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Compound Specific Isotope Analysis (CSIA) refers to measurement of the isotopic signatures (typically, the stable isotopes of carbon, hydrogen, oxygen, nitrogen or sulfur) of individual compounds from a complex environmental mixture. The approach provides information about source differentiation, a quantitative means to delineate reaction pathways, including biodegradation, and an additional line of evidence for remediation monitoring of sites contaminated with hydrocarbons.


CONTRIBUTOR(S): Dr. Barbara Sherwood Lollar, F.R.S.C.


Key Resource(s):

Introduction

Compound Specific Isotope Analysis (CSIA) refers to measurement of the isotopic signatures (typically, carbon, hydrogen, oxygen, nitrogen or sulfur in an environmental context) of individual compounds from a complex environmental mixture of components. The approach was developed most extensively during the post-WWII era for application to source rock identification and hydrocarbon exploration, and remains a foundation of the oil and gas industry. In the 1980s, John M. Hayes at Indiana University Bloomington and collaborators[2][3] introduced the era of continuous flow compound specific mass spectrometry by interfacing a gas chromatograph via a sample preparatory oxidation system to a stable isotope ratio mass spectrometry system. By thus lowering detection limits by up to 5 orders of magnitude and reducing analytical and sample preparation time from hours to minutes, continuous flow techniques allowed CSIA to become widely applied by providing the ability to measure stable isotope ratios for compounds of environmental concern at various spatiotemporal scales in environmental chemistry, biogeochemistry, and contaminant hydrogeology.

Carbon Isotope CSIA

The element carbon has two stable isotopes, 12C and 13C. Typically occurring under natural conditions in the ratio of 99:1, the small relative differences in the ratio of 13C/12C for a given compound (called stable isotope ratio) provide a wealth of information relevant to the investigation and remediation of contaminated sites and the environment[1]. The measured 13C/12C ratio is normalized with respect to international isotopic standards and expressed in delta notation (e.g. δ13C), in units of permil (parts per thousand or per mille). Isotopic standards have been administered centrally since the 1950s through the International Atomic Energy Agency, and in the U.S. through the National Institute of Standards and Technology, ensuring that all stable isotope laboratories worldwide are cross-calibrated to the same standards ensuring global consistency of results within both public and private laboratories[1].

Applications to Environmental Remediation and Restoration – Forensics

Both naturally sourced contaminants (e.g. petroleum hydrocarbons) and man-made industrial organic contaminants (e.g. organochlorides, CFCs, gasoline additives such as methyl tert-butyl ether (MTBE) ) can have different δ13C values due to differences in mechanisms and source materials used in their synthesis. This provides the basis for using CSIA as a forensic science tool[4]. Both different contamination sources and different spills from the same source may potentially have distinct CSIA signatures that can be applied to attribute responsibility in a mixed contaminant plume, such as in a situation where there is off-site migration and impact. In most cases, knowledge of the initial spill material is not possible, but the approach is not dependent on that precondition. Differentiating different potential “source” areas at a site relative to each other can be achieved by comparing different areas of the site in the context of the geologic and hydrogeologic conceptual models to test source zone apportionment[1]. CSIA does not provide a silver bullet, as there can be significant overlap in carbon isotope signatures, but successful applications have taken advantage of the additional certainty afforded by coupling carbon isotope variations with additional constraints provided by dual isotope (2D) or triple isotope (3D) signatures (usually hydrogen isotope signatures and/or chlorine isotope signatures)[1][5][6][7][8][9][10]. Forensic applications of CSIA can include nitrogen isotope signatures (e.g. nitroanilines and other N-containing compounds Kartenbach et al., 2006[11][12] or other elements relevant to the contaminant of concern.

Quantifying and Monitoring Remediation Processes

Many processes that act on contaminants once they enter the environment can affect the relative abundance of isotopes in the compound of interest and hence the 13C/12C ratio. This effect is called isotope fractionation and is the key to CSIA providing insight and information on those processes[13]. While both degradative (e.g. chemical or biological transformation of contaminant to degradation products) and non-degradative processes (e.g. phase changes such as volatilization, sorption, diffusion) have the potential to result in carbon isotope fractionation, to date the largest fractionation signals are related to processes in which bonds are broken[1]. This results from the kinetic isotope effect and the fact that bonds containing a single heavy stable isotope (e.g. 13C) have a lower zero point energy and corresponding larger activation energy than bonds containing exclusively light isotopes (e.g. 12C). Effectively this means bonds containing a heavy isotope are harder to break, and the rate of transformation then of compounds containing exclusively light isotopes is faster than the rate of transformation of compounds containing a heavy isotope[13].

