Chemstation software.
BPCAs were identified by comparison of their retention times with those obtained
for a commercially available mixture and were verified using GC/MS. All methylated BPCAs were quantified relative to the biphyenl-2,2’-dicarboxylic acid internal
standard.
4.3.2
Radiocarbon analysis of isolated samples
To avoid cross contamination from previously injected samples (e.g.: memory), the
compounds collected from the first 10 injections were disposed of and the U-tube
was replaced with a clean, baked tube. Unless otherwise noted, trapped samples
were collected from 50 injections of each sample. To avoid possible isotope fractionation of isolates {Zencak et al., 2007}, care was taken to trap the entire peak.
After PCGC isolation, the U-tubes containing trapped samples were rinsed with
700 µL of CH2 Cl2 into pre-baked GC autosampler vials. Samples were evaluated
by GC-FID for purity and yield. Samples were then transfered to 6 mm quartz tubes
using an additional 700 µL of CH2 Cl2 and the solvent was removed in a stream of
UHP nitrogen. CuO and silver wire were added and the sample tube was evac62
uated to 10-6 Torr and flame-sealed under vacuum. Tubes were then heated to
850o C for 2 hours. The resulting CO2 was purified, quantified and reduced to
graphite according to standard procedures. Measurements of
14
C were made at
the Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory at University of
California Irvine. In all cases, radiocarbon analysis are reported as fraction modern, which is the deviation of a sample from 95 % of the activity in 1950 AD, of
National Bureau of Standards (NBS) oxalic acid 1 normalized to δ13 C = -25 h with
respect to Pee Dee Belemnite {Olsson, 1970; Stuvier and Polach, 1977}. All fraction modern values reported within this paper have been corrected for combustion
and graphitization and mass dependent isotope fractionation by reporting all data
to a common δ13 C value of -25 h {Stuvier and Polach, 1977}.
4.4
Results
4.4.1
Carbon mass balance and corrections
The mass of carbon graphitized in CSRA samples (Creported ) isolated via PCGC that
have been corrected for graphitization and combustion, originate from at least four
sources:
Creported = Csample + Cderivative + Cchemistry + CPCGC
(4.1)
the mass of carbon in the compound of interest isolated from the sample (Csample ),
the mass of added derivative carbon (Cderivative ) and the mass of extraneous carbon
added during chemical extraction (Cchemistry ) and subsequent isolation via PGCG
(CPCGC ).
The compounds of interest in this study, BPCAs, contain functional groups that
63
require derivatization to adjust their polarity and volatility to enable separation by
PCGC. The derivatization adds a methyl group (-CH3 ) to each carboxylic acid group
and this additional carbon affects the
14
C of the sample. Since the isotopic com-
position of the derivative carbon is assumed to be
14
C free (FMderivative = 0), the
reported isotopic signature is known and the amount of added derivative carbon is
known, the radiocarbon composition of the parent BPCA compound can be calculated:
FMsample+chemistry+PCGC =
FMreported − (FMderivative ∗ fderivative )
fsample+chemistry+PCGC
(4.2)
where FMsample+chemistry+PCGC is the FM of the underivatized BPCA, FMreported is
the FM of the BPCA methyl ester corrected for graphitization and combustion,
FMderivative is the FM of the derivative carbon, fsample+chemistry+PCGC is the fraction of
underivatized BPCA in Creported and fderivative is the fraction of derivative carbon in
Creported . Tests with process standards of known 14 C signatures, confirmed that the
derivative carbon is radiocarbon dead.
When samples are corrected for Cderivative (Equation 4.2), Equation 4.1 is simplified
to:
Csample+chemistry+PCGC = Csample + Cchemistry+PCGC = Csample + Cex
(4.3)
To provide accurate isotopic values of Csample+chemistry+PCGC , the mass and isotopic
composition (FM) of Cex must be determined. Here we evaluated two sources
of Cex : added during chemical extraction (Cchemistry ) and during PCGC isolation
(CPCGC ). Reported values of CSRA samples (Creported ) need to be corrected for
Cex . For the purposes of estimating the Cex via process materials, samples had
to be corrected for derivative carbon before estimating Cex , which assumes that all
Cex has been derivatized.
64
4.4.2
Extraneous carbon added during PCGC isolation (CPCGC )
Two methods were used to evaluate the mass and FM of Cex originating from
PCGC isolation. First, the direct approach was used to collected a sample over
a seven minute retention time window (Figure 4.1) from dry 400 injections (direct
CPCGC ). No solvent was injected during the dry injections, that is, there was no
needle in the autosampler, and all other GC parameters (i.e. carrier gas, oven
temperature) were maintained. This sample yielded 7.6 ± 0.4 µg C and had a
FMPCGC of 0.125 ± 0.034. Because the sample collection window varies with sample type (Figure 4.1), we normalized the amount of Cex (µg C) to collection duration
(in minutes) and number of injections using the equation:
normalized Cex =
Cex
(collection duration)(number of injections)
(4.4)
Normalizing the Cex to time assumes the majority of Cex is due to column bleed
(sample history and/or breakdown of the GC-column stationary phase) and that
the bleed does not change over time. To standardize this non-specific background
correction, all subsequent collections maintained the same injection volume and
number of injections; only the collection time and injected materials varied for the
samples reported here. While 50 injections was the standard number used in this
study, periodically fewer injections were made due to technical difficulties. Normalizing the Cex to both the time and number of injections enables one to apply this
corrections to samples that were collected for different durations and or different
number of total injections. We normalized all samples that evaluated Cex , even
samples that included the Cchemistry . The Cex was normalized to µg C per minute
of collection for 50 injections. Thus, evaluated directly the CPCGC added in the dry
injections was 0.1 ± 0.05 µg C min-1 50 injections-1 .
65
Figure 4.1: The magnitude of column bleed (indicated by the magnitude of the
baseline signal) and oven temperature as a function of retention time. The retention
time windows for the isolation of vanillin and BPCAs are marked.
The second method of evaluating the mass and FM of CPCGC used various sizes of
isolated process standards of known FM values. It was assumed that the sample
was diluted with a constant mass and isotopic signature of Cex and the presence
of Cex would cause a deviation in the consensus
14
C value. Vanillin, the process
standard used to estimate CPCGC added during PCGC processing, does not contain
carboxylic acid groups and is not derivatized; thus it thus does not require correction for derivative carbon (Cderivative ). The FM values of samples can be expressed
by the following equation:
FMsample =
FMreported Creported − FMPCGC CPCGC
Csample
(4.5)
where FMsample is the radiocarbon value of the sample corrected for CPCGC , FMreported
is the measured radiocarbon value of the sample uncorrected for CPCGC , and FMPCGC
66
the radiocarbon value of the extraneous carbon added during PCGC isolation. Typically, Cex is assessed as a combination of both dead and modern material. Thus,
we would expect the FMex to be between 0 and 1. Therefore, small samples of
modern isotopic composition isolated by PCGC will become more depleted and
samples of 14 C-depleted composition isolated by PCGC will become more enriched
in radiocarbon (e.g.:Figure 4.2).
