Ion Chromatography
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Ion chromatography (IC) is an instrumental technique that can be used to detect anions (negatively charged atoms or molecules such as Cl–) and cations (positively charged species such as Na+). IC has been applied in forensic science for the analysis of gunshot residue (GSR) and explosives. The ions of interest include ammonium (NH4+), nitrate (NO3–), and chlorate (ClO4–), species that are often detected using color change or presumptive tests. The advantages of IC in these cases include specificity (presumptive tests are subject to false positives and false negatives) and increased sensitivity, down to the part-per-billion (ppb) range. A part per billion is 1 microgram (μmg) per liter of water, and a microgram is 1/1,000,000 of a gram.
IC instrumentation works on the basis of ion exchange, the same principle used in water softeners. To perform an ion exchange, the sample is forced through a column bed that contains an exchange resin. In a water softener, the goal of ion exchange is to remedy hard water problems by removing the calcium and magnesium ions (Ca2+, Mg2+). To do this, the water supply is directed over a bed charged with sodium ions (Na+). In the column, the calcium and magnesium will displace the sodiums at a 1:2 ratio to maintain charge balance. For every magnesium or calcium ion that is removed by the resin, two Na+ cations will be released into solution. Water hardness is removed by trapping calcium and magnesium in the bed, but as a result, soft water has a higher concentration of sodium, which can present problems to people with high blood pressure. Thus, ion exchange does not remove ions from solution, it only replaces one ion with others of the same overall charge.
There are two modes of ion exchange used in ion chromatographs, single ion exchange and suppression ion exchange. Single ion exchange works very much like a water softener, using resins that are specific for different cations, depending on what is of interest to the analyst. Suppression ion systems are used to reduce the ion concentration of the solution using an additional step. An example of ion suppression would be to create a column that replaces cations (positively charged ions) with the H+ ion and anions (negatively charged ions) with the OH– ion. When these two ions combine, they form water (H2O), which is not ionic. Ion exchange still occurs, but by careful selection of resins, ions combine to form covalently bonded molecules (also called molecular compounds) that do not form ions in water. Regardless of the type of ion exchange column used, the detection system is most often a conductivity detector that relates concentrations of ions to the conductivity of the solution.
