Simultaneous determination of the (2)h/(1)h, (17)o/(16)o, and (18)o/(16)o isotope abundance ratios in water by means of laser spectrometry

We demonstrate the first successful application of infrared laser spectrometry to the accurate, simultaneous determination of the relative (2)H/(1)H, (17)O/(16)O, and (18)O/(16)O isotope abundance ratios in water. The method uses a narrow line width color center laser to record the direct absorption spectrum of low-pressure gas-phase water samples (presently 10 μL of liquid) in the 3-μm spectral region. It thus avoids the laborious chemical preparations of the sample that are required in the case of the conventional isotope ratio mass spectrometer measurement. The precision of the spectroscopic technique is shown to be 0.7‰ for δ(2)H and 0.5‰ for δ(17)O and δ(18)O (δ represents the relative deviation of a sample's isotope abundance ratio with respect to that of a calibration material), while the calibrated accuracy amounts to about 3 and 1‰, respectively, for water with an isotopic composition in the range of the Standard Light Antarctic Precipitation and Vienna Standard Mean Ocean Water international standards.     

Conversion of O(2) into CO(2) for High-Precision Oxygen Isotope Measurements

An improved procedure of (18)O/(16)O ratio measurements by means of oxygen conversion to CO(2) is developed, which allows one to obtain the true δ(18)O values with a precision of ±0.05‰ in oxygen samples down to 7 μmol. The isotopic exchange between quartz glass and oxygen gas was measured in the temperature range of 600-900 °C, and it was found to be less than 0.2%.     

Stable isotope ratios using cavity ring-down spectroscopy: determination of 13C/12C for carbon dioxide in human breath

We have constructed a cavity ring-down spectrometer employing a near-IR external cavity diode laser capable of measuring 13C/12C isotopic ratios in CO2 in human breath. The system, which has a demonstrated minimum detectable absorption loss of 3.2 x 10(-11) cm(-1) Hz(-1/2), determines the isotopic ratio of 13C16O16O/12C16O16O by measuring the intensities of rotationally resolved absorption features of each species. As in isotope ratio mass spectrometry (IRMS), the isotopic ratio of a sample is compared to that of a standard CO2 sample calibrated to the Pee Dee Belemnite scale and reported as the sample's delta13C value. Measurements of eight replicate CO2 samples standardized by IRMS and consisting of 5% CO2 in N2 at atmospheric pressure demonstrated a precision of 0.22/1000 for the technique. Delta13C values were also obtained for breath samples from individuals testing positive and negative for the presence of Helicobacter pylori, the leading cause of peptic ulcers in humans. This study demonstrates the ability of the instrument to obtain delta13C values in breath samples with sufficient precision to serve as a useful medical diagnostic.     

An On-Line Technique for the Determination of the δ(18)O and δ(17)O of Gaseous and Dissolved Oxygen

Few studies have used the stable isotopic composition of O(2) as a tracer of gas transport or biogeochemical processes in environmental research. Here we demonstrate field sampling techniques for gaseous and dissolved O(2) and describe an analytical method for measuring δ(18)O and δ(17)O values of O(2) in air, soil gas, and water samples using continuous-flow isotope-ratio mass spectrometry (CF-IRMS). A Micromass CF-IRMS was altered to accommodate a sample gas injection port prior to a CO(2) and H(2)O trap and GC column. The GC column was a 1-m, 5-? molecular sieve column held at 35 °C. The resolved sample O(2) was introduced to the IRMS via an open split. δ(18)O and δ(17)O values were determined by measurement of O(2) isotopes at m/z 34/32 and 33/32 and comparison to a reference pulse of O(2). Repeated injections of atmospheric oxygen yielded a repeatability (±SD) of ±0.17‰ for δ(18)O and ±0.5‰ for δ(17)O. IRMS source linearity was excellent for O(2) over a sample size range of 60-400 μL. The smallest sample for routine δ(18)O and δ(17)O determinations was ～80 μL of O(2), with a sample analysis time of 180 s. Preliminary results from a riverine and soil gas study illustrate natural oxygen isotope fractionation processes.     

