19. Hydride generation for speciation analyses using CG/AAS

Abstract As mentioned in Chapter 1, most of the techniques used for speciation analysis of environmental samples are based on the coupling of different analytical steps involving extraction, derivatization, separation and detection. Derivatization of organometallic compounds is often required when the separation is performed by gas chromatography but may also be used after liquid Chromatographic separation. The major advantage of the derivatization step is the separation of the analytes from the matrix which reduces the occurrence of possible interferences during the subsequent analytical steps and particularly at the detection stage. This procedure also allows pre-concentration of the analytes to be achieved either in the solvent or by cryogenic trapping (cryo-condensation). Hydride generation is one derivatization procedure which has been extensively used since its introduction by Holak [1] for the determination of total content of arsenic by flame atomic absorption spectrometry. Hydride generation can be performed on-line or off-line and allows both the determination of organometallic compounds and elements with different oxidation states. This procedure may be applied to the determination of a wide variety of chemical species in environmental samples (water, sediment, biological tissues); hydride generation with NaBH 4 can be used for the determination of total contents of elements such as antimony, arsenic, bismuth, germanium, lead, mercury, tin, selenium and tellurium or their species [2], depending upon the exact conditions used. An example of the hydride generation reaction applied to tin speciation is given below: R n S n ( 4 − n ) + → N a B H 4 , H + R n S n H ( 4 − n ) + H 2 w i t h n = 1 , 2 , 3 R i s m e t h y l , e t h y l o r b u t y l Hydride generation conditions, e.g. concentration of the reductant solution (NaBH 4 ), pH and types of acids used, must be selected according to the element determined and the quality of the sample [3], as discussed below. Whereas the advantages of this procedure are well recognized, it is often subject to major drawbacks when difficult matrices are to be analyzed. This chapter aims to give an overview of the advantages and limitations of hydride generation when applied to speciation studies.