Analytical Methods for Speciation Analysis
Ion chromatography (IC) is based on the differences in the attraction of the solute ions to the charged sites on the chromatographic column (stationary phase). It provides tremendous separation power and by changing the pH and ionic strength of the eluant (mobile phase), a variety of ionic species can be separated in a single run. The most common detectors for ion chromatography is conductivity and UV spectrophotometry. Even though these detectors are useful for some species, they can not provide enough sensitivity and selectivity especially for metal(loid)s.
Inductively coupled plasma - mass spectrometry (ICP-MS) has been the choice for the determination of total metals and metal(loid)s because it's a mature technique with incredible sensitivity and selectivity. As with every analytical technique, it is prone to interferences but most of these interferences have been well documented. In fact, with the introduction of second generation ICP-MS instruments that are equipped with dynamic reaction cells, almost interference-free analysis at trace levels is becoming reality.
Coupling the separation power of ion chromatography to the detection power of ICP-MS is extremely beneficial for the determination of metals and metalloids. Although, it has been an “old” analytical technique with hundreds of scientific articles in the literature, it has not been utilized in regulatory methods. To our knowledge, the only accepted method that acknowledges and utilizes IC-ICP-MS is ASTM Method D6994-04 for cyanide speciation analysis. This specific method was developed by our senior scientist, Dr. Hakan Gürleyük as part of a joint ASTM and EPA Inter-Laboratory Collaborative for the validation of a Cyanide Speciation Analysis method.
IC-ICP-MS can not only provide detection limits in the range of 1-10 ppt (ng/L) for various analytes, it can also eliminate false positives since most analytes are identified according to their isotopic ratios in addition to retention time matching. Determination of a variety of analytes simultaneously is also possible in some circumstances since many mass-to-charge ratios can be monitored in a single run. In fact, in our laboratory, this feature has been routinely used to monitor interfering species to provide the most accurate results.
In our laboratory, we utilize a Dionex ion chromatography system and couple it to a Perkin Elmer ICP-MS system equipped with a Dynamic Reaction Cell. We have developed various methods for the speciation analysis of a variety of analytes including arsenic, selenium, chromium, metal-cyanide complexes and transition metals. Each of these methods are described in following sections.
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