DETERMINATION OF DISTRIBUTION COEFFICIENTS FOR
CATION EXCHANGE RESIN AND OPTIMISATION OF ION
EXCHANGE CHROMATOGRAPHY FOR CHROMIUM
SEPARATION FOR GEOLOGICAL MATERIALS
A thesis submitted to The University of Manchester for the degree of
Master of Philosophy
in the Faculty of Engineering and Physical Sciences
2012
CATHERINE MARY DAVIES
School of Earth, Atmospheric and Environmental Sciences
TABLE OF CONTENTS
Title page
1
Table of contents
2
List of Tables
5
List of Figures
7
Abstract
8
Declaration
9
Copyright Statement
10
Acknowledgements
11
Chapter 1: Introduction
1.1 Chromium
12
1.2 Chromium isotopes and applications
13
1.2.1 Separation and measurement of chromium isotopes
1.3 Aims and objectives of this study
15
17
1.4 Thesis structure and rationale for submitting as alternative
format
18
1.5 References
19
Chapter 2: Paper 1 - Determination of Distribution Coefficients
for Cation Exchange Resin
2.1 Introduction
20
2.1.1 Principles of ion exchange chromatographic chemistry
21
2.1.2 Distribution coefficient determinations
23
2.1.3 Aim of this study
24
2
2.2 Analytical procedures
24
2.2.1 Reagents and materials
24
2.2.2 Experimental procedures for distribution coefficient
27
determinations
2.2.2.1 Inductively coupled plasma mass spectrometer
(ICP-MS)
29
2.2.2.2 Determination of element concentrations by ICP-MS
31
2.3 Results and Discussion
32
2.4 Conclusions
43
2.5 References
45
Paper to be submitted to Analytical Chemistry
Chapter 3: Separation and Purification of Chromium from
Geological Materials
3.1 Introduction
46
3.2 Geological materials and standards
48
3.3 Digestion procedures
50
3.3.1 Digestion methods of previous work
51
3.3.2 Digestion methods used in this study
52
3.3.2.1 Reagents
52
3.3.2.2 Hotplate digestion
53
3.3.2.3 Microwave digestion method
53
3.3.3 Selection of the preferred digestion method
55
3.4 Chromium speciation assay
57
3.4.1 UV spectrophotometer
58
3.4.2 Results and discussion of the chromium assay
59
3
3.5 Development of chromium purification and separation
3.5.1
60
Analytical methods for cation exchange column
procedures
65
3.5.2 Multi collector-inductively coupled mass spectrometer
80
3.5.3 Chromium measurements by MC-ICPMS
85
3.6 Results and discussion
88
3.7 Conclusion
95
3.8 References
98
Chapter 4: Conclusion
101
Word count = 24906
4
LIST OF TABLES
Table 2.1 Hydrogen ion concentrations in various nitric acid solutions
27
Table 2.2 Distribution coefficients (Kd) for Standard A and B, in
various nitric acid solutions using
Bio-Rad AG® 50W-X8 resin
33
Table 2.3 Distribution coefficients (Kd) for duplicate samples of
Standard A in 0.4M, 0.5M, 6M nitric acid using
Bio-Rad AG® 50W-X8 resin
35
Table 2.4 Distribution coefficients (Kd) for duplicate samples of
Standard B in 0.4M, 0.5M, 6M nitric acid using
Bio-Rad AG® 50W-X8 resin
36
Table 3.1 Chromium concentrations of geological materials and
standards used in this study
49
Table 3.2 Hexavalent chromium concentrations, determined
by colorimetric assay, of analysed sample dissolutions and
Cr standard solutions
60
Table 3.3 Ion exchange procedure developed by (Trinquier et al. 2008)
62
Table 3.4 Chromium concentration (µg/ml) in sample stock
dissolutions; aliquots of which were removed for
column procedures A – L
66
Table 3.5 Column Procedure A: Initial two-step cation
exchange column procedure trialled in this study,
adapted from Trinquier et al. (2008)
5
69
Table 3.6 Summary of the optimised two-step cation
exchange column procedure developed in this study.
Significant alterations from Column Procedure A are
highlighted in italics.
79
Table 3.7 Summary of alterations made to each column procedure
80
Table 3.8 Nu Plasma typical instrumental operating parameters
85
Table 3.9 Chromium yields from samples processed through twelve,
two-step cation exchange procedures A – L
Table 3.10 Optimised cation exchange procedure for the separation of Cr
6
89
95
LIST OF FIGURES
Figure 2.1 Log of distribution coefficient (Kd) for 38 elements
(most prevalent oxidation state) as a function of nitric
acid molarity with cation resin (Bio-Rad AG® 50W-X8)
44
Figure 3.1 Schematic diagram of the two-stage cation exchange
procedure (after Trinquier et al. (2008)). The Cr fraction
eluted from column 1 is further purified through column 2
63
Figure 3.2 Schematic diagram of Nu Plasma MC-ICPMS
(from Nu Plasma Manual)
Figure 3.3
82
Analysis plot of 52Cr peak from the Nu Plasma
MC-ICPMS
87
7
Catherine Mary Davies - The University of Manchester – Master of Philosophy
Determination of Distribution Coefficients for Cation Exchange Resin and
Optimisation of Ion Exchange Chromatography for Chromium Separation for
Geological Materials.
ABSTRACT
Presented in this study is an improved separation technique for the purification of
chromium from geological materials. Variations in the stable isotope ratios of
chromium (Cr) provide a formidable means of tracing and quantifying Cr(VI)
contamination either from anthropogenic impact or natural attenuation from redox
reactions of Cr from mineral constituents. In order to detect slight changes in the
isotopic composition of Cr, high precision analysis by MC-ICPMS requires
careful isolation of Cr from elevated concentrations of matrix elements present in
geological materials. It is essential that interfering elements with isotopes of
similar masses, such as Ti, Fe and V, must be removed from sample dissolutions.
In comparison with previous studies, a two-step cation exchange separation
procedure has been optimised for the isolation of Cr, for subsequent isotope
analysis by Multiple Collector - Inductively Coupled Plasma Mass Spectrometry,
with improved recoveries and purity. The cation exchange procedure relies on Cr
within the digested materials being trivalent; therefore, a hexavalent speciation
assay was employed to confirm the absence of Cr(VI) in dissolutions of chosen
geological materials and standards, prior to cation exchange.
In order to evaluate improvements made to the separation procedure, numerous
geological materials were digested and processed through the cation exchange
separation chemistry. Effective dissolutions of USGS silicate basalt standards:
BHVO-2 and BCR-2, Cody Shale (SCo-1); Chromite from the Bushveld Igneous
Complex and the carbonaceous chondrite Allende, were achieved by microwave
assisted methods. Chromium was separated from matrix elements and analysed by
MC-ICPMS, in conjunction with isotopically certified Cr standards (NIST 3112a
and SRM 979). The chosen samples possess diverse matrices combined with
varying Cr concentrations (16 – 764,000 ppm) in order to assess the limits of the
exchange procedure. Modifications were effected to twelve sequential two-step
cation exchange procedures, in order to improve the purity and yield of the
separated Cr. Through this fast separation method, low procedural blanks
(typically < 20 ng) are achievable in conjunction with improved purity and
procedural yields of up to 97% for chemically separated Cr.
In addition, to facilitate improvements to the ion exchange procedure, extensive
series of distribution coefficients were determined for 38 elements, across a range
of nitric acid molarities with Bio-Rad AG® 50W-X8 cation resin. Precise
distribution coefficients, with improved detection limits, were obtained providing
an invaluable insight for the sorption properties of Cr and matrix elements as a
function of nitric acid molarity with Bio-Rad AG® 50W-X8 cation resin.
8
DECLARATION
No portion of work referred to in this thesis has been submitted in support of an
application for another degree or qualification of this or any other university or
other institute of learning.
9
COPYRIGHT STATEMENT
The author of this thesis (including any appendices and/or schedules to this thesis)
owns certain copyright or related rights in it (the “Copyright”) and she has given
The University of Manchester certain rights to use such Copyright, including for
any administrative purposes.
Copies of this thesis, either in full or in extracts and whether in hard or electronic
copy, may be made only in accordance with the Copyright, Designs and Patents
Act 1988 (as amended) and regulations issued under it or, where appropriate, in
accordance with licensing agreements which the University has from time to time.
This page must form part of any such copies made.
The ownership of certain Copyright, patents, designs, trade marks and other
intellectual property (the “Intellectual Property”) and any reproductions of
copyright works in the thesis, for example graphs and tables (“Reproductions”),
which may be described in this thesis, may not be owned by the author and may
be owned by third parties. Such Intellectual Property and Reproductions cannot
and must not be made available for use without the prior written permission of the
owner(s) of the relevant Intellectual Property and/or Reproductions.
Further information on the conditions under which disclosure, publication and
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IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in
any relevant Thesis restriction declarations deposited in the University Library,
The
University
Library’s
regulations
(see
http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s
policy on Presentation of Theses.
10
AKNOWLEDGEMENTS
I would like to thank my supervisor Dr. Maria Schonbachler and also Dr. Gina
Moseley and Dr. Karen Theis for their invaluable guidance, encouragement,
support and patience.
Thank you to the University of Manchester; School of Earth, Atmospheric and
Environmental Sciences, the Isotope Cosmochemistry Group and notably the
Science and Technology Facilities Council for funding this research.
I am indebted to many people for their help and would like to thank: Richard
Cutting, Ruth Carter, John Cowpe, Jon Fellowes, Richard Hartley, Dave
Blagburn, Steve Olivier, Paul Lythgoe, Hugh Coe, Nicky Snook, Ann Webb,
Mike Turner, Dave Polya, Bev Clementson, Katherine Harrison, Henriette
Uckermann, Nicola Ashton, Waheed Akram, Alastair Bewsher, Niel Williams,
Patricia Clay, Lisa Abbott, Chris Boothman, Paul Wincott, Jim Brydie, Dan Lea
and Steve Stockley.
Words cannot express how grateful I am to my friends (indeed) Karen and Gina
for help with this thesis, kindness, and friendship – thank you.
A heartfelt thank you is expressed to all my family. Thank you and love to my
parents, for their support, their belief in me and always being there for me.
Most importantly, thank you to my daughter, Lauren, for her constant support and
caring. You are my sunshine and I love you to the moon and back.
11
Chapter 1:
Introduction
1.1 Chromium
In the following introduction an overview of the element chromium (Cr) is given
together with an outline of the importance of Cr isotope determinations for
geological materials and reasoning for an optimised separation procedure for the
isolation of low levels of Cr from such materials which possess varying matrices.
Chromium is the 21st most abundant element on Earth. It is a transition metal,
atomic number 24, with a partially filled outer d orbital and as such can form
numerous compounds that are often highly coloured. The presence of Cr in rubies
creates the distinctive red colouration and the vivid green colour of emeralds.
Natural occurrence of Cr in various minerals, such as chromite (FeCr2O4) and
crocite (PbCrO4) are exploited for the production of various dyes, pigments and
extensively in the leather tanning industry. Chromium metal is extremely hard and
is passivated in oxygen thereby preventing any further oxidation of the metal. As
such, it is added to various metals to prevent corrosion and is used extensively in
the electroplating industries. It is also combined with iron to produce stainless
steel (Schoenberg et al. 2008).
In aqueous solution, Cr exhibits various oxidation states with the hexavalent Cr
(VI) and trivalent Cr (III) being the most energetically stable. Hexavalent Cr
species are, in general, anionic and trivalent Cr species typically cationic.
Hexavalent Cr species include: HCrO4- and Cr2O72-. Chromate salts and chromic
acid (H2CrO4) used during electroplating processes, dye production and tanning
12
industries, result in elevated amounts of Cr (VI) in the environment (Blowes
2002). Owing to the high solubility, powerful oxidising nature and ability to pass
easily through cell walls, Cr (VI) is extremely toxic and carcinogenic. However,
due to the reduced solubility of Cr(III) in water (forming insoluble precipitates),
and larger Cr(III) complexes, Cr(III) is much less toxic in the environment (Ellis
et al. 2004).
Many species of aqueous trivalent Cr(III) exist as the hexaaquachromium ion,
[Cr(H2O)6]3+. Hexaaquachromium ionic complexes contain the Cr3+ ion in the
centre of an octahedral coordination complex surrounded by water ligands.
Anions in solution can substitute for the water ligands in the Cr(III) complex,
thereby forming new complexes with differing ionic charge, some of which even
display an overall neutral charge (Housecroft and Constable 2005). The varying
stability of Cr(III) complexes in solution is shown in the brief but distinct colour
changes of Cr bearing solutions. Such variation of Cr species in solution is
particularly challenging for the analyst, especially for the analysis of total Cr in
solution and isolation of Cr in its entirety present in samples by ion exchange
processes.
1.2 Chromium isotopes and applications
Chromium has four naturally occurring isotopes:
50
Cr,
52
Cr,
53
Cr and
54
Cr, with
abundances of: 4.35 %; 83.79 %; 9.50 % and 2.36 % respectively.
The radiogenic isotope of Cr, 53Cr, is primarily produced from the decay of 53Mn.
The decay of
53
Mn to
53
Cr has a half-life of 3.7 million years and
53
Mn is now
extinct (Birck and Allegre 1988). The accurate determination of Cr isotopes has
13
numerous applications therefore, to ascertain various processes/events in the
environment. Of particular importance is identifying the processes which occurred
during the formation of the solar system. The 53Mn-53Cr isotope variation present
in meteorites can be used as a chronometer to date the formation of the planets
and the moon. Most meteorite samples investigated have shown an excess in 53Cr
compared to terrestrial samples (Birck and Allegre 1988; Lugmair and
Shukolyukov 1998). Terrestrial samples formed after the decay of
consequently all terrestrial samples display the same
53
53
Mn and
Cr/52Cr ratio (Frei and
Rosing 2005). Anomalies in the abundance ratio of 53Cr/52Cr relative to terrestrial
standards have been extensively determined to date early solar processes. Whilst
54
Cr/52Cr ratios display anomalies due to nucleosynthetic processes and can be
used to trace the provenance of meteorites (Moynier et al. 2007; Trinquier et al.
2008, {Qin, 2010 #5; Qin et al. 2010).
Stable isotope mass dependent fractionation can lead to small variations in the
terrestrial Cr isotope abundances as a result of chemical and biological processes
(Bonnand et al. 2011). Chromium stable isotope ratios may be used to quantify
reduction of Cr(VI) in the terrestrial environment. It is therefore extremely useful
to monitor transport processes occurring from contamination of mobile, toxic
Cr(VI) to the less harmful and insoluble Cr(III) in the environment (Ellis et al.
2002). The Cr isotopic signature is being utilised as a tool to track reduction of
Cr(VI) contamination and remediation especially as a result of microbial
reduction of Cr(VI) from bacteria and iron oxide materials (Du et al. 2012).
Attenuation of Cr(VI) from contamination can be quantified by Cr stable isotopes
ratios, as abiotic reduction of Cr(VI) results in enrichment of lighter Cr isotopes,
despite any sorption effects (Blowes 2002; Ellis et al. 2004).
14
Determinations of stable Cr isotope ratios in natural waters (Ball and Bassett
2000); in banded iron formations to determine oxygenation of the ancient oceans
(Frei et al. 2009) and in contaminated groundwater (Blowes 2002) provide
valuable information for numerous geochemical and biogeochemical processes.
1.2.1
Separation and measurement of chromium isotopes
There are two techniques available for the determination of Cr isotope abundances
namely: Thermal ionisation mass spectrometry (TIMS) and Multiple collector
inductively coupled mass spectrometry (MC-ICPMS). Both methods ionise the
sample, with efficient thermal ionisation sources, to produce gas phase
monovalent ions of all species present. Ions are magnetically separated according
to mass to charge ratios (m/Z) and the output is recorded as a mass spectrum from
signal intensities received at the detector(s). With the advent of high resolution
mass spectrometry, the ability to distinguish between ions of very similar masses
is possible. The multiple Faraday collectors of the MC-ICPMS permit the rapid
simultaneous determination of ion beams from
50
Cr+,
52
Cr+,
53
Cr+, and
54
Cr+.
Therefore, other ions present with similar masses to ions (isobaric ions) of Cr
isotopes, need to be removed from the analysed sample, as this can affect the
signal intensities of the required ion. As MC-ICPMS requires smaller sample sizes
for analyses this is the preferred technique for Cr isotopic determinations of
geological materials in this study (Bonnand et al. 2011).
Effectual separation of Cr, in its entirety from matrix components, is of uttermost
importance for isotopic analysis. As variations to the isotopic signature are minor,
complete separation and purification of the entire Cr from geological materials is
a prerequisite for precise high resolution mass spectrometric analysis. Matrix
15
elements present of similar masses to the isotopes of chromium, such as
54
Fe on
54
Cr, affect the accuracy of analyses due to isobaric interferences (Bonnand et al.
2011).
Many chromatographic methods used for separation rely on the valence duality of
Cr, employing both anion and cation exchange resin. Discrepancies can occur
from the proliferation of Cr(III) species with varying overall ionic charge. In order
to isolate Cr to the extent and purity required for the determination of stable
isotope ratios by mass spectrometric techniques, it is essential that the separation
techniques are thoroughly developed. As ion exchange chromatographic processes
rely on specific ionic properties, correlation with distribution coefficients for ion
exchange resins is beneficial. Determination of distribution coefficients performed
with specific resin employed during ion exchange separations quantitatively
reveal the extent of adsorption or desorption for Cr or matrix species in solution to
the solid exchange medium. We have improved on previous experiments
performed by Strelow et al. (1965), by defining the distribution coefficient (Kd)
for 38 elements using Bio-Rad AG® 50W-X8 cation resin with a suite of nitric
acid solutions of varying molarities and equilibrium periods.
As Cr in geological materials and particularly chromite, is present as Cr(III), the
method developed by Trinquier et al. (2008) was chosen for optimisation as this
involves separation of Cr cations by a two-stage cation resin exchange technique.
Furthermore, industrially Cr is mainly sourced from chromite ores. Chromium
isotope studies on chromite from the major chromitite intrusions of the Bushveld
igneous complex are invariable in 53Cr/52Cr composition to that of igneous silicate
Earth reservoirs, - 0.124 ± 0.101 ‰ relative to NIST SRM 979 (Schoenberg et al.
16
2008). This signature can then be utilised to reveal the extent of Cr(VI) reduction
to Cr(III) in environmental solutions as a result of anthropogenic impact or from
natural mineralogical processes (Schoenberg et al. 2008).
