Sci. Rep. Kanazawa Univ.
Vol. 56, pp. 45–58, 2012

Stable carbon and oxygen isotope analyses ofcarbonate using a continuous flow isotope ratio mass
spectrometry
Kazuyoshi MORIYA1, Akiko S. GOTO2, Takashi HASEGAWA3
Faculty of Natural System, Institute of Science and Engineering, Kanazawa University
Kanazawa 920-1192, JAPAN
Email: 1 kmoriya@staff.kanazawa-u.ac.jp, 2 akigoto@staff.kanazawa-u.ac.jp,
3 jh7ujr@staff.kanazawa-u.ac.jp(Received February 4, 2013 and accepted in revised form February 23, 2013)

Abstract Basic performance of a continuous-flow isotope ratio mass spectrometer(Thermo Scientific GasBench II and Delta V Advantage) installed in the laboratory of
evolution of earth environment of Kanazawa University is examined by multiple analyses of standard reference materials (NBS19, LSVEC, etc.). Internal precision of Delta V
Advantage determined by 10 consecutive analyses of online reference CO2 gas is 0.01
‰ for both δ 13 C and δ 18 O values. Internal precision of GasBench II inlet system coupled to Delta V Advantage examined by four CO2 gas aliquots, which is evolved by areaction of 300 µg of NBS19 and anhydrous phosphoric acid, is 0.06 ‰ and 0.08 ‰for δ 13 C and δ 18 O, respectively. External precision of the system determined by 20
consecutive NBS19 analyses over 16 hours within the same day represents 0.02 ‰ and0.04 ‰ for δ 13 C and δ 18 O, respectively. A peak area of the first sample aliquot on achromatogram detected on Delta V Advantage significantly correlates with amount ofcarbonate reacted (r = 0.99, p << 0.01). This positive correlation indicates that amountof carbonate within a given sample can be estimated from the chromatogram within anerror of ±30 µg, which enable us to determine carbonate content of the bulk samples inat least better than 10 % precision.
Key words: stable isotopes, carbonate, CF-IRMS, GasBench

1

Introduction

Carbon and oxygen stable isotope analyses of carbonate materials are widely applied ingeoscience, especially in paleoceanography, paleoclimatology, paleontology, etc. Carbon
isotopic composition potentially represents biogeochemical cycles (e.g., Hasegawa andSaito, 1993; Hasegawa, 1997; McCarren et al., 2008), and oxygen isotope compositionis utilized for paleothermometry (e.g., Moriya et al., 2003, etc), ice-volume estimation

46

Carbonate stable isotope analyses on CF-IRMS(Lisiecki and Raymo, 2005; Moriya et al., 2007; Zachos et al., 2008), as well as the otherpaleoclimate analyses.
Stable isotope analyses of carbonate materials have predominantly been achieved by
the dual-inlet isotope ratio mass spectrometry (DI-IRMS) with cryotrap(s). Some automated inlet systems are designed for DI-IRMS, for example, Thermo Scientific Kiel IV
Carbonate Device and Isoprime MultiPrep. Since the development of the continuousflow isotope ratio mass spectrometry (CF-IRMS), it is widely and actively applied tomany fields of science. CF-IRMS technique is also introduced for isotope analyses ofcarbonate materials (e.g. Révész and Landwehr, 2002; Paul and Skrzypec, 2006), evenfor analyses of ultra small amount of carbonate (Ishimura et al., 2004; Ishimura et al.,2008).
Here, we report basic performance of carbon and oxygen isotope ratio measurements
on CF-IRMS in the laboratory of evolution of earth environment of Kanazawa University.
Internal and external precisions are reported on the basis of multiple analyses of standard
reference materials. The possible application of CF-IRMS for carbonate content estimation in bulk samples is also discussed.