A gas chromatographic separation for the h and C stable isotope ratio determination of coal compounds

A new, completely automated gas chromatography technique has been developed to separate the different gaseous compounds produced during underground coal gasification for their (13)C/(12)C and D/H isotope ratio measurements. The technique was designed for separation and collection of H(2), CO, CO(2), H(2)O, H(2)S, CH(4), and heavier hydrocarbons. These gaseous compounds are perfectly separated by the gas-phase chromatograph and quantitatively sent to seven combustion and collection lines. H(2), CO, CH(4), and heavier hydrocarbons are quantitatively oxidized to CO(2) and/or H(2)O. The isotopic analyses are performed by the sealed-tube method. The zinc method is used for reduction of both water and H(2)S to hydrogen for D/H analysis. Including all preparation steps, the reproducibility of isotope abundance values, for a quantity higher than or equal to 0.1 mL of individual components in a mixture (5 mL of gases being initially injected in the gas chromatograph), is ±0.1‰ for δ(13)C(PDB) and ±6‰ for δD(SMOW).     

Isotopic analyses based on the mass spectrum of carbon dioxide

Measurements of carbon and oxygen isotopic abundances are commonly based on the mass spectrum of carbon dioxide, but analysis of that spectrum is not trivial because three isotope ratios (17O/16O, 18O/16O, and 13C/12C) must be determined from only two readily observable ion-current ratios (45/44 and 46/44). Here, approaches to the problem are reassessed in the light of new information regarding the distribution of oxygen isotopes in natural samples. It is shown that methods of calculation conventionally employed can lead to systematic errors in the computed abundance of 13C and that these errors may be related to incorrect assessment of the absolute abundance of 17O. Further, problems arising during the analysis of samples enriched by admixture of 18O-labeled materials are discussed, and it is shown (i) that serious inaccuracies arise in the computed abundance of 17O and 13C if methods of calculation conventionally employed in the analysis of natural materials are applied to material labeled with 18O but (ii) that computed fractional abundances of 18O are always within 0.4% of the correct result. Methods for exact calculation of two isotope ratios when the third is known are presented and discussed, and a more exact approach to the computation of all three isotope ratios in natural materials is given.     

Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method

We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (15N/14N) analysis of nitrate. (1) Here, we address the additional issues associated with 18O/16O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N20 analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N2O. Experiments with 18O-labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N20 product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured 18O/16O ratios of samples for both effects. This is the first method tested for 18O/16O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 microM NO3-), lack of interference by other solutes, and ease of sample preparation.     

Comparative analysis of cellulose preparation techniques for use with 13C, 14C, AND 18O isotopic measurements

A number of operationally defined methods exist for pretreating plant tissues in order to measure C, N, and O isotopes. Because these isotope measurements are used to infer information about environmental conditions that existed at the time of tissue growth, it is important that these pretreatments remove compounds that may have exchanged isotopes or have been synthesized after the original formation of these tissues. In stable isotope studies, many pretreatment methods focus on isolating "cellulose" from the bulk tissue sample because cellulose does not exchange C and O isotopes after original synthesis. We investigated the efficacy of three commonly applied pretreatment methods, the Brendel method and two variants of the Brendel method, the Jayme-Wise method and successive acid/base/acid washes, for use on three tissue types (wood, leaves, roots). We then compared the effect of each method on C and O isotope composition (13C, 14C, 18O), C and N content, and chemical composition of the residue produced (using 13C nuclear magnetic resonance (NMR)). Our results raised concerns over use of the Brendel method as published, as it both added C and N to the sample and left a residue that contains remnant lipids and waxes. Furthermore, this method resulted in 18O values that are enriched relative to the other methods. Modifying the Brendel method by adding a NaOH step (wash) solved many of these problems. We also found that processed residues vary by tissue type. For wood and root tissues, the 13C NMR spectra and the 18O and 13C data showed only small differences between residues for the Jayme-Wise and modified Brendel methods. However, for leaf tissue, 13C NMR data showed that Jayme-Wise pretreatments produced residues that are more chemically similar to cellulose than the other methods. The acid/base/acid washing method generated 13C NMR spectra with incomplete removal of lignin for all tissues tested and both isotopic, and 13C NMR results confirmed that this method should not be used if purified cellulose is desired.     

