Demonstration of Compound-Specific Isotope Analysis of Hydrogen

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Demonstration of Compound-Specific Isotope Analysis of Hydrogen Isotope Ratios in Chlorinated Ethenes Tomasz Kuder* and Paul Philp School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, SEC 710, Norman, Oklahoma 73019, United States S Supporting Information *

ABSTRACT: High-temperature pyrolysis conversion of organic analytes to H2 in hydrogen isotope ratio compound-specific isotope analysis (CSIA) is unsuitable for chlorinated compounds such as trichloroethene (TCE) and cis-1,2dichloroethene (DCE), due to competition from HCl formation. For this reason, the information potential of hydrogen isotope ratios of chlorinated ethenes remains untapped. We present a demonstration of an alternative approach where chlorinated analytes reacted with chromium metal to form H2 and minor amounts of HCl. The values of δ2H were obtained at satisfactory precision (±10 to 15‰), however the raw data required daily calibration by TCE and/or DCE standards to correct for analytical bias that varies over time. The chromium reactor has been incorporated into a purge and trap−CSIA method that is suitable for CSIA of aqueous environmental samples. A sample data set was obtained for six specimens of commercial product TCE. The resulting values of δ2H were between −184 and +682 ‰, which significantly widened the range of manufactured TCE δ2H signatures identified by past work. The implications of this finding to the assessment of TCE contamination are discussed.

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INTRODUCTION Compound-specific isotope analysis (CSIA) combines gas chromatography for separation of target analytes from sample matrix and mass spectrometry for determination of isotope ratios in individual chromatographic peaks. A major application of CSIA is the study of environmental contaminants, with the major share of such applications devoted to the study of chlorinated ethenes (CEs).1 Isotope ratios determined by CSIA provide valuable information on the fate of the contaminants. Characteristic changes of isotope ratios, e.g., 13C/12C, result from different reaction rates during the degradation of molecules substituted by different isotope species. CSIA has been applied successfully for assessment of in situ biodegradation and abiotic degradation, for a range of organic contaminants, including CEs, benzene, and MtBE.1−6 CSIA data may also provide more specific information, useful in corroborating degradation pathways, with potential benefits for testing conceptual models for contaminated sites undergoing remediation.7,8 So called 2D-CSIA combines 2-element isotope assessment of a reaction pathway. 2D-CSIA has shown great potential in pathway identification or discrimination between alternative scenarios of degradation.5,9,10 In CEs studies, past examples of 2D-CSIA utilized a combination of carbon and chlorine isotope ratios. Isotope ratios may also be used as “fingerprints” for sample individualization. The dimensions for sample individualization are provided by their isotope ratios, inherited from the manufacturing processes and feedstocks for a given chemical product.1 Historically, most CEs work to date has centered on determination of carbon isotope ratios.1 In recent years, several methods for chlorine CSIA have been developed,11,12 and an © 2013 American Chemical Society

increasing number of studies have combined carbon and chlorine isotope ratio determination.13−15 By analogy with nonhalogenated hydrocarbon studies, hydrogen isotope ratios may be highly informative in the identification of reaction pathways. Hydrogen isotope ratios might also provide important evidence on the provenance of the environmental CEs, however, no established methodology exists for compound-specific determination of hydrogen isotope ratios in CEs.16 A few studies presented hydrogen isotope ratios of CEs, from bulk preparative techniques or from preparative chromatography.17−19 Neither of those techniques would be applicable for typical environmental samples, due to insufficient detection limits and/or lack of chromatographic resolution. Hydrogen CSIA commonly relies on high-temperature pyrolysis for conversion of analytes to the H2 surrogate.20 However, pyrolysis of CEs tends to produce HCl as well as or instead of H2.21,22 HCl is detrimental for following reasons: (1) HCl production reduces the yields of H2; (2) HCl is corrosive and frequent exposure would result in the need of timeconsuming instrument maintenance; and (3) hydrogen isotope disproportionation between H2 and HCl may cause significant isotope ratio artifacts. While the pyrolysis approach may be successful for target analytes with stoichiometric excess of hydrogen over chlorine,21 no successful compound-specific applications have been presented to date for compounds without stoichiometric excess of H over Cl, such as the Received: Revised: Accepted: Published: 1461

