LIMNOLOGY
and
OCEANOGRAPHY: METHODS
Limnol. Oceanogr.: Methods 7, 2009, 730–739
© 2009, by the American Society of Limnology and Oceanography, Inc.
Rapid and precise δ13C measurement of dissolved inorganic
carbon in natural waters using liquid chromatography coupled
to an isotope-ratio mass spectrometer
Jay A. Brandes
Skidaway Institute of Oceanography, Savannah, GA, USA
Abstract
The measurement of the carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) in natural
systems has provided a tool for examining a variety of processes, from net primary production to anthropogenic
signatures of CO2 uptake in the oceans. Over the past decade, new δ13C-DIC methods have steadily decreased
the sample analysis time and volumes for this measurement. The development of a new interface capable of inline acidification and extraction of CO2 from a liquid stream (LC-Isolink™; Thermo Electron) provides breakthroughs in sample size, ease of sample handling, and speed of analyses. Typical sample size injections of 25 µL
provide precision and accuracy of 0.04‰, comparable to that obtained by off-line extractions/dual inlet analyses on much larger samples. Single-sample injection runs can be accomplished within 4 min, whereas sample
preparation is limited to filtration to remove particulates. Thus combined runs of 200 samples over a 20-h period are possible. Highest-precision triplicate injection analyses of single samples require 11 min. Manually injected marine sample sizes of 2.5 µL provide nearly the same precision and accuracy as the larger loop sizes. For
maximum precision, care must be taken to match sample and standard sizes to correct for linearity effects. Over
wider concentration ranges typically found within estuarine systems, the overall precision of the method
decreases to 0.06‰ with such correction.
Introduction
than direct pCO2 and DIC concentration measurements
(Takahashi et al. 2006).
Measurement of the isotopic composition of oceanic DIC
requires meeting several challenges. Oligotrophic mixed layer
δ13C-DIC values typically exhibit a 0.25‰ seasonal cycle (Gruber et al. 1998; Quay and Stutsman 2003). The uptake of
anthropogenically released CO2 (the 13C Suess effect) into the
oceans has generated slightly larger shifts (1.0‰) over the
past 30 years toward lighter δ13C-DIC values (Gruber et al.
1999; Quay et al. 1992, 2007; Sonnerup et al. 2007). Thus
investigations into both of these processes require measurement techniques of very high precision, generally better
than 0.05‰. On the other hand, measurement of porewater
δ13C-DIC values, useful for constraining the balance between
organic C remineralization, methane oxidation, and carbonate dissolution (Gehlen et al. 1999; Hu and Burdige 2007;
McCorkle and Klinkhammer 1991), have wide isotopic shifts
but generally are limited to volumes less than 2–3 mL for fineresolution profiling. And in all these cases, sample throughput
is a challenge, as detailed basin-wide or seasonal sampling can
generate thousands of individual samples.
Measurement of δ13C-DIC has evolved with the development of new isotope ratio mass spectrometry (IRMS) interfaces,
much as the measurement of isotopes within other sample
Oceanic dissolved inorganic carbon (DIC) represents the
largest active pool of C on Earth’s surface (Hedges and Keil
1995). Thus a great deal of attention has gone into examining different aspects of DIC cycling, especially in the areas of
the uptake of fossil fuel–derived C, the interaction between
DIC and organic carbon pools, and sediment burial/diagenesis and dissolution processes. Radioisotopic and
stable isotopic measurements have proven useful in constraining various aspects of all of these processes (Butler et al.
2009; Sonnerup et al. 1999), but their use is less widespread
*Corresponding author: E-mail: [email protected]
Acknowledgments
This material is based on work supported by the National Science
Foundation under grants NSF OCE-0722604 and GEO-0807387. Any
opinions, findings, and conclusions or recommendations expressed in
this material are those of the author and do not necessarily reflect the
views of the National Science Foundation. The author thanks Dr. Paul
Work and the students of the inaugural Introduction to Coastal
Oceanography class at Georgia Tech for assistance in sampling and data
reduction.