The mass and FM of Cex added during PCGC processing was assessed indirectly
using a two-component approach. The PCGC isolation size-series of modern
vanillin (FMsample = 1.052, Table 4.1) samples revealed that the amount of CPCGC
added (FMPCGC =0.0) was 0.4 ± 0.2 µg C min-1 50 injections-1 . The PCGC isolation of a series of different sized samples of 14 C-free vanillin (FMsample = 0.002) revealed an additional 0.2 ± 0.1 µg C min-1 50injection-1 was added with an assumed
FMPCGC =1.0. These two blanks were added to obtain the total indirect CPCGC of 0.6
± 0.3 µg C with an average FMPCGC =0.2 (Table 4.2).
The difference of 0.5 µg C of Cex added to isolated vanillin samples calculated
using standard materials (0.6 ± 0.3 µg C) as compared to the dry injections (0.1
µg C) may be due to several factors. First, no solvent was injected into the GC
column during dry injections. It is possible that when solvent is present in the GC
column more Cex is mobilized than during the absence of solvent. The FMex value
for vanillin (FMex = 0.2 ± 0.1) and that for the dry injections (FMex = 0.125 ± 0.034)
was similar suggesting the same source of Cex . Another possible explanations are
that CPCGC and its isotopic signature may vary with time, sample memory and / or
contamination of the injector port. Therefore, we estimate that for each minute of
collection on the PCGC, 0.6 µg C with a FM = 0.2 is being added to samples due
to contamination from the PCGC.
67
68
1
dry injection
modern vanillin
dead vanillin
total indirect PCGC
process blank
grass char
hexane soot
total indirect chemistry
Direct
Indirect
Indirect
Direct
Indirect
Indirect
X
X
X
x
x
x
BPCA1
X
X
X
x
X
X
CH2 Cl2
X
X
X
X
X
X
PCGC
1.1 ± 0.2
0.200 ± 0.054
0.80 ± 0.40
0.0
0.15 ± 0.08
1.0
1.0 ± 0.5
0.15 ± 0.08
extraneous carbon, Cex
µg C
FM
0.1
0.125 ± 0.034
0.4 ± 0.2
0.0
0.2 ± 0.1
1.0
0.6 ± 0.3
0.2 ± 0.1
BPCA includes the chemical oxidation of BC into BPCAs and their subsequent derivatization, see text for details.
material
evaluation
Table 4.2: Type and treatment of samples that were evaluated for Cex and FMex during chemical oxidation and PCGC
isolation. The uncertainty of the mass of extraneous carbon was estimated to be 50 % of the sample mass. The uncertainty
of FMex was estimated to be 50 % of the FM value.
4.4.3
Extraneous carbon added during chemical oxidation and
PCGC isolation (Cchemistry+PCGC )
CSRA samples are typically subjected to extensive chemical extraction procedures
prior to isolation by PCGC and consequently it is likely that extraneous carbon is
added during these procedures. Similar to the evaluation of Cex added during
PCGC isolation, we evaluated the mass and FM of Cex added during the chemical
methods and PCGC isolation using both an indirect and direct approach. To evaluate Cex directly, the chemical oxidation and PCGC isolation steps were carried
out but no sample material was added. Direct analysis of the Cex added during
chemical oxidation, derivatization and PCGC isolation with no sample added was
1.1 ± 0.2 µg C min-1 50 injections-1 and FM = 0.200 ± 0.054 (Table 4.2).
The Cex was evaluated indirectly by quantifying the deviation in FMs+ex from the unprocessed material for radiocarbon dead (hexane soot) and modern (grass char)
of different sizes. Samples of modern grass char were chemically oxidized, derivatized and isolated by PCGC. The samples isolated by PCGC (e.g.: CCSRA ) ranged
from 2 to 16 µg C. We found that 0.80 ± 0.40 µg C min-1 50 injections-1 of an
assumed FMex =0.0 was added in chemical oxidation and PCGC isolation. Fossil
hexane soot revealed 0.15 ± 0.08 of an assumed FMex = 1.0 was added in sample
processing. The total indirect method Cex was then calculated to be 1.0 ± 0.5 µg
C min-1 50 injections-1 and FMex = 0.15.
When evaluated directly and indirectly, the mass and isotopic composition of the
Cex added during sample processing and isolation was the same. If the Cex for
indirect assessment was much larger than the direct method, the source of the Cex
may be a matrix effect of the oxidation process. The agreement of the two methods
suggests that the Cex is not associated with any matrix effects in the processing of
69
a sample.
The magnitude of the Cex added during chemical oxidation (Cchemistry = 0.5 µg C) is
approximately equal to that added during PCGC isolation (CPCGC = 0.6 µg C). This
suggests that only half of the non-specific background is originating from within the
PCGC, supposedly column bleed. The remainder is likely from the reagents and
solvents used in the oxidation and derivatization processes. Because reagents
and solvents can become contaminated over time and with use, it is essential to
frequently evaluate the Cex (e.g. every 2 to 5 samples).
4.4.4
Correcting for extraneous carbon and associated uncertainties
Radiocarbon measurements are typically reported with an uncertainty of the AMS
measurement alone. As we have shown above, the corrected radiocarbon value of
a CSRA sample is dependent on of the mass and FM of the Cex . If the sample is
large enough (≥ 50 µg C), the Cex will be insignificant. However, the FM of small
CSRA samples will require a correction for the presence of Cex . The uncertainties
of all these terms must be considered when reporting the uncertainty of the CSRA
FM value. To determine the propagated total mathematical uncertainty of FMsample
(e.g. Equation 4.5), we applied the following equation:
σ2FMsample
!2
∂FMsample
+
σ2FMex
∂FMex
!2
!2
∂FMsample
∂FMsample
2
+
σmreported +
σ2mex
∂mreported
∂mex
∂FMsample
=
∂FMreported
!2
σ2FMreported
(4.6)
where σFMreported is the uncertainty of FMreported measured on the AMS (machine uncertainty), σFMex is the uncertainty for FMex , σmreported is the uncertainty for Creported
70
(uncertainty in graphitization) and σmex is the uncertainty for Cex . The total uncertainty of the direct process blank (Cchemistry+PCGC in Table 4.2) was used for FMex
and Cex .
For grass char, a modern BC standard {Hammes et al., 2007}, the measured
FMreported values for 7 small samples without Cex correction (average FMreported =
0.824 ± 0.128, Table 4.3) are significantly lower than the FM value of the unprocessed material (FM = 1.058 ± 0.002, Figure 4.2). After correction for Cchemistry+PCGC ,
the FMsample (average 1.098 ± 0.221) agrees with that of the unprocessed material.
For hexane soot, a dead BC standard, the measured FM values without correction
for Cex (average FMreported = 0.061 ± 0.55, Table 4.3) are significantly enriched in
14
C in comparison to the unprocessed material (FM = 0.005 ± 0.001). After correc-
tion for Cchemistry+PCGC , the FMreported (average 0.036 ± 0.056) is more comparable
to the FM of the unprocessed material.
These results demonstrate that the uncertainties associated with the preparation
and isolation of samples by CSRA are significantly larger than the machine error.
Propagated total uncertainty of processed
than processed
14
14
14
C modern materials is much higher
C depleted materials, due to the nature of radioactive decay of
C and that in our system the FMex was more
14
C depleted than modern. Not all
systems will have the same FMex and each user needs to evaluate the Cex and
FMex values specific for their system.
