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.     

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.     

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.     

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.     

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.     

Development of N-acetyl methyl ester derivatives for the determination of delta13C values of amino acids using gas chromatography-combustion- isotope ratio mass spectrometry

A novel derivatization procedure, N-acetyl methyl (NACME) esterification, was developed to improve the accuracy and precision of amino acid delta13C value determination using gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS). Standard mixtures of 15 protein amino acids were converted to NACME and N-acetyl-isopropyl (NAIP) esters; the latter established derivative was employed for comparison purposes. Both procedures yielded baseline-resolved peaks for all 15 amino acids when GC columns coated with polar stationary phases were employed. For NACME esters, the methylation conditions governed reaction yields, with highest yields observed when a 1 h, 70 degrees C methylation procedure (anhydrous MeOH/acetyl chloride, 25:4, v/v) was performed. The mean derivatization yields expressed relative to an underivatized coinjected standard (n-nonadecane) for both NACME and NAIP esters were identical. Likewise, the mean kinetic isotope effects (KIEs) were not significantly different (KIE(NACME) = 1.036; KIE(NAIP) = 1.038) and were shown in both cases to be reproducible. The mean reproducibility obtained from 15 replicates (3 x batches of 5) of both derivatives was strong (mean STDV(NACME) = 0.3 per thousand and STDV(NAIP) = 0.4 per thousand). The isotopic robustness of both derivatization procedures was observed over a concentration range of 52,500 microg of amino acid. NACME esters displayed low errors (+/-0.6 per thousand for phenylalanine to +/-1.1 per thousand for serine) due to the higher sample-to-derivative carbon ratio of this derivative. Finally, the integrity of the new NACME procedure was confirmed through analysis of diet and bone collagen amino acids of rats reared on C3 or C4 diets, which indicated the high degree of both accuracy and precision of the delta13C values obtained for individual amino acids.     

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.     

Detection of synthetic testosterone use by novel comprehensive two-dimensional gas chromatography combustion-isotope ratio mass spectrometry

We report the first demonstration of comprehensive two-dimensional gas chromatography combustion-isotope ratio mass spectrometry (GC×GCC-IRMS) for the analysis of urinary steroids to detect illicit synthetic testosterone use, of interest in sport doping. GC coupled to IRMS (GCC-IRMS) is currently used to measure the carbon isotope ratios (CIRs, δ(13)C) of urinary steroids in antidoping efforts; however, extensive cleanup of urine extracts is required prior to analysis to enable baseline separation of target steroids. With its greater separation capabilities, GC×GC has the potential to reduce sample preparation requirements and enable CIR analysis of minimally processed urine extracts. Challenges addressed include online reactors with minimized dimensions to retain narrow peak shapes, baseline separation of peaks in some cases, and reconstruction of isotopic information from sliced steroid chromatographic peaks. Difficulties remaining include long-term robustness of online reactors and urine matrix effects that preclude baseline separation and isotopic analysis of low-concentration and trace components. In this work, steroids were extracted, acetylated, and analyzed using a refined, home-built GC×GCC-IRMS system. 11-Hydroxyandrosterone and 11-ketoetiocolanolone were chosen as endogenous reference compounds because of their satisfactory signal intensity, and their CIR was compared to target compounds androsterone and etiocholanolone. Separately, a GC×GC-quadrupole MS system was used to measure testosterone (T)/epitestosterone (EpiT) concentration ratios. Urinary extracts of urine pooled from professional athletes and urine from one individual that received testosterone gel (T-gel) and one individual that received testosterone injections (T-shots) were analyzed. The average precisions of δ(13)C and Δδ(13)C measurements were SD(δ(13)C) approximately ±1‰ (n = 11). The T-shot sample resulted in a positive for T use with a T/EpiT ratio of >9 and CIR measurements of Δδ(13)C > 5‰, both fulfilling World Anti-Doping Agency criteria. These data show for the first time that synthetic steroid use is detectable by GC×GCC-IRMS without the need for extensive urine cleanup.     

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.     

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.     

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.     