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important CEs contaminant, trichloroethylene (TCE), or its degradation product, cis-1,2-dichloroethylene (DCE).16 As an alternative to high-temperature pyrolysis, conversion to H2 by metal reductants has been used for analysis of water and organic compounds. Chromium (Cr) appears to be the most widely used reductant and a number of off-line23−25 and continuous-flow applications (elemental analyzer or CSIA)22,26−29 have been published using this metal. For chlorinated compounds, the reaction with Cr offers a potential benefit of eliminating the HCl product, due to formation of nonvolatile chlorides instead of HCl.23 An elemental analyzer with a Cr-packed reactor was successfully applied to determine hydrogen isotope ratios of several chlorinated compounds.22 Here, we present results for hydrogen CSIA for two key CEs, TCE and DCE, involving a Cr metal reactor. The ultimate objective of the work is to provide a means for H CSIA in environmental samples. Therefore, we had to consider the requirements of reasonable detection limits and chromatographic separation. In environmental contaminant studies, the CEs may be present in groundwater, soil gas, or air matrix. Analysis of such samples typically requires analyte preconcentration. For CEs present in groundwater samples, the best CSIA detection limits have been reported from purge and trap applications.30 For vapor phase samples, the same applies for preconcentration on adsorbent followed by thermal desorption.15 The principles of the two methods are relatively similar. We present results for a purge and trap−CSIA application that can be easily adapted to adsorption/thermal desorption for CEs vapor analysis. To illustrate the method performance, hydrogen isotope ratios were determined by the purge and trap−CSIA in a group of samples of consumer products containing TCE of unknown hydrogen isotope composition. The results from that data set are discussed in terms of the information potential of H CSIA in CEs contamination assessment.

Figure 1. CSIA instrumentation configured with a purge and trap concentrator: (1) desorption and GC column #1 carrier He pressure regulator; (2) purge and trap concentrator; (3) aqueous VOCs sample in sparge vessel; (4) GC column #1, used for water separation; (5) switching valve; (6) vent; (7) cryotrap (LN2); (8) GC column #2 carrier He pressure regulator; (9) GC column #2; (10) backflush outlet with an on−off valve; (11) Cr reactor; (12) HCl trap; (13) backflush He inlet with an on−off valve; (14) open split; (15) capillary feed to the IRMS.

MtBE column, 60 m × 0.32 mm (J&W). The GC oven was programmed from 40 °C (hold time 0.5 min), with a fast heating rate of 20 °C/min to 100 or 70 °C, for TCE and DCE, respectively, and then held isothermal for the period of data acquisition. The results presented herein were obtained at carrier gas flow rate of 1 mL/min. Different carrier flow rates were tested, from 0.5 to 3 mL/min (Supporting Information Figure S1). The Cr reactor exerts significant back-pressure to the carrier gas flow that tends to vary between individually packed tubes. Each time a new reactor tube was installed, the carrier gas flow was measured on a capillary extending from the HCl trap (Figure 1, Item 12). Typically, a GC−IRMS interface is operated with a backflush program used to vent unwanted parts of the GC effluent. In backflush mode, the back pressure exerted by the reactor tube is eliminated. To maintain a relatively constant carrier gas flow, the back pressure effects were factored in by using a ramped carrier gas flow mode (actual column flow was 1.5 mL/min while in backflush; actual column flow was 1 mL/min after backflush termination). It is possible that back pressure may change over the course of reactor operation if the chromium particles shift or the reactor diameter becomes blocked with carbon residue. While such a phenomenon was not yet observed, it would be readily evident as a shift of the analyte retention times. For aqueous samples, an OI 4660 purge and trap device (OI Analytical) was interfaced to the GC−IRMS unit (Figure 1). The purge and trap samples were analyzed following the CSIA protocols in use by the OU laboratory.31,32 In summary, the purge and trap transfer line was connected to a polar phase precolumn (DB-Wax, 30 m × 0.25 mm, 0.5 μm film; Figure 1, Item 4) to separate water from the analytes of interest, then the analytes were focused on a liquid nitrogen-cooled trap operated at −160 °C (Smartcryo, Weatherford Laboratories; Figure 1, Item 7), and finally the analytes were injected onto the GC column. Both the precolumn and the main separation column were present in the same GC oven. The precolumn, cryofocuser, and the GC column were interfaced through a 6port switching valve to allow splitless transfer of the purge and trap effluent onto GC column. The GC oven was programmed from 40 °C (6 min to transfer the purge and trap effluent to the