Thus, when considering CSRA applications, one must consider the magnitude of
uncertainty requirement to provide useful information about the system being studied. For example, our interest in CSRA of BPCAs is to examine the BC in marine
dissolved organic carbon (DOC). Bulk DOC, which is comprised of a wide range of
organic molecules of varying 14 C ages, typically ranges from FM = 0.8 to 0.5 {Loh
et al., 2004}. The BC in marine DOC has been postulated to be more depleted in
71
72
2
Cex (µg C)
1.32 ± 0.36
4.07 ± 1.11
1.32 ± 0.36
2.31 ± 0.53
2.31 ± 0.63
0.44 ± 0.12
0.44 ± 0.12
0.660 ± 0.180
0.660 ± 0.180
0.990 ± 0.270
0.990 ± 0.270
0.990 ± 0.270
FMreported 1
0.90 ± 0.02
0.91 ± 0.02
0.86 ± 0.04
0.78 ± 0.07
0.82 ± 0.09
0.81 ± 0.24
0.69 ± 0.43
0.824 ± 0.128
0.004 ± 0.010
0.049 ± 0.044
0.054 ± 0.077
0.139 ± 0.090
0.349 ± 0.100
0.061 ± 0.055
Creported (µg C)
16.3
12.0
8.7
5.2
4.6
2.4
1.9
9.4
6.6
4.1
3.5
2.4
after diazomethane correction
determined using Equation 4.5
1
UCID duration (min)
11782
1.2
11801
3.7
11779
1.2
grass char 11777
2.1
11780
2.1
11778
0.4
11781
0.4
isolate average ± std dev
bulk value
11711
0.6
11723
0.6
hexane soot 11713
0.9
11710
0.9
11712
0.9
isolate average ± std dev
bulk value
type
FMsample 2
0.96 ± 0.03
1.27 ± 0.15
0.98 ± 0.06
1.24 ± 0.27
1.45 ± 0.39
0.95 ± 0.30
0.84 ± 0.56
1.098 ± 0.221
1.052 ± 0.002
0.000 ± 0.012
0.032 ± 0.049
0.007 ± 0.103
0.115 ± 0.125
0.454 ± 0.178
0.036 ± 0.056
0.005 ± 0.001
Table 4.3: Radiocarbon values (fraction modern) and associated uncertainty of black carbon reference materials before
and after correction for Cex . Duration (minutes) is the time the collection window is left open. The Cex (Cchemistry+PCGC ) is
assumed to be 1.1 ± 0.2 µg C per minute of collection for a 50 injection run with a FM = 0.2 ± 0.054 (see Table 4.2). The
uncertainty associated with the FMreported is the AMS machine uncertainty and the uncertainty associated
with FMsample is
P
the propagated uncertainty. The collection window duration was varied to collect individual BPCA or BPCA.
(a) Modern grass char
(b)
14
C dead hexane soot
Figure 4.2: (a) Grass char and (b) hexane soot before (open symbols) and after
(filled symbols) Cex correction. The radiocarbon value of the unprocessed material
is indicated by the bolded line: grass char FM=1.056 ± 0.002 and hexane soot
FM=0.005 ± 0.001.
73
radiocarbon. Provided that BC extracted from marine DOC has a propagated total
uncertainty less than FM = 0.10, the results should provide valuable information
about this pool of recalcitrant carbon. However, if we were interested in studying
the removal process of BC from soils over a few centuries, we would require much
larger samples than those presented here in order to ensure that the contribution
of Cex to the FMreported is insignificant, which would in turn minimize the propagated
total uncertainty. Regardless of the application, it is equally important that CSRA
users assess their ability to duplicate CSRA measurements, as in some cases the
duplication of CSRA samples may be larger than the propagated total uncertainty.
The mass and isotopic composition of Cex should ideally be evaluated with each
batch of samples, as we found the mass of Cex to vary by over 50 % over the
course of six months {Ziolkowski, 2009}.
4.5
Conclusions
Extraneous carbon added during PCGC isolation of CSRA samples was found to
be a function of collection duration on the GC. Half of the Cex was added during
PCGC isolation and half was added during the chemical oxidation and derivatization. The estimates of extraneous or non-specific background carbon presented
here are specific to this chemical isolation technique. Another facility using the
same chemical extraction technique would need to determine the extraneous carbon introduced to samples that they process. Different GC columns, solvents and
users may produce more or less Cex carbon, with unique FMex values.
74
Acknolwedgements
The authors would like to thank John Southon, Guacaria dos Santos, Sheila Griffin,
Dachun Zhang and Xiaomei Xu for their technical assistance and comments. This
work was funded by the National Science Foundation Chemical Oceanography
program.
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5
Black carbon in marine dissolved organic carbon
5.1
Abstract
Black carbon (BC) enters the ocean through aerosol and river deposition before its
eventual incorporation into the sediment. It has been postulated that BC resides
in the marine dissolved organic carbon (DOC) pool before sedimentary deposition.
Here we report the concentration and radiocarbon content of BC in high molecular
weight DOC (UDOM). BC exported from rivers has modern levels of
open ocean samples contained BC with an average
14
14
C, while
C age of 20,000 ± 3,000
years. BC represents 0.5 to 3.5 % of UDOM. If marine DOC contains 4 to 22
% BC, as suggested from studies of BC in the sediments {Masiello and Druffel,
1998}, the low molecular weight DOC is rich in BC and possibly the repository of
the missing BC.
79
5.2
Introduction
Black carbon (BC) particles encompass a large range of chemical and physical properties that are produced during biomass burning and fossil fuel combustion. In the atmosphere, BC can lead to increased atmospheric temperatures
and decreased precipitation {Ramanathan and Carmichael, 2008}. BC is stored
in soils where it may be slowly degraded over time before being transported to
rivers {Czimczik and Masiello, 2005; Hockaday et al., 2007}. BC enters the ocean
through aerosol and river deposition {Flores-Cervantes et al., 2009; Masiello and
Druffel, 2001; Dickens et al., 2004}. BC isolated from open ocean sediments is up
to 14,000
14
C years older than non-BC sedimentary organic carbon, suggesting
that BC resides in an intermediate pool, such as marine dissolved organic carbon
(DOC), before sedimentary deposition {Masiello and Druffel, 1998}.
Marine DOC, operationally defined as the material that passes through a 0.2 - 1.0
µm filter, is the largest exchangeable pool of organic carbon in the ocean. Over
80 % of the marine DOC cannot be characterized at the molecular level {Benner,
2002}. High molecular weight ultrafiltered DOM (UDOM, ≥1000 Da) has been
found to contain a small portion of aged lipid like material {Loh et al., 2004}, which
may include some BC. Studies directly assessing the BC and BC-like material
using traditional BC (CTO-375) and FTIR-ICR-MS techniques, in riverine, coastal
and open ocean DOC estimate that BC could be up to 5 % of DOC {Mannino and
Harvey, 2004; Kim et al., 2004; Dittmar and Paeng, 2009}.