CSIA Signals of Transformation and Remediation

The net outcome of fractionation is that a contaminant that has been undergoing degradation can provide a dramatic signal of transformation, as its isotopic signature can have a higher 13C/12C ratio than before transformation[14][15][16]. The obvious corollary is that the products of degradation will be preferentially enriched in the light isotopes (lower 13C/12C ratios) than the parent compound from which they are derived. This principle holds for both chemical transformation and biologically mediated transformation reactions, and the principles described above apply to other elements such as hydrogen, nitrogen, oxygen, and chlorine as well.

Laboratory experiments have shown that not only does fractionation during transformation provide a strong signal of degradation, but also that signal is highly reproducible[1]. For many organic contaminants of interest, the relationship between the change in carbon isotope signature and the degree of degradation is governed by a quantitative relationship – the Rayleigh equation[17]. Specifically, for a given compound and degradation pathway or mechanism, the measured difference in carbon isotope signatures can be quantitatively related to a specific degree of transformation (e.g. fraction or percentage of remaining contaminant) by the equation:

Equation 1: Rt = R0 f(α-1)

where Rt is the stable isotope ratio (13C/12C) of the compound at time t, R0 is the initial isotope value of the compound and f is the fraction of remaining contaminant expressed as f = 0. The stable isotope fractionation factor is the factor alpha (α) where α = (1000 + δ13Ca)/(1000+ δ13Cb). Subscripts a and b may represent a compound at time zero (t0) and at a later point (t) in a reaction; or a compound in a source zone, versus a compound in a downgradient well for instance.

Equation 1 can be rearranged to produce Equation 2 (see Hunkeler et al. 2008 for details)[1]:

Equation 2: Lollar-Article 1-Equation 2.PNG

where δ13Cgroundwater is the measure of the isotope ratio in the organic contaminant in the sample of groundwater, where δ13Csource is the isotopic ratio in the un-fractionated organic contaminant before biodegradation has occurred, and epsilon (ε), the stable isotope enrichment factor, is defined as ε = (α-1) * 1000.

Implications for Remediation

The quantitative relationships controlling carbon isotope fractionation during transformation means that CSIA not only provides a signal of whether transformation is taking place, but can provide a quantitative measure of the extent of transformation independent of conventional metrics based on changes in concentration. In some cases, due to signal sensitivity, changes in stable carbon isotope fractionation can be identified in advance of definitive reduction in contaminant concentrations, or before appearance of daughter products, providing an “early warning system” for confirmation of remediation[18]. This is particularly advantageous for field studies since changes in contaminant concentration result not only from transformation processes, but from contaminant transport and dispersal. For this reason, decreasing concentrations of contaminants alone are insufficient evidence that a site is undergoing transformation towards clean-up goals[19][20]. In contrast, as transport and dispersal processes are largely neutral with respect to carbon isotope signals, a carbon isotope enrichment signal in the contaminants of concern provides a direct line of evidence that transformation is occurring and as outlined above, a second independent quantification of transformation rates that can provide constraints on conventional approaches to derive remediation rates and timelines[21][18][22].

CSIA provides additional value to environmental investigation and remediation – the ability to pinpoint which of a variety of possible degradation mechanisms may be dominating at a contaminated site – because the degree of fractionation is reaction specific. A specific example of this is 1,2-dichloroethane, an industrial chemical used in PVC production, production of furniture, upholstery and automobile parts and a common environmental contaminant of concern. Microbial biodegradation of this compound by in the environment is common, but different organisms degrade the compound via different pathways (e.g. involving a C-Cl bond cleavage, or a C-H bond cleavage). As a result, CSIA can be used to positively identify which of the two biodegradation pathways is operative at a site – information that can be critical to optimizing a remediation strategy[23][24]. In other examples, CSIA has been a critical tool in deciphering the biodegradation potential and remediation mechanisms for benzene[5]Mancini et al. 2002[25][26], methyl tert-butyl ether (MTBE) [9][27][28][8] and other priority pollutants. In related applications, where abiotic and biotic transformation of a compound occurs via different pathways and mechanisms, CSIA can contribute to differentiating between the relative contributions of chemical versus biological transformation[29][30].

Not all transformation processes result in significant fractionation. Fractionation factors can be small to non-existent simply due to the decreased significance of fractionation related to one carbon in a molecule with many carbon atoms (naphthalene paper), or due to the highly stable nature of, for instance, carbon atoms in an aromatic ring structure[31]. In such cases the development of models to determine intrinsic versus apparent kinetic isotope effects[32]; use of multi-isotope analysis (2D or 3D)[11] or novel techniques related to position specific isotope analysis targeted to specific individual atoms on the compound McKelvie et al., 2010[33][34] can be applied. The presence of additional rate-limiting steps in the transformation reaction can suppress the observed fractionation in ways which may complicate the above quantification of transformation governing Equation 1, yet yield other important information for instance about transport effects Nijenhuis et al., 2005 or the efficiency of the enzymes involved in biodegradation[35][36].