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EXPERIMENTAL SECTION CSIA Instrumentation. A gas chromatograph−isotope ratio mass spectrometer (GC−IRMS; Agilent 6890 with Thermo-Finnigan Delta XL) was operated in hydrogen mode, tuned following the standard Delta XL procedure for high linearity performance. A diagram of the instrumentation, including the purge and trap peripheral needed for aqueous sample analysis, is shown in Figure 1. A detailed description of the modified reactor used for conversion of the analytes to H2 is given in the following section. Similarly as in the instrument used by Chartrand et al.,21 a cryogenic trap was included to intercept potential HCl product (Figure 1, Item 12). The trap was a 50-cm section of inert 0.32 ID silica tubing extending from the reactor outlet, coiled and immersed in liquid nitrogen. The loop was cleaned, typically at the end of the day, by removing the liquid nitrogen flask and allowing the condensate to backflush through the reactor. We decided against venting the condensate through the open split (it would lead to corrosion of the interface plumbing) or through disconnected outlet of the trap (increased potential of operator error). While the chosen cleaning method increases the Cl burden of the reactor, the observed good yields of H2 suggest that the yield of HCl condensate is minor in comparison with the total amount of Cl run though the reactor. Direct injections of samples (neat TCE or solution of cDCE in p-xylene) into the Agilent 6890 were made in the split mode (split ratio was 40 or 120 for TCE; all DCE samples were injected at split ratio of 20). The GC was equipped with a DB1462

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cryofocuser, followed by a hold time of 5 min after the cryogen shutoff), with a heating rate of 6 °C/min to 150 °C, followed with 15 min column bake at 220 °C. The precolumn was dried during the 220 °C event. The foreflush mode carrier flow rate was 1 mL/min. Chromium Reactor. A reactor tube was prepared by packing a section of a 1/16 in. alumina tube (ID 0.5 mm) with chromium metal granules (Elementar Americas, Inc.). GC column effluent was directed into the reactor tube through a section of inert silica capillary (0.32 mm ID). The tube and the capillary were joined by a stainless steel 1/16 in. union with polyimide/Valcon ferrules (VICI Valco Instruments). A section of 1 cm of the capillary was inserted into the tube. An additional 2.5 cm section of the alumina tube was left unpacked, followed by a 13 cm section packed with Cr. For reactor settings of 850 °C, the temperature in the unpacked section was as high as 250 °C near the contact with the bed of Cr. The temperature then increased to 800 °C over a distance of 5 cm, while 8 cm of the Cr bed was heated above 800 °C, up to a maximum of 860 °C. The remaining 15.5 cm section of the tube was left unpacked. Of that section, approximately 10 cm resided in the hot reactor furnace (Figure S5 shows a temperature profile of the reactor determined with a thermocouple probe). At the reactor outlet, the flow continued into inert silica capillary connected with another 1/16 in. union. The Cr granules were crushed and sieved to 99.5%. Hydrogen isotope ratios of TCE and cDCE were determined by off-line conversion to hydrogen.34 To assess analytical uncertainty by injection onto GC in split mode, neat TCE and a p-xylene solution of DCE were used. To 1463