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Brandes
Rapid isotopic analysis of DIC
normally used for persulfate oxidant, was not used. The
Isolink reactor was set to 80°C.
Samples were first collected into plastic syringes from 10-L
Rosette Niskin bottles. Samples and standards were subsequently filtered through 0.2-µm pore-size nylon syringe filters
into 2.0-mL glass crimp-seal vials. A needle was fitted onto the
end of the syringe filter and samples filled from the bottom of
vials slowly and with a minimum of turbulence. The first 1 mL
was used as a filter/needle wash and discarded. Vials were
overfilled, then sealed with red butyl rubber/PTFE septa/
aluminum ring tops (Discovery Sciences cat. no. 73070) taking
care to not capture any bubbles within the vial. Vials so prepared were isotopically unchanged over a period of >1 month
at room temperature.
Analytical runs were performed with three injections per
vial, spaced 225 s apart (Fig. 1). The three peaks were typically
averaged, although if one peak was found to differ in peak area
by more than 5% from the mean of the other two, it was discarded. Three CO2 gas injections by the Isolink interface were
also used to provide internal references for the mass spectrometer’s Isodat software. Total run time was 11 min per sample. Overall precision of this method is ±0.04‰ (SD) on replicate samples (n = 10). An internal standard seawater solution,
consisting of a filtered, autoclaved coastal seawater sample
allowed to equilibrate with the atmosphere at room temperature for 1 week before encapsulation in vials, was used periodically to provide a reference. This internal standard was calibrated to NBS 19 CaCO3 standard defined as having an
isotopic composition of 1.95‰ versus vPDB (Coplen 1996)
using the method of Torres et al. (2005).
types has changed over the years. The earliest efforts involved
extraction of CO2 from large-volume (50-mL) samples using
purge and trap systems after acidification (Kroopnick 1974;
Quay et al. 1992). Purified CO2 from such samples was then
measured via dual-inlet IRMS systems, resulting in high precision (±0.025‰ SD; Quay et al. 1992) but this is overall a lowthroughput and highly labor-intensive process. Smaller samples could be measured this way (Mccorkle and Klinkhammer
1991) but require still more care in extraction and analysis. The
advent of automated headspace extraction interfaces in the late
1990s provided an alternative with much smaller sample size
requirements. However, significant sample preparation is still
required for these methods, including vial purging, addition of
concentrated H3PO4, and equilibration times of many hours
after acid addition (Torres et al. 2005).
Here I present a simple, non–labor intensive method to rapidly (4–11 min/sample) and precisely (<0.05‰ SD) measure
δ13C-DIC within aqueous samples on sizes as small as 2.5 µL.
The technique takes advantage of a relatively new liquid chromatography gas extraction interface developed primarily for
the analysis of nonvolatile organic solutes. The combination
of the new interface with modern IRMS instrumentation provides significant improvements in sample throughput, minimizing sample size requirements for porewater and other volume-limited samples, and attaining the precision needed for
surface ocean δ13C-DIC analyses.
