Compound Specific Isotope Analysis (CSIA)

From Enviro Wiki
Jump to: navigation, search

Compound specific isotope analysis (CSIA) usually refers to the measurement of the isotope ratios (typically, the stable isotope ratios of carbon, hydrogen, oxygen, nitrogen, sulfur or chlorine) of individual volatile and semi-volatile compounds extracted from complex environmental mixtures. These compounds (or their derivatization products) are generally separable by conventional gas or liquid chromatography and amenable to high-temperature oxidation or pyrolysis to produce simple gases (H2, CO2, CO, N2, O2, SO2, CH3Cl) suitable for analysis by a gas-source isotope-ratio mass spectrometer. The CSIA methodology provides a quantitative means to differentiate reaction pathways for abiotic and biotic degradation, including different pathways of biodegradation, and may serve as a basis for identification of distinct pollutant sources. CSIA can also provide a powerful line of evidence for monitored natural attenuation of sites contaminated with a wide variety of common organic pollutants including petroleum hydrocarbons, fuel oxygenates, BTEX, chlorinated solvents, and nitroaromatics.

Related Article(s):

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

Key Resource(s):


Compound Specific Isotope Analysis (CSIA) refers to the measurement of the isotope ratios (typically carbon, hydrogen, oxygen, nitrogen, sulfur or chlorine) of individual organic compounds extracted from complex environmental mixtures. The approach was developed initially 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[3][4] 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, which lowered detection limits by up to 5 orders of magnitude and reduced analytical and sample preparation time. This continuous flow technique allowed CSIA to become widely applied by providing the ability, with instrumentation that became commercially available around 1990, to measure and compare stable isotope ratios for compounds of environmental concern at various spatiotemporal scales in environmental chemistry, biogeochemistry, and contaminant hydrogeology.

The CSIA Method

The first isotope ratio to which CSIA was widely applied was that of carbon’s stable isotopes. 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 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 standard reference materials and expressed in delta notation (e.g. δ13C), in units of permil (parts per thousand or per mille). Isotopic standard reference materials for stable isotope ratio measurements of carbon and other light elements (H, N, O, S, Cl and others) have been administered since the 1950s through several sources including the International Atomic Energy Agency, the National Institute of Standards and Technology and the U.S. Geological Survey. All stable isotope laboratories worldwide must report their isotopic data normalized to the appropriate standard reference materials to ensure global consistency and inter-comparability of results[1][5].

Applications to Environmental Remediation and Restoration – Forensics

Both naturally sourced contaminants (e.g., petroleum hydrocarbons) and man-made industrial organic contaminants (e.g., organochlorides, chlorofluorocarbons (CFCs), nitroaromatics, and gasoline additives such as methyl tert-butyl ether (MTBE)) can have distinct isotopic compositions due to differences in the raw materials and industrial synthesis processes used in their manufacture. This provides the basis for using CSIA as a forensic science tool[6]. Different contamination sources or different spills from the same source may have distinct isotopic compositions that can be used to apportion responsibility in a mixed contaminant plume, such as in a situation where there is off-site migration and impact. In many cases, knowledge of the isotopic composition of the initial spill material is not available, but the forensic utility of the isotope ratios does not depend on that precondition. Distinguishing between potential source areas at a site can be done by comparing isotopic data from different areas of the site in the context of the geologic and hydrogeologic conceptual models to test source zone apportionment [1]. Although there may be significant overlap in isotopic compositions, successful forensic applications have taken advantage of the additional constraints afforded by coupling carbon isotope ratio measurements with other isotopes (usually hydrogen, nitrogen and/or chlorine isotope ratios)[1][7][8][9][10][11][12][13][14][15].

Quantifying and Monitoring Remediation Processes

Abiotic and biotic degradation reactions that transform contaminant compounds in the environment generally affect their isotopic compositions in systematic ways. This overall process is called isotope fractionation and is the key to CSIA providing insight and information on transformation pathways[16][2]. 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 degradative processes in which bonds are broken[1]. This results from the kinetic isotope effect and the fact that bonds involving a heavy stable isotope (e.g. 13C-12C bond) have a lower zero-point energy and a larger activation energy than bonds containing exclusively light isotopes (e.g. 12C-12C bond). Effectively this means that the rate of transformation of molecules containing exclusively light isotopes at the reactive site is faster than the rate of transformation of compounds containing a heavy isotope at the reactive site [16][2].