1.3 Aims and objectives of this study
Chromium is generally present at trace levels in geological materials. For high
precision measurements of Cr isotopes by MC-ICPMS, complete dissolution and
separation of the Cr from matrix elements is required. The aims of this study are,
therefore, to effectually dissolve selected geological materials and optimise a twostep cation exchange separation procedure, developed by Trinquier et al. (2008),
for the improved isolation and purification of Cr.
In order to achieve this, two dissolution procedures (hotplate and microwave
assisted) are performed on selected geological materials and the most successful
digestion procedure, with minimum blank contribution, shall be implemented for
the Cr purification procedures. An extensive set of distribution coefficients will be
determined for numerous analytes using cation resin (Bio-Rad AG® 50W-X8), to
establish selectivity of Cr and matrix cations for such resin, in a suite of nitric acid
solutions.
A hexavalent Cr speciation assay is performed to determine the
oxidation state of aqueous Cr in sample dissolutions before ion exchange
chromatographic procedures are undertaken. Successive cation exchange column
procedures are carried out to optimise the separation of Cr for geological
materials. Chromium isotope standards and procedural blank solutions are also
processed. After each column procedure, the purity and percentage yield of the
17
separated Cr is determined by MC-ICP-MS analysis. An optimised procedure will
be presented.
1.4 Thesis structure and rationale for submitting as alternative format
This thesis consists of 4 chapters. One of the chapters is written as a manuscript
which will be submitted to Analytical Chemistry. The structure and brief contents
of each chapter is as follows:
Chapter 1 gives a brief introduction and states the aims, objectives and structure
of this thesis.
Chapter 2 is a paper entitled: Determination of Distribution Coefficients for
Elements with Cation Exchange Resin Bio-Rad AG® 50W-X8, which presents an
extensive set of distribution coefficients for 38 elements, in various molarities of
nitric acid using Bio-Rad AG® 50W-X8 cation exchange resin. This paper is
currently in preparation and will be submitted to Analytical Chemistry.
Collaborators contributions are as follows:
C. Davies – principal author, experimental set-up, analyses and data processing;
M. Schönbächler – conceptual guidance and extensive manuscript review; G.
Moseley – conceptual guidance and extensive manuscript review.
Chapter 3 details the procedures used to make improvements and subsequent
optimisation of Cr separation from geological materials by cation exchange
chromatography. This chapter also provides a review of previous research
associated with the ion exchange techniques for achieving isolation of Cr.
Additionally, the methodology, results and discussion are presented in a format
suitable for submission to a peer-reviewed journal at a later date.
Chapter 4 details conclusions drawn from both previous chapters.
18
1.5 References
Ball, J. W. and R. L. Bassett (2000). "Ion exchange separation of chromium from
natural water matrix for stable isotope mass spectrometric analysis."
Chemical Geology 168(1-2): 123-134.
Birck, J.-L. and C. J. Allegre (1988). "Manganese-chromium isotope systematics
and the development of the early Solar System." Nature 331(6157): 579584.
Blowes, D. (2002). "Tracking Hexavalent Cr in Groundwater." Science
295(5562): 2024-2025.
Bonnand, P., I. J. Parkinson, R. H. James, A.-M. Karjalainen and M. A. Fehr
(2011). "Accurate and precise determination of stable Cr isotope
compositions in carbonates by double spike MC-ICP-MS." Journal of
Analytical Atomic Spectrometry 26(3): 528-535.
Du, J., J. Lu, Q. Wu and C. Jing (2012). "Reduction and immobilization of
chromate in chromite ore processing residue with nanoscale zero-valent
iron." Journal of Hazardous Materials 215–216(0): 152-158.
Ellis, A. S., T. M. Johnson and T. D. Bullen (2002). "Chromium Isotopes and the
Fate of Hexavalent Chromium in the Environment." Science 295(5562):
2060-2062.
Ellis, A. S., T. M. Johnson and T. D. Bullen (2004). "Using Chromium Stable
Isotope Ratios To Quantify Cr(VI) Reduction: Lack of Sorption Effects."
Environmental Science & Technology 38(13): 3604-3607.
Frei, R., C. Gaucher, S. W. Poulton and D. E. Canfield (2009). "Fluctuations in
Precambrian atmospheric oxygenation recorded by chromium isotopes."
Nature 461(7261): 250-253.
Frei, R. and M. T. Rosing (2005). "Search for traces of the late heavy
bombardment on Earth--Results from high precision chromium isotopes."
Earth and Planetary Science Letters 236(1-2): 28-40.
Housecroft, C. E. and E. C. Constable (2005). Chemistry, Pearson.
Lugmair, G. W. and A. Shukolyukov (1998). "Early solar system timescales
according to 53Mn-53Cr systematics." Geochimica et Cosmochimica Acta
62(16): 2863-2886.
Moynier, F., Y. I. N. Qing-Zhu and B. Jacobsen (2007). "Dating the first stage of
planet formation." Astrophysical Journal Letters 671(2 PART 2): L181L183.
Qin, L. P., C. M. O. Alexander, R. W. Carlson, M. F. Horan and T. Yokoyama
(2010). "Contributors to chromium isotope variation of meteorites."
Geochimica et Cosmochimica Acta 74(3): 1122-1145.
Schoenberg, R., S. Zink, M. Staubwasser and F. von Blanckenburg (2008). "The
stable Cr isotope inventory of solid Earth reservoirs determined by double
spike MC-ICP-MS." Chemical Geology 249(3–4): 294-306.
Trinquier, A., J.-L. Birck and C. J. Allegré (2008). "High-precision analysis of
chromium isotopes in terrestrial and meteorite samples by thermal
ionization mass spectrometry." Journal of Analytical Atomic Spectrometry
23: 1565–1574.
19
Chapter 2: Determination of Distribution Coefficients for Cation Exchange
Resin
2.1 Introduction
Ion exchange chromatography is an extremely useful technique for the separation
and isolation of elemental ions in solution. This is particularly relevant for
geological materials, as the dissolution of rock samples leads to solutions with
elevated total dissolved solid contents compared to the elements of interest, which
often occur in low concentrations. The purification of the elements of interest
from such sample matrices provides an improved sensitivity for measurements by
analytical instrumentation such as, multiple collector inductively coupled plasma
mass spectrometry (MC-ICPMS) since matrix suppression effects are minimized
(Makishima and Nakamura 1997). Element purification and careful sample
preparation is uttermost in such mass spectrometric techniques, since accurate
results can be compromised by spectral interferences in the presence of major
element concentrations. Effective separation of the target elements from other ions
in solution significantly reduces these effects and ensures that background analyte
levels are at a minimum, so that instrumental detection limits are improved and
polyatomic (molecular) interferences are reduced or eliminated.
For low level determinations of analytes from samples with minor amounts of
elements of interest, the ion exchange chromatographic separation procedures
necessitate minimal blank contribution. This can be achieved with the use of
highly purified chemical reagents, careful cleaning of the sample containers and
performing the analytical procedures in a cleanroom facility. The precision of
20
subsequent analyses can be improved by minimizing the blanks to ultra-trace
levels (parts per trillion) such that the blank contribution becomes negligible and
negates the need for any blank correction of the samples.
The analysis of specific elements requires thorough development of effectual
isolation by ion exchange chromatographic procedures, whilst simultaneously
aiming to achieve virtually complete separation from matrix elements together
with near quantitative yields. Matrix elements remaining after purification of
sample fractions for isotope analyses can also affect mass bias. Maximum yields
can be challenging to obtain for elements with multiple oxidation states, such as
Cr, whereby the affinity of particular ions for the ion exchange resins is dependent
on the oxidation state of the ions and charge of complexes formed. In order to
develop such procedures, experiments were performed in this study to determine
how ions distribute to the cation exchange resin Bio-Rad AG® 50W-X8 in a suite
of nitric acid solutions.
2.1.1 Principles of ion exchange chromatographic chemistry
Commercially available ion exchange resins are insoluble, organic polymer
materials in the form of very small beads (approx. 0.1mm diameter for 200 - 400
mesh resins) (Korkish 1989). Cation resins have negatively charged functional
groups, which adsorb positively charged ions. Conversely, the positively charged
functional groups of anion resins adsorb negatively charged ions. These ions are
referred to as counterions. The resin used in this study was Bio-Rad AG® 50WX8, hydrogen form, 200 - 400 mesh cation exchange resin. This is a strong cation
exchanger with sulphonic acid functional groups attached to a styrene
divinylbenzene copolymer lattice. Cations in solution are covalently bond to the
21
negatively charged sulphonic acid groups on the resin. The mesh value of the
resin refers to the size of the resin beads. In the case, 200 - 400 mesh, the diameter
of each resin bead is between 0.075 mm and 0.038 mm, thereby having a
relatively large total surface area compared to larger beads, which maximise the
separation resolution. The resin also contains 50 – 56% water content (moisture)
(Bio-Rad Documentation). The amount of crosslinkage between resin beads (X8)
is 8%; thereby having an approximate exclusion limit, for globular molecules of
molecular weight (MW) no greater than 1000 i.e. allowing smaller ions to pass
through and react with the resin but not larger molecules such as large metal
complex ions or proteins (Bio-Rad Documentation). The pH range in which the
resin functions is pH 2 – 12 (Bio-Rad Documentation). Switching between
solutions of extreme pH values should be avoided as it may cause osmotic shock
to the resin beads, which physically alters the resin and therefore its functionality.
Of further importance is the ion exchange capacity of a resin, which is defined as
the number of exchangeable groups per unit volume of resin, as milli-equivalents
per ml of resin (meq/ml) (Johnson 1986).
(Equation: 2.1)
The ion exchange capacity for AG® 50W-X8 is 1.4 meq/ml (Bio-Rad
Documentation); therefore the amount of total ions to be exchanged should not
exceed this amount or effective separation will not be attained.
22
2.1.2 Distribution coefficients determinations
The distribution or partition coefficient (Kd) is defined as the ratio of a
concentration of a substance, as a single definite form, in solution (mobile phase)
and the concentration of the same substance in the solid (stationary phase) at
equilibrium:
(Equation: 2.2)
Where:= concentration of analyte at equilibrium (µg/g)
= concentration of analyte at equilibrium (µg/ml)
Distribution coefficients are constant for a particular element in a given set of
conditions, which include: pH; temperature; equilibrium and the presence or
absence of chelating agents. Distribution coefficients have to be determined and
compared for specific acid solutions since different acid matrices influence the ion
exchange behaviour by modifying specific properties such as oxidising ability,
dissociation constant (pKa) and therefore number of moles of hydrogen ions
present. Large Kd values, for a given acid molarity, are indicative of ions that are
distributed more onto the resin and therefore fewer ions remain in solution and
vice versa for small Kd values.
Distribution coefficients are a very useful parameter in ion exchange
chromatography because they provide information as to how certain elements
distribute between the solution and ion exchange resin at specific molarities for
23
particular acids; thereby enabling the choice of acid and strength, necessary for
effective separation of elements in solution.
2.1.3 Aim of this study
Numerous studies have investigated the determination of distribution coefficients
(Strelow et al. 1965; Strelow 1988; Pourmand and Dauphas 2010) for elements in
various acids and resins. The published distribution coefficient data set for cation
exchange resin in nitric acid is incomplete (Strelow et al. 1965); data is not
available for elements in nitric acid solutions of molarities greater than 4 M. Also,
Sb, Pr, Nd, Eu, Tb, Dy, Ho, Tm or Lu have not been included in previous studies.
Therefore, in this study, the sorption behaviour of a comprehensive suite of
elements in nitric acid onto cation exchange resin (AG® 50W-X8) was reevaluated to obtain a more complete data set. A suite of experiments was
performed to determine the partitioning of 38 elements in various nitric acid
solutions utilizing Bio-Rad AG® 50W-X8 cation exchange resin, at room
temperature. To assess whether the distribution coefficients are constant for these
conditions; the quantity of resin, solution volume and concentration of analytes
were varied.
2.2 Analytical procedures
2.2.1 Reagents and materials
The experiments were performed in a Class 100 trace metal cleanroom. Analytical
reagent grade acids were purified by sub-boiling distillation and were used
throughout. Deionised water produced by reverse osmosis in a Milli-Q Element
24
water purification unit (Millipore Corporation), with a resistivity of 18 M ohm,
was also used throughout this study.
Multi-element commercial standard solutions were utilized for two sets of
experiments:
i)
standard (A), is a 1000 ppm multi-element ICP-MS calibration
standard in 5% nitric acid (manufactured by VWR) containing the
elements Al, Sb, As, Ba, Be, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn,
Mo, Ni, K, Se, Ag, Na, Tl, Th, U, V, Zn.
ii)
standard (B), a 1000 ppm multi-element standard in 5% nitric acid
(manufactured by ROMIL) containing Ce, Dy, Er, Eu, Gd, Ho, La,
Lu, Nd, Pr, Sc, Sm, Tb, Tm, U, Y and Yb. Other elements of
interest were added to standard (B) from commercial single
element standard solutions: 1000 ppm Zr (Alfa Aesar); 998 ppm Ti
(Sigma-Aldrich); 10,000 ppm Sr (Alfa Aesar) and 1000 ppm Rb
(Alfa Aesar).
The commercial multi-element solutions were further diluted to achieve 10 ppm
concentrations for each element. In addition, a suite of nitric acid solutions were
prepared by diluting aliquots of concentrated sub-distilled nitric acid,
volumetrically, with deionised water. The prepared acid solutions were
standardised against sodium carbonate to determine accurate starting molarities
(Table 2.1). The pH of the acid solutions was verified with a Hanna pH checker at
18oC and the molar concentration of the hydronium ion calculated from the
following equation:
25
(Equation: 2.3)
therefore:
(Equation: 2.4)
Calculated molar concentrations of hydronium ions present in each of the
prepared nitric acid solutions used throughout the experiments are presented in
Table 2.1.
Table 2.1: Hydronium ion concentrations in various nitric acid solutions
Molarity (M)
0.1
0.2
0.3
0.4
0.5
1.0
2.0
4.0
6.0
9.0
[H+]
0.017
0.035
0.052
0.069
0.117
0.178
0.257
0.427
0.603
0.977
pH
1.78
1.46
1.28
1.16
0.95
0.75
0.59
0.37
0.22
0.01
Before the ion exchange resin was used for experiments and column procedures,
the resin was cleaned and preconditioned as described in the following. A large
batch of cation exchange resin was transferred from a commercial bottle to a 1
litre Teflon© FEP bottle with a plastic spatula. An amount of water equivalent to
at least twice the volume of resin was added to the bottle, the mixture shaken
thoroughly, allowed to settle, and then the solution was decanted and discarded.
This was repeated until the supernatant appeared clear, thereby removing most of
the fine particulate matter until no more particles went into suspension with the
new addition of fresh water. The same procedure was subsequently repeated in
triplicate using 6 M HCl. This was followed by three sequential wash cycles,
26
which involved thorough rinsing in deionised water. Finally, the resin was stored
in deionised water ready for use.
2.2.2 Experimental procedures for distribution coefficient determinations
Two sets of experiments were performed, one set with multi-element Standard A
and a second set with Standard B (2.2.1). Between 0.5 g and 1 g of Bio-Rad AG®
50W-X8 200 – 400 mesh cation resin was accurately weighed into 50 ml
polypropylene centrifuge tubes. Aliquots of 10 ppm multi-element standards were
added to the tubes and diluted to a total solution volume of 5 ml or 50 ml, with the
suite of nitric acid solutions of varying strengths from 0.1 M to 9 M.
The initial concentration of analytes added was carefully selected so that they
could be precisely analysed by ICP-MS before and after the partitioning. Since the
amount that would be adsorbed onto the resin was undetermined at this point,
sufficiently high concentrations (100 - 1000 ppb) of total analytes were added. In
the case that little or no exchange occurred, the concentration of elements
remaining in solution would be below maximum detection limits, and thus could
still be analysed safely. The concentrations for accurate ICP-MS analysis are
ideally kept within the measured calibration range (10 ppt to 100 ppb) for this
study. In addition, to avoid saturation or overloading of the resin, the
concentration of ions added to the resin were kept considerably below the
maximum ion exchange capacity of 1.4 meq/ml for the Bio-Rad AG® 50W-X8
resin (i.e. Standard A = 0.007 meq/ml and Standard B= 0.002 meq/ml).
The distribution coefficient experiments were performed at room temperature (16
– 19oC) using the batch method. In order to ensure complete equilibrium, the total
27
reaction time was set to 8 hours and during this period the mixtures were agitated
at regular intervals with an IKA MS3 basic vortex mixer at 1000 rpm. Duplicate
samples, for experiments in 0.5 M and 6 M, were reacted for 24 hours to validate
that equilibrium was achieved after 8 hours.
After the resin and solutions were equilibrated for 8 hours, the solutions were
collected in Teflon© PFA Savillex© vials by filtering the resin mixture through
empty 10 ml Bio-Rad polypropylene columns fitted with frit. Subsequently, the
solutions were evaporated to dryness on a Teflon© PFTE coated hotplate at 110oC.
The residues were redissolved with 0.2 ml of concentrated HNO3 and then diluted
to 10 ml with water to produce a resultant solution of 2 % HNO3. The solutions
were sealed tightly and gently refluxed at 60oC for 2 hours. After cooling, the
solutions were transferred to 12 ml polycarbonate ICP-MS autosampler tubes and
were analysed immediately by ICP-MS. After the concentrations of each analyte
were determined, precise distribution coefficient ratios were calculated using
equation 2.2.
Blank samples were prepared containing acid and resin only, without any multielement standard added, to determine the blank contribution from reagents, resin
and containers. For all analysed elements, the blank concentrations were
negligible (< 0.1 ppb), except for Zr and Th, where they were < 3 ppb. A blank
correction was not applied for these elements because erratic signal variations
were observed at constant element concentrations, which were most likely due to
ineffective wash cycles after samples that contained high element concentrations.
28
2.2.2.1 Inductively coupled plasma mass spectrometry (ICP-MS)
A short overview of ICP-MS is presented here and specifically the instrument
employed for the analyses of processed standard solutions for distribution
coefficient determinations, the Agilent 7500 series quadrupole ICP-MS. It follows
the description published in Agilent Technologies Primer (2005).