Conclusions

Applications of CSIA are dependent on background information about the degree of fractionation associated with a specific chemical reaction or biodegradation pathway. While fractionation can be calculated ab initio (from the beginning), fractionation factors are typically empirically derived from laboratory experiments and other approaches. Recent guidance documents and review papers provide an essential resource with database compilations of this knowledge to date[37][1]).

References

  1. ^ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T. C. and Wilson, J. T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. Report pdf
  2. ^ Merritt, D.A., Brand, W.A. and Hayes, J.M., 1994. Isotope-ratio-monitoring gas chromatography-mass spectrometry: methods for isotopic calibration. Organic Geochemistry, 21(6-7), pp.573-583. doi: 10.1016/0146-6380(94)90003-5
  3. ^ Brand, W.A., 1996. High precision isotope ratio monitoring techniques in mass spectrometry. Journal of Mass Spectrometry, 31(3), pp.225-235. <225::aid-jms319>3.0.co;2-l doi: 10.1002/(SICI)1096-9888(199603)31:3<225::AID-JMS319>3.0.CO;2-L
  4. ^ Mancini, S.A., Lacrampe-Couloume, G. and Lollar, B.S., 2008. Source differentiation for benzene and chlorobenzene groundwater contamination: A field application of stable carbon and hydrogen isotope analyses. Environmental Forensics, 9(2-3), pp.177-186. doi: 10.1080/15275920802119086
  5. ^ 5.0 5.1 Hunkeler, D., Andersen, N., Aravena, R., Bernasconi, S.M. and Butler, B.J., 2001. Hydrogen and carbon isotope fractionation during aerobic biodegradation of benzene. Environmental Science & Technology, 35(17), pp.3462-3467. doi: 10.1021/es0105111
  6. ^ Mancini, S.A., Ulrich, A.C., Lacrampe-Couloume, G., Sleep, B., Edwards, E.A. and Lollar, B.S., 2003. Carbon and hydrogen isotopic fractionation during anaerobic biodegradation of benzene. Applied and Environmental Microbiology, 69(1), pp.191-198. doi: 10.1128/AEM.69.1.191-198.2003
  7. ^ Shouakar-Stash, O., Frape, S.K. and Drimmie, R.J., 2003. Stable hydrogen, carbon and chlorine isotope measurements of selected chlorinated organic solvents. Journal of Contaminant Hydrology, 60(3), pp.211-228. doi: 10.1016/S0169-7722(02)00085-2
  8. ^ 8.0 8.1 Kuder, T., Wilson, J.T., Kaiser, P., Kolhatkar, R., Philp, P. and Allen, J., 2005. Enrichment of stable carbon and hydrogen isotopes during anaerobic biodegradation of MTBE: microcosm and field evidence. Environmental Science & Technology, 39(1), pp.213-220. doi: 10.1021/es040420e
  9. ^ 9.0 9.1 Zwank, L., Berg, M., Elsner, M., Schmidt, T.C., Schwarzenbach, R.P. and Haderlein, S.B., 2005. New evaluation scheme for two-dimensional isotope analysis to decipher biodegradation processes: Application to groundwater contamination by MTBE. Environmental Science & Technology, 39(4), pp.1018-1029. doi: 10.1021/es049650j
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  11. ^ 11.0 11.1 Penning, H. and Elsner, M., 2007. Intramolecular carbon and nitrogen isotope analysis by quantitative dry fragmentation of the phenylurea herbicide isoproturon in a combined injector/capillary reactor prior to GC separation. Analytical Chemistry, 79(21), pp.8399-8405. doi 10.1021/ac071420a
  12. ^ Meyer, A.H., Penning, H., Lowag, H. and Elsner, M., 2008. Precise and accurate compound specific carbon and nitrogen isotope analysis of atrazine: critical role of combustion oven conditions. Environmental Science & Technology, 42(21), pp.7757-7763. doi: 10.1021/es800534h
  13. ^ 13.0 13.1 Faure, G. and Mensing, T.M., 2005. Isotopes: principles and applications. John Wiley & Sons Inc.
  14. ^ Meckenstock, R.U., Morasch, B., Warthmann, R., Schink, B., Annweiler, E., Michaelis, W. and Richnow, H.H., 1999. 13C/12C isotope fractionation of aromatic hydrocarbons during microbial degradation. Environmental Microbiology, 1(5), pp.409-414. doi: 10.1046/j.1462-2920.1999.00050.x
  15. ^ Hunkeler, D., Aravena, R. and Butler, B.J., 1999. Monitoring microbial dechlorination of tetrachloroethene (PCE) in groundwater using compound-specific stable carbon isotope ratios: microcosm and field studies. Environmental Science & Technology, 33(16), pp.2733-2738. doi: 10.1021/es981282u