CSIA Signals of Transformation and Remediation

The net outcome of isotope fractionation is that a contaminant that has been undergoing degradation can provide a significant signal of transformation, as its isotopic composition generally (but not always) increases to heavier values as a function of reaction progress[17][18][19][2]. The obvious corollary is that the products of degradation will generally be preferentially enriched in the lighter isotopes relative to the instantaneous isotopic composition of 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, sulfur, and chlorine as well. The largest isotope effects have always been found at the reactive sites where bonds are broken or formed.

Laboratory experiments have shown that not only does fractionation during transformation provide a strong signal of degradation, but also that the signal is highly reproducible[1]. For many organic contaminants of interest, the relationship between the change in isotopic composition and the degree of degradation is governed by a quantitative relationship – the Rayleigh equation[20]. Specifically, for a given compound and degradation pathway or mechanism, the measured difference in isotopic composition can be quantitatively related to the extent of transformation (e.g. fraction or percentage of contaminant remaining) 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 ratio of the compound and f is the fraction of contaminant remaining where f = 1 at t = 0 and decreases to f = 0 when the reactant compound is fully transformed to products. The stable isotope fractionation factor (α) is defined as:

Equation 2:      α = (1000 + δ13Ca)/(1000 + δ13Cb)

where 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 the compound in a downgradient well for instance.

Equation 1 can be rearranged to produce Equation 3[1]:

Equation 3:      f = e(δ13Cgroundwater - δ13Csource) / ε

where δ13Cgroundwater is the measure of the isotope ratio in the organic contaminant in the sample of groundwater, δ13Csource is the isotopic ratio in the un-fractionated organic contaminant before biodegradation has occurred, and epsilon (ε) is the stable isotope enrichment factor, defined as:

Equation 4:      ε = (α -1) * 1000

Implications for Remediation

The quantitative relationships controlling isotope fractionation during transformation enable CSIA to provide a diagnostic signal that transformation is taking place, as well as 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[21]. This is particularly advantageous for field studies since changes in contaminant concentration result not only from transformation processes, but from physical transport and dispersal. For this reason, decreasing concentrations of contaminants alone are insufficient evidence that a site is undergoing transformation towards clean-up goals[22][23]. In contrast, as physical transport and dispersal processes without concomitant degradation produce negligible changes in isotopic composition, any significant isotope effects measured in the contaminants of concern provide a direct line of evidence that transformation is occurring, and also yields independent quantification of the rate of the transformation[24][21][25].

CSIA provides additional value to environmental investigation and remediation in that the degree of fractionation is reaction specific, giving researchers the ability to pinpoint which of a variety of possible degradation mechanisms may be dominating at a contaminated site. 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 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 biodegradation pathways is operative at a site – information that can be critical to optimizing a remediation strategy[26][27]. In other examples, CSIA has been a critical tool in deciphering the biodegradation potential and remediation mechanisms for benzene[7][28][29][30], methyl tert-butyl ether (MTBE)[11][31][32][10] and other priority pollutants. In related applications, where abiotic and biotic transformations of a compound occur via different pathways and mechanisms, CSIA can differentiate between the relative contributions of chemical versus biological transformations[33][34].

Not all transformation processes necessarily result in measurable isotopic fractionation. Fractionation factors can be small simply due to isotope dilution, e.g., where a C isotope effect occurs at a single reactive site in a molecule that has many carbon atoms. In such cases there is need for the development of models to disambiguate intrinsic versus apparent kinetic isotope effects [35], for use of multi-isotope analysis[14], and for the use of novel techniques such as 13C NMR that can allow position-specific isotope analysis[36][37][38]. The presence of additional rate-limiting steps in the transformation reaction can complicate the measurement of isotope fractionation in ways that may obscure the extent of transformation, yet may also yield other important information about transport effects[39] or the efficiency of the enzymes involved in biodegradation[40][41].


Applications of CSIA are dependent on background information about the isotope fractionation factors associated with specific chemical reactions or biodegradation pathways. While fractionation can be calculated ab initio (from the beginning) by using molecular modeling methods, 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[42][1][43].