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perform an analytical train memory test, TCE was mixed with pentane at a mole ratio of 1:5 and injected into the GC in split mode. This produced a chromatographic peak of pentane with a petroleum-like 2H-depleted hydrogen isotope ratio that was about twice in amplitude as the closely following 2H-enriched peak of TCE. Aqueous standards for purge and trap method were prepared from concentrated methanol stock solutions of DCE and TCE, respectively. Benzene was included in the purge and trap standard to provide a nonhalogenated reference compound. Aliquots of the methanol stock solutions were dissolved in water to produce a secondary stock solution, combining DCE, TCE, and benzene at concentrations of 7.5, 20.5, and 2.0 mg/L, respectively. The concentrations were calculated to deliver identical amounts of hydrogen in each of the three chromatographic peaks. The secondary stock solution was used to make test samples for the CSIA and/or daily standards used for quality control in analysis of samples with CEs of unknown isotope composition (see captions of Figures 3 and S2 for final concentrations of the analytes). To test the final purge and trap−CSIA method configuration, a set of TCE-containing consumer products was used. The same samples were previously analyzed for their carbon and chlorine isotope ratios.15 The samples were prepared for purge and trap as discussed previously,15 except for increasing the TCE concentrations in the aqueous sample to 150 μg/L. Reporting of Isotope Ratios. All data are reported in delta notation, where δ2H = (2H/1Hsample − 2 H/1Hstandard)/2H/1Hstandard. Note that the commonly included multiplier of 103 is omitted from the equation, for consistency with IUPAC recommendations.35

Figure 4. Mass 2 chromatogram and 3/2 ratio of a DCE, benzene, and TCE standard analyzed by purge and trap−CSIA. Analyte concentrations are as in Figure 3.

correlation with increasing analytical bias, the δ2H precision remained unaffected. The interim conclusions based on the hydrogen yields from the three analytes are as follows: (1) H2 yields in Cr reactor conversion are not significantly affected by the stoichiometric ratio of Cl to H; and (2) the yields of H2 appear nearquantitative. However, conversion of chlorinated compounds results in limited production of HCl. The HCl byproduct was detected in the postreactor cryogenic trap condensate after conversion of 0.3 μmoles of TCE. The condensate was thawed and allowed to reach the mass spectrometer. HCl was observed as mass 36 on the Faraday cup that is normally used for monitoring mass 44 in carbon isotope ratio mode. While it was not practical to precisely quantify HCl using an uncalibrated IRMS response, a comparison of the mass 36 peak area with that of known amounts of CO2 suggests that as much as several percent of the analyte hydrogen might have been converted to HCl rather than to H2. It is not clear whether H2 and HCl are produced in competing or sequential reactions. It is possible that partial pyrolysis of CEs to HCl occurs in the lower temperature section at the reactor inlet, prior to the onset of CEs reduction to H2 at the maximum temperature (cf. Figure S5). It can be speculated that HCl may then react to form H2 or the yield of HCl is relatively low to start with. The relatively small contribution from HCl product may be responsible for the analytical bias of the method, discussed in the following section. Major hydrogen isotope fractionation between H2 and HCl was observed in a preliminary study that utilized the CSIA instrument configuration adapted after Chartrand et al.21 (Supporting Information). The kinetics and the spatial distribution of H2 and HCl production would ultimately control the yield of HCl and the analytical bias. A detailed study of the pyrolysis and/or reduction kinetics would be necessary to better understand that issue. Analytical Uncertainty. The values of δ2H of the standard CEs obtained with the Cr reactor were always enriched in 2H relative to the actual isotope compositions of the analytes. Figure 2 shows the results from the direct injection of CEs standards (the purge and trap step was omitted). For the reactor temperatures in the 800−900 °C range, the δ2H bias for daily data subsets varied from +6 to +64‰ for peaks with

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RESULTS AND DISCUSSION Products of Analyte Conversion. Figure 3 shows hydrogen peak areas resulting from conversion of DCE, benzene, and TCE, injected via purge and trap peripheral. Under the assumption that the purge and trap analyte recovery was near 100% and the complete mass of the analyte was directed into the GC column, the resulting H2 peak areas can be interpreted in terms of the efficiency of the conversion of the organic compounds to H 2 . In the present case, the configuration of the purge and trap−GC−IRMS system permitted a complete transfer of the analyte recovered from the sample into the analytical column and then into the Cr reactor (Figure 1) and the instrument was operated in constant flow mode, so that the peak areas of different analytes present in the sample were not selectively affected by the injector discrimination or by postreactor split ratio. The obtained chromatographic peaks were relatively sharp, indicating that the adverse effect of the reactor packing does not prevent satisfactory chromatographic performance that is critical in analysis of environmental samples (Figure 4). The H2 peak areas were virtually identical for DCE, benzene, and TCE over the period of reactor operation (Figure 3). The peak area (peak amplitude in volts integrated over peak width in seconds, Vsec) normalized to the mass of hydrogen delivered on the reactor was approximately 0.2 Vsec/ng, a result that compares favorably to the typical hydrogen yields from the same instrument operated with conventional high-temperature pyrolysis reactor. A trend of peak size reduction was observed over time (Figure 3A). It is unclear whether it was caused by the purge and trap or the Cr reactor performance deterioration. While the trend of peak size decrease showed an apparent 1464

