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Fate of Inorganic Contaminants

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.

One such technique identified and developed only recently for perchlorate ions, is Ion Chromatography combined with Triple Quadrupole Mass Spectroscopy (Figure above). Ion Chromatography is a liquid chromatography technique used to separate ions based on their interaction with the ion-exchange resins. The IC separation of ions is followed by a first high-resolution mass spectrometer (MS), which provides the ion’s characteristic mass spectrum. Then, a collision cell located in-between the 2 quadrupole MS detectors, allows the trapping and further ionization of selected ions in the spectrum. Finally, a second high resolution high resolution MS fingerprints the isotopic abundance of the selected ion fragments. This analysis is known as Multiple Reaction Monitoring (MRM).
The procedure yields mass-to-charge (m/z) ratio peaks of the transition of the monitored analyte ion from one form to another (e.g. ClO4- into ClO3-) and compare it to the isotopic abundance of the atom. For example, in the case of perchlorate, the transition of 35ClO4-,37ClO4- into 35ClO3- and 37ClO3- is used to quantify the main analyte and examine the isotopic abundance ratio of 37Cl/35Cl, and evaluate the internal standard, respectively. Thus, this method is unique because of its analytical capabilities in terms of its very low detection limits and high selective detection.




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.