Materials and procedures
The instrumentation used was a Thermo Electron SurveyorLite autosampler equipped with a 25-µL sample loop, a Surveyor MS HPLC pump, and a LC Isolink interface coupled to a
Thermo Scientific Delta V plus stable isotope mass spectrometer. The Isolink is an integrated chemical reactor/gas extraction manifold and standard reference gas injection device. In
normal operation, an incoming liquid stream from the HPLC
is mixed with both phosphoric acid and persulfate oxidant
(not used here), and the resulting fluid passed through a
heated stainless-steel capillary column reactor to convert
organic compounds to CO2. The mixture is cooled and subsequently passed through a gas–liquid separation manifold,
allowing the CO2 to be extracted into a He stream. This He +
CO2 stream passes through two Nafion driers and an open
split before introduction to the IRMS. For further details on
the Isolink’s design, see Krummen et al. (2004). The system
was run in isocratic mode with no separation column. The
mobile phase was degassed (90 min under vacuum in a 40°C
ultrasonic bath) deionized water with 1 drop of 85% H3PO4
added (Fisher Scientific HPLC grade). The same solution composition was used as the autosampler wash solution. Mobilephase flow rate was 300 µL min–1. A phosphoric acid solution
(10% vol/vol of 85% H3PO4 in distilled water, followed by
degassing under vacuum as described for the mobile phase)
was continuously mixed into the mobile phase within the
Isolink at a rate of 50 µL min–1. The second Isolink LC pump,
Assessment
Although the Isolink LC interface was developed primarily
as a tool for measuring the isotopic composition of organic
compounds, its design also provides for the rapid and simple
measurement of dissolved inorganic carbon. Indeed, without
the blank associated with the oxidation of residual DOC in
mobile phases, separation column bleed, and reagents, the
system as described above routinely achieved baseline values
of 20 mV on mass 44, less than 5% of the specified background level in organic analysis mode. This provides much
higher signal-to-noise ratios for sample measurements. In
addition, the lack of an oxidant means that O2 production and
concomitant mass spectrometer sample filament longevity
issues are insignificant (Hettmann et al. 2007).
High-precision measurements place a special emphasis on
sample storage and processing. Previous studies have indicated that standard polytetrafluoroethylene (PTFE)/silicone
septa allow CO2 diffusion (Nelson 2000; Taipale and Sonninen
2009), and measurements of marine samples using such septa
showed distinctive isotopic shifts after 3–4 h of storage at
room temperature. However, use of butyl rubber/PTFE septa
with crimp seal caps effectively provided isolation from
atmospheric CO2 over periods of at least 1 month (Table 1).
731
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Rapid isotopic analysis of DIC
Fig. 1. Sample chromatogram. Upper traces are isotopic ratios 46:44 and 45:44; bottom trace is mass 44 signal versus time. Square peaks are reference CO2 gas injections.
Blank concentrations after 2 weeks were <0.5% of a typical
seawater sample, rising to about 4% after 35 days. The isotopic
value of the blank vial DIC was –0.1 ± 0.5‰ at the end of this
time period. Several other septa types were tested including
black fluoroelastomer, butyl rubber without PTFE backing,
PTFE/chlorobutyl, and Finneran Versa-Vials with PTFE/gray
chlorobutyl plugs. None of these alternatives proved as resistant to leakage as the red butyl/PTFE seals. This is in keeping
with the results for storage of air samples for CO2 analysis
(Knohl et al. 2004). In addition, it is recommended that only
standard crimp-top vials be used, as wide-mouth and snap
ring vials exhibited higher leakage rates. Screw-top septa were
also unacceptable, as the autosampler had a tendency to push
the septa into the sample rather than puncturing it.
Samples, after filtration, were isotopically unchanged after
35 days of storage at room temperature (Table 1). In contrast
to blanks, no change in DIC concentrations was noticeable
after this time. However, samples which are far from equilibrium or far from “normal” isotopic values may be more
strongly affected by CO2 diffusion.
Optimization of the DIC isotopic method required examination of flow rates, acid concentrations, and other parameters. The
intrinsic precision (SD) of the Isolink for repeated analyses of a
single DIC sample is 0.04‰ (Table 2) for samples with peak
heights >4 V. This peak height translates into an injected amount
of C of roughly 17 nmol C. However, precisions of better than
0.1‰ were achievable for samples as small as 5 nmol (Table 3).
For seawater, it is possible to achieve this level of precision on
injections of only 2.5 µL, and it may be possible to achieve this
on 1-µL injections done manually. Indeed, the sample blank measured after 35 days was roughly 50 µM, corresponding to an
injected amount of <1.1 nmol C, and exhibited a precision of
±0.5‰. For such small samples, of interest in studies of pore
waters and other volume- or concentration-limited environments, the difficulty is in finding an effective storage container;
they also possibly require the use of an injector designed specifically for small samples. Over longer terms, the accepted method
of storage in glass-stoppered bottles with HgCl2– poison is recommended, and such treatment did not exhibit any noticeable effect
on the isotopic values of samples analyzed by LC-IRMS.