80
5.3
Approach
BC is a heterogenous material that is aromatic in nature. Soot BC, formed during
high temperature combustion, is characterized by a condensed aromatic structure
(e.g. Figure 2.9, BPCA distributions of BC references materials). In contrast char
BC, formed at lower combustion temperatures, is more oxidized and has a less
condensed aromatic structure. These characteristics can be determined using
NMR {Czimczik et al., 2003}, elemental analysis {Hammes et al., 2008} and the
benzene polycarboxylic acid (BPCA) method {Glaser et al., 1998}. We used the
BPCA method to quantify and characterize BC in marine DOM {Ziolkowski, 2009}.
The abundance of radiocarbon (14 C) in BC will be indicative of its source. Fossil fuel
produced BC contains no radiocarbon (is isotopically “dead”) and thus has a
14
C
age of greater than 50,000 years (the detection limit). In contrast BC produced from
biomass burning has a
14
C value of the contemporary biosphere C. Here we use
radiocarbon measurements of BC markers using the BPCA method to determine
the cycling and residence time of BC in the marine DOC pool.
BC was extracted from a series of UDOM samples (Figure 5.1, Table 5.1) and was
analyzed using the BPCA method {Glaser et al., 1998; Brodowski et al., 2005} as
described in Chapter 4. These samples represent a wide range of ocean locations,
sources and ages of DOC and one river sample. UDOM samples were digested in
concentrated nitric acid to oxidize BC to BPCAs (marker molecules) and were subsequently isolated and purified via pcGC before radiocarbon analysis (see Chapter
2). Individual and nitrated BPCAs were pooled for radiocarbon analysis. No B6CA
were collected for radiocarbon analysis.
81
82
11783, 11784
11921, 11924
10879, 10880
11923, 11924
11925, 11929
11919, 11920
UCIDs
30.7
8.4
37.9
-12.0
19.6
32.5
lat
N
82.5
55.6
73.7
-9.4
-156.0
123.5
long
W
1
2
2
2
20
1000
depth
m
34.180
30.972
33.614
36.071
salinity
6
27
19
26
approx. temp.
oC
3,270
78
74
91
68
38
TOC
µM
-24.6
-22.2
-20.0
-21.6
δ13 CUDOM
h
47.7
16.2
16.3
14.9
13.7
C:N
152.7 ± 1.7
-99 ± 3
2.9 ± 2.6
-90 ± 3
-311 ± 3
-445 ± 2.8
∆14 CUDOM
h
2
obtained from the International Humic Society catalog #1R101N, May 1999, not acidified
MP08-56, May 2003
3 DF20606, June 2006, not acidified
4 SE Atlantic, stn95, April 16, 2003
5 Calcofi 0610, stn80.100, Nov 2006, [TOC] based on Beaupré et al. {2007}, using sal and temp from 500m
6 NEHLA 20m pipe, [TOC] based on D. Hansell’s website, http://www.rsmas.miami.edu/groups/biogeochem/index.html. Accessed on
July 3, 2009.
2,3,4,5 Collected by L. Aluwihare.
6 Collected by M. McCarthy.
1 NOM
Suwanee River1
Amazon influenced2
Mid-Atlantic Bight3
SE Atlantic4
N. Central Pacific6
NE Pacific5
Sample
Table 5.1: Sample information for UDOM samples in this study. Isotopic values were determined on UDOM.
Figure 5.1: Map illustrating sample locations. Filled dots indicate surface (1 or 2
m) or near surface (20 m) depth. Open dot indicates deep sample (1000 m).
5.4
Results
The high proportion of B5CA and B6CA products formed from BC in DOM concentration from the Suwannee River illustrates that terrigenous BC is condensed
in its aromatic structure. In contrast, BPCAs formed from open ocean UDOMBC has a uniformly smaller and less condensed aromatic structure because of
the higher proportion of B3CA and B4CAs formed and absence of B6CAs (Figure 5.2). Suwannee River DOC exhibited the largest BC structure (average BPCA
size = 4.71 acids) and the distribution of BPCAs formed resembled the distribution
of BPCAs formed from charred BC (see Figure 2.9). In the Amazon influenced
sample, that contains a mixture of marine and riverine DOC, the average BC structure (average BPCA size = 4.04 acids) is less condensed than the Suwanee River
structure. The structure of the BC in the open ocean samples (average BPCA size
= 3.5 - 3.92 acids) is less condensed and unvarying in composition, regardless of
depth or ocean location.
83
Figure 5.2: BPCA distribution and 14 C of BC for the samples. For each sample,
the distribution of BPCAs is calculated by relating the total carbon of an individual
BPCA (including nitrated peaks) to the total BPCA carbon.
The ∆14 C values of bulk UDOM ranged from +152 h in the Suwannee River to
-445 h in the the deep NE Pacific Ocean, respectively (Table 5.1). BPCAs formed
from UDOM-BC in Suwannee River were significantly more depleted in
14
C. The
Suwannee River DOM is mostly bomb carbon (due to nuclear weapons testing),
while BPCAs formed from BC are pre-bomb, though mean averages are likely
less than a century old (Table 5.2). In contrast, the BPCAs formed from oceanic
UDOM-BC were 14 C depleted. The ∆14 C values of collected BPCAs correlate with
the BPCA distributions (Figure 5.2). That is, the younger precursor of BPCAs, BC,
is a more condensed aromatic than the older precursor of BPCAs. If the BC in the
Amazon influenced sample is conservative with salinity, a mass balance calculation
reveals that the
14
C age of BC exported from the river is also modern (∆14 C ∼ 0
h).
84
85
11803,11804
11956
10878, 11721
11971
11958
11955
4.71
4.04
3.75
3.68
3.92
3.50
average BPCA
-30 ± 4
-33 ± 4
-23 ± 4
-33 ± 4
-30 ± 4
-28 ± 4
δ13 C
-49 ± 33
-727 ± 44
-858 ± 19
-897 ± 55
-880 ± 38
-918 ± 31
∆14 CBC
C age
410 ± 280
10,400 ± 1300
15,680 ± 1100
18,300 ± 4300
17,000 ± 2500
20,100 ± 3000
14
∆∆14 C1
-202
-629
-861
-807
-569
-473
BC in DOC
29.4 µM
300 nM
560 nM
330 nM
90 nM
330 nM
∆∆14 C is ∆14 CUDOM - ∆14 CBC
2
Blank corrected with the average of process blanks UCID 11701, 11702, 11703, 11704, 11705 and 11706.
3
Blank corrected with process blank UCID 11954.
1
Suwannee River2
Amazon influenced3
Mid-Atlantic Bight2
SE Atlantic3
N. Central Pacific3
NE Pacific3
UCID
Table 5.2: Measurements of black carbon isolated from UDOM. δ13 C are AMS measured values, reported assuming derivative carbon has a δ13 C of -12 h. The estimation is an approximate minimum value of BC in DOC because ultra-filtered
material was analyzed, which constitutes only a portion of the DOC pool. Average BPCA is the average number of acid
functional groups produced in the oxidation of BC.
