(Boro)Hydride Techniques in Trace Element Speciation

Abstract

A number of applications of the sodium tetrahydroborate(iii) (sodium borohydride) reagent in the determination of arsenic, selenium, sulfur and tin species are reviewed. The reaction of an analyte species with aqueous tetrahydroborate(iii) is most frequently employed to yield a volatile hydride product which can be readily removed from the bulk matrix. This results in the isolation of the analyte from interferents and gives a species which can be readily concentrated and separated from other species. Some simple speciation analyses, such as the differentiation of arsenite and arsenate, can be performed by careful control of reaction conditions. With more complex molecules some of the original chemical structure of the target species (such as a C–As bond) can be conserved in the reaction products, permitting the deduction of further speciation information. Arsenic oxy-anions [R_nAsO(OH)_3-n, where R=alkyl or aryl] and alkyltin species, for example, yield different arsines (R_nAsO(OH)_3-n) and stannanes, respectively, which can be cryogenically trapped and subsequently separated by distillation or gas–liquid chromatography. More recently, analyte reactions with tetrahydroborate(iii) have been employed to provide a powerful link between the components employed in hyphenated instrumentation. One of its most significant roles is in the development of HPLC instrumentation where it can be employed to generate gas phase analyte species which are compatible with sensitive and highly selective gas/vapour detection systems such as AAS, AES and ICP-MS. Examples are given in which the tetrahydroborate(iii) reaction link is employed in the coupling of HPLC with AAS. The addition of photochemical oxidation or microwave digestion steps prior to the tetrahydroborate(iii) reaction stage further extends the range of detectable compounds. Whilst the tetrahydroborate(iii) reagent is largely associated with the generation of volatile analyte hydrides, this is not always the case. Cold-trap methods, for example, can be employed to measure some trimethylarsenic compounds by the production of trimethylarsine and alkyltin hydrides can be preconcentrated by solvent extraction. HPLC instrumentation is also described which employs the tetrahydroborate(iii) gas phase/liquid phase link to generate volatile sulfur species which are compatible with flame photometric detection.

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