Inductively coupled plasma mass spectrometry is an analytical technique used for
the determination of major and trace elements in solution (solid samples may be
analysed when the ICP-MS is coupled to a laser ablation system). The instrument
comprises an inductively coupled plasma ionisation source, which is produced in
a quartz torch with argon gas. The plasma consists of highly ionised argon gas,
which has been heated to extremely high temperatures (~7500 K). Plasma
generation takes place within an inductively coupled copper coil, through which a
high frequency electrical current is passed, thereby creating a strong
electromagnetic field, causing electrons to collide with argon atoms forming
monovalent argon ions. The liquid sample enters the plasma via a nebuliser;
which aspirates the sample with argon gas to produce an aerosol. The aerosol then
passes into a spray chamber, in this instrument specifically a Scott double pass
quartz spray chamber, which removes larger droplets from the argon sample
aerosol. This enables a more streamlined flow of sample mist into the plasma, also
removing a large proportion of the solvent matrix and maintaining a stable plasma
temperature. Once the sample enters the centre of the plasma, the aerosol is
rapidly vaporised and atomised. The atoms are then ionised due to the high
temperature of the argon plasma, which is high enough to remove electrons from
the outermost shell of most atoms with a first ionisation potential lower than that
of argon. The singly charged positive ions are subsequently drawn through a small
29
hole in the nickel (sampler) cone under vacuum (~ 2 mbar). Most of the argon
ions do not pass through the second (skimmer) cone, which is the entrance to a
section that is at an even lower pressure (10-4 mbar) (Jarvis 1992). Once passed
the cones, the generated ion beam is further focused and guided with a series of
electrostatic lenses removing most of the photons and neutral particles. The
positive ions are attracted to the negative fields and the unwanted negative ions
are repelled from the positive fields. Also located in the ion focusing stage of this
ICP-MS, is the collision/reaction cell (CRC), which further reduces spectral
interferences. The CRC can be pressurized with a gas thereby enabling reactions
or collisions of gas molecules with polyatomic ions in the ion beam, significantly
reducing isobaric interferences. In reaction mode the gas used in the cell is
normally hydrogen, which reacts with interfering species and converts them into
different species. When the CRC is in collision mode the gas typically used is
helium, which collides with larger polyatomic species resulting in loss of energy
and prevents them from entering the mass spectrometer due to a kinetic energy
bias.
The ion beam, having had the majority of interfering ions removed, now enters
the mass analyser part of the instrument, the mass spectrometer. The mass
spectrometer in this particular ICP-MS is a quadrupole. The ions are separated
according to their mass to charge ratio. This is achieved by varying electric fields
across a set of four parallel rods. When ions of a certain mass to charge ratio are
the same as the electric potential between the rods, they can pass through to the
detector. The voltages across the rods are changed sequentially and at high speed
through the mass range, allowing ions of specific mass to charge ratio to
selectively pass through to the detector.
30
The detector is an electron multiplier, such that when a positive ion impacts the
detector, several free electrons from each dynode surface are released, multiplying
or cascading as they travel along the many dynodes in the detector. This generates
a measurable signal pulse as an ion count per second (cps) and per unit
concentration of ions. The electron multiplier exhibits a high sensitivity and a
wide linear dynamic range, enabling analytes of high concentration (up to 1000
mg/L) to be measured. This is achieved by the detector electronics being able to
operate in dual mode switching from pulse counting with low count rates (parts
per trillion concentrations) to analogue mode for high count rates (parts per
million concentrations). In analogue mode, rather than measuring the pulse from
individual ion impacts, the current is measured from the electron stream. It is
advantageous to be able to switch modes of the detector when measuring solutions
of unknown concentrations as the analogue mode prevents the detector becoming
saturated with ions.
2.2.2.2 Determination of element concentrations by ICP-MS
A set of five calibration standards, ranging in concentration from 1 ppb to 100 ppb
for the required elements, were analysed on the Agilent ICP-MS. All solutions
including a blank solution for the baseline counts and detection limits were
measured in a matrix of 2 % HNO3. A concentric glass nebuliser was operated at
an uptake rate of 0.1 ml/min with 1.5 ml of sample solution consumed during a
quantitative analysis sequence for each sample. For the majority of the analysed
elements, the CRC was operated in collision mode with a helium gas flow of 5
ml/min. The exceptions were Be, Mg, Al and Se, which were analysed in reaction
mode with 3.5 ml/min of hydrogen gas to reduce isobaric interferences.
31
Signals of analyte ions present were measured by the detector in pulse counting
mode with the output as counts per second (cps) for sample, unprocessed standard
and blank solutions. Each analyte peak was measured for a period of 0.3 s (dwell
time), in triplicate (number of sweeps), with an integration time of 0.1 s.
Calibration curves were obtained at the start and at the end of each analysis
sequence to check for signal stability. The sample analyte concentrations were
determined relative to the calibration standards. All measured element
concentrations were within the calibration range of the standards analysed on the
ICP-MS.
2.3 Results and discussion
The calculated experimental distribution coefficients are presented in Table 2.2
and they are reported for the most prevalent oxidation state. In order to normalise
the Kd for each cation in a given acid molarity, they are calculated per gram of
resin and per ml of solution using equation 2.2. Figure 2.1 illustrates the
logarithms of the distribution coefficients against the molarity of the nitric acid. In
addition, unprocessed standard solutions were measured to verify (i) dilution
factors; (ii) potential co-precipitation; (iii) volatilization of elemental ions during
evaporation; (iv) adsorption to the surface of the polypropylene tubes and (v)
initial concentrations of the standard solutions. Both standard solutions processed
through the experimental procedures and those analysed directly by ICP-MS yield
identical results within the uncertainties and this demonstrates that the above
mentioned issues (i) to (v) do not significantly influence the results.
32
Table 2.2: Distribution coefficients (Kd) for Standard A and B, in various nitric
acid solutions using Bio-Rad AG® 50W-X8 resin.
Element
0.1M
0.2M
0.4M*
0.5M
1M
2M
4M
6M
9M
Standard A
Be (II)
1780
429
*in 50 ml
62
57
15
5.1
2.5
2.1
2.0
Mg (II)
927
595
96
90
22
6.6
3.1
2.4
2.2
Al (III)
428
943
1420
344
114
18
4.2
2.1
2.2
V (V)
1480
356
50
51
14
5.3
3.1
2.8
3.0
Cr (III)
277
262
930
199
96
19
4.4
2.9
2.7
Mn (IV)
3490
991
129
135
33
9.8
4.7
3.9
4.2
Fe (III)
1290
2550
2370
762
167
24
6.3
4.4
5.0
Co (III)
3730
901
120
118
29
8.2
3.4
2.6
2.4
Ni (II)
1270
751
117
115
29
8.5
3.6
2.8
2.6
Cu (II)
2140
836
118
111
26
7.0
2.6
2.0
1.8
Zn (II)
289
390
92
77
21
7.0
2.4
1.8
1.6
Mo (VI)
5.8
5.3
2.5
3.2
2.9
2.3
2.2
2.3
2.4
Ag (I)
146
13
52
8.1
6.0
5.7
5.7
5.6
8.5
Cd (II)
3980
1010
131
126
29
7.6
2.8
2.1
1.7
Sb (V)
17
14
5.9
7.8
4.9
2.9
1.9
1.7
1.6
Ba (II)
43810
20750
722
2430
383
67
30
26
26
Tl (III)
439
210
49
62
23
7.5
2.0
1.3
0.8
Pb (IV)
8840
4580
444
430
58
8.3
2.5
2.1
1.9
Th (IV)
2100
6250
6330
3880
4980
110
37
30
U (VI)
2900
728
115
104
29
210
0
8.7
5.2
5.4
6.3
Standard B
Ti (IV)
2590
383
50
46
14
4.3
2.9
2.6
3.3
Rb (I)
195
96
26
30
12
4.7
2.3
1.7
1.0
Sr (II)
10740
3140
315
311
57
10
2.5
1.5
1.0
Y (III)
696300
186100
7150
6160
559
57
8.8
4.5
3.2
Zr (IV)
368400
83330
40960
8340
678
37
4.3
0.6
0.4
La (III)
417200
569600
22930
16310
1310
100
9.9
4.1
2.7
Ce (IV)
70040
180600
15400
12900
1020
79
8.5
3.8
2.7
Pr (IV)
425800
319200
12900
11030
839
65
7.5
3.6
2.7
Nd (III)
276300
292700
11480
10230
735
57
7.2
3.7
2.8
Sm (III)
447300
220000
9350
7910
587
47
6.4
3.5
3.0
Eu (III)
808600
205900
8630
7600
558
45
6.0
3.4
2.8
250400
8610
7280
586
50
6.8
3.6
3.0
Gd (III)
Tb (IV)
581800
194000
8160
7040
576
50
7.0
3.5
2.7
Dy (III)
515100
218500
8010
6830
585
56
7.7
3.7
2.7
Ho (III)
630900
179900
7430
6470
565
53
7.9
3.9
2.8
Er (III)
791100
195800
7210
6420
565
57
8.2
4.1
2.7
Tm (III)
668400
166600
6720
6080
526
53
8.4
4.2
2.9
155500
6630
6000
517
55
9.0
4.1
2.9
577800
151000
6320
5560
493
50
8.3
4.2
3.0
3660
911
111
118
29
9.9
5.7
5.8
6.3
Yb (III)
Lu (III)
U (VI)
33
The reproducibility of the Kd values was assessed with a range of duplicate
samples (Table 2.3 and 2.4). Various experiments were performed in, 0.5 M and 6
M HNO3, with varying conditions (i.e. equilibration time of 8 and 24 hours, 0.5
and 1 g of resin, see Table 2.3 and 2.4). Results for the majority of cations, in
particularly those for the experiments using 6 M HNO3, show little variation,
thereby demonstrating the reproducibility of the data, +/- 1%. However,
exceptions are observed for Th and Zr, where the Kd values vary significantly
(Table 2.3 and 2.4). Thorium and Zr also displayed elevated blank concentrations,
potentially due to wash-out problems, and this introduces a larger analytical
uncertainty on the distribution coefficients, which can partly explain the observed
scatter. Moreover, Zr is also prone to hydrolysis of the solvated complexes at low
acid concentrations and thus tends towards the formation of anionic species,
which do not exchange with cation resin. Hydrolysis can be circumvented by
stabilizing the cationic complexes with trace amounts of hydrogen peroxide or
hydrofluoric acid (Strelow and Bothma 1967), however, this was not performed in
this study. Hydrolysis and elevated blank values entail that the reported
distribution coefficients for Zr and Th represent lower limits.
34
Table 2.3: Distribution coefficients (Kd) for duplicate samples of standard A in 0.4,
0.5 and 6 M nitric acid using Bio-Rad AG® 50W-X8 resin.
0.4M
0.5M
0.5M
0.5M
0.5M
6M
6M*
6M
6M
Standard A (ml)
Resin (g)
Time (hours)
Element
Be (II)
0.5
0.5
8
0.5
0.5
8
0.5
0.5
24
0.5
1.0
8
0.25
0.5
8
.
0.5
0.5
8
0.5
0.5
8
0.5
1.0
8
0.5
0.5
24
61.6
56.8
57.8
62.0
64.9
2.1
2.2
2.0
2.0
Mg (II)
96.2
89.9
85.6
79.9
83.1
2.4
2.3
2.3
2.3
Al (III)
1420
344
1670
935
1340
2.1
1.8
2.7
2.9
V (V)
49.7
51.3
49.0
50.0
52.8
2.8
2.8
2.1
2.3
Cr (III)
930
199
226
132
235
2.9
2.9
3.0
3.4
Mn (IV)
129
135
126
129
134
3.9
3.8
3.1
3.3
Fe (III)
2370
762
10630
5540
5510
4.4
4.3
5.3
6.2
Co (III)
119
118
116
118
125
2.6
2.5
2.5
2.7
Ni (II)
117
115
115
115
122
2.8
2.7
2.4
2.5
Cu (II)
118
111
115
116
124
2.0
1.9
2.2
2.3
Zn (II)
92.2
77.4
66.8
64.8
57.1
1.7
1.9
2.1
2.0
Mo (VI)
2.5
3.2
2.9
2.6
3.0
2.3
2.1
1.6
1.8
Ag (I)
52.1
8.1
18.0
8.5
37.1
5.6
5.5
6.5
7.7
Cd (II)
131
126
127
127
141
2.1
1.9
1.9
2.1
Sb (V)
5.9
7.8
5.9
6.2
7.5
1.7
1.6
1.3
1.4
Ba (II)
722
2425
676
648
671
25.8
25.5
1.3
1.5
Tl (III)
49.4
61.4
56.7
57.8
58.7
1.3
1.2
1.5
1.4
Pb (IV)
444
430
403
424
441
2.1
1.9
1.3
1.3
Th (IV)
6330
3880
2450
2460
249
36.5
34.7
57.7
44.0
115
*Duplicate for ICP-MS analysis.
104
116
117
120
5.4
5.5
6.7
6.1
U (VI)
35
Table 2.4: Distribution coefficients (Kd) for duplicate samples of standard B in 0.4,
0.5 and 6 M nitric acid using Bio-Rad AG® 50W-X8 resin.
0.4M
0.5M
0.5M
0.5M
0.5M
6M
6M
6M
Standard B (ml)
0.5
0.5
0.5
0.5
0.25
0.5
0.5
0.5
Resin (g)
0.5
0.5
0.5
1.0
0.5
0.5
1.0
0.5
8
8
24
8
8
8
8
24
Sr (II)
315
311
302
303
311
1.5
1.3
1.6
Y (III)
3.9
3.8
3.8
3.9
3.8
0.7
0.7
0.7
Zr (IV)
40960
8340
31480
67700
3770
0.6
4.4
2.6
La (III)
19380
16310
22930
19830
11910
4.1
4.1
4.2
Ce (IV)
15400
12900
15150
17540
9470
3.8
3.8
4.0
Pr (IV)
12900
11030
13530
19830
9240
3.6
3.6
3.6
Nd (III)
11480
10230
11600
20870
8400
3.7
3.6
3.9
Sm (III)
9350
7910
9480
13010
6760
3.4
3.5
3.7
Eu (III)
8630
7600
8480
12880
6980
3.3
3.3
3.5
Gd (III)
8610
7280
8590
12210
6660
3.6
3.6
3.8
Tb (IV)
8160
7040
7810
9910
6400
3.5
3.6
3.7
Dy (III)
8010
6830
7930
12150
6410
3.7
3.6
3.8
Ho (III)
7430
6470
7200
9970
6310
3.9
3.9
4.0
Er (III)
7210
6420
6540
8630
5730
4.1
4.0
4.0
Tm (III)
6720
6080
6480
8390
5690
4.2
4.3
4.3
Yb (III)
6630
6000
6630
7890
5640
4.1
4.6
4.4
Lu (III)
6320
5560
5990
7390
5300
4.2
4.3
4.3
U (VI)
111
118
116
125
126
5.8
5.9
6.0
Time (hours)
Element
36
Overall the data presented here and those from (Strelow et al. 1965) are in good
agreement. However, some discrepancies occur in dilute nitric acid (0.1 M and 0.2
M), where the Kd values vary by 50 – 70 % between the two studies. In these
dilute solutions (0.25 ml aliquot of standard A and B), almost all of the analytes
were absorbed onto the resin and therefore extremely low concentrations (< 1
ppb) remained in the solution to be analysed. With the advent of more advanced
instrumentation, it is now possible to analyse such dilute solutions more precisely
and accurately than in previous studies and this most likely explains the
discrepancy between the two studies. At higher nitric acid molarities (0.5 M; 1 M;
2 M and 4 M) the results of both studies are in very close agreement (~3 %).
Equivalent distribution coefficients were obtained for most elements (Table 2.3
and 2.4) for experiments in which: (i) the amount of resin was doubled, whilst
keeping the same concentration of analytes in solution or (ii) reduced
concentration of analytes were added in relation to resin. This is valid for
experiments in 0.5 M and 6 M HNO3 and corroborates the evidence that
adsorption is not increased through a more abundant availability of ion exchange
sites and that the Kd remain constant over the investigated ranges of element
concentrations and resin amounts. It also verifies that the maximum loading
capacity of the resin was not exceeded as was predicted based on the reported
maximum ion exchange capacity (1.4 meq/ml; Bio-Rad Documentation). This is
critical since experiments performed at high concentrations (close to the
maximum ion exchange capacity), yield ambiguous distribution coefficients due
to the saturation of the ion exchange sites, which prohibit further ion exchange of
analytes from solution.
37
Identical Kd values were also observed for duplicate experiments left to
equilibrate for 8 and 24 hours, respectively with the exception of the Kd values for
Al, Cr, Fe and Ba in 0.5 M HNO3 (Table 2.3). This validates that equilibrium was
achieved for most ions between resin and analyte solution during the experiments
left to equilibrate for 8 hours. The results for Al, Cr, Fe and Ba are likely to be
influenced by kinetics of complex formation and stability.
All elements analysed (Fig. 2.1; Table 2.2) show the general trend of high Kd
values (often > 1000) at low molar concentrations of nitric acid (0.1 M),
demonstrating that the majority of the ions investigated are partitioned onto the
resin at these concentrations. The Kd values decrease steeply from 0.1 M to 0.5 M
and further decline from 0.5 M to 6 M. Subsequently, the Kd values plateau and
remain constant from 6 M to 9 M, implying that significantly fewer cations are
adsorbed onto the resin with increasing acid concentration. This most likely
reflects the presence of higher concentration of hydronium ions [H3O]+ in more
concentrated acid (Table 2.1), which compete against other cations for the ion
exchange sites. At higher acid concentrations (≥ 6 M HNO3), the available
exchange sites on the resin become saturated with excess positive H3O+ ions,
which render the sites unavailable for the exchange with other cations in solution
and thus generate the relative flat pattern observed for most elements. These
findings are in good agreement with the study of (Strelow et al. 1965), which
inferred that the cation exchange resin should not be used or stored in strong acids
or alkalis, due to hydronium and hydroxyl ions occupying the active exchange
sites. Moreover, strong nitric acid may attack the physical nature of the resin
beads; which affects resin functionality (Bio-Rad Documentation).
38
This study extends the previously published data set (Strelow et al. 1965) to
experiments utilising strong nitric acid solution (6 M and 9 M) to determine the
ion exchange behaviour at high nitric acid concentrations. The results indicate that
for nitric acid solutions with molarities ≥ 6 M, the adsorption behaviour remains
virtually the same for all analysed elements yielding low Kd of < 1 - 8. This
successfully demonstrates that cations of most elements can be quantitatively
removed from cation exchange resin with 6 M HNO3, thereby avoiding the use of
stronger nitric acid solutions.
