Development of isotope dilution cold vapor inductively coupled plasma mass spectrometry and its application to the certification of mercury in NIST standard reference materials

An isotope dilution cold vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) method featuring gaseous introduction of mercury via tin chloride reduction has been developed and applied to the quantification and certification of mercury in various NIST standard reference materials: SRM 966 Toxic Metals in Bovine Blood (30 ng x mL(-1)); SRM 1641d Mercury in Water (1.6 microg x mL(-1)); and SRM 1946 Lake Superior Fish Tissue (436 ng x g(-1)). Complementary mercury data were generated for SRMs and NIST quality control standards using cold vapor atomic absorption spectroscopy (CVAAS). Certification results for the determination of mercury in SRM 1641d using two independent methods (ID-CV-ICPMS and CVAAS) showed a degree of agreement of 0.3% between the methods. Gaseous introduction of mercury into the ICPMS resulted in a single isotope sensitivity of 2 x 10(6) counts x s(-1)/ng x g(-1) for 201Hg and significantly reduced the memory and washout effects traditionally encountered in solution nebulization ICPMS. Figures of merit for isotope ratio accuracy and precision were evaluated at dwell times of 10, 20, 40, 80, and 160 ms using SRM 3133 Mercury Spectrometric Solution. The optimum dwell time of 80 ms yielded a measured 201Hg/202Hg isotope ratio within 0.13% of the theoretical natural value and a measurement precision of 0.34%, on the basis of three replicate injections of SRM 3133.     

Stable carbon and oxygen isotopic analysis of atmospheric carbon monoxide using continuous-flow isotope ratio MS by isotope ratio monitoring of CO

We have developed a rapid and simple measurement system for both content and stable isotopic compositions (13C and 18O) of atmospheric CO, using continuous-flow isotope ratio mass spectrometry by simultaneously monitoring the CO+ ion currents at masses 28, 29, and 30. The analytical system consisted sequentially of a sample trapping port (liquid nitrogen temperature silica gel and molecular sieve 5A), a gas dryer, a CO purification column (molecular sieve 5A), a cryofocusing unit, and a final purification column using a GC capillary. Analytical precision of 0.2 per thousand for 13C and 0.4 per thousand for 18O can be realized for samples that contain as little as 300 pmol of CO within 40 min for one sample analysis. Analytical blanks associated with the method are less than 1 pmol. The extent of analytical error in delta13C due to mass-independent fractionation of oxygen in natural CO is estimated to be less than 0.3 per thousand. Based on this system, we report herein a kinetic isotopic effect during CO consumption in soil.     

Determination of Intramolecular delta13C from incomplete pyrolysis fragments. Evaluation of pyrolysis-induced isotopic fractionation in fragments from the lactic acid analogue propylene glycol

Intramolecular carbon isotope ratios reflect the source of a compound and the reaction conditions prevailing during synthesis and degradation. We report here a method for determination of relative (Deltadelta13C) and absolute (delta13C) intramolecular isotope ratios using the volatile lactic acid analogue propylene glycol as a model compound, measured by on-line gas chromatography-pyrolysis coupled to GC-combustion-isotope ratio mass spectrometry. Pyrolytic fragmentation of about one-third of the analyte mass produces optimal fragments for isotopic analysis, from which relative isotope ratios (Deltadelta13C) are calculated according to guidelines presented previously. Calibration to obtain absolute isotope ratios is achieved by quantifying isotope fractionation during pyrolysis with an average fractionation factor, alpha, and evaluated by considering extremes in isotopic fractionation behavior. The method is demonstrated by calculating ranges of absolute intramolecular isotope ratios in four samples of propylene glycol. Relative and absolute isotope ratios were calculated with average precisions of SD(Deltadelta13C) <0.84 per thousand and SD(delta13C) <3.0 per thousand, respectively. The various fractionation scenarios produce an average delta(13)C range of 2 per thousand for each position in each sample. Relative isotope ratios revealed all four samples originated from unique sources, with samples A, B, and D only distinguishable at the position-specific level. Regardless of pyrolysis fractionation distribution, absolute isotope ratios showed a consistent pattern for all samples, with delta13C(3) > delta13C(2) > delta13C(1). The validity of the method was determined by examining the difference in relative isotope ratios calculated through two independent methods: Deltadelta13C calculated directly using previous methods and Deltadelta13C extracted from absolute isotope ratios. Deviation between the two Deltadelta13C values for all positions averaged 0.1-0.2 per thousand, with the smallest deviation obtained assuming equal fractionation across all fragment positions. This approach applies generally to all compounds analyzed by pyrolytic PSIA.     