Realtime stable isotope monitoring of natural waters by parallel-flow laser spectroscopy

A parallel-flow H(2)O(liquid)-H(2)O(vapor) equilibration and laser spectroscopy method provides a new way to monitor the hydrogen and oxygen stable isotopic composition of water from rivers or lakes or in hydrologic tracer tests in real time. Two custom-built equilibrator devices and one commercial membrane device were tested to determine if they could be used to convert natural water samples (lakes, rivers, groundwater) to a H(2)O gas phase for continuous online δ(18)O and δD isotopic analysis by laser spectroscopy. Both the commercial minimodule device and the marble-filled equilibrator produced water vapor in isotopic equilibrium with the flowing liquid water, suggesting that unattended field measurement using these devices is possible. Oxygen isotope disequilibrium was indicated using the minimodule device at low temperatures.     

Determination of δ(18)O and δ(15)N in Nitrate

The analyses of both O and N isotopic compositions of nitrate have many potential applications in studies of nitrate sources and reactions in hydrology, oceanography, and atmospheric chemistry, but simple and precise methods for these analyses have yet to be developed. Testing of a new method involving reaction of potassium nitrate with catalyzed graphite (C + Pd + Au) at 520 °C resulted in quantitative recovery of N and O from nitrate as free CO(2), K(2)CO(3), and N(2). The δ(18)O values of nitrate reference materials were obtained by analyzing both the CO(2) and K(2)CO(3) from catalyzed graphite combustion. Provisional values of δ(18)O(VSMOW) for the internationally distributed KNO(3) reference materials IAEA-N3 and USGS-32 were both equal to +22.7 ± 0.5‰. Because the fraction of free CO(2) and the isotopic fractionation factor between CO(2) and K(2)CO(3) were constant in the combustion products, the δ(18)O value of KNO(3) could be calculated from measurements of the δ(18)O of free CO(2). Thus, δ(18)O(KNO)((3)) = aδ(18)O(free)(?)(CO)((2)) - b, where a and b were equal to 0.9967 and 3.3, respectively, for the specific conditions of the experiments. The catalyzed graphite combustion method can be used to determine δ(18)O of KNO(3) from measurements of δ(18)O of free CO(2) with reproducibility on the order of ±0.2‰ or better if local reference materials are prepared and analyzed with the samples. Reproducibility of δ(15)N was ±0.1‰ after trace amounts of CO were removed.     

The carbon isotope composition of ancient CO2 based on higher-plant organic matter

