Speciation of Specific Elements

Some metals are of particular interest in environmental samples, and specific methods for these have been developed. Arsenic, chromium, and mercury are all important in this respect, having very differing toxicities in different forms. 

The two major species in which chromium exists are the Cr(III) cation and the chromate and dichromate anions, Cr(VI). The Cr(VI) is considered to be considerably more dangerous in environmental samples of soil or water because it is more toxic and also considerably more soluble, and therefore more mobile in the environment. A pH 8, 0.05 M ammonia buffer is used with ultrasonication to extract the Cr(VI) from soil samples, and then the Cr(VI) is sorbed from the extract on an anion-exchange column (Dowex 1-X8). The sorbed analyte is eluted with 10 mL of pH 8, 0.5 M ammonia buffer. The Cr(III) is determined by difference from the total Cr measure- 

Methyl mercury is of much greater concern when health effects are considered, as it is much more toxic than ionic mercury or free mercury. Methyl mercury is also much more likely to be bioaccumulated, leading to serious contaminations, especially of fish. The speciation for mercury can be accomplished by derivatizing the methyl mercury and Hg2+ with sodium tetraethylborate, NaBEt4. The volatile MeHgEt, from methyl mercury, and HgEt2, from Hg2+, species formed are purged from the sample solution and separated in a GC column. An atomic emission spectrometer is used as a detector. 

Samples of freeze-dried fish tissues are extracted with 25% tetramethy-lammonium hydroxide using a microwave digester. After extraction, the pH is adjusted to 4 with acetic acid buffer. A 1% solution of NaBEt4 is added, with some hexane. The solution is shaken for 5 minutes. A fresh portion of NaBEt4 is added, the shaking is repeated, and finally, a third portion is added and allowed to react. The sample is centrifuged and an aliquot of the supernatant hexane is taken for injection into the GC [132], 

Strong acids used in digestion can also be a source of contamination when substantial quantities of acid are evaporated in the process of digestion. Ultrapure acids are required in wet digestion processes if traces of 

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