Anion-exchange chromatographic separation of Hg for isotope ratio measurements by multicollector ICPMS

A procedure is described for precise Hg isotope ratio measurements by solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Hg was released from geological samples using aqua regia extraction and then separated from other matrix elements with the aid of anion-exchange chromatography using strongly basic Dowex 1-X8 anion-exchange resin. Performance of the chromatographic procedure was evaluated using various types of replacement anions for elution of mercury, including l-cysteine, thiourea, NO3-, and SO42-. A solution of 0.15% l-cysteine in 0.06 M HCl was found to be the most convenient eluent for subsequent MC-ICPMS measurements. The optimized procedure provides separation of Hg from virtually all concomitant matrix elements while maintaining quantitative (>95%) recovery. In addition, band displacement chromatographic experiments were conducted to assess whether the anion-exchange purification can produce Hg isotope fractionation artifacts. No isotope fractionation between the Hg(II)-l-cysteine complex in aqueous solution and Hg ions in the anion-exchange resin was observed. Hg isotope ratio measurements were performed using the bracketing standards approach and on-line correction for instrumental mass discrimination using Tl spiking and normalization to the 205Tl/203Tl ratio. The absence of spectral interference during Hg isotope ratio measurements was verified using a three-isotope plot. Uncertainties of Hg isotope ratio measurements for replication of the entire procedure, expressed as two standard deviations, are better than +/-0.08 per thousand/amu. The described procedure facilitates study of variations in the isotopic composition of Hg in nature.     

Simultaneous mass bias and fractionation corrections utilizing isotopic solid standards and laser ablation ICPMS

Homogeneous incorporation of analytes of known isotopic abundance into sol-gel-derived standards that mimic important mineral systems has the potential to contribute significantly to providing solid standard benchmarks for a range of applications. This preliminary study reports on the synthesis of solid glass standards produced via the sol-gel method and their doping with Standard Reference Material (SRM) 981 Common Lead Isotopic Standard and SRM 982 Equal-Atom Lead Isotopic Standard. Custom isotopic materials were also prepared using mixtures of the two isotopic SRMs. Particles from these solid samples were then introduced into an inductively coupled plasma mass spectrometer via laser ablation to determine whether materials of suitable homogeneity could be developed as isotopic reference materials. Preliminary results for Pb isotope ratios show that these solid isotopic reference standards are capable of correcting for instrumental mass bias and laser ablation-induced bias due to fractionation simultaneously. Correction factors generated from the quotient of the certified and measured Pb isotopic ratios in sol-gel disks spiked with SRMs 981 and 982 were successfully applied to produce accurate isotope ratios using comparative control/unknown checks. These correction factors were also used to assign Pb isotopic ratios in NIST SRM 612 Trace Elements in Glass that were in excellent agreement with published measurements, suggesting that tunable matrix sol-gel disks can serve as adequate control matrixes for evaluation of isotope ratios in glass samples.     

Observations of large mass-independent fractionation occurring in MC-ICPMS: implications for determination of accurate isotope amount ratios

Multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) suffers large bias in isotope amount ratio determinations which has to be properly accounted for. The choice of the proper discrimination model is crucial. Over the last few decades, the exponential mass-bias correction model (Russell's law) has become a standard curriculum in isotope amount ratio measurements. In nature, however, isotopic fractionation that deviates significantly from the exponential model has been known for a long time. Recently, such fractionation was also observed in MC-ICPMS. This phenomenon is termed mass-independent fractionation. In this study, significant departure from the mass-dependent fractionation model is reported for germanium and lead with the most dramatic occurring for germanium-73 and lead-204 isotopes wherein, on average, close to a half percent bias was evidenced from the Russell's law.     