Our data also shows that generally lower distribution coefficients were obtained
for elements of low atomic number and with high ionisation energies, such as Be,
Mg, Al, Zn and Ag, in comparison to other elements such as the rare earth
elements (REE) (Table 2.2). The low reported distribution coefficients for
magnesium cations (Mg2+) verify that these cations do not have a strong affinity
for cation resin. The chemical properties of Mg are similar to Li, and as such
displays similar behaviour to these smaller cations, such as Li+ and Na+ which
have relatively low selectivity for cation resin (Bio-Rad Documentation).
Beryllium cations display an anomalous high distribution coefficient in 0.1 M
HNO3. Beryllium cations are distinctly different to other Group 2 elements; the
Be2+ cation possesses a high charge density. It forms [Be(H2O)4]2+ in strongly
acidic solutions, but exists as structural variations of {Be4O}6+ in very weak acids
(Housecroft and Constable 2005). The existence of higher charged complexes in
weak acids, for Be, is clearly validated in the high distribution coefficient reported
for Be in 0.1 M HNO3 and the strong decline in Kd with increasing acid
concentrations (Table 2.2).
39
Ions with lower ionisation potentials, for instance the rare earth elements (REE),
display the highest distribution coefficients (Table 2.2). This strong absorption of
highly charged cations is well documented in the literature (Strelow et al. 1965).
Within the REE, yttrium displays a higher Kd than La (Table 2.2 and 2.4). Both
elements form trivalent ions in solution, however, Y is more electronegative. This
supports the general rule that the selectivity of specific cations to exchange with
cation resin, in preference to other cations, is related to the oxidation state and
degree of ionisation. Owing to the similarities in the chemical behaviour of the
REE in aqueous solution, the determined distribution coefficients exhibited
practically no variation and it is therefore challenging to separate individual REE
from other f-block elements utilising AG® 50W-X8 resin in HNO3.
A few elements, such as Mo and Sb, exhibit unusually low distribution
coefficients (Table 2.2). Molybdenum and Sb both form oxyanions in acidic
solution. However, anions do not adsorb to cation resin, which explains the low
Kd for these elements.
In addition, anomalously high distribution coefficients were obtained for Cr, Al
and Fe species in 0.4 M HNO3, while Ba exhibited an unusual low Kd value
compared to the other analysed elements at this molarity (Table 2.2). The
experiments performed in 0.4 M HNO3 had a total solution volume of 50 ml
(compared to 5 ml for all the other experiments) and thus contained an order of
magnitude lower total concentration of analytes. For Ba cations, minimal
exchange with the resin in 0.4 M HNO3 may be due to the formation of Ba(OH)2.
Chromium, Al and Fe are transition metal elements which form numerous ions
and complexes in solution. The extent of ionisation and oxidation state of these
40
ions in solution varies with acidic strength. In aqueous acidic solutions, Cr, Al and
Fe all form hexaaqua octahedral complexes. Whereby, the central metal ion will
retain the same oxidation state but the overall ionic charge on the coordination
complex may change depending on the substitution and nature of the ligands
bonded to the central metal ion. With increased water molecules, at 0.4 M HNO3,
equilibrium may be favoured towards the more positively charged species. The
rate of formation and stability of these complexes at certain pH can also vary and
will influence the ion exchange behaviour.
Interestingly, the distribution coefficients obtained for Cr, Al and Fe in
experiments with 0.5 M HNO3 with different equilibration periods (8 and 24
hours; Table 2.3) yielded significantly different Kd. The Kd were generally higher
(equating to increased sorption onto the resin) for the 24 hour experiment. Again,
Ba shows the opposite trend. This suggests that equilibration time is critical for
the reproducibility of experiments involving these elements. Therefore, optimum
reaction times need to be determined to ensure either complete transformation to
the desired complex formation or to prevent redox reactions from occurring, for
instance oxidation of Cr3+ to Cr6+, which results in anionic species such as the
Cr2O72-; CrO4- and HCrO7-.
Increased absorption of Cr, Al and Fe to the resin is not only observed with longer
reaction time but also with larger total volumes of aqueous solution (Table 2.3).
This is likely related to the lower ionic strength of the solution based on the
activities of the analyte ions, which increases the degree of exchange to the resin
as the degree of ionisation increases. Additionally, the elevated adsorption may
also be mediated through the solvated state of the ionic complexes in more dilute
41
analyte solution, which can alter the ionisation potentials and therefore change the
affinity of the ion for the resin. The hexaaqua complex formation of transition
metals in acidic solution, for example [Cr(H2O)5OH]2+, is unstable in certain
conditions (~ pH 7) but with increased [H3O]+ stable [Cr(H2O)6]3+ is formed
(Housecroft and Constable 2005). In weaker acid solutions, equilibrium is
favoured towards [Cr(H2O)5OH]2+ but may also continue to substitute to form
monovalent and even neutral complexes, leading to decreased selectivity of
transition metal cations.
42
2.4 Conclusions
An extensive set of distribution coefficients for 38 elements is presented for BioRad AG® 50W-X8 cation resin used in conjunction with nitric acid. For most
elements, high Kd values were obtained for 0.1 M HNO3, confirming strong
sorption to the resin, while conversely, the Kd decrease with increasing acid
strength and plateau at ~ 6 M HNO3. This plateau validates that all of these
elements virtually do not distribute onto the resin in strong nitric acid solutions.
This is attributed to the excess of hydronium cations, present in strong nitric acid
solutions, which compete with analyte cations for available exchange sites.
Equilibrium between resin and analyte solution was achieved after 8 hours
reaction time, except for a number of redox sensitive elements such as Al, Cr and
Fe. A comprehension of the oxidation state of the analytes is vital (particularly
with transition metals) and consequently determines whether ionic species are
neutral, cationic or anionic, which significantly influences the Kd. It is imperative
to allow reaction times to reach equilibrium, in order to achieve optimum
distribution coefficients for a given set of conditions, which can then be employed
for ion exchange separation procedures.
The presented extensive results complete previous work (Strelow et al. 1965) and
are extremely useful for developing ion exchange procedures for the separation of
cations and enhancing procedural yields. In particular, the distribution coefficients
obtained for high nitric acid concentrations illustrate that generally cations can be
quantitatively removed (eluted) from cation exchange resin by elution with 6 M
nitric acid.
43
Figure 2.1: Logarithms of distribution coefficients (Kd) for 38 elements (most
prevalent oxidation state), as a function of nitric acid molarity with cation resin
(Bio-Rad AG® 50W-X8).
44
2.5 References
Agilent Technologies. (2005). ICP-MS : Inductively Coupled Plasma Mass
Spectrometry : A Primer.
Bio-Rad laboratories, Inc. AG® 50W-X8 Cation Exchange Resin Instruction
Manual.
Housecroft, C. E. and E. C. Constable (2005). Chemistry, Pearson.
Jarvis, K. E. (1992). Handbook of inductively coupled plasma mass spectrometry.
Johnson, E. L., Ed. (1986). Handbook of Ion Chromatography. California, Dionex
Corporation.
Korkish, J. (1989). Handbook of Ion Exchange Resins: Their Application to
Inorganic Analytical Chemistry, CRC Press.
Makishima, A. and E. Nakamura (1997). "Suppression of Matrix Effects in ICPMS by High Power Operation of ICP: Application to Precise
Determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U at ng g-1 Levels in
Milligram Silicate Samples." Geostandards Newsletter 21(2): 307-319.
Pourmand, A. and N. Dauphas (2010). "Distribution coefficients of 60 elements
on TODGA resin: Application to Ca, Lu, Hf, U and Th isotope
geochemistry." Talanta 81(3): 741-753.
Strelow, F. W. E. (1988). "Comparative Distribution Coefficients for Some
Elements with a Macroporous Cation Exchange Resin in HNO3 and HCl."
Solvent Extraction and Ion Exchange 6(2): 323-334.
Strelow, F. W. E. and C. J. C. Bothma (1967). "Anion exchange and a selectivity
scale for elements in sulfuric acid media with a strongly basic resin."
Analytical Chemistry 39(6): 595-599.
Strelow, F. W. E., R. Rethemeyer and C. J. C. Bothma (1965). "Ion Exchange
Selectivity Scales for Cations in Nitric Acid and Sulfuric Acid Media with
a Sulfonated Polystyrene Resin." Analytical Chemistry 37(1): 106-111.
45
Chapter 3:
Separation and Purification of Chromium from Geological
Materials.
3.1 Introduction
Determination of Cr isotopes in geological materials requires isolation of the
entire Cr present, which is often very low in concentration compared to the bulk
of the sample material. Effectual separation of the Cr requires complete
dissolution of the sample, though this is often particularly challenging owing to
the matrices of geological samples. Upon dissolution of samples, the Cr can be
separated and subsequently purified by means of ion exchange chromatography,
to remove most of the total dissolved solids. It is imperative for the analysis of Cr
by mass spectrometry, such as MC-ICPMS or TIMS, that the separation
procedure is thoroughly developed to ensure purification is maximised and
interfering elements are minimised.
As previously stated, successful separation of Cr from geological materials
requires complete dissolution. This is particularly important for chromite,
FeCr2O4, as significant amounts of Cr are present, 46.5 %, in addition to
containing no manganese or silicate matrix. Chromite is a highly refractive
mineral and as such is thermodynamically stable at high temperatures and thereby
extremely difficult to dissolve. Modifications will need be made to established
microwave dissolution methods to accomplish the complete dissolution of
chromites without having to use traditional fusion procedures (Totland et al.
1992). This study will evaluate the most effective dissolution of the chosen
geological materials resulting from two dissolution procedures.
46
The range of geological materials processed in this study, were chosen for having
varying matrices together with differing chromium concentrations so that the
effectiveness of the dissolution procedure and subsequent ion exchange separation
procedures could be assessed.
Various different methodologies have been employed for the chemical separation
and purification of Cr from rocks, meteorites and natural waters. Many of the
methods take advantage of Cr forming an oxyanion in solution to separate Cr(VI)
anions from matrix cations in solution by anion exchange chromatography (Ball
and Bassett 2000; Halicz et al. 2008). Chromium is initially separated from matrix
cations, by reductively eluting Cr(VI) with 2 M HNO3 using anion exchange resin
(Götz and Heumann 1988). The separated Cr is then further purified by
converting Cr(VI) to positively charged Cr(III) cations, which are then exchanged
from solution utilising cation exchange resin. The advantage of such dual stage
exchange methods, is that the Cr is firstly separated from the majority of matrix
cations by the initial anion exchange process and thus quantitative separation can
be achieved from samples with high total dissolved solids. By comparison,
separation of Cr cations from such samples by cation exchange alone can be close
to the maximum ion exchange capacity for the resin, making it difficult to isolate
appreciable amounts of pure Cr for analysis. However, as discussed in Chapter 2,
owing to the proliferation of Cr species (with varying charge), complete
separation of Cr is not achieved using anion exchange resin with 2 M HNO3,
leading to reduced yields.
The redox reaction of Cr(VI) to Cr(III) also requires the use of strong oxidising
agents such as ammonium persulphate, (NH4)2S2O3, (Götz and Heumann 1988;
47
Frei and Rosing 2005), leading to elevated solution matrices for the cation
exchange procedures. This can also result in increased amounts of sulphate anions
in the purified Cr fractions, which is problematic for mass spectrometric
techniques such as TIMS.
Sequential ion exchange separation procedures will be performed in this study, in
order to optimise the two-step cation exchange procedure developed by Trinquier
et al. (2008) for the purification of Cr from dissolutions of chosen geological
materials (Table 3.3). Following each procedure, the separated Cr will be analysed
by MC-ICPMS and progressive alterations made to each successive procedure.
3.2 Geological materials and standards
In order to investigate the effectiveness of the Cr separation procedure for
geological materials, specific materials were chosen with high to low Cr
concentrations in conjunction with varying matrices consisting of silicate;
sediment; organic and high Fe concentration (Table 3.1). They include various
Standard Reference Materials (SRM’s), chromite from the Bushveld Complex, the
carbonaceous chondrite Allende, and a Cr standard solution of known
concentration from the National Institute of Standards and Technology (NIST).
48
Table 3.1: Chromium concentrations of geological materials and standards used
in this study
Standard Ref
Type
Origin
BHVO-2
Basalt
Hawaiian volcanic observatory,
USA.
USGS Reference material.
BCR-2
Basalt
Columbia river, Oregon. USA.
USGS Reference material.
16
SCo-1
Cody Shale
Upper Cretaceous silty marine shale.
Wyoming, USA.
USGS Reference material.
68
Allende
Carbonaceous chondrite
(CV3)
Iron chromium oxide
FeCr2O4
Meteorite, fall Mexico, 1969.
Bushveld Igneous Complex, South
Africa.
464,600
Cr metal in 10% nitric
acid.
Cr(NO3)3
NIST certified reference material.
10,000
NIST certified reference material.
1000
Chromite
NIST3112a
SRM979
Chromium
content
(ug/g)
280
2530
Chromium is a hard, corrosion resistant metal that is passivated in air, forming
chromium oxides, Cr2O3 or CrO3. In water, Cr is insoluble and passivation of the
metal contributes to lowered reactivity with oxidising acids and thereby reduced
dissolution. Chromium is predominantly found in nature, as Cr(III), in chromite
ore. Chromite, FeCr2O4 is a dark brown, iron chromium oxide mineral, which
belongs to the spinel group. It is common in all meteorites except carbonaceous
chondrites. It is highly refractive and as such retains its structure even at high
temperatures. The chromite sample used in this study was sourced from the
Bushveld Igneous Complex, South Africa (Table 3.1). The Bushveld Igneous
Complex is a large outcrop (around 70,000 km2) of layered igneous intrusion,
formed by the fractional crystallization of mafic magma from the earth’s mantle
around two billion years ago. Chromite is one of the first minerals to crystallize
during slow cooling of magma and since it is denser (specific gravity of 4 g/ml)
than most of the other minerals, concentrates in the lower parts of the magma
49
chamber. Chromite seen in the Bushveld outcrop appears as large dark bands of
chromitite between bands of pyroxenite and anorthosite. Chromite is also present
in metamorphic rocks, which are formed by the high temperature and high
pressure alteration of ultrabasic rocks, demonstrating the resistance of chromite to
alteration (Roeder and Reynolds 1991). It is also concentrated by sedimentary
processes to form placer deposits of economic value.
NIST 3112a and SRM 979 are primary Cr standards of known Cr isotopic
composition for the quantitative determination of Cr and have been used in
numerous published methodologies for the isotopic determination and separation
of Cr.
BHVO-2 and BCR-2 are basic igneous silicate mantle materials that are of known
mineral composition. Cody shale (SCo-1) is a silty marine shale containing iron
oxides, organic and silicate components and is also a standard reference material
of known composition.
3.3 Digestion procedures
Complete dissolution of geological materials for analysis of major and trace
elements can be difficult owing to silicate and acid/heat resistant refractory
mineral constituents. Dissolution of such materials is achieved by reaction with
hydrofluoric acid which destroys the silicate portions, in conjunction with high
temperatures and pressures. Digestion is traditionally achieved by heating samples
with acidic solution mixtures during: open dish methods; alkali fusion methods;
microwave or Parr Bomb procedures. For the separation of elements from
50
dissolutions and subsequent analysis of trace elements by MC-ICPMS, the
preferred methods are hotplate, microwave and Parr Bomb. These methods
involve hot hydrofluoric and nitric acid mixtures for reactions to proceed.
However, during hotplate methods, although heated in sealed Teflon© PFA
Savillex© containers (as opposed to open dish methods) reaction mixtures do not
reach the same elevated temperature and pressures as during Parr Bomb and
microwave assisted methods.
3.3.1 Digestion methods of previous work
Many of the previous studies for the separation of Cr from geological materials
have utilised the lithium metaborate flux method (Qin et al. 2010) for dissolution.
Dissolution is achieved by melting samples in a furnace at high temperatures
(1100oC) with lithium metaborate thereby forming a molten sample glass. The
glass is subsequently dissolved in strong nitric acid solutions. The disadvantages
with this method of digestion are that the total dissolved solids of the resulting
sample solution are elevated thereby increasing the subsequent sample load on the
ion exchange columns. This can reduce the effectiveness of Cr separation by ion
exchange as there is greater competition for exchange sites. In addition, the risk of
contamination from extra reagents and heating at high temperatures in open
crucibles, is greater, leading to higher analyte concentrations in the blank and
sample solutions (Totland et al. 1992). Additionally, the combination of Cr with
excess borate ions in a solution or glass is likely to form Cr(VI) in chromium
borate salts (Merkle et al. 2004). This is not conducive to effective Cr separation
by cation exchange chemistry since Cr(VI) anions will not exchange with cation
resin.
51
Previous methods for the dissolution of geological materials, notably Allende,
using hotplate methods, have resulted in incomplete digestion, whereby fine black
material has remained after dissolution (Schönbächler et al. 2007). Allende
contains significant amounts of organic material and dissolution is not effective
by mineral acids alone with hotplate heating, to attack the carbon rich fractions.
Additional stronger oxidants would be required for hotplate dissolutions, for
instance ammonium persulphate (Frei and Rosing 2005). Consequently, the use of
such additional oxidising agents leads to increased ionic species in the solution
matrix. Accordingly, microwave digestion of geological materials especially those
with higher organic contents and more refractory components, is more favourable
(Halicz et al. 2008).
3.3.2 Digestion methods used in this study
The dissolution of samples was investigated in this study by means of two
procedures: i) hotplate (3.3.2.2) and ii) microwave assisted reaction system
(3.3.2.3). Preparation of total procedural blanks was undertaken in both
procedures. Each digestion procedure shall be described in the following.
3.3.2.1 Reagents
All hydrochloric and nitric acid solutions used in the digestion and column
procedures were purified by sub-boiling distillation of concentrated analytical
reagent grade acids. High purity 18 MΩ deionised water, produced by a Milli-Q
Element water purification system manufactured by the Millipore Corporation,
was used throughout. All sample vials and containers used during the procedures
were thoroughly cleaned by refluxing in 6 M HCl for 72 hours, followed by
52
refluxing in 6 M HNO3 for 72 hours and finally rinsed and soaked in 18 MΩ
deionised water (Milli-Q) for 24 hours.