Carbon isotope ratios in higher-plant organic matter (delta(13)C(plant)) have been shown in several studies to be closely related to the carbon isotope composition of the ocean-atmosphere carbon reservoir, and, in particular, the isotopic composition of CO(2). These studies have primarily been focused on geological intervals in which major perturbations occur in the oceanic carbon reservoir, as documented in organic carbon and carbonates phases (e.g. Permian-Triassic and Triassic-Jurassic boundary, Early Toarcian, Early Aptian, Cenomanian-Turonian boundary, Palaeocene-Eocene Thermal Maximum (PETM)). All of these events, excluding the Cenomanian-Turonian boundary, record negative carbon isotope excursions, and many authors have postulated that the cause of such excursions is the massive release of continental-margin marine gas-hydrate reservoirs (clathrates). Methane has a very negative carbon isotope composition (delta(13)C, ca. 60 per thousand ) in comparison with higher-plant and marine organic matter, and carbonate. The residence time of methane in the ocean-atmosphere reservoir is short (ca. 10 yr) and is rapidly oxidized to CO(2), causing the isotopic composition of CO(2) to become more negative from its assumed background value (delta(13)C, ca. -7 per thousand ). However, to date, only the Early Toarcian, Early Aptian and PETM are well-constrained chronometric sequences that could attribute clathrate release as a viable cause to create such rapid negative delta(13)C excursions. Notwithstanding this, the isotopic analysis of higher-plant organic matter (e.g. charcoal, wood, leaves, pollen) has the ability to (i) record the isotopic composition of palaeoatmospheric CO(2) in the geological record, (ii) correlate marine and non-marine stratigraphic successions, and (iii) confirm that oceanic carbon perturbations are not purely oceanographic in their extent and affect the entire ocean-atmosphere system. A case study from the Isle of Wight, UK, indicates that the carbon isotope composition of palaeoatmospheric CO(2) during the Mid-Cretaceous had a background value of 3 per thousand, but fluctuated rapidly to more positive (ca. +0.5 per thousand ) and negative values (ca. 10 per thousand ) during carbon cycle perturbations (e.g. carbon burial events, carbonate platform drowning, large igneous province formation). Hence, fluctuations in the carbon isotope composition of palaeoatmospheric CO(2) would compromise our use of palaeo-CO(2) proxies that are dependent on constant carbon isotope ratios of CO(2).     

A specialized isotope mass spectrometer for noninvasive diagnostics of Helicobacter pylori infection in human beings

A specialized isotope mass spectrometer for noninvasive diagnostics of Helicobacter pylori infection in human beings based on the carbon-13 isotope breath test has been designed and constructed. Important stages of the work included (i) calculating a low-aberration mass analyzer, (ii) manufacturing and testing special gas inlet system, and (iii) creating a small-size collector of ions. The proposed instrument ensures ¹³C/¹²C isotopic ratio measurement to within 1.7‰ (pro mille) accuracy, which corresponds to requirements for a diagnostic tool. Preliminary medical testing showed that the mass spectrometer is applicable to practical diagnostics. The instrument is also capable of measuring isotopic ratios of other light elements, including N, O, B (for BF₂+ ions), Ar, Cl, and S.     

New guidelines for delta13C measurements

Consistency of delta13C measurements can be improved 39-47% by anchoring the delta13C scale with two isotopic reference materials differing substantially in 13C/12C. It is recommended that delta13C values of both organic and inorganic materials be measured and expressed relative to VPDB (Vienna Peedee belemnite) on a scale normalized by assigning consensus values of -46.6 per thousand to L-SVEC lithium carbonate and +1.95 per thousand to NBS 19 calcium carbonate. Uncertainties of other reference material values on this scale are improved by factors up to two or more, and the values of some have been notably shifted: the delta13C of NBS 22 oil is -30.03 per thousand.     

Method for simultaneous oxygen and hydrogen isotope analysis of water of crystallization in hydrated minerals

The isotopic composition of water in hydrated minerals, such as gypsum and jarosite, has numerous applications in studies of recent climate change, ore formation, and soil development. However, oxygen and hydrogen isotope analysis of water of crystallization is currently a complex procedure. Commonly used techniques involve offline extraction of water from hydrated minerals and subsequent isotope analysis. Such methods are time-consuming, require relatively large sample sizes, and the stepwise procedure has to be carried out with extreme caution to avoid erroneous results. We present a novel online method for the oxygen and hydrogen isotope analysis of water of crystallization in hydrous minerals. Gypsum (CaSO 4.2H 2O) samples, 2 mg in size, are reacted in a simply modified carbon reducing furnace connected to a continuous-flow mass spectrometer system. Analysis time is less than 10 min/sample. The precision (2 std dev mean) of our method for 2-mg gypsum (30 mumol of H 2O) samples is 0.3 per thousand for oxygen and less than 1.4 per thousand for hydrogen isotope measurements. For oxygen isotope analysis alone, samples as small as 0.2 mg of gypsum can be analyzed with a precision of 0.3 per thousand.     