Evaluation of a microfabricated thermal modulator for comprehensive two-dimensional microscale gas chromatography

A microfabricated thermal modulator (μTM) designed for ultimate use in a comprehensive two-dimensional microscale gas chromatography (μGC × μGC) system is evaluated. The 2-stage device measures 13 mm (l) × 6 mm (w) × 0.5 mm (h) and consists of two interconnected serpentine etched-Si microchannels suspended from a thin Pyrex cap and wall-coated with PDMS (polydimethylsiloxane). The chip is mounted within a few tens of micrometers of a thermoelectric cooler that maintains both stages at a baseline temperature between -35 and -20 °C in order to focus analytes eluting from an upstream separation column. Each stage is heated to 210 °C sequentially at a rate as high as 2400 °C/s by independent thin-film resistors to inject the analytes in consecutive fractions to a downstream column, and then cooled at a rate as high as -168 °C/s. The average power dissipation is only ～10 W for heating and 21 W for cooling without using consumable materials. In this study, the outlet of the μTM is connected directly to a flame ionization detector to assess its performance. Following a demonstration of basic operation, the modulated peak amplitude enhancement (PAE) and full-width-at-half-maximum (fwhm) are evaluated for members of a series of n-alkanes (C(6)-C(10)) as a function of the rim and stage temperatures; modulation period, phase, and offset; analyte concentration; and carrier-gas flow rate. A PAE as high as 50 and a fwhm as narrow as 90 ms are achieved for n-octane under optimized conditions.     

Coupling a high-temperature catalytic oxidation total organic carbon analyzer to an isotope ratio mass spectrometer to measure natural-abundance delta13C-dissolved organic carbon in marine and freshwater samples

The stable isotope composition of dissolved organic carbon (delta(13)C-DOC) provides powerful information toward understanding carbon sources and cycling, but analytical limitations have precluded its routine measurement in natural samples. Recent interfacing of wet oxidation-based dissolved organic carbon analyzers and isotope ratio mass spectrometers has simplified the measurement of delta(13)C-DOC in freshwaters, but the analysis of salty estuarine/marine samples still proves difficult. Here we describe the coupling of the more widespread high-temperature catalytic oxidation-based total organic carbon analyzer to an isotope ratio mass spectrometer (HTC-IRMS) through cryogenic trapping of analyte gases exiting the HTC analyzer for routine analysis of delta(13)C-DOC in aquatic and marine samples. Targeted elimination of major sources of background CO2 originating from the HTC analyzer allows for the routine measurement of samples over the natural range of DOC concentrations (from 40 microM to over 2000 microM), and salinities (<0.1-36 g/kg). Because consensus reference natural samples for delta(13)C-DOC do not exist, method validation was carried out with water-soluble stable isotope standards as well as previously measured natural samples (IAEA sucrose, Suwannee River Fulvic Acids, Deep Sargasso Sea consensus reference material, and St. Lawrence River water) and result in excellent delta(13)C-DOC accuracy (+/-0.2 per thousand) and precision (+/-0.3 per thousand).     

IR laser extraction technique applied to oxygen isotope analysis of small biogenic silica samples

An IR-laser fluorination technique is reported here for analyzing the oxygen isotope composition (delta18O) of microscopic biogenic silica grains (phytoliths and diatoms). Performed after a controlled isotopic exchanged (CIE) procedure, the laser fluorination technique that allows one to visually check the success of the fluorination reaction is faster than the conventional fluorination technique and allows analyzing delta18O of small to minute samples (1.6-0.3 mg) as required for high-resolution paleoenvironmental reconstructions. The long-term reproducibility achieved with the IR laser-heating fluorination/O2 delta18O analysis is lower than or equal to +/-0.26 per thousand (1 SD; n = 99) for phytoliths and +/-0.17 per thousand (1 SD; n = 47) for diatoms. When several CIE are taken into account in the SD calculation, the resulting reproducibility is lower than or equal to +/-0.51 per thousand for phytoliths (1 SD; n = 99; CIE > 5) and +/-0.54 per thousand (1 SD; n = 47; CIE = 13) for diatoms. A minimum reproducibility of +/-0.5 per thousand leads to an estimated uncertainty on delta18Osilica close to +/-0.5 per thousand. Resulting uncertainties on reconstructed temperature and delta18Oforming water are, respectively, +/-2 degrees C and +/-0.5 per thousand and fit in the precisions required for intertropical paleoenvironmental reconstructions. Several methodological points such as optimal extraction protocols and the necessity or not of performing two CIE prior to oxygen extraction are assessed.     

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.     