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and trap−CSIA method, the linearity remained unaffected for the whole range of peak amplitudes. Memory effects have been reported in the past in several studies utilizing Cr reduction reactors for analysis of δ2H of water. It appears that the memory problem resulted primarily from water retention on active sites of the syringe, instrument tubing, etc., and may bear no direct significance for analysis of CEs that are not adsorbed strongly on surfaces.28 It was suggested that memory may also result from postconversion retention of hydrogen on the reactor.36 A test was conducted to determine the effect of analyzing closely spaced chromatographic peaks with strong contrast in their δ2H values. Such a scenario is particularly relevant to TCE analysis, because its δ2H can be highly enriched as opposed to the typical depleted δ2H values of organic compounds that may be present in the same samples and could pass through the reactor shortly before the TCE peak.18 A sequence of samples of pure TCE (δ2H = +506‰), and the same TCE combined with pentane was injected and the TCE peak eluted 3 min after that of pentane. While the δ2H of pentane was not precisely determined by offline method, it was approximately −150‰ (uncalibrated values from the instrument), i.e., the difference between pentane and TCE was approximately 650‰. No significant effect could be attributed to the presence of the depleted hydrogen peak preceding TCE (Figure S4). There are practical implications of the significant bias observed. Daily fluctuations of the accuracy have to be accounted for by bracketing the samples with runs of calibration standard (a standard prepared with the target analytes with known isotope ratios). The presence of outliers (for example, in Figure 3, one of the DCE results for day 8 exceeded the ±10‰ error limit) suggests that occasionally, a bracketing standard may be compromised. Two approaches for bracketing frequency and number of standards per bracket are proposed. One option is to increase the number of standard runs at the beginning of each daily sequence, to two or even three, to eliminate a chance of a single standard being an outlier. This approach, however, will result with significant reduction of time available for analysis of samples. The alternative is to rely on a single standard run to open and close a bracket, with a contingency of reanalyzing the samples from any problematic bracket. An excessive difference between the δ2H values in the opening and the closing standard runs would imply that one of the standards is compromised. The latter approach may be more efficient, since the probability of outliers is low. The more stringent approach with multiple standards per bracket may be required if sample availability is limited and reanalysis is not practical. A minimum of three standard runs is recommended for any sample sequence. Analysis of samples in duplicate is recommended to reduce the potential of a single sample suffering from excessive analytical error. It is also recommended to adjust the amplitudes of target analyte peaks by dilution to match those of the calibration standard peaks and thus avoid a problem of poor linearity (this is the conservative approach, while a more optimistic conclusion on the linearity could be drawn from Figures S2 and S3). One element of the performance that was not experimentally tested was the accuracy for a wider range of isotope ratios. While both CEs used as standards have strongly positive values of δ2H, the isotope composition of environmental samples can be expected to span a wide range from strongly positive to strongly negative values of δ2H. The latter can be expected in