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Rapid isotopic analysis of DIC
Table 1. Effect of storage on blank (degassed deionized water) and Skidaway River estuarine samples.
Time, h
0
19
42
192
240
840
Area, V-s
Blank
Amount injected, nmol
δ13C
Area, V-s
Dock standard
δ13C
0.14
0.24
0.43
0.82
0.78
4.25
0.04
0.06
0.11
0.21
0.20
1.11
20
33
23
9.8
10.2
–0.1
134.9
134.5
135.2
138.8
137
134.2
–1.03
–1.03
–1.03
–1.02
–1.07
–1.07
Table 2. Number of injections per vial versus precision (SD).
Injections/vial
SD isotope, ‰
SD area, %
Vials, n
0.020
0.036
0.026
0.025
0.79
0.39
0.31
0.13
4
4
4
4
1
2
3
4
Precisions are given as SD of measurement divided by sample peak area.
Table 3. Comparison of loop volume on isotopic precision, manual injections.
Loop volume, µL
2.5
5
10
20
25
Height, mV
Area, V-s
Amount C injected, nmol
δ13C precision, ‰
Injections, n
1202
2149
4178
6267
7289
18.2
34.3
67.3
102.5
124.5
4.8
9.0
17.7
27.0
32.7
0.05
0.025
0.05
0.05
0.05
5
5
5
5
4
autosampler to analyze a series of standards with different
injection volumes to bracket sample peak areas.
Changing mobile-phase flow rates has an effect on isotopic
composition, peak height, peak area, and linearity. Peak areas
were inversely dependent on total flow (Table 4), suggesting
incomplete extraction within the gas manifold from the sample stream. Peak widths were negatively correlated with total
solution flow rate (mobile phase + acid) (Table 4). Note that
the chief limiting factor in determining peak widths is the
Isolink gas extraction manifold, as calculated peak widths
assuming instantaneous extraction would be on the order of
15 s based on injector loop volume and reagent flow rates.
Peak heights were influenced by both total flow and acid flow
rates, with lower acid flows at a given total flow rate producing higher peaks. The acid flow rates chosen for the method
provide a large excess (~150 times) compared to the DIC
within injected samples, and no significant sample area or
isotopic shift was noted with acid flow rates as low as 10 µL
min–1. However, higher flow rates and lower acid flow rates,
while producing better apparent chromatography, resulted in
worse linearity (e.g., 400 µL min–1 data in Fig. 2a). In all cases,
the total flow rate affects the isotopic composition of the
For high-precision analyses, the primary concern therefore was the linearity of the system and in particular the
Isolink interface. Tests over a wide range of DIC injection
amounts, starting from seawater amended with NaHCO3 to
triple present-day equilibrium levels, indicated that the
Isolink interface did not deviate from the intrinsic linearity
of the Delta plus IRMS from peaks of >35 V on mass 44 to
about 3 V peaks (or about 100 V-s peak areas). From 3 to 1.5
V (areas of 100 to 50 V-s) the system exhibited a stronger
fractionation, approximately a shift of –0.5‰ (Fig. 2a). This
shift was repeatable and reproducible day to day and thus
can be corrected for in samples by running standards that
bracket the concentration range of samples analyzed.
Although these tests were primarily conducted using the Surveyor autoinjector’s partial loop injection mode, this pattern
was verified by manually injecting seawater solutions using
different size loops (5, 10, 20, 25 µL; Fig. 2a). For routine
measurement of low DIC concentration samples, it is suggested that a larger sample loop be used to maintain sample
peak areas >100 V-s and that standards be run with concentration values bracketing the concentrations of samples. An
alternative is to use the partial loop injection mode of the
733
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Rapid isotopic analysis of DIC
sample by –0.18‰ for every 100-mL increase in flow rate
(data from Table 2, r 2 = 0.95 linear regression). Therefore the
choice of 300 µL min–1 (mobile phase) and 50 µL min–1 (acid)
was chosen as the best compromise between peak width, sensitivity, and linearity.