Carbon position-specific isotope analysis of alanine and phenylalanine analogues exhibiting nonideal pyrolytic fragmentation

Recent advances in gas chromatography combustion-isotope ratio mass spectrometry (GCC-IRMS) has made compound-specific isotope analysis routine, but reports on position-specific isotopic analysis are still scarce. On-line GC-pyrolysis (Py) coupled to GCC-IRMS is reported here for isolation and isotopic characterization of alaninol and phenethylamine, analogues of alanine and phenylalanine, respectively. Ideally, pyrolytic fragments will originate from unique sites within the parent molecule, and isotope ratios for each position within the parent can either be measured directly or calculated from fragment isotope ratios without substantially degrading the analytical precision. Alaninol pyrolysis yielded several fragments, of which CO and CH4 were used for isotope ratio calculations. Isotope labeling experiments showed that CO derived entirely from the C(1) position, while all three positions of alaninol contributed to CH4 (29.0 +/- 0.3% from C(1), 3.6 +/- 0.2% from C(2), and 66.9 +/- 1.1% from C(3)). We demonstrate iterative use of mass balance to calculate isotope ratios from all positions despite the nonideal positional fidelity of CH4. Pyrolysis of phenethylamine generated benzene and toluene fragments. Benzene derived entirely from C(ring), and toluene was proportionately formed from C(3) and C(ring). Relative intramolecular isotope ratios (Deltadelta13C) were calculated directly from delta13C of fragments or indirectly by mass balance. Though the C(3) isotope ratio was calculated from the benzene and toluene fragments, propagation of errors showed that the final precision of the determination was degraded due to the small contribution that C(3) makes to toluene. Samples of each amino acid from four different vendors showed natural variability between sources, especially at the C(1) position of alaninol (range of Deltadelta13C approximately 50 per thousand). The average precision was SD(Deltadelta13C) < 0.20 per thousand for directly measured positions of alaninol and phenethylamine. The precision of indirectly measured positions was poorer (SD(Deltadelta13C) = 0.94 per thousand for alaninol, 6.54 per thousand for phenethylamine) due to propagation of errors. These data demonstrate that GC-Py-GCC-IRMS data can be used to extract high-precision isotope ratios from amino acids despite nonideal positional fidelity in fragments and that natural intramolecular variability in delta13C can be used to distinguish different sources of amino acids.     

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.     

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).     

Modulator design for comprehensive two-dimensional gas chromatography: quantitative analysis of polyaromatic hydrocarbons and polychlorinated biphenyls

A novel cryogenic modulator was constructed for comprehensive two-dimensional gas chromatography (GC x GC). The modulator is based on two-step cryogenic trapping with CO2 and thermal desorption with electric heating. The GC x GC system included a nonpolar first-dimension column and two semipolar second-dimension columns, one connected to a flame ionization detector and the other one to a electron capture detector. A Matlab-based program, which allowed determination of peak heights and volumes, was written for the data analysis. The GC x GC system was applied for the analysis of polyaromatic hydrocarbons and polychlorinated biphenyls. The functioning of the modulator and the quantitativity of the method were studied with both peak volumes and peak heights from a three-dimensional plot. The separate peak areas from the modulated chromatogram were calculated as a comparison. The quantitative results were compared with those obtained with the same system but without the thermal modulation. The method was found to be repeatable and linear with use of peak volumes as well as peak heights. There was also good agreement with the results obtained by integration of separate peak areas. The developed GC x GC method was applied to the analysis of a Soxhlet extract of a certified sediment sample. The results were compared with the certified values.     

Determination of the S isotope composition of methanesulfonic acid

Sulfur (S) isotopes have been used to apportion the amount of biogenic and anthropogenic sulfate in remote environments, an important parameter that is used to model the global radiation budget. A key assumption in the apportionment calculations is that there is little isotope selectivity as reduced compounds such as dimethyl sulfide (DMS) are oxidized. This paper describes a method to determine, for the first time, the S isotope composition of methanesulfonic acid (MSA), the product of DMS oxidation. The isotope composition of MSA was measured directly by EA-IRMS and was used as an isotope reference for the method. Synthetic mixtures approximating the conditions expected for aerosol MSA samples were prepared to test this method. First, MSA solutions were measured alone and then in combination with MSA and SO4(2-). In synthetic mixtures, SO4(2-) was separated from MSA by precipitating it as BaSO4 prior to preparation of MSA for isotope analysis. The delta 34S value for MSA solutions was -2.6 per thousand (SD +/- 0.4 per thousand), which is not different from the delta 34S obtained from MSA filtrate after precipitating SO4(2-) from the mixture (-2.7 +/- 0.3 per thousand). However, these values are offset from direct EA-IRMS analysis of MSA used as the isotope reference by -1.1 +/- 0.2 per thousand, and this must be accounted for in reporting MSA measurements. The S isotope measurements using this method approach a limiting value above 300 microg of MSA. This is approximately equal to the MSA found in 20,000 m3 of air, assuming ambient concentrations of approximately 15 ng m(-3). Three samples of MSA from the Pacific Ocean measured using this technique have an average delta 34S value of +17.4 +/- 0.7 per thousand.