Determination of the total oxygen isotopic composition of nitrate and the calibration of a delta 17O nitrate reference material

A thermal decomposition method was developed and tested for the simultaneous determination of delta 18O and delta 17O in nitrate. The thermal decomposition of AgNO3 allows for the rapid and accurate determination of 18O/ 16O and 17O/16O isotopic ratios with a precision of +/- 1.5 per thousand for delta 18O and +/- 0.11 per thousand for delta 17O (delta 17O = delta 17O - 0.52 x delta 18O). The international nitrate isotope reference material IAEA-NO3 yielded a delta 18O value of +23.6 per thousand and delta 17O of -0.2 per thousand, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO3 from the Atacama Desert, Chile, was found to have delta 17O = 21.56 +/- 0.11 per thousand and delta 18O = 54.9 +/- 1.5 per thousand, demonstrating a substantial mass-independent isotopic composition consistent with the proposed atmospheric origin of the desert nitrate. It is suggested that this sample (designated USGS-35) can be used to generate other gases (CO2, CO, N2O, O2) with the same delta 17O to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.     

Development of a compound-specific carbon isotope analysis method for atmospheric formaldehyde via NaHSO3 and cysteamine derivatization

A novel method has been developed for the compound-specific carbon isotope analysis of atmospheric formaldehyde using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The method allows the determination of the delta13C value for atmospheric formaldehyde at nanogram levels with higher precision and lower detection limit. In the present work, atmospheric formaldehyde was collected using NaHSO3-coated Sep-Pak silica gel cartridges, washed out by water, then derivatized by cysteamine of known delta13C value, and the delta13C value of its derivative (thiazolidine) determined by GC/C/IRMS. Finally, the delta13C value of atmospheric formaldehyde could be calculated by a simple mass balance equation between formaldehyde, cysteamine, and thiazolidine. Using three formaldehydes with different delta13C values, calibration experiments were carried out over large ranges of formaldehyde concentrations. The carbon isotope analysis method achieved excellent reproducibility and high accuracy. There was no carbon isotopic fractionation throughout the derivatization processes. The differences in the carbon isotopic compositions of thiazolidine between the measured and predicted values were always <0.5 per thousand, within the specifications of the GC/C/IRMS system. The present method was also compared with the previous 2,4-dinitrophenylhydrazine derivatization method, and this method could be performed with lower analytical error and detection limit. Using this method, four 6-h ambient atmospheric formaldehyde samples were consecutively collected from 8 to 9 March 2005. The results showed that the delta13C values of atmospheric formaldehyde were different during the daytime and nighttime. This method proved suitable for the routine operation and may provide additional insight on sources and sinks of atmospheric formaldehyde.     

Reduction of nonpolar amino acids to amino alcohols to enhance volatility for high-precision isotopic analysis

Amino acids are routinely derivatized using carbon-containing groups prior to gas chromatography continuous-flow isotope ratio mass spectrometry (GCC-IRMS). Derivative C contaminates analyte C because the entire derivatized compound is combusted to CO2. Correction procedures are required to extract the analyte isotope ratio. We present a method for reduction of six nonpolar amino acids to their corresponding amino alcohols, demonstrate a GC strategy to produce acceptable peak shapes from the resulting strongly H-bonding analytes, and present isotopic analysis for amino acids and their corresponding amino alcohols to evaluate any possible isotopic fractionation. Alanine, valine, leucine, isoleucine, methionine, or phenylalanine was reduced using NaBH4 in THF with I2 as an electrophile. Reactions were performed with 2 g of analyte to permit isotopic analysis by conventional elemental analysis-IRMS. All reactions were quantitative as assessed by IR spectra, melting points, and GC. Recovery from the reaction mixture was 60-84%. GC separation of a mixture of the six amino alcohols was achieved using a thick stationary-phase (5 microm) capillary column to avoid tailing due to hydrogen bonding to the walls of the fused-silica capillary. The reproducibility of GCC-IRMS determinations of amino alcohols averaged SD(613C) = 0.25 +/- 0.19%. The absolute differences between delta13C of amino acids measured by an elemental analyzer coupled to IRMS and amino alcohols measured by GCC-IRMS was delta613C = 0.14% and showed no general trend. Reactions performed with 2 mg of analyte yielded equivalent chromatograms. These data indicate that the reduction method does not induce isotopic fractionation and can be used for continuous-flow isotopic analysis to avoid addition of contaminating carbon.     