Given the need to prevent sample degassing, vials must be
filled with no headspace. This led to inconsistent peak areas
in replicate samples, especially in refrigerated samples. An
investigation of autosampler performance led to the conclusion that peak area consistency could be improved by overfilling the loop. Peak areas increased up to a doubling of loop
volume (Fig. 2b). This led to a more effective washing of
mobile phase from the loop and a decrease in the influence
of fluctuations in sample peak area in pressurized vials, from
an SD of 5% to an SD of 1%. Peak areas were more consistent
in samples collected from warmer estuarine waters and in laboratory standards made from equilibrated seawater, down to
an SD of 0.5%, which is the reported precision given by the
manufacturer (Table 2). Sample area precision also was
improved in samples warmed to room temperature before
subsampling into sealed vials, but this increases the risk of
bubble formation and subsequent isotopic fractionation in
typical oceanic samples. Sample area precisions might be
improved by the use of a refrigerated sample holder, but the
system used lacked that capability. The system is quite linear
throughout typical estuarine-marine DIC concentrations, with
an r 2 = 0.9999 determined using Na2CO3 standards (Fig. 2c).
This provides the possibility of obtaining both isotopic and
concentration information from samples (e.g., Salata et al.
2000), although much more rapid shipboard methods exist
for DIC concentration determinations (Feely et al. 2006). In
the case of volume-limited samples, however, this capability
will be useful. Again, the use of a refrigerated autosampler,
and in particular conducting duplicate or triplicate measurements using separate vials, may improve such results.
Accuracy may be improved by running standards spaced periodically throughout a run, to account for any changes in
Isolink extraction and ionization efficiency within the mass
spectrometer.
Tests were performed to determine if replicate injections on
the same vial would improve precision. Although there was
little difference in the isotopic precision between single injections and multiple injections (Table 2), the precision of peak
areas, and thus quantification of DIC concentrations, was
improved by having up to three or four replicate injections
(Table 2). Another advantage of replicate injections is that
there occasionally appear to be injections with too small a
peak area or other problems (as noted above), and having several peak injections provides the investigator the ability to statistically discard the erroneous peak. In practice, any peak
with an area diverging by >5% from the mean should be considered suspect. The disadvantages of multiple peak injections
are time and autosampler wear. Sample runs with one injection per vial can require as little as 200 s, plus an additional 40
Fig. 2. Performance tests. (A), Effect of sample size on isotopic value. Circles are samples injected using autosampler at nominal (300/50) flow
conditions. Size adjustments were achieved by changing injection volumes. Triangles are values obtained using the Isolink manual injector and
changing loop volumes. Squares are values obtained using the autosampler at a flow rate of 400 µL min–1 mobile phase and 10 µL min–1 acid.
Error bars represent 1 SD. (B), Effect of overfilling sample loop. Autosampler loop has a nominal volume of 25 µL. (C), Peak area (total sum of mass
44, 45, and 46) versus concentration of Na2CO3 standard.
734
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Rapid isotopic analysis of DIC
Table 4. Effect of different mobile-phase and acid flow rates on DIC peak area, peak width, and isotopic value.
Total flow,
µL min–1
300
300
325
350
350
350
350
350
350
350
400
400
400
450
450
500
500
Mobile-phase
flow rate
Acid
flow rate
Area,
% standard
Isotope, ‰
difference from standard
Peak width,
% standard
250
290
300
250
300
300
325
335
340
345
300
350
390
400
440
400
450
50
10
25
100
50
50
25
15
10
5
100
50
10
50
10
100
50
103
104
103
100
100
100
101
100
101
100
94
96
97
93
92
88
88
0.08
0.08
0.07
–0.01
0
–0.01
0.02
0.01
–0.01
–0.07
–0.2
–0.17
–0.13
–0.24
–0.24
–0.33
–0.34
108
106
103
101
100
100
100
99
99
100
98
95
94
92
90
90
89
Areas and widths are relative to those found for standard conditions of 350/50 mobile-phase/acid flow, whereas isotopic values are those referenced to
values obtained at the standard conditions.