amplitudes ranging from 1000 to 2000 mV to as much as +95‰ for a set of peaks with amplitudes ranging from 300 to 400 mV (Figure 2). The magnitude of bias varied on daily basis. Interestingly, the “heaviest” δ2H values were obtained for the lower loads of TCE injected into the reactor (Figure 2G). That data subset was obtained by injecting TCE at split ratio of 120, while the remaining subsets were injected at split ratio of 40. It is unlikely that increasing the split ratio from 40 to 120 would introduce isotope fractionation of +30‰, to explain the “heavy” values of data in Figure 2G. Moreover, the differences between the remaining data subsets (spanning approximately 60‰) cannot be attributed to the injection technique (it was a constant). This leads to a conclusion that the changes of the analytical bias resulted from corresponding fluctuations of the Cr reactor performance and that data obtained over longer time periods would likely require a bias calibration at daily time resolution. The protocol for such calibration is discussed below. Further tests were performed to ensure that there was no systematic linearity effect leading to 2H enrichment at lower CEs loadings, in direct injection and analysis of aqueous CEs by purge and trap−CSIA (Figures S2 and S3). The results from those linearity tests are discussed below. The analytical precision of δ2H for samples analyzed within a single day was generally within ±10‰ or somewhat larger, depending on the data subset considered and outliers with excessive analytical error were occasionally present (Figures 2 and S2−S4). In comparison with the typical uncertainty of ±5‰ for H CSIA for nonchlorinated hydrocarbons, 1 the analytical uncertainty for TCE and DCE for the majority of the samples is larger. While the analytical bias can be corrected for by utilization of daily calibration standards, the precision bracket remains approximately two times as large as for CSIA of nonchlorinated hydrocarbons. The results for the instrumental configuration without the purge and trap stage reflect primarily the performance of the Cr reactor and can be used as a benchmark for evaluation of the performance of the more complex configuration with added potential uncertainties of sample extraction and desorption in the purge and trap process. Figure 3 shows data from a sequence of standards analyzed by purge and trap−GC−IRMS. The range of analytical bias and the apparent δ2H precision are within the ranges observed for the direct injection data. Longterm trends of δ2H accuracy for TCE and DCE are relatively well-correlated with each other and the net magnitudes for the bias for the two compounds were similar within several ‰ units. On the other hand, the bias of the δ2H values for benzene for the same series of samples is generally lower and poorly correlated with that for CEs. The fluctuations of the analytical bias were significant (similar to or larger in magnitude than the analytical precision) for time intervals of several days or longer, but, similarly as in the case of direct injection runs, were negligible for data acquired within given daily sequence. To assess the linearity of the purge and trap−CSIA method, a series of samples was analyzed at varying concentrations (Figure S2). Unlike the results from direct injection samples (cf. Figure 2), no additional 2H enrichment was observed for the low end of the analyte mass range. The linearity was also verified by analysis of TCE and DCE headspace, to eliminate the potential of isotope fractionation occurring during the purge and trap process (Figure S3). Similarly as for the purge 1465

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CEs formed as dechlorination sequence intermediates.18 The lack of performance data for 2H-depleted CEs implies that extrapolation of the accuracy data from this study to such 2Hdepleted CEs is conditional. Ideally, the δ2H values of the bracketing standard used to calibrate CE sample results and of the calibrated samples should be as close as possible to remove the effect of imperfect linearity in conversion to H2 or in determination of 2H/1H by the IRMS.37 To date, high-purity specimens of 2H-depleted TCE or DCE, suitable for development of a depleted 2H standard, have not been identified. If such standards become available and the accuracy for depleted CEs is found to be an issue, routine standard bracketing will have to include both the depleted and the enriched reference compounds. Analysis of TCE in Commercial Products and Implications to Environmental Studies of CEs. To illustrate the performance of the purge and trap−GC−IRMS with a Cr reactor, a set of retail products containing trichloroethylene was analyzed (Figure 5). Out of the six

Apart from using the H CSIA data in TCE provenance studies, H CSIA may potentially supplement the more established C and Cl CSIA data in identification of the effects of degradation processes in environmental samples. In the majority of proposed reaction pathways, including reductive dechlorination,38,39 biological cometabolic oxidation,40 and abiotic oxidation by permanganate,41 the initial slow reaction step that is responsible for the observed isotope effect does not directly involve C−H bond(s) of the CE reactant. However, secondary7 hydrogen isotope effects are likely to be present at measurable magnitude, with diagnostic potential for mechanism discrimination.