A technique for the determination of 18O/16O and 17O/16O isotopic ratios in water from small liquid and solid samples

We have developed a new technique in which a solid reagent, cobalt(III) fluoride, is used to prepare oxygen gas for isotope ratio measurement from water derived either from direct injection or from the pyrolysis of solid samples. The technique uses continuous flow, isotope ratio monitoring, gas chromatography/mass spectrometry (irmGC/MS) to measure the delta18O and delta17O of the oxygen gas. Water from appropriate samples is evolved by a procedure of stepped pyrolysis (0-1000 degrees C, typically in 50 degrees C increments) under a flowing stream of helium carrier gas. The method has considerable advantages over others used for water analysis in that it is quick; requires only small samples, typically 1-50 mg of whole rock samples (corresponding to approximately 0.2 micromol of H2O); and the reagent is easy and safe to handle. Reproducibility in isotope ratio measurement obtained from pyrolysis of samples of a terrestrial solid standard are delta18O +/- 0.54, delta17O +/- 0.33, and delta17O +/- 0.10/1000, 1sigma in all cases. The technique was developed primarily for the analysis of meteorites, and the efficiency of the method is illustrated herein by results from water standards, solid reference materials, and a sample of the Murchison CM2 meteorite.     

Compound-specific chlorine isotope analysis: a comparison of gas chromatography/isotope ratio mass spectrometry and gas chromatography/quadrupole mass spectrometry methods in an interlaboratory study

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of (37)Cl/(35)Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ(37)Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1‰), to GC/qMS (1σ ≈ 0.2-0.5‰ for Agilent GC/qMS and 1σ ≈ 0.2-0.9‰ for Thermo GC/qMS). Nonetheless, δ(37)Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.     

High-precision laser spectroscopy D/H and 18O/16O measurements of microliter natural water samples

Newly available gas analyzers based on off-axis integrated cavity output spectroscopy (OA-ICOS) lasers have been advocated as an alternative to conventional isotope-ratio mass spectrometers (IRMS) for the stable isotopic analysis of water samples. In the case of H2O, OA-ICOS is attractive because it has comparatively low capital and maintenance costs, the instrument is small and field laboratory portable, and provides simultaneous D/H and 16O/18O ratio measurements directly on H2O molecules with no conversion of H2O to H2, CO, or H2/CO2-water equilibration required. Here we present a detailed assessment of the performance of a liquid-water isotope analyzer, including instrument precision, estimates of sample memory and sample mass effects, and instrumental drift. We provide a recommended analysis procedure to achieve optimum results using OA-ICOS. Our results show that, by using a systematic sample analysis and data normalization procedure routine, measurement accuracies of +/-0.8 per thousand for deltaD and +/-0.1 per thousand delta18O are achievable on nanoliter water samples. This is equivalent or better than current IRMS-based methods and at a comparable sample throughput rate.     

pH-Dependent Equilibrium Isotope Fractionation Associated with the Compound Specific Nitrogen and Carbon Isotope Analysis of Substituted Anilines by SPME-GC/IRMS

Solid-phase microextraction (SPME) coupled to gas chromatography/isotope ratio mass spectrometry (GC/IRMS) was used to elucidate the effects of N-atom protonation on the analysis of N and C isotope signatures of selected aromatic amines. Precise and accurate isotope ratios were measured using polydimethylsiloxane/divinylbenzene (PDMS/DVB) as the SPME fiber material at solution pH-values that exceeded the pK(a) of the substituted aniline's conjugate acid by two pH-units. Deviations of δ(15)N and δ(13)C-values from reference measurements by elemental analyzer IRMS were small (<0.9‰) and within the typical uncertainties of isotope ratio measurements by SPME-GC/IRMS. Under these conditions, the detection limits for accurate isotope ratio measurements were between 0.64 and 2.1 mg L(-1) for δ(15)N and between 0.13 and 0.54 mg L(-1) for δ(13)C, respectively. Substantial inverse N isotope fractionation was observed by SPME-GC/IRMS as the fraction of protonated species increased with decreasing pH leading to deviations of -20‰ while the corresponding δ(13)C-values were largely invariant. From isotope ratio analysis at different solution pHs and theoretical calculations by density functional theory, we derived equilibrium isotope effects, EIEs, pertinent to aromatic amine protonation of 0.980 and 1.001 for N and C, respectively, which were very similar for all compounds investigated. Our work shows that N-atom protonation can compromise accurate compound-specific N isotope analysis of aromatic amines.     

