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.     

Measuring methane and its isotopes 12CH4, 13CH4, and CH3D on the surface of Mars with in situ laser spectroscopy

In light of the recent discovery of methane on Mars and its possible biological origin, a strategy is described for making in situ measurements of methane and its isotopes on the surface of Mars by laser spectroscopy in the 3.3-microm wavelength region. An instrument of reasonable mass (approximately 1 lb) and power (few watts) is capable of measuring mixing ratios down to 0.1 part per 10(9) by volume, a hundred times lower than recently reported observations. Making accurate measurements of 13CH4 and CH3D will be more difficult. For measuring delta13C to 10/1000 and deltaD to 50/1000, sample preconcentration will be required to approximately 3 parts per 10(6) by volume for delta13C and to approximately 40 parts per 10(6) by volume for deltaD. This need would be mitigated by the discovery of larger local abundances of methane near the source regions.     

An analytical system for determining delta17O in CO2 using continuous flow-isotope ratio MS

We developed a simple measurement system for delta17O in nanomole quantities of CO2 using continuous flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted of a sample injection system, a helium-purged CO2 purification line, a capillary GC, a combustion unit, and CF-IRMS. A unique feature of the system is that we use molecular CO2 to determine the isotopic compositions including delta17O. The delta17O of CO2 in a sample is calculated from the mass ratios of both 45/44 and 46/44 of two different kinds of CO2, which have been purified quantitatively from different aliquots of a sample. While one aliquot (rCO2) flows into IRMS directly, the other (eCO2) flows through a CuO unit (900 degrees C) prior to injection into IRMS, to exchange oxygen atoms in the sample CO2 molecules with those in CuO for which we can assume Delta17O = 0. In our system, we introduce both rCO2 and eCO2 alternately to IRMS repeatedly by using an automatic multianalytical system to improve analytical precision statistically. The standard deviation of 0.35 per thousand for Delta17O can be realized using as little as 8.7 nmol CO2 in a approximately 3-h analysis. Based on this system, we have quantified delta17O in the stratospheric CO2 over Japan.     

On-line technique for the determination of the delta37Cl of inorganic and total organic Cl in environmental samples

Here we describe an on-line method for measuring delta(37)Cl values of chloride bearing salts, waters, and organic materials using multicollector continuous-flow isotope ratio mass spectrometry (CF-IRMS). Pure AgCl quantitatively derived from total Cl in water, inorganic Cl salts, and biological samples was reacted with iodomethane in evacuated 10-mL stopper sealed glass vials to produce methyl chloride gas. A GV Instruments Multicollector CF-IRMS with CH(3)Cl optimized collector geometry was modified to accommodate a headspace single-sample gas injection port prior to a GC column. The GC column was a 2-m Porapak-Q packed column held at 160 degrees C. The resolved sample CH(3)Cl was introduced to the IRMS source in a helium stream via an open split. delta(37)Cl values were calculated by measurement of CH(3)Cl at m/z 52/50 and by comparison to a reference pulse of CH(3)Cl calibrated to standard mean ocean chloride. Sample CH(3)Cl analysis time was approximately 6 min. Injections of 40 microL of pure CH(3)Cl gas yielded a repeatability (+/-SD) of +/-0.06 per thousand for delta(37)Cl (n = 10). Combined GC and IRMS source linearity for CH(3)Cl was <0.2 per thousand/nA (V) peak height. External repeatability, based on processing of seawater and NaCl reference solutions, was better than +/-0.08 per thousand. The smallest sample for delta(37)Cl analysis by this method was approximately 0.2 micromol of Cl. Selected results from a river basin and biological samples study illustrate the potential of on-line chlorine isotope assays in environmental pollution studies.     

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.     

Stable isotopic analysis of atmospheric methane by infrared spectroscopy by use of diode laser difference-frequency generation

An infrared absorption spectrometer has been constructed to measure the stable isotopic composition of atmospheric methane samples. The spectrometer employs periodically poled lithium niobate to generate 15 microW of tunable difference-frequency radiation from two near-infrared diode lasers that probe the nu3 rotational-vibrational band of methane at 3.4 microm. To enhance the signal, methane is extracted from 25 l of air by use of a cryogenic chromatographic column and is expanded into the multipass cell for analysis. A measurement precision of 12 per thousand is demonstrated for both delta13C and deltaD.     

