Unveiling the mechanisms behind the chemical vapor generation of plumbane for trace analysis of lead.

The mechanisms controlling the generation of PbH by reaction of inorganic Pb(II) with aqueous NaBH were investigated both in the presence and in the absence of the additive KFe(CN). For the first time PbH has been identified in analytical chemical vapor generation (CVG) by using gas chromatographic mass spectrometry (GC-MS), which allows the use of deuterium labelled experiments. In the absence of the additive, under reaction conditions typically employed for trace lead determination by CVG, Pb(II) is converted to solid species and no volatile lead species can be detected by either atomic or mass spectrometry for Pb(II) concentration up to 100 mg L. In alkaline conditions Pb(II) substrates are unreactive towards NaBH. In the presence of KFe(CN), deuterium labelled experiments clearly indicated that the generated PbH is formed by the direct transfer of hydride from borane to lead atoms. Kinetic experiments were carried out to evaluate the rate of reduction of KFe(CN) by NaBH, the rate of hydrolysis of NaBH both in the presence and in the absence of KFe(CN), and the rate of dihydrogen evolution following NaBH hydrolysis. The effect of delayed addition of Pb(II) to NaBH-HCl- KFe(CN), and KFe(CN) to NaBH-HCl-Pb(II) reaction mixtures on the efficiency of plumbane generation was investigated by continuous flow CVG coupled with atomic fluorescence spectrometry. The collected evidences, complemented with thermodynamic considerations and literature data, have made it possible to clarify long-standing controversial aspects related to the mechanism of plumbane generation and the role of KFe(CN) additive.