2

Methodology and equipment

The isotope ratio mass spectrometer (Thermo Scientific Delta V Advantage) equippedwith Thermo Scientific GasBench II inlet system including autosampler (CTC GC PAL)
was installed in our laboratory on February, 2010. GasBench II is an online gas preparation and introduction system for the isotope ratio mass spectrometer using continuous
flow techniques (Figure 1). This preparation system can be used for isotope ratio analyses of headspace gas samples including CO2 evolved from carbonate and phosphoricacid reaction, CO2 equilibrated with water, and atmospheric gases.
For carbonate analyses, 300 µg of carbonate sample is loaded on a 12 ml borosilicate vial capped with a rubber septum (A in Figure 1). The rubber septum keeps an
airtight seal after being punctured with a syringe for acid dosing or a needle for gas transfer. Those vials with samples are placed on the temperature-stabilized tray and kept at
70.0 ± 0.1 ℃. Prior to the acid dosing, residual air is removed from the sample vial byautomated autosampler-assisted flushing with ultra-pure helium (> 99.99995% He), using a flow of 100 ml/min of He for 5 minutes. After flushing all vials, 0.1 ml of anhydrous
phosphoric acid, which is pre-heated at 70 ℃, is manually injected into each vial with adisposable syringe. CO2 gas evolved in the headspace of the vial is delivered to a sample
loop (D in Figure 1) by the ultra-pure He flow with automated CTC GC PAL autosampler
through a water removal system (B in Figure 1) and a multi-port valve (C in Figure 1)(shown by a solid line in Figure 1). The sample loop aliquots the sample CO2 gas into
four fractions, and is repetitively emptied onto a GC column (E in Figure 1) (shown by
a dashed line in Figure 1). The sample CO2 gas is chromatographically separated from

K. Moriya et al.

47

Figure 1: Schematic diagram showing GasBench II inlet system and Delta V Advantage mass spectrometer.
A. Autosampler (96 vials); B. Water removal system 1; C. Loop injection system; D. 100 µl sample loop; E.
Isothermal gas chromatograph; F. Water removal system 2.

other gas species, and the peak of CO2 is delivered to the mass spectrometer through thesecond water removal system (F in Figure 1). Abundance of each CO2 molecule of mass44, 45, and 46 in each pure CO2 aliquot is determined by the mass spectrometer (Figure 1
and 2).
In our laboratory, phosphoric acid is manually injected into all vials at the sametime after the He flushing was completed. After acid injection, the sample CO2 gas
evolved is delivered to the mass spectrometer through GasBench II with CTC GC PAL
autosampler. While isotopic composition of the first sample is analyzed at 60 minutes after the acid injunction, other vials are kept on the temperature-stabilized tray at
70 ℃ until analyses. Therefore, reaction time between carbonate samples and phosphoric acid is not constant, i.e., longer in the sample analyzed in the later in a batch.
We analyzed NBS19 (δ 13 C = +1.95 ‰, δ 18 O = −2.20 ‰, distributed by International
Atomic Energy Agency), LSVEC (δ 13 C = −46.6 ± 0.2 ‰, δ 18 O = −26.7 ± 0.2 ‰, distributed by International Atomic Energy Agency), W-lab-std (δ 13 C = +2.16 ± 0.05 ‰,δ 18 O = −1.76 ± 0.05 ‰, courtesy of Prof. Weissert), and Lab-std (pure carbonate,> 99.95 %, of δ 13 C and δ 18 O unknown, produced by Wako Pure Chemical Industries,Ltd.) over consecutive 16 hours within the same day for this report (see chapter 3.2
for detailed description of W-lab-std and Lab-std). All δ values are reported relative to
VPDB in this report.

3 Analytical results
A set of a single sample analysis is composed of three reference gas injections and four
sample gas aliquots injections (Figure 2). For ideal analyses, it is desirable that intensity

48

Carbonate stable isotope analyses on CF-IRMS[mV] of reference gas and each sample gas aliquot would be equivalent. In the analyses on GasBench II, as shown in Figure 2, intensity of each sample aliquot decrease
through retention time, so we should have known the dynamic range (and linearity) of
the GasBench II system (Goto et al., 2012). Goto et al. (2012) analyzed various amountof carbonate samples and reported δ 13 C and δ 18 O values against an area [V·sec] of eachaliquot on the chromatogram (Figure 2). On the basis of those analyses, they discussed
the dynamic range of the GasBench II system in our laboratory. They concluded that bothδ 13 C and δ 18 O are constant within the range of the area from 10 thorough 70 [V·sec](Goto et al., 2012). All the analyses described in this report are conducted under thiscondition, and we report accuracy and precision of isotope ratio analyses of standard reference materials, including NBS19 and LSVEC, in this report.