Fast gas chromatography combustion isotope ratio mass spectrometry

We report here the first coupling of fast GC to IRMS, in a system capable of 250 ms peak widths (fwhm) at 1 mL/min flow rates, one-fifth as narrow as any previously reported GCC-IRMS system. We developed an optimized postcolumn interface that results in minimal peak broadening, using a programmable temperature vaporization injector in place of a rotary valve or backflush system to divert solvent, a narrow capillary combustion reactor followed by a cryogenic water trap with narrow-bore (<0.20 mm i.d.) transfer lines, and a narrow i.d. open split to the IRMS directly inserted into the column effluent. Quantitative combustion was demonstrated with CH4 injections. A comparison of CO2 injections with different fwhm peak widths (250, 2500, and 7500 ms) showed similar precisions, SD(delta13C)=0.2-0.3 per thousand, for injections of >600 pmol C on column; precision for the narrow peaks (250 ms) was considerably better for injections<150 pmol C on column. SD(delta13C)<1 per thousand was achievable for injections of 5-15 pmol on column for 250 ms wide peaks, 10-fold better precision than 2500 ms wide peaks, and within a factor of 3 of the counting statistics limit. For a mixture of 15 fatty acid methyl esters (FAME), 1.5 nmol C of each on column yielded typical SD(delta13Cpdb)=0.4 per thousand for fast GC and 0.5 per thousand for conventional GC. For 14 of the 15 FAME, delta13C values between the two systems were within+/-1.5 per thousand and not significantly different. Fast GCC-IRMS required one-third the run time (450 s vs 1400 s) to achieve comparable resolution. Mean peak widths for fast GCC-IRMS of the FAME were 720 ms, compared to 650 ms by fast GC with flame ionization detection. At a 15-fold dilution (100 pmol C on column for each FAME), fast GCC-IRMS achieved approximately 2-fold better precision and accuracy than similar injections on conventional GCC-IRMS. Finally, a mixture of 10 steroids (approximately 7 nmol C (100 ng) each on column) was analyzed with mean precision of SD(delta13C)=0.2 per thousand in 620 s by fast GCC-IRMS, while conventional GCC-IRMS required 1200 s and achieved poorer resolution. delta13C values for the two system were similar (Deltadelta13C1 nmol C) and achieves modest precision (approximately 1 per thousand) near the counting statistics limit on low level components.     

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.     

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.     

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.     

Medusa: a sample preconcentration and GC/MS detector system for in situ measurements of atmospheric trace halocarbons, hydrocarbons, and sulfur compounds

Significant changes have occurred in the anthropogenic emissions of many compounds related to the Kyoto and Montreal Protocols within the past 20 years and many of their atmospheric abundances have responded dramatically. Additionally, there are a number of related natural compounds with underdetermined source or sink budgets. A new instrument, Medusa, was developed to make the high frequency in situ measurements required for the determination of the atmospheric lifetimes and emissions of these compounds. This automated system measures a wide range of halocarbons, hydrocarbons, and sulfur compounds involved in ozone depletion and/or climate forcing, from the very volatile perfluorocarbons (PFCs, e.g., CF(4) and CH(3)CF(3)) and hydrofluorocarbons (HFCs, e.g., CH(3)CF(3)) to the higher-boiling point solvents (such as CH(3)Cl(3) and CCl(2)=CCl(2)) and CHBr(3). A network of Medusa systems worldwide provides 12 in situ ambient air measurements per day of more than 38 compounds of part per trillion mole fractions and precisions up to 0.1% RSD at the five remote field stations operated by the Advanced Global Atmospheric Gases Experiment (AGAGE). This custom system couples gas chromatography/mass spectrometry (GC/MSD) with a novel scheme for cryogen-free low-temperature preconcentration (-165 degrees C) of analytes from 2 L samples in a two-trap process using HayeSep D adsorbent.     

Compound-specific stable isotope analysis of soil mesofauna using thermally assisted hydrolysis and methylation for ecological investigations

Stable isotope mass spectrometric approaches are proving to be valuable tools in unravelling biotic interactions in complex ecosystems, yielding information on trophic preferences and functional roles of individual species. Gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) provides considerable opportunities to assist in studies concerned with ecosystem processes mediated by soil invertebrates and microorganisms by determination of delta(13)C values of individual compounds, for example, lipids, amino acids etc. However, techniques conventionally adopted for "wet" chemical extractions and derivatizations necessary for compound-specific stable isotope determinations restrict the size of soil organism that can be studied and can limit investigations of individuals or even parts of individuals. We demonstrate here that individual soil mesofauna can be probed directly for their fatty acid stable isotope signatures by pyrolysis-GC/C/IRMS. A thermally assisted hydrolysis and methylation (THM) reaction is described for the determination of delta(13)C fatty acid values using trimethylsulfonium hydroxide (TMSH). Authentic fatty acids, acyl lipids, and individual Collembola (Folsomia candida) raised on C(3) and C(4) isotopically labeled yeast were analyzed initially by py-GC/MS with TMSH and then by py-GC/C/IRMS. A kinetic isotope effect (KIE) observed with the THM reaction prevents direct calculation of the fatty acid delta(13)C values by simple mass balance equations. However, the KIE is shown to be both reproducible and robust and can therefore be accounted for by the use of correction factors. The fatty acid methyl ester compositions of individual F. candida and their respective delta(13)C values were determined and shown to agree with those obtained by conventional "wet" chemical procedures applied to much larger numbers of Collembola, thus enhancing the scope to which stable isotopes can be applied to the study of invertebrates in complex food webs in any environment.     