Contemporary
14
C ages of riverine and riverine-influenced BC suggests a non-
fossil fuel source of BC exported from these rivers, but is difficult to reconcile with
a biomass combustion source. The BPCAs distribution of riverine and riverineinfluenced BC resembles that of charred BC, which agrees with the findings that
char is mobilized in watersheds {Hockaday et al., 2007}. When this material reaches
the ocean, it appears that the UDOM quickly loses its aromatic character. This loss
of aromaticity could be due to photochemical degradation. In estuaries the abundance of aromatic compounds exposed to ultraviolet radiation has been observed
to decrease {Tremblay et al., 2007; Gonsior et al., 2009} and / or microbial utilization {Carlson, 2002}. Since only aromatic materials form BPCAs, it is unlikely that
this decreased aromatic character of UDOM is due to dilution with non-aromatic
material.
The BPCA distributions of the marine samples suggest that BC cycling in the open
ocean is distinct from the BC that is exported from the Suwannee and Amazon
Rivers. If the BC exported from rivers remained unaltered in the UDOM pool,
one would expect the chemical composition of BC from the Atlantic Ocean to be
more similar to terrestrial BC. However, this is not the case. The ∆14 C value and
BPCA distribution of BC isolated from the Mid-Atlantic Bight and SE Atlantic do
not appear to resemble the aromatically condensed modern 14 C exported from the
rivers. Additionally, the ∆14 C values of BPCAs isolated from UDOM-BC from the
Pacific were not significantly different from the Atlantic samples.
Atmospheric inputs of BC reveal a wide variety ∆14 C values (-220 to -600 h), in-
dicating a variety of sources {Eglinton et al., 2002; Gustafsson et al., 2009}. Amazon Basin atmospheric BC had a mean particle size of 0.175 µm {Echalar et al.,
1998}, smaller than the size cutoff for DOC, indicating that more than half of the
86
BC aerosols could be incorporated into the UDOM. Unless soot, originating from
fossil fuel combustion, is chemically solubilized via atmospheric oxidation, drastically changing its chemical composition, before deposition to the surface ocean
{Decesari et al., 2002} it is unlikely that the isolated BC originated from recent fossil
fuel emissions. The distribution of BC oxidation products in marine samples do not
resemble the condensed aromatic character of soot particles (see Figure 2.9).
BPCAs extracted from UDOM-BC had more depleted 14 C values than bulk UDOM,
suggesting that the BC is more recalcitrant or has other sources compared to other
components of the bulk UDOM. The ∆14 C off-set between UDOM-BC and bulk
UDOM is not consistent ( ∆∆14 C ranged from -202 to -861 h, Table 5.2), suggest-
ing that UDOM-BC cycles on longer times scales than any chemical components
of DOC identified to date. If BC was produced in situ (e.g. mid-water column production {Yamashita and Tanoue, 2008} or from bacterial production {Ogawa et al.,
2001}), the ∆14 C of newly produced “BC” would reflect that of bulk DOC, that is
being consumed. However, little variation of the 14 C age of UDOM-BC is observed
and it is consistently older than bulk DOC (Figure 5.3). While the two river systems
studied, Suwannee and Amazon, contain modern
14
C levels and are condensed
in aromatic character, it is possible that other river systems could export older
graphitic material (e.g.: Dickens et al. {2004}). However, graphitic black carbon is
of a condensed aromatic structure and upon oxidation would likely produce more
substituted BPCAs (B6CA, B5CA), such as that observed for soot.
These data represent BC in UDOM, which typically represents 25 to 35 % of the
bulk DOC {Benner, 2002}. Assuming a conversion factor of 4 (see Chapter 2.4.5),
BC in my samples ranged from 0.5 to 3.5 % (on a carbon basis). This corresponds
to minimum marine BC concentrations of 85 – 500 nM. It is likely that the low
87
molecular weight (LMW) fraction of marine DOM contains a higher proportion of
BC than that in the UDOM, as suggested by an isotopic mass balance study {Loh
et al., 2004} and the decreasing size of BPCAs with ∆14 C value (Figure 5.2). The
concentrations presented here are in agreement with BC concentrations (600 to
800 nM) in solid-phase extracted DOC determined in the S. Atlantic and S. Ocean
{Dittmar and Paeng, 2009}. The LWM DOM would contain aromatic molecules
from polycyclic aromatic hydrocarbons to larger molecules such as fullerenes (e.g.:
C60 ). The UDOM-BC studied here are likely macromolecules or more likely, smaller
molecules that are complexed thus making them larger than the 1000 Da size cutoff. If the size of BC in the DOC pool, as inferred by the observed BPCA distribution, is a function of
14
C age, it is likely that the BC in the LMW fraction of DOC is
older than the values presented here.
From the age differences between BC and non-BC sedimentary organic carbon
(SOC), Masiello and Druffel {1998} suggested that BC resides in the DOC pool
from 2400 to 13,900
14
C years before deposition. Assuming the average annual
flux of pre-industrial BC to the world oceans is 10 Tg per year {Suman et al., 1997}
and a marine DOC pool of 685 x 1015 g C {Hansell and Carlson, 1998}, Masiello
and Druffel {1998} calculated that BC could be up to 4 to 22 % of the total deep
ocean DOC. Therefore 27 – 151 x 1015 g could be BC, corresponding to a concentration of BC in marine DOC between 1.7 and 9.4 µM. In the deep NE Pacific,
we found that the UDOM was 3.5 % BC or 0.29 µM (see Chapter A.2 for calculation details). Thus, if DOC is 4 to 22 % BC, then the lower molecular weight DOC
contains a substantial proportion of BC (1.4 – 9.1 µM).
Our measurements of the radiocarbon of BPCAs extracted from UDOM-BC from
the deep NE Pacific Ocean had a
14
C age of BC of 20,000 ± 3,000 years. This
residence time calculation assumed that the source(s) of BC to the ocean are 14 C
88
Figure 5.3: ∆14 C of black carbon and marine DOC as a function of depth. The
depth profiles of DOC are from the Sargasso Sea (SS) and North Central Pacific
(NCP) from Druffel et al. {1992}.
89
modern. However, with industrialization fossil fuel and coal combustion (soot) has
increased the amount of 14 C depleted BC {Bond et al., 2004}. If BC from fossil fuel
or coal combustion was incorporated into the UDOM-BC, it would decrease the
14
C signature. Stable carbon isotope (δ13 C) measurements of BPCAs extracted
from UDOM-BC may help pinpoint the source of the BC. However δ13 C measurements of BC would only distinguish a marine or terrestrial (forest fire and fossil
fuel combustion) source. Since the BPCA distribution of UDOM-BC does not resemble soot, it is unlikely that
14
C depleted soot is present in the deep NE Pacific
Ocean sample. This does not exclude the presence of other yet to be discovered
source(s) of 14 C depleted aromatic compounds, resembling BC, in the UDOM.
BC has a much lower ∆14 C value than DOC (-900 h versus -500 h for the deep
Pacific Ocean). If BC is up to 22 % of the deep DOC (i.e. 9.4 µM), then the
remaining 78 % (28.6 µM) would have an average 14 C of -388 h (4000 14 C years),
which is significantly younger than the deep bulk DOC value (6000
14
C years).