as the density profile at this location was homogeneous. A
plot of δ13C-DIC versus salinity (Fig. 5) shows a near-linear
relationship between these two parameters. However, a two
end-member mixing model (Fry 2002) for δ13C-DIC indicates
that the observed trend cannot be described simply as an abiotic process. Savannah River DIC concentrations and δ13CDIC were determined at the head of tides to be 570 µM and
–10.0‰, respectively, and using these plus the seawater endmember at stations S0 and W0 results in the mixing curve
shown in Fig. 5. Sample δ13C-DIC values deviate rapidly from
this line, indicating the input of significant amounts of
organic carbon remineralization within the estuary.
Although many estuarine systems are assumed to be conservative with respect to DIC (Fry 2002), a number of studies
have shown deviations of >1‰ in δ13C-DIC values from conservative mixing lines (Chanton 1999; Coffin and Cifuentes
1999; Kaldy et al. 2005). With rapid, precise measurement of
δ13C-DIC, such “small” signals may be used to examine the
contributions of organic matter remineralization within
these systems. Although a calibration of DIC concentrations
was not performed at the time of these estuarine measurements, most of the riverine apparent DIC concentrations
were above those predicted from conservative mixing and
sample salinity values. This is not unexpected, as the Savannah River is highly turbid and passes through a major urban
area. Furthermore, the Wilmington (and Savannah, through
mixing) is heavily influenced by marsh respiration and the
consequent addition of DIC from this source (Cai and Wang
1998; Cai et al. 2003). Future work to better constrain the
seasonal cycling of C within this system could involve much
s between runs for peak centering routines on the mass spectrometer. This 200-s limit is also set by the autosampler;
attempts at analyzing samples faster than this rate led to occasional skipped injections. Likewise, the replicate injection rate
was set to 220 s to avoid skipped injections during multiple
injection analyses.
Sample ocean application
The ability to rapidly and with little effort analyze large
numbers of samples for δ13C-DIC opens up several avenues for
research. One example is the study of estuarine carbon
cycling. Samples were collected in fall 2008 along two transects within the Savannah and Wilmington River estuaries
(Fig. 3). Samples were collected by Rosette into 10-L Niskin
bottles, subsampled into 60-mL prerinsed plastic bottles, and
filtered through 0.2-µm pore size nylon syringe filters into 2mL vials as noted above. Samples were collected roughly every
2 m from surface to 1 m above the bottom. The LC method
used here completed the entire sequence of analyses in less
than 1 day including duplicate analyses of each bottle sample.
Samples collected on the South Atlantic Bight shelf were isotopically homogeneous, as expected from density profiles
(Fig. 4). However, δ13C-DIC values were rapidly influenced by
remineralization at locations very close to the entrance of
both rivers. The Savannah River has a much larger drainage
basin than the Wilmington (which is essentially a tidal estuary), and the influence of this input is reflected in a stronger
surface layer at upstream stations. There is the suggestion of a
benthic respiration signal at station W4 in the bottommost
sample. This was not caused by mixing of another water mass,
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Rapid isotopic analysis of DIC
Fig. 3. Map of Georgia coast sampling locations.
Fig. 4. DIC δ13C values versus depth for stations indicated in Fig. 3. Error bars for isotopic data are within the width of the points in each figure.
736
Brandes
Rapid isotopic analysis of DIC
such as the XYZ autosamplers found on GC and headspace
analyzers, may also avoid this problem. Second, care should be
taken to avoid the entrapment of bubbles in the 2-mL
autosampler vials, as even small bubbles pose the potential for
isotopic exchange with the gaseous phase. Third, replicate
(duplicate or triplicate) vials should be analyzed wherever possible for high-precision applications. This is especially critical
because the autosampler appears to occasionally mis-inject or
skip injections on random vials, and re-analyzing such vials at
a later time introduces the possibility of outgassing or other
contamination.