3.3.2.2 Hotplate digestion method
Between 50 and 300 mg of each sample were weighed into 60 ml Teflon© PFA
Savillex© jars fitted with screw top lids. A small aliquot (0.5 ml) of deionised
water was added to wet the powdered sample (chromites were not powdered) and
to prevent passivation of metals upon the addition of strong acids. Hydrofluoric
acid (7 ml) and 2.5 ml of 15.7 M HNO3 were added to the samples. The jars were
tightly sealed and heated on a Teflon© PTFE coated hotplate at 110oC for 72
hours. The samples were uncapped and evaporated to almost dryness at 130oC.
Additional aliquots of acids were added and evaporated for a further three cycles.
To the sample residue, 5 ml of deionised water and 5 ml of 15.7 M HNO3 was
added and refluxed at 110oC overnight. The sample solutions were evaporated to
incipient dryness to ensure removal of hydrofluoric acid. To the precipitate, 3 ml
of deionised water and 3 ml of 10.6 M HCl were added; the vials were tightly
sealed and refluxed overnight at 60oC.
3.3.2.3 Microwave digestion method
Samples were digested using a MDS-2000 microwave system. Between 50 and
300 mg of each sample were accurately weighed into 100 ml Teflon© PFA
vessels. To each vessel, 1 ml of deionised water; 3 ml of 15.7 M HNO3 and 6 ml
of hydrofluoric acid (40 %) was added. The vessels were placed inside Kevlar©
jackets and Teflon© PFA lids (incorporating pressure relief valves) were securely
fitted to the vessels and then placed in the microwave oven. All samples and
blanks were subjected to a heating cycle involving a ramp for 15 minutes from
53
room temperature to 150oC followed by an isothermal step at 150oC for a period
of 45 minutes. During this procedure, the microwave energy was supplied at 100
% power of 800W. Upon heating, the pressure inside the sealed vessels increases
to approximately 100 psi (measured with a pressure sensor attached to the
pressure relief valve), as a result of increased vapour pressure from the acid
mixtures. In order to ensure complete dissolution of the samples, the vessels were
subjected to further heating cycles if necessary. For the dissolution of chromites,
the heating program was restarted immediately at the end of each run and was
repeated 15 times over a period of 2 – 3 days.
The digested sample solutions and blanks, were thoroughly rinsed out of the
microwave vessels with deionised water and transferred into 60 ml Teflon© PFA
Savillex© vials. In order to remove the hydrofluoric acid and transient products,
the sample solutions were evaporated on a hotplate at 130oC to incipient dryness.
Subsequently, 5 ml of deionised water and 5 ml of concentrated HNO3 were
added to the evaporated sample residues and refluxed at 110oC overnight. The
sample solutions were again evaporated to incipient dryness, followed by the
addition of 3 ml of deionised water and 3 ml of 12 M HCl, the vials were tightly
sealed and refluxed overnight at 60oC. The sample solutions were visually
inspected to ensure complete dissolution; if the solutions were transparent they
were stored in the resultant 6 M HCl in preparation for column chemistry
procedures.
54
3.3.3 Selection of the preferred digestion method
Upon visual inspection of the sample solutions after each digestion method,
dissolutions resulting from the microwave assisted reaction system appeared
much clearer than those from the hot plate digestion. This was markedly visible
for the carbonaceous chondrite Allende, as a film of extremely fine black material
remained floating on the surface of the solution following the hotplate digestion
procedures.
Moreover, the microwave assisted dissolution method was a much
faster method (2 days compared to 10 days for the hot plate dissolution).
Chromites were only successfully digested by the microwave assisted reaction
system, while the hot plate dissolution left the chromites intact. The complete
dissolution of chromites by the microwave assisted reaction system was
accomplished by continuously heating the vessels, with no cooling period
employed. This was to simulate conditions present during Parr Bomb dissolution
procedures (Schonbachler et al 2004), whereby samples are digested at high
temperatures and pressures for long periods of time in a furnace. It is noteworthy
to state that it is not suitable to digest samples for Cr determinations in Parr
Bombs, as they are stainless steel jacketed, which contains greater than 10 % Cr.
All of the sample dissolutions resulting from microwave digestion methods were
clear, transparent solutions and most were bright orange in colour whilst the
chromite dissolutions were bright green in colour.
As many geological materials consist of a silicate matrix, the dissolution
procedure ensues by reaction with hydrofluoric acid to form various fluorides.
Most importantly, with an excess of hydrofluoric acid present during the reaction,
volatile silicon tetrafluorides (SiF4) and water soluble hexafluorosilicic acid
(H2SiF6) are produced upon heating. Such decomposition products result from the
55
destruction of silicate material by reaction with hydrofluoric acid and
consequently all silicon is removed from the sample as gaseous products.
Hydrofluoric acid possesses a lower dissociation constant (pKa) than other mineral
acids and is therefore, in terms of acidic strength, a weaker acid. However,
fluorine has the strongest electronegativity of all the elements, and thus has a huge
capacity to attract electrons and break chemical bonds, thereby explaining the
strong reactivity of hydrofluoric acid. Although hydrofluoric acid has a low
boiling
point
and
high
vapour
pressure
at
room
temperature,
high
temperatures/pressure are required to ensure dissolution reactions go to
completion, particularly those involving silicates and the subsequent removal of
SiF4 gaseous products from the digested sample solution.
Additionally, the resultant procedural blank solutions from the microwave and
hotplate methods were analysed by ICP-MS and blank contributions were
negligible for Cr and other major elements (<20 +/- 2ppt). Low blank values were
achieved during the microwave and hotplate digestion procedure as for most of
the process the samples are completely sealed and fewer cycles of extra reagent
additions are needed compared to hotplate digestion. However, dissolution by
microwave systems is much more effectual than hot plate digestion due to
constant heating in a closed system and pressurization of the Teflon© PFA vessels.
This increases the vapour pressure and boiling points of the acid mixtures and
therefore increases rates of enthalpy of solution (kJ/mol).
Consequently, as complete dissolution of all of the chosen samples together with
low blank contributions were easily achieved using the microwave assisted
reaction system; this was the preferred chosen digestion method for this study.
56
3.4 Chromium speciation assay
The oxidation state of Cr influences its behaviour during cation exchange
separation procedures and therefore the relative Cr yields. The cation exchange
procedure is most effective if all of the dissolved Cr species are cationic (trivalent)
and not as anionic hexavalent species, as Cr(VI) will not distribute to cation resin.
For this reason the oxidation state of Cr in sample and standard solutions prior to
ion exchange procedures was evaluated by performing a hexavalent Cr assay. The
Diphenylcarbazide (DPC) Colorimetric EPA method 7196A (US EPA 1992 –
Rev. 1) was the assay utilised to detect any Cr(VI) present in sample, standard and
blank solutions, a description of which is as follows.
A stock solution of DPC was prepared by dissolving 37.5 mg of 1,5diphenylcarbazide in 1.25 ml 3 M HCl, 7.5 ml acetone and 7.5 ml deionised
water. A 25 % dilution of the stock solution was prepared by taking a 1 ml aliquot
of the stock solution and diluting to 25 ml with deionised water. Both stock and
diluted solutions were freshly prepared and stored in the dark as they degrade with
time and light.
A set of five Cr(VI) calibration standards was prepared from potassium chromate,
ranging from 0.052 mg l-1 Cr(VI) to 10.4 mg l-1 Cr(VI). For the analysis, 0.1 ml of
the sample or standard solutions was added to a 1 cm3 curvette and 1.4 ml of
diluted DPC solution added. The solutions were left for 15 minutes for the purple
coloured Cr complex (the formula and structure of which is unknown) to develop.
The stability of the Cr(VI) complex is also unknown; therefore the
spectrophotometric measurements were performed immediately after the addition
57
of DPC (and development time) to the sample aliquots. The absorbance of the
targeted purple Cr(VI) complex was then measured photometrically at a
wavelength of 540 nm on a UV/VIS spectrophotometer. This is the wavelength of
maximum absorbance for measurement of the complex.
3.4.1 UV Spectrophotometer
A single beam spectrophotometer (Camspec model M501) was used for this
procedure. This spectrophotometer consists of five parts: (1) a halogen or
deuterium lamp to supply the light; (2) a monochromator to isolate the wavelength
of interest; (3) a sample compartment for the sample solution; (4) a detector to
receive the transmitted light and convert it to an electrical signal and (5) a digital
display to indicate absorbance or transmittance (Laqua et al. 1988). The
wavelength of maximum absorption (lambda-max) depends on the presence of
particular chromophores (light-absorbing groups) in a molecule. Different
materials absorb different wavelengths of light. The intensities of the light
entering and exiting a sample are measured and the two intensities compared.
Information about the two intensities can be expressed as transmittance (ratio of
exiting light intensity relative to the entering light) or percent transmittance (%T)
(Equation 3.1) (Laqua et al. 1988).
Percent transmittance (%T) can be related to the absorbance (A) by:
(Equation 3.1)
58
Beer's law states that the absorbance is directly proportional to the concentration
of an analyte in a solution: (Equation 3.2, 3.3).
(Equation 3.2)
(Equation 3.3)
Where e = molar absorptivity or molar absorption coefficient; l = cell path length
and c = analyte concentration.
Absorbance readings were measured for blank solutions and the values were
subtracted from absorbance readings for sample and standard solutions. For
calibration of the assay, the absorbance readings for known Cr(VI) concentrations
(calibration standards) and blanks were plotted, in order to calculate Cr(VI)
concentrations for samples. The detection limit for this method is 0.052 mgL-1
hexavalent Cr.
3.4.2 Results and discussion of the chromium assay
The expected concentration of Cr in the analysed sample solution must be above
the detection limit to confirm the presence or absence of Cr(VI). If the expected
total Cr concentration was below the detection limit, the absence of Cr(VI) should
not be assumed. All sample solutions analysed in this study were expected to be
above the detection limit.
Analysis of Cr bearing solutions, utilising the DPC spectrophotometric assay,
indicated the absence of hexavalent Cr in all of the solutions, except for the 25
59
and 40 ppm solution of NIST 3112a in 2 % HNO3, in which all the Cr present was
shown to be Cr(VI). This confirms that Cr present in most of the samples and
standards analysed is trivalent (Table 3.2). A 40 ppm solution of NIST 3112a in 6
M HCl was analysed to validate that Cr metal dissolved in HCl, is trivalent, as no
Cr(VI) was detected. We have therefore utilised this method to confirm the
absence of Cr(VI) in sample dissolutions and standards, prior to cation ion
exchange column procedures. It is important to note that Cr in NIST 3112a with a
HNO3 solution matrix is present as Cr(VI) species whilst Cr in NIST 3112a in
HCl solution matrix is present as Cr(III) species.
Table 3.2: Hexavalent chromium concentrations, determined by colorimetric
assay, of analysed sample dissolutions and Cr standard solutions.
Sample
Matrix
Cr(VI) (mgL-1)
Blank
Blank
NIST 3112a 40 ppm
NIST 979 4.94 ppm
Cr(NO3)3 98.04 ppm
NIST 3112a 25 ppm
BHVO-2 dissolution
BHVO-2 column load
Allende dissolution
Allende column load
Dissolution blank
NIST 3112a 40 ppm
NIST 3112a 8 ppm column load
2 % HNO3
1.2 M HCl
6 M HCl
2% HNO3
2% HNO3
2% HNO3
2% HNO3
1.2 M HCl
2% HNO3
1.2 M HCl
2% HNO3
2% HNO3
1.2 M HCl
0.00
0.00
0.00
0.00
0.00
27.95
0.00
0.00
0.00
0.00
0.00
45.42
0.00
3.5 Development of chromium purification and separation
The separation method developed by Trinquier et al. (2008) was selected as a
starting point for optimisation of Cr separation from geological material, as this
method is an effectual method for the separation of Cr(III) from samples using
60
cation exchange procedures. The rationale of the two-stage cation exchange
procedure employed by (Trinquier et al. 2008) was to maximize Cr yields from
samples by an efficient and rapid ion exchange separation process thereby
reducing the volume of reagents used and hence lowering blank contribution.
Reported yields of the Trinquier et al. (2008) method are ~ 70 % from the first
column and ~100 % Cr recovery from the second column. Notwithstanding, the
separation procedure was optimized for analysis of Cr isotopes by TIMS and not
for analysis by MC-ICPMS. Trinquier et al. (2008) processed a range of samples:
terrestrial basalts including rock standard BCR-2; chromites; clays; limestone and
meteorite separates. The samples were digested on a hotplate at 150oC in Teflon©
vials. Silicate and oxide samples were reacted with hydrofluoric acid and HNO3;
metal matrices with aqua regia and clays/limestone with 6 M HCl. Subsequent
separation of trivalent Cr from matrix elements ensued by a two-stage column
cation exchange chromatographic procedure (Table 3.3). This was achieved by
ensuring all the Cr present in the samples was converted to Cr(III), by reduction
with 6 M HCl at 100oC. After Cr reduction the samples were processed through
an initial column to separate Cr from most of the matrix elements. The Cr
fraction from this column was then further purified by processing through a
second column to remove traces of Al, Ti, Fe and Mn. Once separated and
purified the Cr isotope ratios in the Trinquier study, were measured by TIMS.
This two stage column procedure developed by (Trinquier et al. 2008) detailing
the volumes and molarity of eluants, is presented in Table 3.3. Both steps utilise
Bio-Rad AG® 50W-X8 200-400 mesh cation exchange resin.
61
Table 3.3: Ion exchange procedure developed by (Trinquier et al. 2008)
Step
Acid volumes (ml)
Acid
Column 1 (1ml resin bed)
Load sample
1.2
1 M HCl
Collect Cr
4
1 M HCl
Elute matrix
6
6 M HCl
Column 2 (0.33ml resin bed)
Load sample
Elute residual Al, Ti
Elute residual Fe, Ni, Mn
Collect Cr
2
2.5
8
2.5
0.15 M HNO3
0.5 M HF
1 M HCl
2 M HCl
Improvements to the Trinquier et al. (2008) procedure were formulated in this
study, whereby dissolution of geological materials (Table 3.1) were effected by
microwave assisted reaction systems, in contrast to dissolution in Teflon© vials,
employed by Trinquier et al. (2008). Moreover a cleaner separation of Cr through
the first cation exchange column was targeted in order to reduce contamination
from matrix elements, notably Fe and Ti. Improved purity of the Cr fractions from
the first column would further enhance separation on the second much smaller
column. This would be advantageous for minimizing isobaric interferences which
are more problematic for MC-ICPMS analyses as opposed to TIMS. Additionally,
sample amounts loaded onto the first column were assessed, so as not to exceed
the maximum ion exchange capacity for the resin volume in order to make
improvements to the percentage yields.
Each modification made to the original Trinquier et al (2008) procedure during
this study was evaluated by analysing the chemically separated Cr by MCICPMS, thereby assessing yield and purity. In order to analyse trace quantities and
isotopic ratios of Cr, the sample solution needs to be free of interfering elements
(Ti; V and Fe) for high precision measurements by MC-ICPMS. An outlined
description of this instrument is described in section 3.5.2.
62
The ion exchange chromatographic separation procedure utilizes cation exchange
resin in columns, which are eluted with acid solutions under gravity, in series. The
majority of matrix elements from the samples are removed on the first column.
Subsequently the Cr fraction from the first column is then further purified on the
second column, removing any traces of matrix elements, which did not exchange
with the resin in the first column (Figure 3.1).
Figure 3.1: Schematic diagram of two-stage cation exchange procedure (after
Trinquier et al. (2008)). The Cr fraction eluted from column 1 is further purified
through column 2.
Both column cation exchange processes rely on the speciation of Cr in solution to
be present as reduced Cr(III) cations. If the Cr present in the sample solution is
not fully trivalent (cationic) will therefore, lower the percentage yields obtained,
as hexavalent (anionic) Cr will not distribute onto cation exchange resin, as
previously discussed. Furthermore, consultation with the distribution coefficients
63
obtained in this study and that of (Strelow et al. 1965) proved advantageous in
predicting elution curves for Cr and major matrix elements.
The distribution coefficients provide invaluable information necessary for
improving the separation technique to purify Cr from matrix elements since an
indication can be made as to what extent Cr would be distributed to the resin with
known acidic strengths whilst eluting other elements and vice versa. As certain
samples have much lower Cr concentrations compared to the matrix elements
present, the total milliequivalents (meq) of expected analytes in the sample
loading aliquots were calculated (Equation. 2.1). The maximum ion exchange
capacity for the resin volume and the total amount of sample that theoretically
could be loaded onto individual columns was taken into account. Column loading
aliquots were assessed for each geological material, as certain materials have
higher total dissolved solid contents than other sample materials used in this
study. For specific resin volumes used in columns, the theoretical maximum ion
exchange capacity was not exceeded when loading aliquots of samples onto the
columns. This is essential for the purity of the separated Cr, as samples processed
through columns that are close to the maximum ion exchange capacity do not
separate from matrix elements effectively.
Numerous column procedures were performed to assess the effectiveness of the
two-stage cation exchange procedure. After each column procedure, each
modification was evaluated through analysis of the eluted fractions, by MC-ICPMS, to determine the purity and percentage yield of the separated Cr. The
percentage yield is equal to the actual yield obtained divided by the theoretical
(expected) yield. Elution curves for Cr were charted to ascertain whether Cr was
64
quantitatively eluted in the desired Cr fractions. If tailing of the Cr elution peak
had occurred, to what degree it was being eluted in the matrix fractions was
assessed as this incurs lowered yields. The necessary alterations for each
subsequent exchange procedure were then formulated. Details of each adaptation
in all of the column procedures are described below and are summarised in Table
3.7.
3.5.1 Analytical methods for cation exchange column procedures
Twelve column procedures (A-L) were performed in total. The inital column
procedures (Column procedure A and B) performed followed the procedure
developed by (Trinquier et al. 2008) (Table 3.3). Each procedure consists of a
two-stage column process using cation exchange resin as previously described.
Systematic changes were made after each procedure, once the purity and the
percentage yield of the separated Cr had been determined by MC-ICPMS.
Between 45 and 75 fractions were eluted and collected for analysis from each
column procedure. The sample dissolutions and procedural blanks that were
previously digested by the microwave assisted reaction system (3.3.2.3) were used
throughout the cation ion exchange separation procedures. Actual sample amounts
(dissolved in 6 ml of 6 M HCl), for each sample stock dissolution are shown in
(Table 3.4). Adequate amounts of each sample were dissolved in order to have
sufficient sample solution to accomplish numerous column procedures and
duplicates. In the order of 8 µg of separated Cr from each sample/standard
processed from the column procedures, was required for precise MC-ICPMS
analyses. Aliquots of NIST 3112a (40 ppm) were also processed with the sample
and blank solutions for each column procedure. This was to ascertain percentage
yield of Cr in all of the eluted fractions; since the Cr standard would be free of
65
interfering elements and therefore all matrix fractions from the standard could be
safely analysed on the MC-ICPMS without the risk of overloading the detectors.