Our hypothesis that a large fraction of old BC is in the LMW fraction of DOC is
at odds with this calculation and needs to be tested. It is not unlikely that old
smaller BC is in the LMW fraction, as the LMW fraction of DOC has been found
to be significantly older than the HMW fraction {Santschi et al., 1995}. Molecular
analysis of BC mobilized within a fire-impact watershed had a peak mass to charge
ratio of 400 {Hockaday et al., 2006}, which is equivalent to the size of a five ring
PAH and would be in the LMW fraction of DOC. Should it be proven that there
is a large pool of aged BC in the LMW fraction of DOC, then this would help to
explain the enigma that has existed in our understanding of the BC cycle. That is,
the sources of BC far outweigh the known sinks. However, marine DOC may be a
temporary reservoir for BC, and processes that are responsible for its breakdown,
(e.g. photochemical oxidation, bacterial remineralization or physical removal to the
sediments) warrant investigation.
90
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6
Conclusions and thoughts on future research
This thesis helped improve the utility of molecular-level compound specific radiocarbon analysis (CSRA) as an approach to understanding the source and cycling
of recalcitrant carbon in marine dissolved organic carbon. The following topics
were addressed.
6.1
BPCA method and its applicability
Measurements of black carbon (BC) in environmental matrices, such as sediment
or soil, require the isolation of BC from non-BC material. This separation is typically
done using chemical or thermal methods. The benzenepolycarboxlic acid (BPCA)
method, a chemical oxidation technique, oxidizes the BC to BPCAs that are markers of BC. Information about the temperature of combustion can be inferred from
the distribution of BPCAs. This method seemed ideal for studying BC in marine
DOC because it provides both quantitative and qualitative characterization of BC.
At the outset of this work, little was known about the chemistry of the BPCA
method. A suite of polycyclic aromatic hydrocarbons (PAHs), compounds that are
96
structurally similar to BC, were studied to identify any systematic oxidation patterns, which may have proven useful for modeling the structure of oxidized BC.
PAHs larger than four rings typically favored the formation of the largest possible
BPCAs than that expected from stoichiometry. Smaller PAHs favored the formation
of smaller BPCAs than that expected from stoichiometry. The pattern of oxidation
products was not systematic enough to model the parent structure of the oxidized
BC, but was predictable enough to provide generalized information about the structure of the BC oxidized.
Preliminary experiments revealed that some of the BPCAs produced from the oxidation of BC were nitrated (contained one or more -NO2 functional groups). Nitrated BPCAs were not quantified in previous versions of the BPCA method. The
oxidation of PAHs produced exclusively nitrated BPCAs. I concluded that the quantification of nitrated BPCAs was essential for the accurate estimation of BC; thus
nitrated BPCAs were quantified for all samples presented within this thesis.
During the nitric acid oxidation process, sample handling and PCGC isolation
losses of sample material occur. The distribution and recovery of the oxidation
products of the PAH anthracene was studied in great detail, as it seemed well
suited to be an internal standard that could be added before oxidation to account
for these losses. Quantitatively producing predominantly B2CA, which is not typically included as oxidation products of BC, anthracene could be used as an internal
standard to correct for losses in sample handling. However, anthracene was only
used in those samples that were passed through the cation column (e.g.: sediments and CNTs in Chapter 3).
Integrating the carbon yield data for the range of BC or BC-like materials studied,
a conversion factor from BPCAs to BC was determined. Generated BPCAs were
converted to BC using the conversion factor of 4 ± 1 (the inverse of 25 ± 6 %).
97
BC yields from reference materials presented here illustrate that this version of
the BPCA method is effective at quantifying both soot-like and char-like BC and
more satisfactory for quantifying BC than either the chemo-thermal oxidation (e.g.:
CTO-375) or previous versions of the BPCA method.
6.2
Evaluating extraneous material added during the
preparation of CSRA samples
Measurements of natural abundance
14
C of individual compounds requires two-
distinct, rigorous sets of laboratory protocols. These steps consist of (i) the careful
separation of pure compounds from a complex mixture by PCGC and (ii) the preparation and analysis of this sample by
14
C-AMS. Uncertainty of
14
C measurements
of individual compounds is a function of both steps in sample preparation. Commonly, the uncertainty of the second step is the only 14 C uncertainty reported and
the uncertainty of the first step is grossly overlooked.
In an attempt to constrain the uncertainty of CSRA measurements of BC, I examined the mass, sources and variability of extraneous carbon added in the first step
of sample preparation. The extraneous carbon added in the separation of pure
compounds by PCGC originate from two discrete sources: the chemical preparation of samples (e.g.: chemical oxidation and solvents) and from within the PCGC
(i.e.: column bleed). When evaluated both indirectly and directly, the mass of extraneous carbon (Cex ) appeared to be a function of collection time. It also became
apparent that only half of the Cex , which could be attributed to the PGCG (e.g.:
column bleed). The other half of Cex likely originates from solvents and reagents
and thus, it could change over time as new batches of chemicals are used. Discon-
98
certingly, I observed that the mass and ∆14 C value of the Cex changed over time,
which means that the blank must be evaluated frequently when processing small
PCGC samples (≤20 µC).
6.3
Black carbon in the marine DOC pool
Based on radiocarbon measurements of BC in marine sediments, Masiello and
Druffel {1998} postulated that BC was 4 to 22 % of deep dissolved organic carbon
(DOC). Using the BC extraction and CSRA methods developed within this thesis
and a suite of UDOM samples from a river and five ocean locations, I tested this
hypothesis.
The BPCA distribution of the Suwannee River resembled char and was the most
condensed aromatic structure of BC measured in these UDOM samples. BC from
open ocean samples produced smaller BPCAs, indicating that the structure of BC
becomes smaller the longer it is in the ocean. The radiocarbon signature of BC
from the Suwannee River was modern, while the BC extracted from open ocean
samples was uniformly depleted in
14
C. BC in the deep Pacific had a ∆14 C = -918
± 31 h, and this value represents the oldest reported compound isolated from
DOC. It is likely that the lipids extracted from UDOM by Loh et al. {2004} contained
depleted BC, similar to the material quantified here. The fraction of carbon UDOM
that is ranged from 0.5 – 5 %. Since UDOM typically only represents about 25 % of
the DOC pool, the proportion of BC in the low molecular weight (LMW) pool must
be much higher if there is 4 – 22 % BC in marine DOC.
99
6.4
Future work
The conclusions of this thesis, as well as the unanswered questions, lead to many
potential avenues of new research. Some of these potential avenues are listed
below.
What is the “black carbon” in marine DOC?
BC cycling in marine DOC is complex. The source(s) of this material were not
well quantified by the CSRA presented within this thesis. The δ13 C of BC extracted from UDOM values were not well defined (-24 to -33 h, UCI Keck AMS
values). Thus, the source of radiocarbon depleted BC in marine UDOM-BC is not
well constrained. Using HPLC to isolate the BPCAs, thereby eliminating the need
to derivatize of BPCAs and thus eliminate potential isotope fractionation associated with incomplete derivatization, could be valuable for determining the δ13 C of
BC in marine DOC.
Chemically, what is BC? Could this material be graphitic BC {Dickens et al., 2004}
or resuspended BC from sediments? Or is it simply old terrestrial material? Can
its composition be described using other techniques or isotopes?