Determination of mercury in coal by isotope dilution cold-vapor generation inductively coupled plasma mass spectrometry

A method based on isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) has been developed for high-accuracy determinations of mercury in bituminous and sub-bituminous coals. A closed-system digestion process employing a Carius tube is used to completely oxidize the coal matrix and chemically equilibrate the mercury in the sample with a 201Hg isotopic spike. The digestates are diluted with high-purity quartz-distilled water, and the mercury is released as a vapor by reduction with tin(II) chloride. Measurements of 201Hg/202Hg isotope ratios are made using a quadrupole ICPMS system in time-resolved analysis mode. The new method has some significant advantages over existing methods. The instrument detection limit is less than 1 pg/mL. The average blank (n = 17) is 30 pg, which is roughly 1 order of magnitude lower than the equivalent microwave digestion procedure. The detection limit in coal is blank limited and is approximately 40 pg/g. Memory effects are very low. The relative reproducibility of the analytical measurements is approximately 0.5% for mercury concentrations in the range 10-150 ng/g. The method has been used to measure mercury concentrations in six coal reference materials, SRM 1632b (77.4 ng/g), SRM 1632c (94.3 ng/g), BCR 40 (433.2 ng/g), BCR 180 (125.0 ng/g), BCR 181 (135.8 ng/g), and SARM 20 (252.6 ng/g), as well as a coal fly ash, SRM 1633b (143.1 ng/g). The method is equally applicable to other types of fossil fuels including both crude and refined oils.     

Determination of bromine stable isotopes using continuous-flow isotope ratio mass spectrometry

A new methodology for bromine stable isotope determination by continuous-flow isotope ratio mass spectrometry (CF-IRMS) was developed. The technique was tested on inorganic samples. Inorganic bromide was precipitated in the form of silver bromide by using silver nitrate in a standard methodology. Bromine stable isotope analysis was carried out on methyl bromide (CH3Br) after converting silver bromide to methyl bromide by reacting it with methyl iodide (CH3I). The system used in this study is an IsoPrime IRMS, with analytical capabilities of both dual-inlet and continuous-flow modes coupled with an Agilent 6890 GC equipped with a CTC Analytics CombiPAL autosampler. This new technique measures samples as small as 0.2 mg of AgBr (1 micromol of Br-). The bromine stable isotope analysis using continuous flow technology showed excellent precision and accuracy. The internal precision using pure methyl bromide gas is better than +/-0.03 per thousand (+/-SD); the external precision using seawater standard is better than +/-0.06 per thousand (+/-SD) for n = 12. Moreover, the sample analysis time is 16 min, as compared to 75 min needed in previous techniques. This allows for 50 samples to be analyzed in 1 day, as compared to 8 samples using the conventional techniques. A series of natural saline formation waters and brines from sedimentary and crystalline rock environments was measured by this new methodology to test the potential natural range of delta81Br. The bromine isotopic composition of the samples ranged from 0.00 to +1.80 per thousand relative to standard mean ocean bromide (SMOB). Initial trends and distinctive isotopic difference were noticed between crystalline shield brines and sedimentary formation brines.     

High-precision position-specific isotope analysis of 13C/12C in leucine and methionine analogues