Five individual columns were processed simultaneously during each column
procedure. Each column procedure is described in detail and thereafter the
significant modifications will be emphasized (Table 3.6 and 3.7).
Table 3.4: Chromium concentration (µg/ml) in sample stock dissolutions;
aliquots of which were removed for column procedures A – L.
Sample
Amount sample (g)
Cr
(µg/ml)
-
40.000
BHVO-2
0.24955
11.646
BCR-2
0.47875
1.277
Allende
0.03275
13.810
Chromite
0.00827
192.112
Cody Shale
0.40451
4.585
NIST 3112a
Column Procedure A
Column (1)
From the sample stock dissolutions (BHVO-2 and Allende), procedural blank and
standard solution, 0.6 ml aliquots were removed and placed into 7 ml Savillex©
vials and evaporated at 110oC, to a volume of approximately 200 µL. This proved
difficult; consequently the 0.6 ml sample aliquots were evaporated to dryness. To
the evaporates were added: 200 µL of 6 M HCl and 1 ml of H2O, the resultant
solutions being 1 M HCl. The vials were capped tightly and refluxed at 80 oC,
overnight.
66
A slurry of pre-cleaned AG® 50W-X8 cation resin (as described in 2.2.1) was
carefully pipetted into 10 ml Bio-Rad polypropylene columns. Deionised water
was added until a 1 ml resin bed, with no air bubbles and a flat resin bed surface
was achieved. The 1 ml resin bed was preconditioned with 10 ml of deionised
water and 10 ml of 1 M HCl.
Aliquots of sample, blank and standard (NIST 3112a) solutions in 1.2 ml 1 M HCl
(column loading solutions), having been refluxed overnight were cooled to room
temperature and pipetted carefully onto each column. For each column, the Cr
fraction was eluted with 4 ml of 1 M HCl and collected into a 7 ml Savillex © vial.
The sample matrix, mostly iron (Fe) cations, was eluted with 6 ml of 6M HCl and
collected in a 7 ml Savillex© vial. A portion (0.5 ml) of the Cr fraction (Cr cut)
was removed, evaporated and 1 ml of 2 % HNO3 added. This solution was then
refluxed overnight at 80oC, in preparation for Cr analysis by MC-ICPMS. The
purpose of removing this aliquot was to check for percentage Cr yield from
column 1. The remainder (4.7 ml) of the Cr fraction was evaporated to dryness at
110oC, in preparation for loading onto column 2. This was repeated for each of the
sample, blank and standard solutions.
The matrix (Fe) fractions were evaporated to dryness at 110 oC. The evaporated
matrix (Fe) fractions from the blank and standard solutions only, were redissolved in 1 ml of 2 % HNO3. These solutions were analysed by MC-ICPMS to
check for any Cr present, as the eluted matrix fractions from the samples could
not be analysed since they would be excessively contaminated with matrix
elements.
67
Column (2)
Approximately 0.5 ml of pre-cleaned AG® 50W-X8 cation resin (as described in
chapter 2) was carefully pipetted into 5 ml handmade PTFE columns (0.5 ml resin
bed volume: approximately 10 mm high). Deionised water was added until a 0.5
ml resin bed, with no air bubbles and a flat surface was achieved. The 0.5 ml resin
bed was preconditioned with: 5 ml of deionised water and 5 ml of 0.5 M
hydrofluoric acid.
The evaporated Cr fractions (minus the 0.5 ml Cr cut) from column (1) were
refluxed overnight in 2 ml 0.15 M HNO3. These were then loaded onto the 0.5 ml
columns, by pipetting the cooled solutions slowly onto the resin. The first matrix
fraction eluted was residual Al and Ti (Trinquier et al. 2008), with 2.5 ml 0.5 M
hydrofluoric acid. The total 4.5 ml (2.0 ml initial sample + 2.5 ml Al and Ti
fraction) was collected in a 7 ml Savillex© and evaporated to dryness. Secondly
residual Fe, Ni and Mn (Trinquier et al. 2008) was eluted with 8 ml of 1 M HCl,
into 15 ml Savillex© vials and evaporated to dryness. Finally, Cr was eluted with
2.5 ml of 2 M HCl into 7 ml Savillex© vials. The purified Cr fraction was
evaporated, re-dissolved in 1 ml of 2 % HNO3, refluxed overnight at 80oC and
subsequently analysed by MC-ICPMS. Evaporated matrix fractions from the
blank and standard solutions only, were re-dissolved in 1 ml of 2 % HNO3 and
also analysed by MC-ICPMS.
68
Table 3.5: Column Procedure A: Initial two-step cation exchange column
procedure trialled in this study, adapted from Trinquier et al. (2008)
#
Step
Column 1
1
Sampling
2
Dry down
3
1 M HCL
4
Reflux
5
Prepare resin
6
7
8
9
10
11
12
Rinsing
Conditioning
Cooling
Loading
Collection
Collection
Apportion
13 Dry down
14 Storage
Column 2
15 Reflux
16 Prepare resin
17
18
19
20
21
22
23
24
25
26
Rinsing
Conditioning
Cool
Loading
Collection
Collection
Collection
Dry down
Storage
Reflux
Details
0.6 ml from sample stock dissolutions, procedural blank and NIST 3112a
Evaporate to dryness on hotplate at 110°C
Take up in 200 µL 6 M HCl + 1 ml MQ
Reflux 80°C overnight
Pipette 1 ml Bio-Rad AG® 50W-X8 cation exchange resin into 10 ml
Bio-Rad polypropylene columns
10 ml MQ
10 ml 1 M HCl
Samples, standard and blank cooled to room temperature
Carefully pipette aliquots in 1.2 ml 1 M HCl onto column
4.0 ml 1M HCl– elute Cr fraction (solutions 1.1)
6.0 ml 6M HCl– elute Fe matrix (solutions 1.2)
Separate 0.5 ml from solutions 1.1 and evaporate to dryness on hotplate
at 80°C – Cr cut (solutions 1.1a)
Evaporate solutions 1.1 and 1.2 to dryness on hotplate at 110 °C
Solutions 1.1a and 1.2 in 1 ml 2% HNO3
Reflux Cr solutions 1.1 overnight in 2 ml 0.15 M HNO 3
Pipette 0.33 ml Bio-Rad AG® 50W-X8 cation exchange resin into 5 ml
Teflon© PTFE columns
5 ml MQ
5 ml 1 M HCl
Cool Cr solutions (1.1) to room temperature
Carefully pipette solutions (1.1) onto column
2.5 ml 0.5 M HF- elute AL, Ti matrix (solutions 2.1)
8.0 ml 1 M HCl- elute Fe, Ni, Mn matrix (solutions 2.2)
2.5 ml 2 M HCl- elute Cr (solutions 2.3)
Evaporate solutions 2.1, 2.2 and 2.3 to dryness on hotplate
Solutions 2.1, 2.2 and 2.3 in 1 ml 2 % HNO3
Reflux all solutions (1.1a, 1.2, 2.1, 2.2 and 2.3) overnight at 80°C prior
to analysis
Column Procedure B
Column (1)
Aliquots of standard (NIST 3112a) and blank solutions only were processed in
this procedure. An aliquot of 0.6 ml was removed from the blank solution and two
separate 0.4 ml aliquots were taken from the standard solution and processed
through column (1) as for column procedure A.
69
Column (2)
The Cr fractions from column (1) were loaded onto preconditioned 0.33 ml (not
0.5 ml) cation resin volume columns, in 2 ml of 0.15 M HNO3. All matrix
fractions were evaporated, re-dissolved in 1 ml of 2 % nitric acid and were also
analysed by MC-ICPMS to check for any Cr present.
Column Procedure C
Column (1)
From the BHVO-2 and Allende sample stock dissolutions and procedural blank
solution, 0.6 ml aliquots were taken for column loading. Two 0.4 ml aliquots were
taken from NIST 3112a solution. The solutions were evaporated to approximately
200 µL followed by the addition of 1 ml of deionised water.
The sample, blank and standard solutions (in 1.2 ml of 1 M HCl), were loaded
onto 1 ml resin beds. For each column, the Cr fraction was eluted with 4 ml of 1
M HCl. However, to the duplicate standard solution, processed to assess Cr
elution, a further 2 ml of 1 M HCl was added to this column and collected in two
separate 1 ml fractions for subsequent analysis. The matrix (Fe) was eluted with 6
ml of 6 M HCl. For the duplicate column processed standard solution, the elution
of the matrix was collected in 3 separate 2 ml fractions. The matrix (Fe) fractions
from the blank and standard solutions only, were re-dissolved in 1 ml of 2 %
HNO3 and analysed for any Cr present by MC-ICPMS.
70
Column (2)
The evaporated Cr fractions from column (1) were similarly processed as per the
previous procedure.
Column Procedure D
Column (1)
From the BHVO-2 and Allende sample stock dissolutions and procedural blank
solution, 0.6 ml aliquots were taken for column loading. Two 0.4 ml aliquots were
removed from the NIST 3112a solution. The solutions were evaporated to dryness
and as in column procedure A, 200 µL of 6 M HCl and 1 ml of H2O were added.
For each column, the Cr fraction was eluted with 5 ml of 1 M HCl, as compared
to 4 ml in previous procedures. The matrix (Fe) fraction was eluted with 6 ml of 6
M HCl as previously, although for the duplicate standard solution, this matrix
elution was collected in 3 separate 2 ml fractions. The evaporated matrix (Fe)
fractions from the blank and standard solutions only, were re-dissolved in 1 ml of
2 % HNO3 and analysed for Cr by MC-ICPMS to check for any Cr present.
Column (2)
The volume of cation resin used for these columns was increased from 0.33 ml
previously used, to 0.50 ml. Consequently, the volume of eluants was
proportionally increased for elution. Additionally, the molarity of HNO3 was
increased from 0.15 M to 0.5 M. The Cr fractions from column (1) were refluxed
71
for 48 hours with 3.32 ml of 0.5 M HNO3 and loaded onto 0.50 ml of
preconditioned resin. The Al, Ti matrix fraction was eluted first with 4.15 ml 0.5
M hydrofluoric acid. The total 7.47 ml was collected in 15 ml Savillex© vials.
Secondly, the residual Fe, Ni and Mn fraction was eluted with 13.28 ml of 1 M
HCl. Finally the Cr fraction was eluted with 4.15 ml of 2 M HCl. The purified Cr
was dissolved in 1 ml of 2 % HNO3 and analysed by MC-ICPMS. Evaporated
matrix fractions from the blank and standard solutions only, were re-dissolved in
1 ml of 2 % HNO3 and analysed for Cr by MC-ICPMS.
Column Procedure E
Column (1)
From the BHVO-2 stock dissolution a 0.6 ml aliquot was removed. Two separate
Allende samples were processed, one 0.6 ml aliquot and a 1.2 ml aliquot. No
blank was processed in this procedure. Two 0.4 ml aliquots of the standard
solution were taken; however, the duplicate standard solution was not processed
through column (1). No Cr cut was removed from the Cr fractions during this
procedure. The matrix (Fe) was eluted with 6 ml of 6 M HCl. The evaporated
matrix (Fe) fraction from the standard solution only was re-dissolved in 1 ml of
2% nitric acid and analysed by MC-ICPMS.
Column (2)
The evaporated Cr fractions from column (1) and the duplicate standard solution
(not processed through column 1) were refluxed for 24 hours at 80oC with 3 ml
0.5 M HNO3 and loaded onto 0.5 ml of resin. The Al, Ti matrix fraction was
72
eluted first with 3.75 ml 0.5 M hydrofluoric acid. Secondly residual Fe, Ni and
Mn matrix was eluted with 12 ml of 1 M HCl. Finally the Cr fraction was eluted
with 3.75 ml of 2 M HCl. The purified Cr was dissolved in 1 ml of 2 % nitric acid
and analysed by MC-ICPMS, together with matrix fractions from the standard
solutions.
Column Procedure F
Column (1)
From the BHVO-2 and Allende stock dissolutions 0.6 ml aliquots were taken. No
blank was processed in this procedure. Three separate aliquots of the standard
solution were taken, 1 ml and two, 0.4 ml; however, one of the 0.4 ml duplicate
standard solutions was not processed through column (1).
The sample and standard column loading solutions (in 1.2 ml 1 M HCl), in this
procedure were not loaded onto the columns immediately. From these solutions,
0.2 ml was removed and analysed immediately for hexavalent Cr by DPC assay
(3.3). The remaining 1 ml solutions were reprocessed as described in Column
procedure A, in preparation for column loading the following day.
The Cr fractions were eluted with 5 ml of 1 M HCl. For MC-ICPMS analysis, 0.5
ml Cr cut was removed from each of the Cr fractions and evaporated. For DPC
analysis a further 0.2 ml was removed. Each of the 5.5 ml Cr fractions were
evaporated, one drop of concentrated HNO3 added and evaporated to dryness at
110oC, in preparation for loading onto column (2). The matrix (Fe) fractions were
73
eluted with 6 ml of 6 M HCl. The evaporated matrix (Fe) fractions from the
standard solutions only were re-dissolved in 1 ml of 2 % HNO3 and analysed for
Cr by MC-ICPMS.
Column (2)
The Cr fractions from column (1) and the duplicate standard solution not
processed through column (1) were refluxed for 24 hours at 80oC with 3 ml 0.5 M
HNO3. The column loading solutions were then treated as column procedure E.
Column Procedure G
Column (1)
From the Allende stock dissolution a 0.6 ml aliquot was removed. Two separate
BHVO-2 samples were processed, one 0.6 ml aliquot and a 1.2 ml aliquot. No
blank was processed in this procedure. Two 0.4 ml aliquots of the standard
solution were taken; however, both solutions were not processed through column
(1), only through column (2).
The sample solutions were processed through column (1) as the previous
procedure. However for BHVO-2 and Allende samples, the matrix (Fe) fraction
was eluted with 6 ml of 6 M HCl in two fractions; the first 0.5 ml collected
separately from the following 5.5 ml fraction.
74
Column (2)
The Cr fractions from column (1) and the standard solutions (not processed
through column (1) were refluxed for 24 hours at 80oC with 3 ml 0.5 M HNO3.
During this procedure, the Al, Ti matrix fractions were eluted with 6.75 ml 0.5 M
hydrofluoric acid in five separate fractions, 2 ml; 1 ml; 0.75 ml; 1 ml and 2 ml.
Secondly residual Fe, Ni and Mn matrix fractions were eluted with 12 ml of 1 M
HCl. Finally the Cr fractions were eluted with 8.5 ml of 2 M HCl in three separate
fractions: 3.75 ml; 1 ml and 3.75 ml. The purified Cr was dissolved in 1 ml of 2 %
HNO3 and analysed for Cr by MC-ICPMS together with matrix fractions from the
standard solutions.
Column Procedure H
Column (1)
From the BCR-2 dissolution, procedural blank solution and standard solution, 0.6
ml aliquots were removed. From the BHVO-2 dissolution, a 2.0 ml aliquot was
taken. The sample solutions were processed through column (1) as the previous
procedure.
Column (2)
The evaporated Cr fractions from column (1) were refluxed for 24 hours at 80oC
with 3 ml 0.5 M HNO3 and loaded onto 0.5 ml resin volume columns. In this
procedure the solutions eluted immediately after the column loading solutions
were added to the resin, were collected separately. The Al, Ti matrix fractions
75
were eluted first with 3.75 ml 0.5 M hydrofluoric acid. Secondly residual Fe, Ni
and Mn matrix fractions were eluted with 12 ml of 1 M HCl. In this procedure, the
Cr fraction was eluted with 8.5 ml of 2 M HCl in three separate fractions: 4.75 ml;
1.25 ml and 2.5 ml. The purified Cr was dissolved in 1 ml of 2 % HNO3 and
analysed for Cr by MC-ICPMS with the matrix fractions from the standard and
blank solutions.
Column Procedure I
Column (1)
From the BCR-2 stock dissolution and procedural blank dissolution 0.6 ml
aliquots were removed. Two separate BHVO-2 samples were processed, a 1.0 ml
aliquot and a 2.0 ml aliquot. NIST3112a was also processed.
The resin volume in this procedure was increased from 1 ml used in previous
procedures, to 2 ml. The column loading solutions were now in 2.4 ml 1 M HCl.
For each column, the Cr fraction was now eluted with double the previous eluant
volume of 5 ml, with 10 ml of 1 M HCl and collected in a 15 ml Savillex© vial.
The eluant volume for eluting the matrix (Fe) fraction was also increased to 12 ml
6 M HCl. The evaporated matrix (Fe) fractions from the standard and blank
solutions only, were re-dissolved in 1 ml of 2 % nitric acid and analysed for Cr by
MC-ICPMS.
76
Column (2)
The Cr fractions from column (1) were loaded onto 0.5 ml resin in 5 ml HNO 3
and the Al, Ti matrix fractions were eluted with 3.75 ml 0.5 M hydrofluoric acid.
Then residual Fe, Ni and Mn matrix was eluted with 12 ml of 1 M HCl. Finally,
the Cr fraction was eluted with 8.5 ml of 2 M HCl in three separate fractions: 4.75
ml; 1.25 ml and 2.5 ml into 7 ml Savillex© vials. The Cr fractions were dissolved
in 1 ml of 2 % HNO3 and analysed for Cr by MC-ICPMS. Evaporated matrix
fractions from the standard and blank solutions only were also analysed for Cr by
MC-ICPMS.
Column Procedure J
Column (1)
From the BCR-2 stock dissolution, one 0.4 ml and two 0.6 ml aliquots were
removed. From the standard solution two 0.6 ml aliquots were taken. All were
processed through this column as described in column procedure I.
Column (2)
The Cr fractions from column (1) were processed in the same manner as described
in column procedure I with the exception of one of the duplicate BCR-2 samples.
Here, for BCR-2 the alteration to the procedure was to split the column loading
solution in 5 ml HNO3, into two 2.5 ml loading solutions, which were loaded onto
two individual 0.5 ml resin bed columns.
77
Column Procedure K
Column (1)
From the Chromite stock dissolution, two 0.2 ml and 0.4 ml aliquots were taken
and 0.6 ml aliquots were removed from the standard and blank solutions. For each
2 ml (resin volume) column, the Cr fraction was eluted with 10 ml of 1 M HCl
and followed the same process as column procedure J.