Precise and traceable (13)C/(12)C isotope amount ratios by multicollector ICPMS

A new method for the measurement of SI traceable carbon isotope amount ratios using a multicollector inductively coupled mass spectrometer (MC-ICPMS) is reported for the first time. Carbon (13)C/(12)C isotope amount ratios have been measured for four reference materials with carbon isotope amount ratios ranging from 0.010659 (delta(13)C(VPDB) = -46.6 per thousand) to 0.011601 (delta(13)C(VPDB) = +37 per thousand). Internal normalization by measuring boron (11)B/(10)B isotope amount ratios has been used to correct for the effects of instrumental mass bias. Absolute (13)C/(12)C ratios have been measured and corrected for instrumental mass bias and full uncertainty budgets have been calculated using the Kragten approach. Corrected (13)C/(12)C ratios for NIST RM8545 (Lithium Carbonate LSVEC), NIST RM8573 (L-Glutamic Acid USGS40), NIST RM8542 (IAEA-CH6 Sucrose) and NIST RM8574 (L-Glutamic Acid USGS41) differed from reference values by 0.06-0.20%. Excellent linear correlation (R = 0.9997) was obtained between corrected carbon isotope amount ratios and expected carbon isotope amount ratios of the four chosen NIST RMs. The method has proved to be linear within this range (from (13)C/(12)C = 0.010659 to (13)C/(12)C =0.011601), and therefore, it is suitable for the measurement of carbon isotope amount ratios within the natural range of variation of organic carbon compounds, carbonates, elemental carbon, carbon monoxide, and carbon dioxide. In addition, a CO2 gas sample previously characterized in-house by conventional dual inlet isotope ratio mass spectrometry has been analyzed and excellent agreement has been found between the carbon isotope amount ratio value measured by MC-ICPMS and the IRMS measurements. Absolute values for carbon isotope amount ratios traceable to the SI are given for each NIST RM, and the combined uncertainty budget (including instrumental error and each parameter contributing to Russell expression for mass bias correction) has been found to be < 0.1% for the four materials. The advantage of the method versus conventional gas source isotope ratio mass spectrometry measurements is that carbon isotope amount ratios are measured as C(+) instead of CO2(+), and therefore, an oxygen (17)O correction due to the presence of (12)C(17)O(16)O(+) is not required. Organic compounds in solution can be measured without previous derivatization, combustion steps, or both, thus making the process simple. The novel methodology opens new avenues for the measurement of absolute carbon isotope amount ratios in a wide range of samples.     

Developing micro-Raman mass spectrometry for measuring carbon isotopic composition of carbon dioxide

We investigated the applicability of micro-Raman spectroscopy for determining carbon isotopic compositions (13C/12C) of minute CO2 fluid inclusions in minerals. This method is nondestructive and has sufficiently high spatial resolution (1 microm) to measure each fluid inclusion independently. Raman spectra of CO2 fluid have 12CO2-origin peaks at about 1285 cm(-1) and 1389 cm(-1) (V(-)([12]) and V+[12]) and a 13CO2-origin peak at about 1370 cm(-1) (V+[13]). The relationship between carbon isotopic compositions and peak intensity ratios of V+[12] and V+[13] was calibrated. Considering several factors affecting the peak intensity ratio, the error in obtained carbon isotopic composition was 2% (20%). The reproducibility of the intensity ratio under the same experimental environment was 0.5% (5%). Within these error values, we can distinguish biogenic CO2 from abiogenic CO2.