Since the BPCA method detects all aromatic structures and the BC isolated from
UDOM was ubiquitously old, could the material detected originate from a noncombustion source? Could asphaltenes, which are remnants of fossil kerogen
found in suspension with crude oil, be dissolved in DOC? Asphaltenes from oil
have been found to be larger than 1000 Daltons and are theorized to have a molecular structure that would produce the BPCA distribution observed in open ocean
100
H2N
S
O=
S
S
S
O
N
N
S
OH
O
S
N
S
S
S
S
S
S
S
OH
Figure 6.1: Proposed chemical structure of asphaltene, adapted from Artok et al.
{1999}.
UDOM-BC (Figure 6.1). When oil undergoes biodegradation, the asphaltene fraction increases {Peressutti et al., 2003} indicating that asphaltenes are resistant to
microbial degradation. Previously, asphaltenes have not been considered to be in
the DOC pool as they may not be soluble in DOC. An oversight of this dissertation
was not studying any kerogen materials, which may have provided more insight
into the asphaltene hypothesis. Analysis of asphaltenes extracted from crude oil
and / or BC in DOC extracted from locations close to oil seeps (i.e.: deep water in
Santa Monica basin) may provide more insight into this hypothesis.
101
Where is the BC?
The loss of highly condensed, B6CA producing, BC exported from rivers suggests
that BC is removed from HMW DOC. But where does the riverine BC go? Are
the smaller BPCAs measured in the ocean, remnants of the condensed aromatic
material exported by rivers? There are at least three possibilities of what happens
to the BC. It could:
1. undergo microbial consumption, removing some or all of the aromatic material.
2. undergo photochemical oxidation BC in the surface ocean, quickly removing
the condensed aromatic character of BC exported from the rivers.
3. flocculate and / or sorb onto particles ultimately leading to its burial in sediments.
The most direct way to quantify the fate of the condensed aromatic BC would be
to quantify the BPCAs in marine DOC isolated by two techniques: ultrafiltration
(UF, size of particles) and reverse osmosis/electrodialysis (RO/ED) {Gurtler et al.,
2008}. If one were to analyze the BC in these two types of isolates of marine
DOC extracted at the same location and time, one could provide a more accurate
estimation of the fate of the BC in marine DOC. Is the concentration of BC higher
in the LMW DOC? Its also very likely that “BC” in LMW DOC is actually a mixture
of PAHs with alkyl side-chains and not complex macromolecules. While chemically
solid phase extraction (SPE) may be well suited for BC extraction, the range of BC
molecules extracted by SPE will not be the sample as those extracted by UF and
RO/ED thereby limiting our ability to compare the extracted BC with the total DOC
composition.
102
Studying the loss of terrestrial BC within an estuarine system may provide insight
into the mechanism(s) that remove BC from the water column. Any such study
should include quantification of the BC in sediment, DOC and particulate organic
carbon (POC). Flores-Cervantes et al. {2009} demonstrated the important role of
POC in the removal of BC from the surface ocean. The major limitation of using
CSRA to study the loss BC from riverine DOC is extracting sufficient amounts of
material for CSRA. Within this study, at least one liter of seawater was required to
provide enough UDOM-BC for one CSRA sample. The volume of water required
to extract enough material for CSRA limits the types of process studies possible
(i.e. photochemical incubation).
Since each BC method quantifies a different type of BC, it is difficult to compare
ages of BC isolated by different BC methods. This version of the BPCA method
appears to quantify the same BC as the chromic acid method used by Masiello
and Druffel {1998}, as indicated by BC standard materials. Would BC isolated from
sediments using the BPCA method have the same radiocarbon age offset between
the BC and non-BC material? Quantifying the BC in sedimentary material would
provide insight into the variability of the time BC spends in an intermediate pool
before sedimentary deposition.
This is the first work quantifying and radiocarbon of BC in marine DOC, therefore
this work leaves an open pathway for future studies on the spatial and temporal
variability of BC in the marine DOC pool. How do these measured concentrations
vary? How variable is the isotopic composition of BC in the ocean? Is there any
part of the ocean where the BC is significantly older (i.e.: where BC is accumulating)?
103
Bibliography
Artok, L., Y. Su, Y. Hirose, M. Hosokowa, S. Murata, and M. Nomura, Structure and
reactivity of petroleum-derived asphaltene, Energy Fuels, 13, 287 – 296, 1999.
Dickens, A., Y. Gelinas, C. Masiello, S. Wakeham, and J. Hedges, Reburial of fossil
organic carbon in marine sediments, Nature, 427(6972), 336–339, 2004.
Flores-Cervantes, D., D. Plata, J. MacFarlane, C. Reddy, and P. Gschwend, Black
carbon in marine particulate organic carbon: Inputs and cycling of highly recalcitrant organic carbon in the gulf of maine, Marine Chemistry, 113(3-4), 172–181,
doi:10.1016/j.marchem.2009.01.012, 2009.
Gurtler, B., T. Vetter, E. Perdue, E. Ingall, J. F. Koprivnjak, and P. Pfromm, Combining reverse osmosis and pulsed electrical current electrodialysis for improved
recovery of dissolved organic matter from seawater, Journal of Membrane Science, 323(2), 328 – 336, doi:10.1016/j.memsci.2008.06.025, 2008.
Loh, A., J. Bauer, and E. Druffel, Variable ageing and storage of dissolved organic
components in the open ocean, Nature, 430(7002), 877–881, 2004.
Masiello, C., and E. Druffel, Black carbon in deep-sea sediments, Science,
280(5371), 1911–1913, doi:10.1126/science.280.5371.1911, 1998.
Peressutti, S. R., H. M. Alvarez, and O. H. Pucci, Dynamics of hydrocarbondegrading bacteriocenosis of an experimental oil pollution in patagonian soil,
International Biodeterioration and Biodegradation, 52(1), 21 – 30, doi:10.1016/
S0964-8305(02)00102-6, 2003.
104
A
Determination of Carbon Yields
A.1
Polycyclic Aromatic Hydrocarbons
How the carbon recovery was calculated for polycyclic aromatic hydrocarbon (PAH)
samples.
The following steps were taken:
1. The peak area relative to internal standard was determined for each BPCA.
[relative peak area]
2. The relative calibration curves were applied to each BPCA. Nitrated BPCAs
were assumed to have the same calibration as non-nitrated BPCAs of the
same number of functional groups (i.e.: B4CA-N1 was calibrated using the
B4CA data). [mg BPCA]
3. The mg BPCA from step (2) were converted to mg C as BPCA using the
percent carbon for each BPCA (see Table A.1). [mg C BPCA for individual
BPCAs].
105
Table A.1: Carbon content of BPCAs.
B#CA
B3CA
B3CA
B4CA
B4CA
B5CA
B5CA
B6CA
N#
N0
N1
N0
N1
N0
N1
% Carbon
51.4
41.7
47.2
40.0
44.3
38.5
42.1
4. The mg C BPCAs of individual BPCAs (including those nitrated) were summed
to get a carbon yield [sum of mg C for all BPCAs].
5. If the relative carbon yield is to be reported, divide (4) by % C in starting
material [mg C as “BC”].
Results from the formation of BPCAs from anthracene are shown in Table A.2 as
an example of how the carbon yields were calculated.
