Derivatization of organometal(loid) species by sodium borohydride: Problems and solutions

Like other derivatization techniques, hydride generation is a chemical reaction that produces side-reactions leading to analytical problems. Demethylation of dimethylarsinic acid was observed to be dependent upon the pH level of the hydride generation reaction mixture. If the reaction mixture was acidic, then in addition to (CH3)2AsH, the monomethyl arsenic hydride [(CH3)AsH2] could be detected. Demethylation and also the formation of an unidentified arsenic species were noted when trimethyl arsonic oxide was used as derivatization educt. All of these effects depend on the pH level of the hydride generation mixture. We observed significant levels of organometal(loid) species of elements such as Ge, As, Sn, Sb, Hg and Bi in blank hydride generation mixtures. The organometal(loid) contamination was irreproducible even during 1 day using a single solution of sodium borohydride in deionized water. We concluded that the organometal(loid) contamination arises directly from the derivatization agent, sodium borohydride, itself. Use of helium purging and various adsorptive materials to decontaminate the sodium borohydride solution prior to analysis did not result in a significant decrease in organometal(loid) contamination levels. Use of a palladium-cluster stabilised with 1,10-phenanthrolin as alternative hydride generation derivatization agent was not found to be suitable, since reaction yields were poor and transmethylation reactions were noted.