Compound-Specific Isotope Analysis (CSIA)
CSIA is an analytical method that measures the ratios of naturally occurring stable isotopes in environmental samples. CSIA can be used to gain information about potential contaminant sources, the extent of degradation, comingling of contaminant plumes, and the origins of some chemicals. This latter application is often valuable for investigating disputed environmental contamination, a field known as environmental forensics.
CSIA works because each element in a compound has a distinct isotopic ratio. That is, every contaminant is made of atoms of various elements. The isotopes of a given element (e.g., carbon, hydrogen, chlorine) have the same number of protons and electrons but a different number of neutrons and thus different atomic mass. Each element has a most-abundant isotope (for example, 12C, or ńcarbon-twelve,î for carbon) and one or more less-abundant isotopes (such as13C, or ńcarbon-thirteen,î for carbon). The less-abundant isotopes are sometimes heavier (i.e., contain one or more extra neutrons). The significant difference between these stable yet heavier, less-abundant isotopes is the increased mass. The ratio between isotopes is, in this example, the ratio between 12C and 13C. For a given element the isotopic ratio is known to within a few percent.
However, that ratio can change in a systematic way during the course of biodegradation and abiotic degradation. CSIA measures these small changes in isotopic ratios. Important information about the source, transport, and fate of a compound can be gained from these changes.
As the mass is increased in a compound, the process is called ńisotopic fractionation.î Isotopic fractionation occurs because it takes slightly less energy to break a bond between a light isotope and another atom than it takes to break a bond between a heavy isotope and that same atom. In a simple example, it takes less energy to break the bond between carbon 12 and hydrogen 1 than it does to break the bond between carbon 13 and hydrogen 1. Thus, over time the percentage of carbon 13 compared to carbon increases in the residual contaminant pool. CSIA measures these changes in isotopic ratio.
Since the isotopic ratio in the compound is a function of the starting material and the manufacturing process as well as the degradation of that compound after it was made, CSIA has applications in environmental forensics, biodegradation, and abiotic degradation. For example, isotopes of carbon and hydrogen are often used to detect changes (through biotic and degradation) in or the source of MTBE. In another example, isotopes of chlorine and two isotopic ratios of oxygen are often used for perchlorate.
Physical processes such as dilution, diffusion, and volatilization do not change the isotopic ratios in compounds to the same extent as chemical or biochemical processes such as degradation. For volatile organic compounds (VOCs) in groundwater, this means that degradation of a compound is, by far, the major cause of significant changes in isotopic ratios. This change in isotopic ratio happens in both biological and abiotic reactions.
Limitations and Concerns
´ Only a limited number of laboratories provide CSIA services.
‡ Though CSIA may theoretically be the way to answer a question at a site, specific isotopic analyses may not be available in some labs. Collaboration with academic laboratories is an alternative.
´ Because of the large number of compounds in petroleum products, there is the potential for interference at petroleum-release sites (as well as other sites with many compounds in the groundwater), especially near the contaminant source.
‡ CSIA is often used as a line of evidence to demonstrate that biodegradation is occurring at a site. While this may provide helpful information in determining whether Monitored Natural Attenuation (MNA) is appropriate, CSIA analysis must be coupled with other lines of evidence to be conclusive.
‡ It is important that a carefully structured analysis takes place in order to make determinations related to the origins of a contaminant. We have seen CSIA used inappropriately to rule out a partyÍs responsibility.
´ In some compounds, fractionation may be so minimal that little or no isotopic enrichment is detected. For example, CSIA of either carbon or hydrogen is unlikely to provide valuable information for many high-molecular-weight polynuclear aromatic hydrocarbons during degradation.
CSIA is used to monitor and assess biological and abiotic degradation of VOCs, especially TCE and PCE. It can also be used to assess the origin of VOCs where there are many potential sources; to assess whether nitrate is from runoff or natural sources; to assess whether methane is of thermogenic origin (e.g., thermogenic methane is formed when organic matter is exposed to extreme heat and pressure deep within the ground) or biogenic origin (biogenic methane is formed from the degradation, by reduction or fermentation, of organic compounds); and to assess whether perchlorate is from a natural source (e.g., Chilean fertilizer), or synthetic perchlorate used in explosives and rocket fuel. It can also be used to distinguish the signature isotopes of mixed plumes, including gasoline and MTBE.
Technology Development Status
The technology is commercialized and several labs are available to do this analysis. This technique is in the early stages of development for discriminating between indoor air contaminants that emanate from soil vapor and those from commercial products used in the home.
Other Resources and Demonstrations
See http://www.clu-in.org/download/contaminantfocus/vi/VI-ER-201025-TR.pdf and http://www.serdp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-201025 for efforts to distinguish between vapor intrusion and indoor sources of volatile organic compounds (VOCs). An Air Force funded project developed a new method for application of CSIA for this purpose. The application required development and validation of an adsorbent sampler method (similar to US EPA Method TO-17) to obtain a sufficient sample. The new sampling method was applied to five residences near Hill Air Force Base (AFB), UT, with potential PCE or TCE vapor intrusion concerns. At two of the residences, the CSIA results indicated an indoor source of PCE, while at two other residences the CSIA results confirmed TCE vapor intrusion. The results for the fifth residence were not definitive. The project results confirmed that subsurface and indoor sources of TCE and PCE often exhibit distinct carbon and chlorine isotope ratios and that CSIA can be used to identify the source(s) of these chemicals when they are present in indoor air.
See http://www.ciaaw.org/ for periodic table of isotopes.
See http://www.wspenvironmental.com/media/docs/newsroom/In_Situ_White_Paper_Matt_Burns.pdf for analysis of In-Situ Chemical Oxidation (ISCO) performance monitoring.
See http://www.clu-in.org/download/newsltrs/tnandt0210.pdf for a write up of several applications of CSIA.