Column (2)
The evaporated Cr fractions from column (1) were loaded in 5 ml 0.5 M HNO 3
and placed onto 0.5 ml columns. In this procedure, the Cr fractions were eluted
with 12 ml of 2 M HCl in two separate fractions: 7 ml and 5 ml.
Column Procedure L
Column (1)
From the Chromite stock dissolution, a 0.3 ml and a 0.6 ml aliquot was taken and
0.6 ml aliquots were removed from the standard and blank solutions. An aliquot
of 0.6 ml was taken from the dissolution of SCo-1 sample. For each 2 ml column,
the Cr fraction was eluted with 10 ml of 1 M HCl and the same process followed
as column procedure J.
Column (2)
The Cr fractions from column (1) were processed as in column procedure J.
78
Table 3.6: Summary of the optimised two-step cation exchange column procedure
developed in this study. Significant alterations from original Column Procedure A
are highlighted in italics.
Step
Details
Column 1
1
Sampling
Take required aliquot from sample, blank and standard stock dissolutions
2a
Dry down
Evaporate to dryness on hotplate at 110°C
2b
HNO3
Add 3.0 ml 15.7 M HNO3 and dry down
2c
HCl
Add 3.0 ml 6 M HCl and reflux at 80°C for 24 hours
3
Dry down
Evaporate to dryness on hotplate at 110°C
4
1 M HCl
Take up in 400 µL 6 M HCl + 2.0 ml MQ
5
Warm
Warm solution for 10 minutes
6
Prepare resin
7
Cooling
Pipette 2 ml Bio-Rad AG® 50W-X8 cation exchange resin into 10 ml BioRad polypropylene columns; rinse with 10 ml MQ and condition with 10
ml 1 M HCl.
Samples, standard and blank solutions are cooled to room temperature
8
Loading
Carefully pipette aliquots (in 2.4 ml 1M HCl) onto each 2 ml column
9
Collection
Elute Cr fraction (solutions 1.1) with 10.0 ml 1 M HCl
10
Collection
Elute matrix (Fe) fraction (solutions 1.2) with 12.0 ml 6M HCl
12
Separation
13
Dry down
Remove 0.5 ml from solutions 1.1 and evaporate to dryness on hotplate at
80 °C – Cr cut (solutions 1.1a)
Evaporate solutions 1.1 and 1.2 to dryness on hotplate at 110 °C
13a
Dry down
14
Storage
Add 1 drop 15.7 M HNO3 to solutions 1.1, 1.1a and 1.2 and dry down on
hotplate
Solutions 1.1a and 1.2 in 1 ml 2 % HNO3
Column 2
15
Reflux
Reflux Cr solutions 1.1 overnight in 5 ml 0.5 M HNO3
16
Prepare resin
Pipette 0.50 ml Bio-Rad AG® 50W-X8 cation exchange resin to 5 ml
Teflon© PTFE columns; rinse with 5 ml MQ and condition with 5 ml 1M
HCl.
19
Cool
Cool Cr solutions 1.1 to room temperature
20
Loading
Carefully pipette solutions 1.1onto column
21
Collection
3.75 ml 0.5M HF- elute Al, Ti matrix fraction (solutions 2.1)
22
Collection
12.0 ml 1M HCl- elute Fe, Ni, Mn matrix fraction (solutions 2.2)
23
Collection
8.50 ml 2M HCl- elute Cr fraction (solutions 2.3)
24
Dry down
Evaporate solutions 2.1, 2.2 and 2.3 to dryness on hotplate 110°C
25
Storage
Solutions 2.1, 2.2 and 2.3 in 1 ml 2 % HNO3
26
Reflux
Reflux all solutions (1.1a, 1.2, 2.1, 2.2, and 2.3) overnight at 80°C prior to
MC-ICPMS analysis.
79
Table 3.7: Summary of alterations made to each column procedure.
Column 1
Column
Sample load Cr elution Fe elution
Resin bed
Procedure
(ml)
(ml)
(ml)
(ml)
1.2 M HCl 1 M HCl 6 M HCl
A
1.0
1.2
4.0
6.0
B
1.0
1.2
4.0
6.0
C
1.0
1.2
6.0
6.0
D
1.0
1.2
5.0
6.0
E
1.0
1.2
5.0
6.0
F
1.0
1.0
5.0
6.0
G
1.0
1.2
5.0
6.0
H
1.0
1.2
5.0
6.0
I
2.0
2.4
10.0
12.0
J
2.0
2.4
10.0
12.0
K
2.0
2.4
10.0
12.0
L
2.0
2.4
10.0
12.0
Matrix 1 = Al + Ti elution; Matrix 2 = Fe, Ni + Mn elution
Column 2
Matrix 1 Matrix 2
Sample load
Cr elution
elution elution
Resin bed
(ml)
(ml)
(ml)
(ml)
(ml)
HNO3
2 M HCl
0.5 M HF 1 M HCl
0.50 2.0 (0.15 M)
2.5
8.0
2.5
0.33 2.0 (0.15 M)
2.5
8.0
2.5
0.33 2.0 (0.1 5M)
2.5
8.0
2.5
0.50 3.32 (0.5 M)
4.15
13.3
4.15
0.50 3.0 (0.5 M)
3.75
12.0
3.75
0.50 3.0 (0.5 M)
3.75
12.0
3.75
0.50 3.0 (0.5 M)
6.75
12.0
8.5
8.5
0.50 3.0 (0.5 M)
3.75
12.0
0.50 3.0 (0.5 M)
3.75
12.0
8.5
0.50 5.0 (0.5 M)
3.75
12.0
8.5
0.50 5.0 (0.5 M)
3.75
12.0
12.0
0.50 5.0 (0.5 M)
3.75
12.0
12.0
3.5.2 Multi collector-inductively coupled mass spectrometer
This study employed a Nu Plasma Multiple collector inductively coupled plasma
mass spectrometer (MC-ICPMS) manufactured by Nu Instruments, which is a
double focusing mass spectrometer, capable of high resolution measurements of
atomic ions with similar mass to charge ratios (m/z) (Rehkämper et al. 2001).
Precise, fast sequential determinations of isotopes of trace quantities of elemental
ions in minimal solution volumes (< 1 ml) can be made at the same time due to
collection in multiple Faraday cups (Rehkämper et al. 2001). Most elements are
fully ionised in the plasma ion source, but the advantage with this system is that a
dry plasma mode (with the use of a desolvator nebuliser system) can be employed
to reduce spectral interferences by removing most of the solvent (Schönbächler et
al. 2007). This significantly reduces isobaric interferences, from polyatomic ions
of equal mass present in the solvent (Rehkämper et al. 2001). The Nu Plasma HR
uses an electrostatic analyser in series with a magnetic sector. The reason for this
is that the trajectory of an ion is dependent on its kinetic energy and the width of a
mass peak is dependent on the energy spread of the ion source. The electrostatic
80
analyser is an energy focusing device, which rejects ions whose energies are not
within a set narrow range, thereby only allowing ions with certain energies to
reach the Faraday collector array and hence a greater separation/resolving power
between peaks of adjacent masses (Gill 1997).
The sample introduction system used with the MC-ICPMS was the Desolvating
Nebulizer System (DSN-100). A short description follows here which is based on
details in the Nu Plasma Manual. Small amounts (100 µL/min) of sample solution
are aspirated with argon gas via a glass nebulizer, into a heated (110oC) Teflon©
PFA spray chamber, where the sample is almost entirely vaporised. The low
aspiration rate and an additional stream of preheated argon gas, ensures fine
aerosols enter the spray chamber with minimal condensation to the chamber walls.
The vaporised sample is then transported through a semi-porous Teflon© PTFE
membrane, which is also heated. This allows the solvent from the sample solution
to diffuse through the membrane wall, the solvent is then removed by a membrane
gas flow to a condensate waste vessel. The desolvated sample then exits the DSN100 and enters the plasma (ion source) through the quartz torch via the sample
outlet. The torch position can be altered, either up/down or forward/backward and
in/out with respect to the cones to optimise the ion beam extracted from the
plasma.
81
Figure 3.2: Schematic diagram of Nu Plasma MC-ICPMS (from Nu Plasma
Manual).
In the plasma, argon atoms and sample atoms are ionised at high temperatures
(6000 K) and atmospheric pressure, the plasma is a very efficient ionisation
source. Under these conditions, an ion beam of mainly singly charged positive
ions is produced. The resultant ion beam is accelerated and extracted from the
plasma, through the nickel cones, source slit and the alpha slits by means of an
applied high voltage under high vacuum (Fig. 3.2). This imparts a kinetic energy
to all ions present; therefore the velocity will vary depending on the mass of given
ions. The emerging ion beam contains many ions with a wide range of energies.
82
For high mass resolution, it is necessary to limit the spread of ions with varying
energies from reaching the detectors. The source slit is set at the smallest setting,
0.03 mm, reducing sensitivity for pseudo-high resolution; true high-resolution
would produce separate sharp peaks and stable ion counting of the peak maximum
would not be achieved (Nu Plasma Manual).
The ion beam is then passed through the alpha slits, which are adjusted to prevent
a diverging beam from entering the magnet. The alpha slits are used to reduce
transmission, to allow only 50% of the ions produced in the ion source to reach
the detector and thereby minimise any aberrations or blurring of the image
arriving at the detectors. Optimisation of beams from ions of similar masses, are
partially resolved/separated to obtain one overlapping peak, of two adjacent mass
signals, such as 52Cr and 36Ar 16O, with the analyte plateau on the lower mass side
of the combined interference analyte peak (Fig. 3.3). This resolution produces
wide flat topped square peak shapes with reduced peak tailing, which are typical
from MC-ICPMS, where in the centre of this plateau stable ion counting of the
analyte signal can be made.
The ion beam then enters the electrostatic analyser, which has a 90o Nier-Johnson
geometry, where a radial field exists between two curved parallel plates which are
held at equal but opposite electric potentials (Jarvis 1992). The resultant radial
force generated by this electric field ensures that the radius of curvature of ions
through the electrostatic analyser depends on their kinetic energy and not mass.
The energy dispersion of ions results in ions following different trajectories
through the field. Ions with higher energies will travel a wider radius than ions
with lower energies. Only ions with certain energies will travel in a uniform
83
circular motion through the electrostatic analyser on a set trajectory whilst other
ions will collide with the walls and will not exit at the correct angle (angular
dispersion). Subsequently, the exiting ion beam is directed and focused with
extraction lenses according to the energy dispersion of the ions, into the magnetic
sector.
The convergences of the ion beams enter the magnetic sector from the curved
electrostatic analyser tube. Poles of a large electromagnet are held above and
below the analyser tube. The ion beam is deflected through the strong magnetic
field according to the mass to charge ratio of the ions (Gill 1997).
Once passed through the magnet the beam is further focused by the zoom lenses
(Fig. 3.2). The zoom lenses are held at various set voltages so that the ion beam
can be precisely channelled into specific Faraday collectors. There are twelve
Faraday collectors (cups) in the Nu Plasma HR MC-ICPMS. When ion beams of
singly charged positive ions (in a vacuum) strike the conductive metal cup
(Faraday cup), electrons from the surface are displaced, neutralise the charge and
this flow of electrons is directly measured as a current. An output of 1 V from a
Faraday cup, is the result of 62.42 million ions hitting the metal surface per
second (with a 1011Ω resistor). For instance, a 1 ppm Cr solution typically
produces a 3 V 52Cr signal. A Faraday cup should not be set to mass 40 as there
are so many
40
Ar ions from the plasma that this would swamp the ion counters.
The Faraday collectors measure all positive ions and the signals produced are
therefore energy and mass dependant. They can simultaneously collect several ion
beams in different Faraday cups over a narrow range of mass to charge ratios. For
Cr isotope measurements, the collectors are set at 0.25 atomic mass units (amu)
84
apart. The axial collector is set on the mass of the most abundant isotope to be
analysed and collectors either side can be used to analyse for other isotopes or
interferences of similar masses. The amplified direct current signal is then
integrated by the computer to obtain a mean ion current from each collector. This
is directly compared to signals, in volts, taken from standards containing known
quantities of analyte ions. Typical operating parameters of the MC-ICPMS are
presented in Table 3.8.
Table 3.8: Nu Plasma typical instrumental operating parameters
Units
Forward power
1300
W
Reflected power
<1
W
Cool gas
13.0
L min-1
Auxiliary gas
0.8
L min-1
Plasma isolation pressure
1.3
mbar
Chamber 1 pressure
2.5 x 10-4
mbar
-9
Mass spectrometer pressure
1.0 x 10
mbar
DSN-100
Argon inlet pressure
Spray chamber temperature
Membrane temperature
Hot gas flow
Membrane gas flow
Nebuliser pressure
Sample uptake rate
50
110
116
0.18 to 0.22
2.6 to 3.2
25 to 33
100
PSI
°C
°C
L min-1
L min-1
PSI
μL min-1
3.5.3 Chromium isotope measurements by MC-ICPMS
Before the samples were analysed on the MC-ICPMS, optimisation of the signal
obtained from analysing a 200 ppb Cr standard was carried out for each analytical
session. The measurements were obtained in high resolution mode, which means
that the source slit was set to 0.03 mm. Most of the isobaric interferences from
the solvent such as,
40
Ar
14
N,
36
Ar
16
O and
85
38
Ar
16
O have been removed by the
DSN-100, therefore, the majority of polyatomic interferences have been removed
prior to entering the ion source. Once a stable signal is obtained, the extraction
and acceleration optics are finely adjusted to achieve optimal peak signal from a
200 ppb Cr standard solution, on the axial cup detector set at mass 52, the most
abundant (84%) isotope of Cr (52Cr). Generally, this produces a signal of 200
mV. Also, small adjustments are made to the magnet position, within 0.25 amu, to
ensure that total ion beam for
52
Cr is being captured into the required Faraday
collector. This is done by calculating the measured voltage (signal) for the
concentration of ions analysed in the Cr standard and with the corresponding
abundance for 52Cr, the optimum voltage for 52Cr is attained. The magnetic field is
precisely set in the centre of the shoulder plateau of the flat-topped mass peak
during a specific analysis session and the position of such, together with signal
intensity is checked throughout the session as instrumental drift occurs with
changing plasma and ion optics conditions. The shoulder plateau is the analyte
only, resolved from the analyte plus interferences (from Ar polyatomic ions) of
the main flat topped peak (Figure 3.3). A significant shoulder width on the low
mass side of the (52Cr and interferences) resolved peak, with optimisation of the
plasma through positioning of the torch, fine tuning of the lenses and adjustments
to the alpha slits, should be obtainable during each session for accurate, precise
and stable measurements. For flat-topped peak measurements, the low mass side
of the peak should not be significantly tailing and should be near vertical.
86
54
Cr + 40Ar14N
54
Cr on H6
40
Ar14N
52
Cr on axial cup
Figure 3.3: Analysis plot of 52Cr peak from the Nu Plasma MC-ICPMS
Once an optimum plateau width and stable signal was achieved, the samples were
analysed simultaneously for Cr,
50
Ti,
51
V and
54
Fe isotopes using the standard
sample bracketing method. A blank solution of 2% HNO3 (identical solution
matrix as the samples) was analysed after each sample/standard solution. The
blank solution was analysed to ensure complete wash out of the sample from the
sample injection system and also to ensure adequate time for memory signal
effects from previous sample/standard solutions with high ion counts, to diminish
to background voltages. The bracketing standard used throughout the analysis
session was a 1 ppm Cr (NIST 3112a) solution in 2% HNO3. The signals obtained
from this standard in the axial cup, set at mass 52 for
52
Cr, was used to calculate
Cr concentrations in the samples. The 1 ppm Cr (NIST 3112a) solution produced
a signal of 1000 ± 50 mV. Simultaneous signals from Faraday cups set to record
masses either side of the main Cr isotopic peak were also monitored:
87
54
Cr;
53
Cr,
and 50Cr. Also, elevated counts in these cups were monitored to check for isobaric
interferences from
50
Ti and
51
V. Any high amounts of
54
Fe were detected by the
offset of the 54Cr/52Cr ratios and high 50Ti by the offset of the 50Cr/ 52Cr ratios. If
significant voltages are measured in these cups other than the expected isotopic
ratio for Cr, then the samples are contaminated with Ti, V, Mn or Fe.
3.6 Results and discussion
In the following, the results from each column procedure (A – L) shall be
presented and discussed and the reasoning for subsequent changes to the original
procedure explained. The percentage yield for Cr from samples processed during
each column procedure is presented in Table 3.9. Theoretical Cr yields (µg) were
calculated from the amount of sample dissolved and accordingly from the
proportion (aliquot) of stock dissolutions taken for each column loading (Table
3.4). Theoretical yields assume 100% recovery of Cr from each column. The
actual amounts of Cr eluted, obtained from MC-ICPMS analyses, provide actual
Cr yields for each column. The % Cr yields expressed here were calculated by
dividing the actual (measured) yield by the theoretical yield for each sample
processed (Table 3.9).
Several procedural blanks were processed throughout the column procedures and
Cr concentrations of the blanks ranged from, 0.1 ng to 7.7 ng (+/- 0.25 ng), as this
was negligible compared to analysed samples, no blank corrections were made.