Determination of atmospheric methyl bromide by cryotrapping-gas chromatography and application to soil kinetic studies using a dynamic dilution system

Methyl bromide (CH(3)Br) is considered to be a major source of stratospheric Br, which contributes to the destruction of ozone. It is therefore necessary to understand the natural sinks of this compound and to accurately measure ambient mixing ratios. Methodology is described for the measurement of atmospheric CH(3)Br by cryotrapping-gas chromatography and its application to soil kinetics. A 2-propanol/dry ice cryotrap was used to preconcentrate CH(3)Br in standard and air samples, with subsequent detection using a gas chromatograph equipped with an O(2)-doped electron capture detector (GC-ECD). The GC-ECD cryotrapping method had a detection limit of 0.23 pmol of CH(3)Br. This is equivalent to the amount of CH(3)Br in a 500 mL sample of ambient air at the estimated northern hemisphere atmospheric mixing ratio of 11 parts per trillion by volume (pptv). A dynamic dilution system was developed to produce mixing ratios of CH(3)Br ranging between 4 and 1000 pptv. Calibrated mixing ratios of CH(3)Br produced with the dilution system were used to determine soil uptake kinetics employing a dynamic soil incubation method.     

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.     

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.     

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.     

Automated measurement of nitrogen trifluoride in ambient air

We present an analytical method for the in situ measurement of atmospheric nitrogen trifluoride (NF(3)), an anthropogenic gas with a 100-year global warming potential of over 16,000. This potent greenhouse gas has a rising atmospheric abundance due to its emission from a growing number of manufacturing processes and an expanding end-use market. Here we present a modified version of the "Medusa" preconcentration gas chromatography/mass spectrometry (GC/MS) system of Miller, B. R.; Weiss, R. F.; Salameh, P. K.; Tanhua, T.; Greally, B. R.; Mühle, J.; Simmonds, P. G. Anal. Chem.2008, 80 (5), 1536-1545. By altering the techniques of gas separation and chromatography after initial preconcentration, we are now able to make atmospheric measurements of NF(3) with relative precision <2% (1σ) for current background clean air samples. Importantly, this method augments the currently operational Medusa system, so that the quality of data for species already being measured is not compromised and NF(3) is measured from the same preconcentrated sample. We present the first in situ measurements of NF(3) from La Jolla, California made 11 times daily, illustrating how global deployment of this technique within the AGAGE (Advanced Global Atmospheric Gases Experiment) network could facilitate estimation of global and regional NF(3) emissions over the coming years.     

Precision trace gas analysis by FT-IR spectroscopy. 2. The 13C/12C isotope ratio of CO2

We report the development of a method of carbon stable isotope ratio analysis based on 1-cm-1 resolution Fourier transform infrared (FT-IR) spectroscopy, deployable in both laboratory and field applications. We demonstrate the determination of the 13C/12C ratio of CO2 (i.e., delta 13CO2) in air with an analytical precision of the order of +/- 0.1/1000 (i.e., +/- 0.01%). The FT-IR method relies on calibration using synthetically calculated absorbance spectra and a multivariate calibration algorithm. The method requires no sample preparation other than optional drying of the sample and may be applied directly to ambient air samples containing approximately 350 mumol mol-1 CO2 (molar mixing ratio). It may also be applied to samples more concentrated in CO2, such as human breath, approximately 5% CO2. We demonstrate the utility of the technique to the analysis of delta 13CO2 in air during an experimental field campaign and to the laboratory-based analysis of human breath. A similar method could also be used to determine the H/D ratio in atmospheric water vapor.     

Compound-specific nitrogen and carbon isotope analysis of nitroaromatic compounds in aqueous samples using solid-phase microextraction coupled to GC/IRMS

Solid-phase microextraction (SPME) coupled to gas chromatography/isotope ratio mass spectrometry was used to determine the delta15N and delta13C signatures of selected nitroaromatic contaminants such as the explosive 2,4,6-trinitrotoluene (TNT) for derivation of isotopic enrichment factors of contaminant transformation. Parameters for efficient extraction of nitroaromatic compounds (NACs) and substituted anilines from water samples were evaluated by SPME-GC/MS. delta13C signatures determined by SPME-GC/IRMS and elemental analyzer IRMS (EA-IRMS) were in good agreement, generally within +/-0.7 per thousand, except for 2,4-dinitrotoluene (2,4-DNT) and TNT, which showed slight deviations (<1.3 per thousand). Limits of detection (LODs) for delta13C analysis by SPME-GC/IRMS were between 73 and 780 microg L-1 and correlated with the extraction efficiencies of the compounds determined by SPME-GC/MS. Nitrogen isotope measurements by SPME-GC/IRMS were of similar precision (standard deviations <0.8 per thousand) for all NACs except for TNT. delta15N signatures matched the reference values obtained by EA-IRMS within +/-1.3 per thousand (+2.5 per thousand for TNT), but no systematic trend was found for the deviations. LODs of delta15N measurements ranged from 1.6 to 9.6 mg L-1 for nitrotoluenes, chlorinated NACs and DNTs (22 mg L-1 for TNT). The SPME-GC/IRMS method is well suited for the determination of isotopic enrichment factors of various NAC transformation processes and provides so far unexplored possibilities to elucidate behavior and degradation mechanisms of nitroaromatic contaminants in soils and groundwaters.     

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‰.