We report an automated method for high-precision position-specific isotope analysis (PSIA) of carbon in amino acid analogues. Carbon isotope ratios are measured for gas-phase pyrolysis fragments from multiple sources of 3-methylthiopropylamine (3MTP) and isoamylamine (IAA), the decarboxylated analogues of methionine and leucine, using a home-built gas chromatography (GC)-pyrolysis-GC preparation system coupled to a combustion-isotope ratio mass spectrometry system. Over a temperature range of 620-900 degrees C, the characteristic pyrolysis products for 3MTP were CH4, C2H6, HCN, and CH3CN and for IAA products were propylene, isobutylene, HCN, and CH3CN. Fragment origin was confirmed by 13C-labeling, and fragments used for isotope analysis were generated from unique moieties with > 95% structural fidelity. Isotope ratios for the fragments were determined with an average precision of SD(delta13C) < 0.3% per thousand, and relative isotope ratios of fragments from different sources were determined with an average precision of SD(delta(delta)13C) < 0.5% per thousand. Delta(delta)13C values of fragments were invariant over a range of pyrolysis temperatures. The delta(delta)13C of complementary fragments in IAA was within 0.8% per thousand of the delta(delta)13C of the parent compounds, indicating that pyrolysis-induced isotopic fractionation is effectively taken into account with this calibration procedure. Using delta(delta)13C values of fragments, delta(delta)13C values were determined for all four carbon positions of 3MTP and for C1, C2, and the propyl moiety of IAA, either directly or indirectly by mass balance. Large variations in position-specific isotope ratios were observed in samples from different commercial sources. Most dramatically, two 3MTP sources differed by 16.30% per thousand at C1, 48.33% per thousand at C2, 0.37% per thousand at C3, and 5.36% per thousand at C(methyl). These PSIA techniques are suitable for studying subtle changes in intramolecular isotope ratios due to natural processes.     

Stable nitrogen isotope analysis of amino Acid enantiomers by gas chromatography/combustion/isotope ratio mass spectrometry

The analysis of the stable nitrogen isotope compositions of individual amino acid stereoisomers through the use of gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is presented. Nitrogen isotopic compositions of single amino acids or of their enantiomers is possible without the labor-intensive and time-consuming preparative-scale chromatographic procedures required for conventional stable isotope analysis. Following hydrolysis and derivatization, single-component isotope analysis is accomplished on nanomole quantities of each of the stereoisomers of an amino acid, utilizing the effluent stream of gas chromatographic separation. Nitrogen isotope fractionation is minimal during acylation of the amino acid, with no additional nitrogen being added stoichiometrically to the derivative. Thus, the isotopic composition of the nitrogen in the derivative is that of the original compound. Replicate stable nitrogen isotope analyses of 11 amino acids, and their trifluoroacetyl (TFA)/isopropyl (IP) ester derivatives, determined by both conventional isotope ratio mass spectrometry (IRMS) and GC/C/IRMS, indicate that the GC procedure is highly reproducible (standard deviations typically 0.3-0.4‰) and that isotopic differences between the amino acid and its TFA/IP derivative are, in general, less than 0.5‰.     

Generation of O2 from BaSO4 using a CO2-laser fluorination system for simultaneous analysis of delta18O and delta17O

With the observation of mass-independent isotopic anomalies in numerous atmospheric molecules, the ability to measure both delta17O and delta18O in a range of samples is needed. Sulfate oxygen isotopic studies conventionally report only delta18O values. Recent findings indicate that sulfate delta17O and delta18O values, particularly the delta17O value (= delta17O - (0.52)(delta18O)), can provide independent information on the origin, mixing, and transformation of sulfate in the atmospheric and surface environments, which is not resolvable by only delta18O measurements. Existing methods for analyzing sulfate delta17O and delta18O are extremely laborious and demand high-purity BrF5. Here we report a novel method of generating O2 directly from Barite (BaSO4) for simultaneous analysis of delta18O and delta17O by isotope ratio mass spectrometry (IRMS). The method utilizes a CO2-laser fluorination system that can also be used to quantitatively generate O2 from silicates and oxides. Partial but consistent oxygen yields from BaSO4 are obtained for samples >4 mg. Correction factors of +9.4% for delta18O and 4.89% for delta17O are obtained, and there is no deviation in the delta17O value due to the nonquantitative O2 generation. The system may process more than a dozen samples per working day, with analytical error of +/-0.05% and +/-0.8% for delta17O and delta18O, respectively. This new method is ideal for studies emphasizing an accurate sulfate delta17O value.     