  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. Report pdf
  2. ^ 2.0 2.1 2.2 2.3 Elsner, M., 2010. Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations. Journal of Environmental Monitoring, 12(11), pp.2005-2031. doi: 10.1039/C0EM00277A
  3. ^ 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), 573-583. doi: 10.1016/0146-6380(94)90003-5
  4. ^ Brand, W.A., 1996. High precision isotope ratio monitoring techniques in mass spectrometry. Journal of Mass Spectrometry, 31(3), 225-235. <225::aid-jms319>;2-l doi: 10.1002/(SICI)1096-9888(199603)31:3<225::AID-JMS319>3.0.CO;2-L
  5. ^ Coplen, T.B., 2011. Guidelines and recommended terms for expression of stable‐isotope‐ratio and gas‐ratio measurement results. Rapid Communications in Mass Spectrometry, 25(17), pp.2538-2560. doi: 10.1002/rcm.5129
  6. ^ Mancini, S.A., Lacrampe-Couloume, G. and Sherwood Lollar, B., 2008. Source differentiation for benzene and chlorobenzene groundwater contamination: A field application of stable carbon and hydrogen isotope analyses. Environmental Forensics, 9(2-3), 177-186. doi: 10.1080/15275920802119086
  7. ^ 7.0 7.1 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), 3462-3467. doi: 10.1021/es0105111
  8. ^ Mancini, S.A., Ulrich, A.C., Lacrampe-Couloume, G., Sleep, B., Edwards, E.A. and Sherwood Lollar, B., 2003. Carbon and hydrogen isotopic fractionation during anaerobic biodegradation of benzene. Applied and Environmental Microbiology, 69(1), 191-198. doi: 10.1128/AEM.69.1.191-198.2003
  9. ^ 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), 211-228. doi: 10.1016/S0169-7722(02)00085-2
  10. ^ 10.0 10.1 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), 213-220. doi: 10.1021/es040420e
  11. ^ 11.0 11.1 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), 1018-1029. doi: 10.1021/es049650j
  12. ^ Sessions, A.L., 2006. Isotope‐ratio detection for gas chromatography. Journal of Separation Science, 29(12), 1946-1961. doi: 10.1002/jssc.200600002
  13. ^ Hartenbach, A., Hofstetter, T. B., Berg, M., Bolotin, J., Schwarzenbach, R. P., 2006. Using nitrogen isotope fractionation to assess abiotic reduction of nitroaromatic compounds. Environmental Science & Technology, 40(24), 7710-7719. doi: 10.1021/es061074z
  14. ^ 14.0 14.1 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), 8399-8405. doi 10.1021/ac071420a
  15. ^ 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), 7757-7763. doi: 10.1021/es800534h
  16. ^ 16.0 16.1 Faure, G. and Mensing, T.M., 2005. Isotopes: principles and applications. John Wiley & Sons Inc.
  17. ^ 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), 409-414. doi: 10.1046/j.1462-2920.1999.00050.x
  18. ^ 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), 2733-2738. doi: 10.1021/es981282u
  19. ^ Sherwood Lollar, B., 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), 813-820. doi: 10.1016/S0146-6380(99)00064-9
  20. ^ 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), 413-430. doi: 10.1007/BF02374138
  21. ^ 21.0 21.1 Morrill, P.L., Lacrampe-Couloume, G., Slater, G.F., Sleep, B.E., Edwards, E.A., McMaster, M.L., Major, D.W. and Sherwood Lollar, B., 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
  22. ^ 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.
  23. ^ 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 groundwater. EPA-600-R-98-128. Report pdf
  24. ^ 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), 261-269. doi: 10.1021/es001227x
  25. ^ McKelvie, J.R., Mackay, D.M., de Sieyes, N.R., Lacrampe-Couloume, G. and Sherwood Lollar, B., 2007. Quantifying MTBE biodegradation in the Vandenberg Air Force Base ethanol release study using stable carbon isotopes. Journal of Contaminant Hydrology, 94(3), 157-165. doi: 10.1016/j.jconhyd.2007.05.008
  26. ^ Hunkeler, D. and Aravena, R., 2000. Evidence of Substantial Carbon Isotope Fractionation among Substrate, Inorganic Carbon, and Biomass during Aerobic Mineralization of 1, 2-Dichloroethane by Xanthobacter autotrophicus. Applied and Environmental Microbiology, 66(11), 4870-4876. doi: 10.1128/AEM.66.11.4870-4876.2000