Figure 2: Typical chromatogram of CO2 isotope ratio analysis. Three reference gas injection pulses at 70,
120, and 170 sec are followed by a set of four sample gas aliquots pulses at 300, 450, 600, and 750 sec foreach sample.

3.1

Internal and external precisions of GasBench II/Delta V Advantage

Internal precision of Delta V Advantage in our laboratory
Pure CO2 gas (>99.995 % CO2 ), of which δ 13 C and δ 18 O are previously determined bydual inlet system with NBS19, is connected on the mass spectrometer as reference gas
in our system (Figure 1). δ 13 C and δ 18 O of 10 pulses of reference gas introduced bycontinuous flow technique are shown in Figure 3. δ 13 C and δ 18 O of the first pulse are
regarded as 0.00 ‰ in this test. Internal precision (1σ ) of these 10 consecutive analyses
is 0.01 ‰ for both δ 13 C and δ 18 O (Table 1).

K. Moriya et al.

49

Figure 3: Carbon and oxygen isotope ratios of multiple analyses of online reference CO2 gas showing
the internal precision of Delta V Advantage in our laboratory. Open symbols show each reference gas
analysis. Filled symbols and whiskers show average and standard deviation (1σ ) of 10 consecutive analyses,
respectively.

Internal and external precisions of GasBench II/Delta V Advantage
20 samples of approximately 300 µg each of NBS19 were analyzed over 16 consecutive
hours (Figure 4). Headspace CO2 gas in a single vial evolved from the reaction between
phosphoric acid and 300 µg of NBS19 was delivered to the sample loop in GasBench
II, and aliquoted into four fractions (Figure 2). Average and precision (1σ ) of thesefour sample aliquots analyses are shown in Figure 4 (open symbols and whiskers). The
precision (1σ ) of these four aliquot analyses is considered as internal precision of the
combination of GasBench II and Delta V Advantage (Table 1). Although minimum precision is surprisingly small, 0.01 ‰ for both δ 13 C and δ 18 O, maximum precision of 0.06
‰ and 0.08 ‰ for δ 13 C and δ 18 O, respectively, is reported as the internal precision of
the system in this report (Table 1).

10
10
4
4
20
20

Reference CO2 gas

Reference CO2 gas

CO2 evolved from a single reaction
between phosphoric acid and 300 µg of NBS19

CO2 evolved from a single reaction
between phosphoric acid and 300 µg of NBS19

CO2 evolved from multiple reactions
between phosphoric acid and 300 µg of NBS19

CO2 evolved from multiple reactions
between phosphoric acid and 300 µg of NBS19

0.01 ‰

δ 18 Oδ 18 Oδ 13 Cδ 18 O

0.04 ‰

0.02 ‰

0.08 ‰

0.06 ‰

0.01 ‰

δ 13 Cδ 13 C

Precision (1σ )Isotope

External precision of
GasBench II/Delta V Advantage

External precision of
GasBench II/Delta V Advantage

Maximum internal precision of
GasBench II/Delta V Advantage

Maximum internal precision of
GasBench II/Delta V Advantage

Internal precision of Delta V Advantage

Internal precision of Delta V Advantage

Remarks

Table 1: Performance data for GasBench II/Delta V Advantage system in the laboratory of evolution of earth environment of Kanazawa University.

Number ofanalyses

Analyte and method

50
Carbonate stable isotope analyses on CF-IRMS

K. Moriya et al.

51

Figure 4: Time series analyses of the standard reference material (NBS19) over 16 consecutive hours.
Open symbols and whiskers show average and standard deviation (1σ ) of quadruplicate aliquots analyses of each sample (cf. Figure 2). Filled symbols and whiskers show average and standard deviation (1σ )of all analyses. Dashed lines represent the certified value by definition of VPDB scale (δ 13 C = +1.95 ‰,δ 18 Ο = −2.20 ‰).