Comprehensive two-dimensional gas chromatography combustion isotope ratio mass spectrometry

We report the first coupling of comprehensive two-dimensional gas chromatography (GC x GC) to online combustion isotope ratio mass spectrometry (C-IRMS). A GC x GC system, equipped with a longitudinally modulated cryogenic system (LMCS), was interfaced to an optimized low dead volume combustion interface to preserve <300 ms full width at half-maximum (fwhm) fast GC peaks generated on the second GC column (GC2). The IRMS detector amplifiers were modified by configuration of resistors and capacitors to enable fast response, and a home-built system acquired data at 25 Hz. Software was home-written to handle isotopic time shifts of less than one bin (40 ms) and to integrate peak slices to recover isotope ratios from cryogenically sliced peaks. The performance of the GC x GCC-IRMS system was evaluated by isotopic analysis of urinary steroid standards. Steroids were separated by a nonpolar GC1 column (30 m x 0.25 mm, 5% phenyl), modulated into multiple 4- or 8-s cryogenic slices by the LMCS, and then separated on a polar GC2 column (1 or 2 m x 0.1 mm, 50% phenyl). GC2 peak widths from a 1-m column averaged 276 ms fwhm. Steroid standard sliced peaks were successfully reconstructed to yield delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.30 per thousand and average accuracies within 0.34 per thousand, at 8 ng on column. Two steroids, coeluting in GC1, were baseline separated in GC2 and resulted in delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.86 per thousand and average accuracies within 0.26 per thousand, at 3 ng on column. Results from this prototype system demonstrate that the enhanced peak capacity and signal available in GC x GC is compatible with high-precision carbon isotope analysis.     

Enzymatic decarboxylation of tyrosine and phenylalanine to enhance volatility for high-precision isotopic analysis

We present a rapid and selective method to increase the volatility of tyrosine and phenylalanine without adding derivative C for high-precision gas chromatography-continuous-flow isotope ratio mass spectrometry (GCC-IRMS) based on enzymatic decarboxylation to yield alkylamines and evaluated for 15N isotopic integrity. Purified tyrosine and phenylalanine were converted to tyramine and phenethylamine by tyrosine and phenylalanine decarboxylases, respectively. GC separation was achieved using a thick stationary phase (5-microm) capillary column. Recoveries were 95 +/- 2%. The reproducibility of delta15N of tyramine and phenethylamine measured by GCC-IRMS averaged SD(delta15N) = 0.33 per thousand. The absolute differences between delta15N of amino acids measured by elemental analyzer-IRMS and the alkylamines measured by GCC-IRMS was not significant. Phenethylamine and tyramine prepared from a mixture of 18 amino acids were extracted by ethanol with 95% recovery, and analysis yielded clean chromatograms and equivalent precision. These data indicate that enzymatic decarboxylation of phenylalanine and tyrosine is a convenient method to increase their volatility for continuous-flow isotopic analysis without introducing extraneous C or significant isotopic fractionation.     

Chromatographic separation and multicollection-ICPMS analysis of iron. Investigating mass-dependent and -independent isotope effects

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02 per thousand /amu at 2sigma. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil CI1 carbonaceous chondrite, which best approximates the solar composition, is indistinguishable from that of IRMM-014 (-0.005 +/- 0.017 per thousand /amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wüstite (FeO) and solar (MgO-Al(2)O(3)-SiO(2)-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of Fe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor alpha equal to (m(1)/m(2)()1/2), where m(1) and m(2) are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 +/- 0.000 67 for the (56)Fe/(54)Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the (57)Fe/(54)Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2epsilon and 0.5epsilon at 2sigma for (56)Fe/(54)Fe and (58)Fe/(54)Fe ratios, respectively (epsilon refers to relative variations in parts per 10 000). For (58)Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz-pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.     

High-precision continuous-flow measurement of delta13C and deltaD of atmospheric CH4

