Stable Isotopes and Anionic Isotopomers as Target Analytes Stable isotopes are naturally-occurring and non-radioactive isotopes of commonly known elements, found as a result of variation in the number of neutrons in an atom’s nucleus. For example, the element chlorine is found in nature as 2 stable isotopes: ³⁷Cl and ³⁵Cl (i.e., chlorine atom with an atomic mass of 37 and an atomic mass of 35). Chlorine’s official atomic mass of 35.5 is an average of the 2 stable isotopes weighted by their relative abundance on the planet. Table 1 below, lists some of the common elemental stable isotopes and their relative abundance on Earth.  

Table 1. Stable Isotopes found in Common Anions and their Relative Abundance.

* Note: Some elements have more >1 heavy stable isotope; only the most abundant one shown.

When atoms form molecules a number of “isotopomers” can result owing to the various combinations of stable isotopic elements. For example, the gas chlorine or Cl2 can have 3 potential isotopomers (i.e.,³⁵Cl³⁵Cl, ³⁵Cl³⁷Cl, and ³⁷Cl³⁷Cl). Similarly, the gas methane (CH4) can have 10 possible isotopomers based on the stable isotopes of carbon (¹²C and ¹³C) and hydrogen (¹H and ²H). Isotopic ratios for atoms in a specific compound can vary as a result of source materials used in its synthesis or because of kinetic effects during compound transformation. The so-called kinetic isotope effect arises during a chemical transformation process, when molecules containing the lighter isotope react at a slightly faster rate than those containing the heavier isotope. Biological transformation processes often display kinetic effects due to the specificity of enzymatic catalysis. In environmental applications, a kinetic isotope effect is evident during compound biodegradation and transformation due to the preferential reaction of compounds composed of lighter isotopes. Hence, the parent compound becomes relatively more enriched in heavy isotopes while transformation products are depleted in heavy isotopes, resulting in fractionation of isotopes. Physical processes such as evaporation and sorption also exhibit fractionation; however, these processes are often too subtle to be detected on short time scales.   

Compound-Specific Stable Isotope Analysis (CSIA) of Anions as a Fingerprint of Natural Water Bodies and Aqueous Wastestreams Conventional CSIA involves the initial separation of chemical compounds using traditional separation methods (most often gas chromatography [GC]), followed by conversion of the separated target compound to an easily measurable surrogate compound (e.g., CO2 for ¹³C/¹²C measurements) in an inline interface or reaction chamber. Finally, the abundance of stable isotopes of the surrogate isotopomers is measured, typically, by isotope ratio mass spectrometry (IRMS) ). A GC separation is favored in CSIA because of the high vacuum, and therefore, low fluid loading requirements of the IRMS. GC produces a lower carrier fluid loading than a liquid chromatograph. Unfortunately, many ionic compounds-of-interest in the environment, such as nitrate (NO3^-), perchlorate (ClO4^-), and borate (B(OH4)^-) are non-volatile and quite soluble in water. Furthermore, unlike the combustion of organic carbon to surrogate CO2 gas, the reaction of relatively recalcitrant inorganic anions to form surrogate compounds cannot be easily achieved. Usually, a complex interface is required to chemically or biologically react the anion and separate it from co-products of the reaction even when the ionic salt is available in purified form. Therefore, CSIA of ions poses significant challenges that require innovative methods and approaches.

 

Applications

 

We foresee 3 or more important applications of the analytical techniques described above. These include:  

1. Natural attenuation of nitrate and perchlorate in natural water systems.
2. Fingerprinting of tracers in RO desalinated waters (with Dr. Arafat).
3. Brine fingerprinting and mixing models for discharge outfall location.