External precision of 20 analyses of NBS19 over 16 consecutive hours is presented
in Figure 4 as well (filled symbols and whiskers). The external precision is 0.02 ‰and 0.04 ‰ for δ 13 C and δ 18 O, respectively (Table 1). Révész and Landwehr (2002)
also reported external precision of δ 13 C and δ 18 O analyses with GasBench II coupled to
Thermo Scientific Delta Plus XL mass spectrometer. As we have done in our laboratory,
they manually injected 0.1 ml of phosphoric acid with a syringe, so their method is quite
comparable to our method. However, acid reaction temperature in their method was 26℃, which is significantly lower than our methods (70 ℃). They described that higheracid reaction temperature (65 ℃) significantly deteriorated the accuracy and precisionof δ 13 C and δ 18 O analyses. In fact, under the condition of overnight reaction at 65℃, they found that δ 18 O of NBS19 showed −8 ± 1.88 ‰ deviation from the certified
value (Révész and Landwehr, 2002). Instead of using the overnight acid reaction at
higher temperature, they employed approximately 24 to 54 hours reaction time at 26 ℃
to improve accuracy and precision. Finally, they achieved the precision of 0.1 ‰ and0.2 ‰ for δ 13 C and δ 18 O, respectively, with this reaction condition. In contrast to theirstruggle, both internal and external precision of our analyses are fairly acceptable even

52

Carbonate stable isotope analyses on CF-IRMS

in higher acid reaction temperature. However, as mentioned above, we should be awarethat reaction time is not constant in our method, which will be discussed below.

3.2

Accuracy and precision of oxygen and carbon isotope analyses of various materials

In addition to NBS19, we used W-lab-std (carrara marble), which is kindly provided
by Helmut Weissert in Swiss Federal Institute of Technology Zurich (Eidgenössische
Technische Hochschule Zürich) for further investigations. Reported values for δ 13 C andδ 18 O of W-lab-std are +2.16 ± 0.05 ‰ and −1.76 ± 0.05 ‰, respectively. We analyzed
10 W-lab-stds over 16 consecutive hours (Figure 5). Average and precision (1σ ) of thoseanalyses are +2.09 ± 0.03 ‰ and −1.76 ± 0.04 ‰ for δ 13 C and δ 18 O, respectively. As
seen in NBS19 analyses, precision of those analyses is fairly small for both δ 13 C andδ 18 O. On the other hand, while δ 18 O is very accurately determined by our analyses,
the average of δ 13 C shows 0.07 ‰ difference from the reported value. This result indicates that carbon isotope analyses in our laboratory might have systematic uncertainty ofless than 0.1 ‰. This uncertainty is currently under reevaluation with other carbonate
reference materials, such as NBS18 and IAEA-CO-8.

Figure 5: Time series analyses of W-lab-std (see text for detail) over 16 consecutive hours. Open symbols
and whiskers show average and standard deviation (1σ ) of quadruplicate aliquots analyses of each sample(cf. Figure 2). Filled symbols and whiskers show average and standard deviation (1σ ) of all analyses.
Dashed lines, and gray shadings represent recommended values and standard deviations (1σ ) of W-lab-std(δ 13 C = +2.16 ± 0.05 ‰, δ 18 Ο = −1.76 ± 0.05 ‰).

K. Moriya et al.

53

LSVEC, the other reference material composed of lithium carbonate, is also analyzed.
Reported isotopic values of LSVEC are −46.6 ± 0.2 ‰ and −26.7 ± 0.2 ‰ for δ 13 C andδ 18 O, respectively (Stichler, 1995; Ghosh et al., 2005). Because isotopic measurement
results become less accurate if unknown samples have significantly negative δ 13 C value
relative to NBS19, such as δ 13 C in organic carbon, it is recommended using two-pointcalibration with NBS19 and LSVEC for δ 13 C analyses of such samples (Coplen et al.,2006). In this report, instead of regarding the average values of δ 13 C and δ 18 O as anchor
points, i.e., normalizing δ 13 C and δ 18 O analyzed to −46.6 ‰ and −26.7 ‰, δ valuesof LSVEC are reported by direct comparison to NBS19 (Figure 6). The δ values determined in our analyses are −46.35 ± 0.06 ‰ and −26.11 ± 0.12 ‰ for δ 13 C and δ 18 O,
respectively. Although both δ 13 C and δ 18 O show systematic uncertainty from the recommended values, standard deviation (1σ ) for δ 13 C is fairly small, indicating that two-pointcalibration with NBS19 (Coplen et al., 2006) would provide precise results for the samples having negative δ 13 C. On the other hand, standard deviation (1σ ) for δ 18 O is significantly larger than any other analyses (±0.12 ‰) (see Table 1). Additionally, it is obviousthat δ 18 O values become more positive with elapsed time (Figure 6). In fact, δ 18 O val-