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ASSOCIATED CONTENT

S Supporting Information *

Cr reactor performance for a range of temperatures and carrier gas flow rates (Figure S1). Linearity of δ2H determination by purge and trap−GC−IRMS for varying analyte concentrations (Figure S2) and linearity determination by headspace injection−GC−IRMS (Figure S3). Reactor memory test (Figure S4); temperature provide of the Cr reactor (Figure S5) and a summary of a preliminary study assessing H CSIA of TCE with the utilization of a conventional pyrolysis (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone 405-325-3253; fax 405-3253140. Notes

Figure 5. Carbon and hydrogen isotope ratios of miscellaneous lots of manufactured TCE. Data from this study (+) and after Shouakar-Stash et al.18 (○) are shown. Carbon isotope ratios for the samples analyzed as part of the present study reported after McHugh et al.15 See the same reference for additional description of the TCE samples.

The authors declare no competing financial interest.

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REFERENCES

(1) U.S. EPA. Guide for Assessing Biodegradation and Source Identification of Organic Ground Water Contaminants using Compound Specific Isotope Analysis (CSIA); National Risk Management Research Laboratory: Ada, OK, 2008. (2) Sherwood Lollar, B.; Slater, G. F.; Sleep, B.; Witt, M.; Klecka, G. M.; Harkness, M.; Spivack, J. Stable Carbon Isotope Evidence for Intrinsic Bioremediation of Tetrachloroethene and Trichloroethene at Area 6, Dover Air Force Base. Environ. Sci. Technol. 2001, 35 (2), 261− 269. (3) Hunkeler, D.; Aravena, R.; Parker, B. L.; Cherry, J. A.; Diao, X. Monitoring oxidation of chlorinated ethenes by permanganate in groundwater using stable isotopes: Laboratory and field studies. Environ. Sci. Technol. 2003, 37 (4), 798−804. (4) Richnow, H. H.; Annweiler, E.; Michaelis, W.; Meckenstock, R. U. Microbial in Situ Degradation of Aromatic Hydrocarbons in a Contaminated Aquifer Monitored by Carbon Isotope Fractionation. J. Contam. Hydrol. 2003, 65, 101−120. (5) Kuder, T.; Wilson, J.; Kaiser, P.; Kolhatkar, R.; Philp, P.; Allen, J. Enrichment of Stable Carbon and Hydrogen Isotopes during Anaerobic Biodegradation of MTBE: Microcosm and Field Evidence. Environ. Sci. Technol. 2005, 39, 213−220. (6) Hirschorn, S. K.; Grostern, A.; Lacrampe-Couloume, G.; Edwards, E. A.; MacKinnon, L.; Repta, C.; Major, D. W.; Sherwood Lollar, B. Quantification of biotransformation of chlorinated hydrocarbons in a biostimulation study: Added value via stable carbon isotope analysis. J. Contam. Hydrol. 2007, 94 (3−4), 249−260. (7) Elsner, M.; Zwank, L.; Hunkeler, D.; Schwarzenbach, R. P. A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants. Environ. Sci. Technol. 2005, 39 (18), 6896−6916.

products analyzed, three showed strongly enriched δ2H values (+533 to +682‰), while the other three (three different lots of the same product) showed depleted values (−139 to −184‰). The depleted results are potentially affected by the uncertainty over the accuracy for depleted CEs, as discussed in the preceding paragraph, but their strong contrast versus the typical enriched values is nevertheless evident. Five samples of the set of six were analyzed in duplicate and the results were within the ±10‰ range or better. The data set offers an interesting comparison with isotope ratios of TCE that were published elsewhere. The highly enriched values of δ2H are relatively similar to the values observed in the past for several TCE products.17−19 Based on the limited data set available, it was suggested that manufactured TCE and TCE produced in dechlorination of tetrachloroethylene can be readily differentiated by their hydrogen isotope ratios, with the δ2H of the former showing major 2H enrichment, and the δ2H of the latter showing 2H depletion, reflecting incorporation of depleted hydrogen during dechlorination process.18 Identification of TCE products with depleted δ2H values complicates that scheme of data interpretation. While a major part of TCE releases may be indeed associated with strongly positive values of δ2H, detection of depleted δ2H values of TCE should not be taken as a sufficient evidence of TCE forming in situ in dechlorination process. 1466

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dx.doi.org/10.1021/es303476v | Environ. Sci. Technol. 2013, 47, 1461−1467