Compound Specific Isotope Analysis (CSIA)

Compound specific isotope analysis (CSIA) is an analytical method that measures the ratio of stable isotopes (e.g. 13C/12C, 2H/1H, or 37Cl/35Cl) of a contaminant.

Is Contaminant Degradation Occurring?

For some compounds, isotopic ratios change in predictable ways (e.g. isotopic fractionation) as the compound is degraded.  Conversely, physical processes like volatilization and dilution generally do not appreciably shift the isotopic ratios.

  • Therefore, CSIA can potentially provide direct evidence of ongoing contaminant degradation including reductive dechlorination of PCE, TCE, and daughter products.
  • The results can also be used to estimate the extent of contaminant degradation (fraction remaining or fraction degraded).

 

Why Does CSIA Work?

Degradation of some compounds can cause a shift in the isotopic ratios of the parent and daughter products in a process referred to as kinetic isotope fractionation.

Using the ratio of 13C /12C as an example…

  • Chemical bonds formed by the heavier isotope (13C) are slightly stronger than bonds formed by the lighter isotope (12C).
  • Therefore, molecules of the contaminant with the lighter isotope (12C) tend to be degraded more quickly than molecules containing the heavier (13C) isotope.
  • Faster or preferential degradation of molecules with the lighter isotope (12C) means that during degradation, the remaining parent compound becomes enrichened in 13C (increased δ13C) while the daughter compound is initially 13C depleted (decreased δ13C).
  • Ultimately when degradation is complete, the isotope ratio of the final product (e.g. ethene) will equal the initial isotope ratio of the parent compound.

 

Applicable Contaminants

Isotopes Analyzed to Assess Contaminant Degradation

Carbon Isotope Ratio (13C/12C):  Analysis of carbon isotopes is the most frequently used approach to assess degradation of a number common groundwater contaminants including chlorinated ethenes, ethanes, and methanes.

Hydrogen Isotope Ratio (2H/H) and Chlorine Isotope Ratio (37Cl/35Cl):  Two dimensional compound specific isotope analysis (2D-CSIA) is simply the analysis of the isotope ratios of multiple elements (e.g. 13C/12C, 2H/1H, or 37Cl/35Cl).  2D-CSIA is more sensitive than single-element CSIA and should be considered for certain applications