The second primary concern when attempting to measure
small isotopic signals is accuracy. Although the system can be
calibrated to secondary standards, which are in turn calibrated
to primaries, there exists no widely available DIC isotopic
standard. Although use of solid standards can calibrate measured 13C-DIC values to within 0.1‰ or better with sufficient
care, there exist difficulties in comparing the isotopic values of
solid standards with those found in solutions (Gillikin and
Bouillon 2007). Given the small signals discussed above for
anthropogenic signals, better and consistent standardization
is needed. Ideally, an analog to the VSMOW/SLAP/GISP water
isotope standards should be produced and made available to
the scientific community, with a range in δ13C and concentrations (possibly characteristic surface and deep oceanic waters).
These could then be used in individual laboratories to calibrate secondary DIC solution isotopic values. Having a range
of concentrations is critical, as at the highest precision
requirements the linearity correction (~0.04‰ V–1) becomes
significant even between surface and deep ocean samples.
Such a set of standards will aid in improving the accuracy of
the reported concentrations as well.
Fig. 5. DIC δ13C values versus salinity for stations indicated in Fig. 3.
Filled circles are stations S0–S4 and open squares are W0–W4. Line represents conservative mixing curve between seawater and riverine endmembers. Inset shows range of model isotopic values including river endmember at –10‰.
more intensive time-series sampling from shore points (docks,
bridges) within the instrument time constraints of this method.
Discussion and recommendations
Several authors (Tans et al. 1993; Quay and Stutsman 2003;
Quay et al. 2007) have noted the potential for using 13C-DIC
as a tracer for oceanic uptake of anthropogenic CO2. However,
the requirements for such a tracer are significant. Anthropogenic signals are on the order of 0.025‰ year–1 in the subtropics to 0.01‰ year–1 in high-latitude oceans (Sonnerup et
al. 1999; Tanaka et al. 2003; Tagliabue and Bopp 2008). The
precision of the LC-IRMS method reported here (±0.04‰)
would therefore dictate that it could be used to observe
changes taking place over roughly a decade (subtropics) to 20
years (high latitude) timescales (S/N = 5). To obtain the highest precision from this technique, the following recommendations should be employed. One primary source of poor precision appears to be sample bottle pressurization. The evidence
for this is that samples collected from near–room temperature
waters and from lab standards have routinely low peak area
and isotopic variability, whereas samples collected from colder
waters exhibit larger variabilities, on the order of 1% and
0.6‰, respectively. Therefore, either first warming samples
before subsampling (if bubble formation can be avoided) or
the use of a refrigerated autosampler tray should act to reduce
this problem. It may be possible that other autosampler types,
Comments
The Isolink interface provides a new avenue for δ13C-DIC
analyses, with high precision, low sample preparation
requirements, rapid analyses, and small sample volume as primary advantages. Challenges include sample storage methods
for small volumes, isotopic linearity in lower concentration
samples, and calibration standards. The operating costs and
student/technician effort required for this application are
low, consisting of 2-mL vials + septa, a syringe + filter
(reusable with rinsing), and a few minutes for sample filling
and capping. The lack of an oxidant allows for the use of lowcost deionized water for reagents, mobile phase, and wash
solutions. If samples are not poisoned, then no hazardous
waste is generated other than an easily neutralizable weak
H3PO4 solution. The automated nature of the analyses means
that long runs are possible, exceeding those from other IRMS
interfaces such as elemental analyzers. Although Isolink
interfaces are uncommon, this technique opens up their use
for high-throughput analyses with no modification required,
extending their utility when not being used for organic compound analyses.
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Rapid isotopic analysis of DIC
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Submitted 9 June 2009
Revised 15 September 2009
Accepted 25 September 2009
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