  16. ^ Lollar, B.S., Slater, G.F., Ahad, J., Sleep, B., Spivack, J., Brennan, M. and MacKenzie, P., 1999. Contrasting carbon isotope fractionation during biodegradation of trichloroethylene and toluene: Implications for intrinsic bioremediation. Organic Geochemistry, 30(8), pp.813-820. doi: 10.1016/S0146-6380(99)00064-9
  17. ^ Mariotti, A., Germon, J.C., Hubert, P., Kaiser, P., Letolle, R., Tardieux, A. and Tardieux, P., 1981. Experimental determination of nitrogen kinetic isotope fractionation: some principles; illustration for the denitrification and nitrification processes. Plant and soil, 62(3), pp.413-430. doi: 10.1007/BF02374138
  18. ^ 18.0 18.1 Morrill, P.L., Lacrampe-Couloume, G., Slater, G.F., Sleep, B.E., Edwards, E.A., McMaster, M.L., Major, D.W. and Lollar, B.S., 2005. Quantifying chlorinated ethene degradation during reductive dechlorination at Kelly AFB using stable carbon isotopes. Journal of Contaminant Hydrology, 76(3), pp.279-293. doi: 10.1016/j.jconhyd.2004.11.002
  19. ^ Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N. and Hansen, J.E., 1995. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. U.S. Air Force Center for Environmental Excellence, Technology Transfer Division, Brooks Air Force Base, San Antonio, Texas]
  20. ^ Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., and Chapelle, F.H. 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. Report pdf
  21. ^ Sherwood Lollar, B., Slater, G.F., Sleep, B., Witt, M., Klecka, G.M., Harkness, M. and Spivack, J., 2001. Stable carbon isotope evidence for intrinsic bioremediation of tetrachloroethene and trichloroethene at area 6, Dover Air Force Base. Environmental Science & Technology, 35(2), pp.261-269. doi: 10.1021/es001227x
  22. ^ McKelvie, J.R., Mackay, D.M., de Sieyes, N.R., Lacrampe-Couloume, G. and Lollar, B.S., 2007. Quantifying MTBE biodegradation in the Vandenberg Air Force Base ethanol release study using stable carbon isotopes. Journal of contaminant hydrology, 94(3), pp.157-165. doi: 10.1016/j.jconhyd.2007.05.008
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  24. ^ Hirschorn, S.K., Grostern, A., Lacrampe-Couloume, G., Edwards, E.A., MacKinnon, L., Repta, C., Major, D.W. and Lollar, B.S., 2007. Quantification of biotransformation of chlorinated hydrocarbons in a biostimulation study: Added value via stable carbon isotope analysis. Journal of Contaminant Hydrology, 94(3), pp.249-260. doi: 10.1016/j.jconhyd.2007.07.001
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  28. ^ Elsner, M., McKelvie, J., Lacrampe Couloume, G. and Sherwood Lollar, B., 2007. Insight into methyl tert-butyl ether (MTBE) stable isotope fractionation from abiotic reference experiments. Environmental Science & Technology, 41(16), pp.5693-5700. doi: 10.1021/es070531o
  29. ^ Elsner, M., Chartrand, M., VanStone, N., Lacrampe Couloume, G. and Sherwood Lollar, B., 2008. Identifying abiotic chlorinated ethene degradation: characteristic isotope patterns in reaction products with nanoscale zero-valent iron. Environmental Science & Technology, 42(16), pp.5963-5970. doi: 10.1021/es8001986
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  34. ^ Gilbert, A., Yamada, K., Suda, K., Ueno, Y. and Yoshida, N., 2016. Measurement of position-specific 13 C isotopic composition of propane at the nanomole level. Geochimica et Cosmochimica Acta, 177, pp.205-216. doi: 10.1016/j.gca.2016.01.017
  35. ^ Mancini, S.A., Hirschorn, S.K., Elsner, M., Lacrampe-Couloume, G., Sleep, B.E., Edwards, E.A. and Sherwood Lollar, B., 2006. Effects of trace element concentration on enzyme controlled stable isotope fractionation during aerobic biodegradation of toluene. Environmental Science & Technology, 40(24), pp.7675-7681. doi: 10.1021/es061363n
  36. ^ Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A. and Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), pp.7498-7503. doi: 10.1021/es101330r
  37. ^ Meckenstock, R.U., Morasch, B., Griebler, C. and Richnow, H.H., 2004. Stable isotope fractionation analysis as a tool to monitor biodegradation in contaminated acquifers. Journal of Contaminant Hydrology, 75(3), pp.215-255. doi: 10.1016/j.jconhyd.2004.06.003

See Also

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