88
Table 3.9: Chromium yields from samples processed through twelve, two-step
cation exchange procedures A - L
Sample
Procedure
NIST 3112a
A
B
B*
C
C*
D
D*
E**
E**
F
F*
F*
G**
G**
H
I
J
J*
K
L
BHVO-2
A
C
D
E
F
G
G*
H
I
I*
Allende
Amount
sample
(mg)
Cr
(µg)
Column 1
(% yield)
Column 2
(% yield)
Total Procedure
(% yield)
8
16
16
16
16
16
16
16
16
16
40
16
16
16
24
24
24
24
24
25.2
44
88
92
90
94
62
67
85
95
97
91
75
89
85
111
92
46
77
73
58
43
74
76
67
69
88
84
85
90
87
92
131
93
102
73
83
21
68
67
52
40
81
51
67
69
74
80
82
90
87
83
98
82
86
81
77
24.96
24.96
24.96
24.96
24.96
24.96
45.92
108.49
54.24
108.49
6.99
6.99
6.99
6.99
6.99
6.99
12.29
30.38
15.19
30.38
40
68
59
136
43
30
115
80
47
48
131
71
60
192
224
49
47
31
19
33
77
71
81
82
67
49
54
25
A
C
D
E
E*
F
G
3.28
3.28
3.28
3.28
6.56
3.28
3.28
12.29
12.29
12.29
12.29
24.59
12.29
12.29
24
90
88
89
91
73
94
86
65
67
77
82
18
84
75
65
67
69
74
BCR-2
H
I
J
J*
J*
47.89
47.89
31.92
47.89
47.89
0.77
0.77
0.51
0.77
0.77
51
49
57
61
56
96
84
98
118
144
49
57
56
72
82
SCo-1
L
40
2.72
58
147
86
Chromite
K
0.046
21.33
57
66
K*
0.046
42.65
59
83
K*
0.046
21.33
61
103
L*
0.1
46.46
63
98
L*
0.05
23.23
69
*denotes duplicate sample; **denotes sample processed through column(2) only
89
38
43
62
62
-
All matrix fractions from processing the NIST 3112a standard were analysed by
MC-ICPMS throughout every column procedure to check for any Cr present, as
the eluted matrix fractions from the samples could not be analysed since they
would contain significant amounts of Fe, Ti and Mn. Measurement of Cr fractions
contaminated with matrix elements, would result in significant voltages measured
in the detector array and would offset the expected isotopic ratio for Cr. This
would result in elevated yields reported for Cr.
Sample aliquots were loaded onto 1 ml resin bed (column (1)) in approximately
the same volume (1.2 ml in 1 M HCl) as the resin volume (Trinquier et al. 2008).
In general during ion exchange chromatography this is to ensure that the sample
solution has sufficient resin and time to completely exchange with the resin:
thereby enabling maximum distribution of matrix elements to the resin. This
amount of resin proved insufficient for complete separation of Cr from major
matrix elements and was increased to a 2 ml resin volume from procedure I. The
volume of eluants was increased proportionally.
The reported maximum ion exchange capacity of the resin (2.1.1) was taken into
account when loading samples, so as to not exceed exchange capacity for exact
resin volumes used. For instance, 0.6 ml aliquots of sample dissolutions were
taken, containing 24.96 mg of BHVO-2; 3.28 mg of Allende and 4 ppm
NIST3112a whilst being sufficient to obtain theoretical yields of Cr of 6.99 µg;
8.29 µg and 8 µg respectively (Table 3.4). The quantities of samples loaded onto
column (1) during all of the procedures (A – L) were extensively tested.
90
For procedures (A – H) a 1 ml resin volume was used for column (1) and for
column (2), 0.33ml resin. Approximately 25 mg of BHVO-2, proved to be too
much sample loaded onto a 1 ml resin volume during column procedures A and B,
as the separation of Cr was ineffective, resulting in low yield and purity of the
analysed Cr fractions (Table 3.9). Owing to the high total dissolved solids in
dissolutions of samples such as, BHVO-2, varying aliquots of BHVO-2 (25 – 100
mg) were loaded onto column 1 during subsequent column procedures to verify
that column overloading had occurred. Elevated yields were observed for column
2 further proving that separation was unsuccessful on column 1 with a 1 ml resin
volume. The resin volume was therefore doubled for column 1 to 2 ml and
increased from 0.33 ml for column 2 to 0.5 ml. This would greatly increase the
number of exchange sites available for matrix elements to exchange with. This is
extremely advantageous for processing samples with high total dissolved solids
such as, BHVO-2 and especially BCR-2 which contains even lower
concentrations of Cr (16 µg/g) and therefore more sample has to be loaded onto
the first column to recover adequate Cr for analysis. Dissolutions of samples with
high Cr concentrations in proportion to the matrix, namely Allende and Chromite,
could be loaded onto the resin in much smaller aliquots to recover similar
amounts of Cr.
In column procedure C, a duplicate NIST standard solution was processed to
assess Cr elution of the 4 ml Cr fraction from column (1). An additional 2 ml of 1
M HCl was added (after the 4 ml elution) and collected in two separate 1 ml
fractions. In the first additional 1 ml added, 9.6 and 9.0% Cr yields were
observed. No significant Cr was detected in the final 1 ml fractions. The Cr
elution volume for column 1 was therefore increased from 4 ml 1 M HCl to 5 ml.
91
Once the resin volume was doubled from Procedure I, this elution volume was
also proportionally doubled to 10 ml.
When the matrix fraction is eluted from the first column with 6 M HCl, it is bright
yellow in colour due to mainly iron cations eluting. It is therefore possible to
assess visually when the colourless Cr fraction has finished eluting and
correspondingly when the mostly Fe cations matrix fraction starts to elute. The 6
ml Fe fraction was collected throughout the procedures to evaluate if any Cr was
co-eluting with the matrix. Although, only Fe matrix solutions eluted from
processed NIST standards were analysed by MC-ICPMS. During column
procedure C, the Fe matrix fraction from column 1 was collected in three separate
2 ml fractions to assess if the Cr, eluted in the previous 1 M HCl fraction, had
been fully eluted and was not significantly tailing into the Fe fraction. Elution
curves confirmed that the Cr elution peak from the NIST standard was not coeluting into the Fe matrix fraction (resulting in loss of Cr), whilst taking into
account that the Cr fraction elution volume needed to be increased by an
additional volume, as described above. Following the first 2 ml elution of the Fe
matrix, in the next 2 ml of 6 M HCl, 1.5% Cr was measured and in the final 2 ml
0.18% Cr. This Fe elution was kept at 6 ml of 6 M HCl for subsequent
procedures.
Cr yields from column 2 were problematic, especially from samples and NIST
3112a processed during Procedures A – D (Table 3.9) due to poor separation and
insufficient resin volumes. Separated Cr fractions from column 2, (from BHVO-2,
BCR-2 and SCo-1) analysed by MC-ICPMS also revealed Fe, Ti and Mn
impurities present. Such interferences from isobaric interferences due to matrix
elements on the
50
Cr and
51
Cr peaks, resulted in elevated Cr yields (Table 3.9).
92
The initial resin volume of 0.33 ml was insufficient, especially if large amounts of
matrix had not been successfully separated on column (1). The resin volume was
increased to 0.5 ml and therefore, the eluant volumes were increased
proportionally. Before the evaporated Cr fractions were loaded onto column (2),
large amounts of white precipitate were visible. The solubility of such Cr fraction
residues in 0.15 M HNO3 from samples such as BHVO-2, was low and for
column loading this was not adequate, as a result the acidic strength was increased
to 0.5 M HNO3, to keep the high amounts of dissolved solids in solution. Whilst
this increase was necessary to keep samples in solution, especially when held at
room temperature, careful consideration was given to results obtained from
distribution coefficients experiments performed in Chapter 2 to ensure that this
increase in acid concentration would not elute Cr cations. Chromium cations have
a high distribution coefficient in 0.5 M HNO3 (although not as high as when in
0.15 M) and is therefore exchanging (sticking) to the resin (Table 2.2).
Chromium yields from column (2) ranged from 43.0 % to 128 % for procedures A
– F; various alterations were subsequently made to improve yields and purity.
Processing of NIST3112a in Procedure F confirmed that loading the Cr fractions
in 0.5 M HNO3 showed no difference in resulting yields. Samples loaded onto
column (2) in 3 ml of 0.5 M HNO3 were, following procedure G, collected
(column loading fractions) immediately after addition to the column (before
matrix elution) to assess complete Cr exchange with the resin. No Cr should be
present in these solutions, however between 0.2 and 6.8 % Cr was found in the
column loading fractions. The column loading volume was increased from 3 ml to
5 ml 0.5 M HNO3. No improvement was found and to counteract this, the volume
of resin should be increased from 0.5 ml to 1 ml and also the size of the column
93
for future work. A longer, smaller diameter column would ensure adequate time
for the Cr to exchange with the resin.
Chromium elution curves performed in Procedure G, following adjustments to
column (1) in preceding procedures, were analysed to verify Cr has ceased eluting
after the 6 ml Cr fraction. No Cr was detected in the first 0.5 ml of the 6 ml 6 M
HCl fraction; therefore this elution volume was optimised at 6 ml and
subsequently at 12 ml for 2 ml resin volume. Secondly, the Al, Ti matrix fraction
volume was increased to 6.75 ml 0.5 M hydrofluoric acid and was eluted in five
fractions (3.4.1) and revealed loss of Cr in the initial 2 ml of this fraction, ranging
from 0.6 % to 1.7 % Cr. The elution volume was kept at the initial 3.75 ml as this
was effective in removing these matrix elements. However, owing to small
amounts of Cr being eluted at the start of this fraction, the column loading volume
was increased to 5 ml, as discussed above.
The Cr fraction eluted from column (2) was assessed with several elution curves
over successive procedures (D – K). The original elution volume of 2.5 ml 2 M
HCl was kept constant during procedures A – C, with average Cr yields of 59.4%
for NIST 3112a; 47.5% for BHVO-2 and 83.5% for Allende (Table 3.9). This
elution volume was increased to 4.15 ml in procedure D and then changed to 3.75
ml during procedures E and F with resulting 77.6% average Cr yields for NIST
3112a; 87.0% for BHVO-2 and 73.8% for Allende. Consequently, from column
procedure G, following the 3.75 ml elution of the Cr fraction a further 1 ml
followed by 3.75 ml was added, 8.50 ml in total and collected for analysis. The Cr
was found to be eluting with up to 6 ml of 2 M HCl added. Accordingly, in
column procedures H, I and J, 4.75 ml of 2 M HCl was added and analysed and a
94
further 1.25 ml (6.0 ml total) added to the columns and analysed to verify
complete Cr elution. Combined average yields were 99.2% for NIST 3112a;
108.6% BHVO-2; 82.0% Allende and 108% BCR-2. Although the elevated Cr
yields for BHVO-2 and BCR-2 were resulting from high sample loads onto
column (1) for determining maximum exchange limits. The eluant volume for
eluting the Cr fraction from column (2) was therefore increased to 7 ml for
procedures K and L.
Negligible blank values, <20 +/- 1 ng Cr, were also obtained from procedural
blank solutions processed through each column procedure.
3.7 Conclusion
An optimised two-stage cation exchange procedure is presented for the improved
separation of Cr from geological materials (Table 3.10). Numerous modifications
have been made to the cation exchange procedure developed by Trinquier et al
(2008), following extensive testing of various samples and standards through
twelve two-step cation exchange procedures.
Table 3.10: Optimised cation exchange procedure for the separation of Cr
Step
Acid volumes (ml)
Eluant
Column 1 (2ml resin bed)
Load sample
2.4
1 M HCl
Elute and collect Cr fraction
10.0
1 M HCl
Elute matrix (Fe)
12.0
6 M HCl
Column 2 (0.50ml resin bed)
Load Cr fraction from column 1
Elute matrix (residual Al, Ti)
Elute matrix (residual Fe, Ni, Mn)
Elute and collect pure Cr
5
3.75
12.0
8.50
95
0.5 M HNO3
0.5 M HF
1 M HCl
2 M HCl
The main advantage of this fast separation method is that the reduced form of Cr,
existing mainly as Cr3+ cations, is separated from interfering matrix cations in
acidic solution using a cation exchange resin. The Cr present in geological
materials and resulting dissolutions exists as Cr(III) cations, therefore, no further
chemical treatment is necessary to reduce any Cr(VI) anions to Cr(III). The
presence of Cr(III) and hence the absence of Cr(VI) in dissolutions prior to cation
exchange was confirmed by performing a hexavalent Cr assay.
The results show that for dissolutions of basaltic standard reference materials,
such as BHVO-2, no more than 20 mg should be processed through an initial 2 ml
volume of cation exchange resin (Bio-Rad AG® 50W-X8) for effective
purification of Cr. However, whilst this is an adequate sample amount to process
for BHVO-2 (280 ppm Cr), to obtain sufficient separated Cr (5 µg) of acceptable
purity for MC-ICPMS analysis for other materials of similar matrices but with
lower Cr concentrations, the amount of sample loaded cannot simply be doubled,
the volume of resin must also be proportionally increased. For example, BCR-2, a
standard containing much less Cr (16 ppm) than BHVO-2 but containing a higher
percentage of matrix material, 20 mg is the maximum amount that can be
processed through the optimised procedure to separate 0.5 µg Cr. For samples
with higher Cr concentrations, such as Allende and Chromite, smaller sample
quantities (scaled down according to Cr content) can be separated using this
procedure.
All stages in each of the twelve two-step cation exchange procedures were
thoroughly investigated to deduce where shortages in Cr yields were occurring.
Subsequent alterations were made after each procedure in order to recover such
96
losses and increase Cr yields without compromising purity of the eluted Cr
fractions.
In this study, purified Cr fractions obtained allow for accurate and precise analysis
of Cr by MC-ICPMS analysis, successfully purified from matrix elements such as
Fe, V, Ti and Mn, significantly reducing isobaric interferences. Improved
separation of Cr obtained during this study has enhanced the purification of Cr
from geological materials, with increased yields, particularly from those with high
dissolved solids. Negligible blank values, <20 +/- 1 ng Cr, were also obtained
throughout this study.
97
3.8 References
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natural water matrix for stable isotope mass spectrometric analysis."
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Birck, J.-L. and C. J. Allegre (1988). "Manganese - chromium isotope systematics
and the development of the early Solar System." Nature 331(6157): 579584.
Blowes, D. (2002). "Tracking Hexavalent Cr in Groundwater." Science
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Bonnand, P., I. J. Parkinson, R. H. James, A.-M. Karjalainen and M. A. Fehr
(2011). "Accurate and precise determination of stable Cr isotope
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Du, J., J. Lu, Q. Wu and C. Jing (2012). "Reduction and immobilization of
chromate in chromite ore processing residue with nanoscale zero-valent
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2060-2062.
Ellis, A. S., T. M. Johnson and T. D. Bullen (2004). "Using Chromium Stable
Isotope Ratios To Quantify Cr(VI) Reduction: Lack of Sorption Effects."
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Frei, R., C. Gaucher, S. W. Poulton and D. E. Canfield (2009). "Fluctuations in
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bombardment on Earth--Results from high precision chromium isotopes."
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Gill, R. (1997). Modern analytical geochemistry : an introduction to quantitative
chemical analysis for earth, environmental, and materials scientists.
Götz, A. and K. G. Heumann (1988). "Chromium trace determination in
inorganic, organic and aqueous samples with isotope dilution mass
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und
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Halicz, L., L. Yang, N. Teplyakov, A. Burg, R. Sturgeon and Y. Kolodny (2008).
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23(12): 1622-1627.
Housecroft, C. E. and E. C. Constable (2005). Chemistry, Pearson.
Jarvis, K. E. (1992). Handbook of inductively coupled plasma mass spectrometry.
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Corporation.
Korkish, J. (1989). Handbook of Ion Exchange Resins: Their Application to
Inorganic Analytical Chemistry, CRC Press.
98
Laqua, K., W. H. Melhuish and M. Zander (1988). "Nomenclature, Symbols,
Units and Their Usage in Spectrochemical Analysis .7. Molecular
Absorption-Spectroscopy, Ultraviolet and Visible (Uv Vis) (Recommendations 1988)." Pure and Applied Chemistry 60(9): 14491460.
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according to 53Mn-53Cr systematics." Geochimica et Cosmochimica Acta
62(16): 2863-2886.
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Determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U at ng g-1 Levels in
Milligram Silicate Samples." Geostandards Newsletter 21(2): 307-319.
Merkle, R. K. W., M. Loubser and P. P. H. Gräser (2004). "Incongruent
dissolution of chromite in lithium tetraborate flux." X-Ray Spectrometry
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on TODGA resin: Application to Ca, Lu, Hf, U and Th isotope
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(2007). "High precision Ag isotope measurements in geologic materials by
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99
Trinquier, A., J.-L. Birck and C. J. Allegré (2008). "High-precision analysis of
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100
Chapter 4: Conclusion
A prerequisite for the precise and accurate analysis of Cr by MC-ICPMS is the
purification of Cr from matrix elements present in dissolutions of geological
materials. To achieve quantitative chemical separation of Cr, often present in trace
concentrations, from such matrix elements by ion exchange, distribution
coefficient determinations (Kd) are invaluable.
The objective was to improve and extend published data sets for the distribution
coefficient of elements in nitric acid on Bio-Rad AG® 50W-X8 cation exchange
resin. Presented is an extensive set of distribution coefficients for 38 elements
using Bio-Rad AG® 50W-X8 cation exchange resin in conjunction with a suite of
nitric acid solutions. Large distribution coefficient values were obtained for
elements in weak HNO3, confirming strong sorption to the resin; while
conversely, the distribution coefficient values decrease with increasing HNO3
strength. Verification is shown that all of the analysed elements do not distribute
onto the resin in strong nitric acid solutions and this can be attributed to an excess
of hydronium cations competing with analyte cations for available exchange sites.
The results confirm that equilibrium is achieved after a reaction time of 8 hours.
The distribution coefficient values presented are in excellent agreement with and
extend previous literature values with improved detection limits. Moreover, they
were extremely useful during the method development for optimising the cation
exchange procedure for the purification of Cr.
An improved and optimised procedure for the chemical separation of Cr has been
developed in this study. Extensive testing and modification of the two-step cation
101
exchange procedure developed by Trinquier et al. (2008) has resulted in the
production of purified Cr fractions from various geological materials with
differing Cr concentrations and matrices. Excellent recoveries were obtained from
samples with typical matrices, such as Allende, with Cr yields of 91% from
column 1 and 82% from column 2. This is in excellent agreement with yields
obtained from the NIST 3112a standard processed of 97% for column 1 and 87%
for column 2.
This study has determined the constraints for quantitative separation of Cr
eliminating potential isobaric interferences from elements such as, Ti, Fe, Mn and
V, to allow for the precise and accurate determination of Cr isotopes by MCICPMS.
Prior to column procedures, sample dissolutions and standards are stored in 6 M
HCl and any Cr aqueous species present has equilibrated as Cr(III) which is
conducive for effective separation by cation exchange chromatography. The
hexavalent assay performed confirmed the presence of anionic Cr(VI) in nitric
acid and therefore sample aliquots should not be evaporated with nitric acid
before loading onto column 1. Additionally, the distribution coefficient data
indicate that it would be preferable to load Cr fractions onto column 2 in 0.4 M
instead of 0.5 M HNO3.
Future work to further increase yields and purity of separated Cr should include
placing the required amounts of resin in columns of reduced diameter and
increased length as this would greatly increase the resolution of the eluted
fractions.
102