  27. ^ Hirschorn, S.K., Grostern, A., Lacrampe-Couloume, G., Edwards, E.A., MacKinnon, L., Repta, C., Major, D.W. and Sherwood Lollar, B., 2007. Quantification of biotransformation of chlorinated hydrocarbons in a biostimulation study: Added value via stable carbon isotope analysis. Journal of Contaminant Hydrology, 94(3), 249-260. doi: 10.1016/j.jconhyd.2007.07.001
  28. ^ Mancini, S.A., Lacrampe-Couloume, G., Jonker, H., van Breukelen, B.M., Groen, J., Volkering, F., Sherwood Lollar, B., 2002. Hydrogen isotope enrichment: An indicator of biodegradation at a petroleum hydrocarbon contaminated field site. Environmental Science & Technology, 36(11), 2464-2470. doi: 10.1021/es011253a
  29. ^ Mancini, S.A., Devine, C.E., Elsner, M., Nandi, M.E., Ulrich, A.C., Edwards, E.A. and Sherwood Lollar, B., 2008. Isotopic evidence suggests different initial reaction mechanisms for anaerobic benzene biodegradation. Environmental Science & Technology, 42(22), 8290-8296. doi: 10.1021/es801107g
  30. ^ Fischer, A., Gehre, M., Breitfeld, J., Richnow, H.-H., 2009. Carbon and hydrogen isotope fractionation of benzene during biodegradation under sulphate-reducing conditions: A laboratory to field site approach. Rapid Communications in Mass Spectrometry, 236, 2439-2447. doi:10.1002/rcm.4049
  31. ^ McKelvie, J.R., Hyman, M.R., Elsner, M., Smith, C., Aslett, D.M., Lacrampe-Couloume, G. and Sherwood Lollar, B., 2009. Isotopic fractionation of methyl tert-butyl ether suggests different initial reaction mechanisms during aerobic biodegradation. Environmental Science & Technology, 43(8), 2793-2799. doi: 10.1021/es803307y
  32. ^ 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), 5693-5700. doi: 10.1021/es070531o
  33. ^ 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), 5963-5970. doi: 10.1021/es8001986
  34. ^ Elsner, M., Lacrampe Couloume, G., Mancini, S., Burns, L. and Sherwood Lollar, B., 2010. Carbon isotope analysis to evaluate nanoscale Fe (O) treatment at a chlorohydrocarbon contaminated site. Groundwater Monitoring & Remediation, 30(3), 79-95. doi: 10.1111/j.1745-6592.2010.01294.x
  35. ^ Elsner, M., Zwank, L., Hunkeler, D. and Schwarzenbach, R.P., 2005. A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants. Environmental Science & Technology, 39(18), 6896-6916. doi: 10.1021/es0504587
  36. ^ McKelvie, J.R., Elsner, M., Simpson, A.J., Sherwood Lollar, B., Simpson, M.J., 2010. Quantitative site-specific 2H NMR investigation of MTBE: Potential for investigating contaminant sources and fate. Environmental Science & Technology, 44(3), 1062-1068. doi: 10.1021/es9030276
  37. ^ Julien, M., Parinet, J., Nun, P., Bayle, K., Höhener, P., Robins, R.J. and Remaud, G.S., 2015. Fractionation in position-specific isotope composition during vaporization of environmental pollutants measured with isotope ratio monitoring by 13C nuclear magnetic resonance spectrometry. Environmental Pollution, 205, 299-306. doi: 10.1016/j.envpol.2015.05.047
  38. ^ Gilbert, A., Yamada, K., Suda, K., Ueno, Y. and Yoshida, N., 2016. Measurement of position-specific 13C isotopic composition of propane at the nanomole level. Geochimica et Cosmochimica Acta, 177, 205-216. doi: 10.1016/j.gca.2016.01.017
  39. ^ Nijenhuis, I., Andert, J., Beck, K., Kastner, M., Diekert, G., Richnow, H-H., 2005. Stable isotope fractionation of tetrachloroethene during reductive dechlorination by sulfurospirillum multivorans and desulfitobacterium sp. Strain PCE-S and abiotic reactions with cyanocobalamin. Applied and Environmental Microbiology, 71(7), 3413-3419. doi: 10.1128/AEM.71.7.3413-3419.2005
  40. ^ 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), 7675-7681. doi: 10.1021/es061363n
  41. ^ Sherwood Lollar, B., 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), 7498-7503. doi: 10.1021/es101330r
  42. ^ 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), 215-255. doi: 10.1016/j.jconhyd.2004.06.003
  43. ^ Elsner, M. and Imfeld, G., 2016. Compound-specific isotope analysis (CSIA) of micropollutants in the environment—current developments and future challenges. Current opinion in biotechnology, 41, pp.60-72. doi: 10.1016/j.copbio.2016.04.014

See Also