Figure 6: Time series analyses of the standard reference material (LSVEC) over 16 consecutive hours.
Open symbols and whiskers show average and standard deviation (1σ ) of quadruplicate aliquots analyses ofeach sample (cf. Figure 2). Filled symbols and whiskers show average and standard deviation (1σ ) of all
analyses. Dashed lines and gray shadings represent recommended values and standard deviations (1σ ) of
LSVEC (δ 13 C = −46.6 ± 0.2 ‰, δ 18 Ο = −26.7 ± 0.2 ‰). Note that y-axis scale is different from the otherfigures. Whiskers showing 1σ in each δ 18 Ο analyses are small enough to be hidden beneath the symbols.

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Carbonate stable isotope analyses on CF-IRMS

ues have the positive correlation with elapsed time (r = 0.91, p << 0.01). The positive
correlation between δ 18 O and elapsed time has also been observed in analyses of Lab-std
which is pure carbonate utilized for laboratory working standard (r = 0.88, p << 0.01)(Figure 7). Average and standard deviation (1σ ) of Lab-std analyses are −5.22 ± 0.03 ‰and −19.03 ± 0.10 ‰ for δ 13 C and δ 18 O, respectively. As seen in LSVEC analyses,
standard deviation (1σ ) of δ 18 O value is significantly larger than that of δ 13 C.

Figure 7: Time series analyses of the Lab-std (pure carbonate) over 16 consecutive hours. Open symbols
and whiskers show average and standard deviation (1σ ) of quadruplicate aliquots analyses of each sample(cf. Figure 2). Filled symbols and whiskers show average and standard deviation (1σ ) of all analyses. Notethat y-axis scale is different from the other figures.

Because all analyses reported here have been conducted within a single batch on thesame day, all analyses should have exactly the same analytical condition except for theacid reaction time. Although they didn’t mention the correlation and/or trend between
δ 18 O and acid reaction time, Révész and Landwehr (2002) also found significantly larger
standard deviation in δ 18 O value in longer acid reaction time. Because, as shown in our
results, the acid reaction time seems to affect δ 18 O only, Révész and Landwehr (2002)
hypothesized that CO2 evolved with phosphoric acid reaction exchanges oxygen isotopes
with water vapor within a vial. Carbonate samples react with phosphoric acid following
the equation of;

CaCO3 + H3 PO4−→CaHPO4 + H2 O + CO2

K. Moriya et al.

55

If CO2 stays in the headspace of a vial for substantially longer time after the reaction
under high temperature, it is expected that oxygen isotope exchange reaction occurs between CO2 and H2 O (Révész and Landwehr, 2002). The positive correlation observed
between δ 18 O and elapsed time, hence reaction time, in LSVEC and Lab-std can be explained by this isotope exchange reaction within the headspace. In theory, this isotope
exchange reaction should be observed in NBS19 and W-lab-std as well. However, strong
correlation has not been observed in NBS19 and W-lab-std. We suspect that δ 18 O valueof CO2 , which exchanged oxygen isotopes with water vapor, would be similar to that oforiginal CO2 evolved in the reaction of NBS19 and W-lab-std. If this is the case, oxygenisotope exchange reaction between CO2 and H2 O can’t be detected in these materials, indicating that δ 13 C and δ 18 O analyses of unknown samples, of which δ 13 C and δ 18 O areclose to NBS19, shouldn’t be affected by the oxygen isotope exchange reaction within
the headspace.