A.2
Calculating the percentage of black carbon in
UDOM
How the carbon recovery was calculated for UDOM-BC samples.
The following steps were taken:
1. The peak area relative to internal standard was determined for each BPCA.
[relative peak area].
106
107
1
Anthracene
mg C
94.3 % C
4.70
5.96
3.09
2.35
4.81
5.01
5.28
7.90
BPCA relative peak area
B2CA-N1
B2CA-N2
45.1 % C
36.9 % C
2.46
0.42
3.10
0.52
1.70
0.31
1.07
0.19
2.44
0.19
2.61
0.35
2.70
0.34
3.27
0.26
1.03
1.30
0.71
0.45
1.03
1.09
1.13
1.81
1.17
1.48
0.81
0.51
1.09
1.21
1.24
2.00
average
sum
∗ %C.
24.9
24.8
26.2
21.7
22.7
24.2
23.5
25.3
24.2 ± 1.6
% C yield
(normalizedpeakarea−0.01495)
1.0672
0.14
0.18
0.10
0.06
0.06
0.12
0.11
0.09
mg C BPCA1
B2CA-N1 B2CA-N2
Calibration of 3-nitrophthalic acid relative to internal standard. mg C BPCA=
94
95
107
108
109
110
112
113
UCIZ
Table A.2: Example calculation of BPCAs in PAHs.
2. The relative calibration curves were applied to each BPCA. Nitrated BPCAs
were assumed to have the same calibration as non-nitrated BPCAs of the
same number of functional groups (i.e.: B4CA-N1 was calibrated using the
B4CA data). [mg of individual BPCAs].
3. The mg of individual BPCAs was converted to mg C as BPCA using the
percent carbon for each BPCA (see Table A.1). [mg C of individual BPCAs].
4. The mg C of individual BPCAs are summed [mg C BPCA].
5. The BPCAs are converted to “BC” by dividing by 25 % (conversion factor)
[mg C BC].
6. The initial carbon used was calculated by multiplying the weight UDOM by
the percentage of carbon in UDOM [mg C UDOM].
7. Then, divide mg C BC by mg C UDOM to get the carbon yield [carbon yield].
108
B
BPCA protocol
This is the generalized protocol used to process samples within this thesis. A more
detailed protocol is available upon request. Each step of the protocol is followed
by an explanation of the logic behind the step.
B.1
B.1.1
Chemical extraction of BPCAs
Cleaning the bomb
Add 2 mL of concentrated HNO3 directly into the Teflon sleeves within the bomb.
Put the bomb in the oven for 4 hours at 180 o C. At the same time, bake all glassware
for 2 hours at 550 o C that will be used for processing the samples.
Cleaning the bomb with HNO3 will oxidize any carbon that may have accumulated
in the Teflon over time. This step should be conducted every few weeks. Baking
the glassware will also remove any residual carbon.
109
B.1.2
Pre-treatment of samples (if required)
This step is required for soil, mineral containing aerosols and sediments containing
cations (i.e.: Fe3+ ). There are two additional steps in the processing of samples
that contain cations: TFA pre-treatment and cation column processing. Both steps
are required to remove the cations. It is recommended to add the anthracene
internal standard at the beginning of this step to account for any losses of sample
through the processing.
Weigh out samples into pre-baked quartz bomb tubes. Add 10 mL of 4 M trifuloroacetic acid to each tube. Cook in the bomb for 4 hours at 104 o C. Filter these
samples and retain the material on the filter. Dry the samples at 45 o C.
If the cations are not removed from the sample, they will interfere with the oxidation
and derivatization of the BPCAs.
B.1.3
Filter sample
Filter the samples after bombing. Rinse with at least 15 mL Milli-Q water. Retain
the filtrate.
Some solid materials are not oxidized and remain. These particles need to be
removed from the BPCA solution.
B.1.4
Cation column (if required)
For soil, mineral aerosol and sediment samples drop the filtrate onto the cation
column when it is in the H+ state.
110
This is the second step specific to process samples that contain cations. The resin
retains the cations.
B.1.5
Dry samples
Freeze samples in the freezer. Then dry for 24 hours in the freezedryer.
Water interferes with the derivatization and can damage the GC. Therefore samples are dried to remove the water.
B.1.6
Derivatize
Add 5 mL of methanol to each dried sample. Add a fixed volume of the derivatization standard (biphenyl-2,2’-dicarboxylic acid). Titrate the sample with diazomethane until the solution remains yellow.
Derivatization converts the acid groups into methyl esters, which increases the
separation of compounds on the GC and makes them more responsive to the detector. Upon GC analysis if the yield of the derivatization standard is low, it is likely
that cations are present in the solution.
B.1.7
Solvent change
Dry the derivatized BPCA solutions under a stream of N2 . Add a fixed volume of
dichloromethane. Transfer the sample to a GC autosampler vial.
Polar solvents, like methanol, cannot be injected onto a GC column. Therefore we
111
must change to a solvent more amenable for use on a GC column. The methylesters of BPCAs (created in the previous step) are most soluble in dichloromethane.
B.2
PCGC settings and parameters
General parameters of the PCGC are given in Table B.1. More detailed information
is available upon request.
Table B.1: PCGC settings and parameters
B.2.1
Column
DB-XLB column
30 m x 53 mm x 1.5 µm film thickness
Oven temperature program
initial temperature 100 o C
ramp to 280 o C
hold, collect all samples
ramp to 320 o C
bake out column (5 minutes)
Determination of sample concentration and retention times
Inject the samples and assess the retention times (RTs) of the compounds of interest. Assess if the concentration of the compounds of interest are sufficient for
collection. If not, concentrate the solution by reducing the volume.
112
B.2.2
Program the preparative fraction collector (PFC) to collect at selected RTs
Take care to trap the entire peak, as isotopic fraction occurs if the whole peak is
not captured.
B.2.3
Prime the PCGC for collection
Inject and collect the sample for 10 injections. Discard the collected material and
replace the U-tubes with clean, baked out, U-tubes.
We have found that the BPCA methyl esters from the previous sample remain in
the sample for a few injections. If this step is not done, the
14
C of the collected
samples will contain an unknown portion of the previous sample.
B.2.4
Collect sample(s)
Inject and collect the sample for 50 injections.
Inject the sample many times to concentrate the compound(s) of interest into an
isolate. We try limiting the number of injections to 50 because it may minimize the
contribution of extraneous carbon added from the PCGC. Our blank corrections for
compound specific radiocarbon analysis are normalized to 50 injections.
113
B.2.5
Check the concentration and purity of the isolate
After the collection has finished, transfer the isolate to the GC autosampler vial
using a known volume of dichloromethane. Run the sample, as in Appendix B.2.1,
to assess the concentration and purity of the sample.
It is important to know if your isolated sample actually contains the only the compounds of interest that you programmed the PFC to collect. It is important to confirm the purity of the isolate. The concentration of this solution will provide yield
information to assess the recovery of the injected samples.
B.2.6
Prepare sample for combustion
Transfer the solution (dichloromethane and isolate) to a clean 6 mm quartz tube.
Dry the dichloromethane under a stream of N2 . Add the combustion chemicals,
evacuate to 10-6 Torr and flame off.
114