Speciation and Hybrid Techniques

From time to time in this chapter, mention has been made of procedures for the determination of particular chemical forms of elements. Sometimes this is achieved by a totally manual fractionation of an element into separate and distinct chemical fractions, and the fractions are then analysed one at a time by spectrometric procedures. However, by the 1980s there was a growing tendency to automate the fractionation procedures and to incorporate the spectrometric detector on-line . It is appropriate at this point to consider a few examples of this approach. 

Tye et al.56 used hydride generation coupled to AAS to quantify organic and inorganic arsenic species in soil pore waters, after pre-concentration on a pellicular anion-exchange column. They were able to detect down to 2 ng of arsenate, arsenite, and monomethyl arsenite and down to 1 ng of dimethyl arsonate. More recently, an argon-hydrogen-entrained air flame fitted with a 

Inorganic, methyl, and n-butyl tin compounds may be converted to volatile hydrides, and the latter separated on a chromatographic column prior to detection of tin by AAS.58 In this particular study, an electrically heated absorption cell was used, although a flame-heated quartz cell could have been employed equally well. Balls59 used on-line cryogenic trapping on a silanized glass-wool column to separate dibutyl tin and tributyl tin in sea water prior to transport to a quartz-tube atomizer for determination by AAS. 

Speciation of lead in air and atmospheric particulates is still a topic of great environmental relevance. Sodium tetrahydroborate may be used to hydrogenate inorganic lead ions and alkyl-lead species.60 As in the study by Balls outlined above, cryogenic trapping may again be used to trap temporarily the plumbane and alkyl- and tetraalkyl-lead compounds, which are then released sequentially by heating for detection by flame AAS. 

It will be clear from the above examples that hydride-forming elements especially are often subjected to such speciation studies. There are, of course, good reasons for this. These are the very elements which tend to form toxic organometallic compounds, and they are elements which may be determined with excellent sensitivity. Moreover, interferences are not usually a problem following a separation process. 

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