We describe our development of a CH4 preconcentration system for use with continuous-flow gas chromatograph combustion isotope ratio mass spectrometry (GC/C/IRMS). Precision of measurement of delta13C-CH4 is 0.05/1000 (1sigma) on multiple 60-mL aliquots of the same ambient air sample. The same front-end on-line CH4 preconcentration system allows us to measure deltaD of CH4 by gas chromatography IRMS when the combustion furnace is replaced with a pyrolysis oven (GC/P/IRMS). Precision of measurement for deltaD-CH4 is 1.5/1000 (1sigma) using 120 mL of ambient air based on multiple aliquots of the same air sample. These are the first reported measurements of atmospheric CH4 using GC/P/IRMS methodology. Each isotope analysis can be made much more rapidly (30-40 min) than they could using off-line combustion of an air sample (1-6 h) followed by conventional dual-inlet IRMS measurements (12-20 min), while requiring much less total volume and retaining a comparable level of precision and accuracy. To illustrate the capabilities of our preconcentration GC/C/IRMS system, we compare the results of measurement of 24 background air samples made using both GC/C/IRMS and conventional vacuum line/dual-inlet IRMS methodology. The air samples were collected on a shipboard air sampling transect made across the Pacific Ocean in July 2000 and are part of an ongoing atmospheric CH4 research program. The average difference between the two methods of IRMS analyses on these 24 samples is 0.01 +/- 0.03/1000 (95% confidence interval) for delta3C-CH4. These are the first measurements to be reported of air samples directly intercompared for delta13C-CH4 using both GC/C/IRMS and dual-inlet IRMS measurement methodology. Measurement of deltaD-CH4 of these air samples is also presented as an illustration of the ability of this system to resolve small isotopic differences in remote air. High-precision measurement of delta13C and deltaD of atmospheric CH4 made using our coupled preconcentration GC/IRMS system will greatly improve our ability to utilize isotopic data in understanding spatial and temporal changes in atmospheric CH4 and the biogeochemistry of its sources and sinks.     

Determination of stable carbon isotopic compositions of low molecular weight dicarboxylic acids and ketocarboxylic acids in atmospheric aerosol and snow samples

We report a new method developed for the determination of stable carbon isotopic composition of homologous alpha,omega-dicarboxylic acids and phthalic acid isolated from environmental samples such as atmospheric aerosols and snow. Dicarboxylic acids are derivatized with BF3/1-butanol to dibutyl esters, which are analyzed for the stable carbon isotopic composition using a capillary GC interfaced to on-line combustion isotope ratio mass spectrometer. The delta13C values for individual dicarboxylic acid are then calculated from delta13C of 1-butanol and butyl ester derivative using a mass balance equation. The accuracy of the delta13C measurement for C2-C10 diacids is within 0.8 per thousand. We report a few examples of the delta13C ratios of saturated C2-C9 alpha,omega-dicarboxylic acids, unsaturated (maleic, phthalic) diacids, and oxocarboxylic acids in the aerosol and snow samples.     

Evaluation of gas chromatographic isotope fractionation and process contamination by carbon in compound-specific radiocarbon analysis

The relevance of both modern and fossil carbon contamination as well as isotope fractionation during preparative gas chromatography for compound-specific radiocarbon analysis (CSRA) was evaluated. Two independent laboratories investigated the influence of modern carbon contamination in the sample cleanup procedure and preparative capillary gas chromatography (pcGC) of a radiocarbon-dead 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) reference. The isolated samples were analyzed for their 14C/12C ratio by accelerator mass spectrometry. Sample Delta14C values of -996 +/- 20 and -985 +/- 20 per thousand agreed with a Delta14C of -995 +/- 20 per thousand for the unprocessed PCB 169, suggesting that no significant contamination by nonfossil carbon was introduced during the sample preparation process at either laboratory. A reference compound containing a modern 14C/12C ratio (vanillin) was employed to evaluate process contamination from fossil C. No negative bias due to fossil C was observed (sample Delta14C value of 165 +/- 20 per thousand agreed with Delta14C of 155 +/- 12 per thousand for the unprocessed vanillin). The extent of isotopic fractionation that can be induced during pcGC was evaluated by partially collecting the vanillin model compound of modern 14C/12C abundance. A significant change in the delta13C and delta14C values was observed when only parts of the eluting peak were collected (delta13C values ranged from -15.75 to -49.91 per thousand and delta14C values from -82.4 to +4.71 per thousand). Delta14C values, which are normalized to a delta13C of -25 per thousand, did not deviate significantly (-58.9 to -5.8 per thousand, considering the uncertainty of approximately +/-20 per thousand). This means that normalization of radiocarbon results to a delta13C of -25 per thousand, normally performed to remove effects of environmental isotope fractionation on 14C-based age determinations, also cor-rects sufficiently for putative isotopic fractionation that may occur during pcGC isolation of individual compounds for CSRA.