4

Possible application for carbonate content estimation

The mass spectrometer produces the mass chromatogram for each analysis (Figure 2).
Because the area [V·sec] of the first sample aliquot is a function of the amount of sample, these two variables should have the liner correlation (Figure 8). In fact, CaCO3[µg] shows the significant positive correlation with total area of the first aliquot peak(r = 0.99, p << 0.01). Using this correlation, CaCO3 [µg] within a given bulk sediment
sample will be predicted from the area of the first aliquot peak [V·sec]. Precision of theestimation of the CaCO3 content within the 95 % prediction interval is better than ±30[µg]. Révész and Landwehr (2002) also reported the correlation between CaCO3 [µg]and the first aliquot peak [V·sec]. However, the correlation coefficient they found is lessthan 0.5. They mentioned that “however, the area of the first injection peak is not simply a linear function of the amount of sample material: the linear correlation coefficient
between the amount of sample and the first peak was only 0.22, suggesting that somefactor in addition to any error in weighing confounded this relationship” (Révész and
Landwehr, 2002). In contrast to their results, we found very strong significant correlation. One possible reason that Révész and Landwehr (2002) haven’t found significant
correlation would be the clogging of capillary with phosphoric acid.When Goto et al.
(2012) reported linearity of our mass spectrometer, they didn’t find reasonable correlation between CaCO3 [µg] and the first aliquot peak [V·sec]. Because headspace CO2
is transported into GasBench II by He flow, phosphoric acid stuck on/around the rubber
septum would also be accidentally delivered into GasBench II. Phosphoric acid contaminating the multi-port valve and capillary would block gas flow within GasBench II. As
a result, the first aliquot peak [V·sec] becomes smaller than expected, and doesn’t show
significant correlation with CaCO3 [µg] reacted. In fact, after we cleaned the multi-portvalve and capillary after a series of analyses made by Goto et al. (2012), we found sig-

56

Carbonate stable isotope analyses on CF-IRMS

nificant positive correlation (Figure 8).

Figure 8: Scatter diagram showing CaCO3 [µg] reacted with phosphoric acid and total area [V·sec] of thefirst sample aliquot peak (sum of areas of mass 44, 45, and 46) (cf. Figure 2). Solid, dotted, and dashed lines
show the least-squares regression, 95 % confidential interval, and 95 % prediction interval for the regression,respectively.

5

Concluding remarks

On the basis of reference CO2 gas and NBS19 analyses, we reported the internal andexternal precisions of δ 13 C and δ 18 O analyses in our system. Those values reported
here are well comparable with those reported by Thermo Scientific, which ensure results
produced in our system. However, analyses of LSVEC and Lab-std indicate the isotope
exchange reaction between CO2 and water vapor evolved by phosphoric acid reaction
within a headspace of a vial. If CO2 evolved is kept in the headspace of the vial for aK. Moriya et al.

57

longer time, i.e., taking a longer reaction time, the isotope exchange reaction is expected
to progress more. Therefore, we recommend that acid reaction time should be consistentthrough the analyses. Alternatively, lower acid reaction temperature should be used as
employed by Révész and Landwehr (2002).
Carbonate content within bulk sediment samples, especially for pelagic sediments,
potentially represents global carbon perturbation. Although analyses of carbonate content
are best achieved by coulometry, it takes considerable amount of time. While the precision is less than that of coulometry, carbonate content estimation by GasBench II/IRMS
is significantly faster than coulometry. In our system, precision of carbonate content estimation is better than ±30 µg within the range from 300 through 600 µg. In addition to
the quicker measuring time, a 12 ml vial allows loading large amount of sample of whichcarbonate content is expected to be low. Therefore, the GasBench II/Delta V Advantage
system has an advantage of analyzing bulk carbonate samples.

6

Acknowledgement

We thank Helmut Weissert for providing his laboratory standard (W-lab-std), Toyoho
Ishimura for his useful discussion. Thanks are extended to the editors of the journal,
Hidekazu Ito and Takahiro Kamiya, and to a reviewer of this paper, Robert G. Jenkins
for their helpful suggestion. All diagrams, except for Figure 1, were generated with
the Generic Mapping Tools (Wessel and Smith, 1998). This work was partly supported
by JSPS KAKENHI Grant Numbers 24654169 (KM), and 23340158, 24403012, and24654168 (TH).

7

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