Determination of thiolic compounds as mercury complexes by cold vapor atomic absorption spectrometry and its application to wines

We report on the application of a commercially available mercury analyzer, which is based on vapour generation of Hg-0 by NaBH4 reduction and atomic absorption detection, to the quantification and characterization of -SH groups and its application to wine samples. The behaviour of Hg(II) and thiol-Hg(II) (RS-Hg) complexes at nanomolar level (RS = L-Cysteine, DL-penicillamine, N-acetyl penicillamine, glutathione, cysteinylglycine, homocysteine) has been studied following their reduction with alkaline NaBH4 to give Hg-0. In the absence of thiol-Hg(II) is quantitatively converted to Hg-0 by stoichiometric amount of NaBH4 (reaction ratio 1/4 mole NaBH4/mole Hg), while the complete reduction of Hg(II)-thiol complexes to Hg-0 requires molar excess of NaBH4 up to six orders of magnitude, depending on the type of complex and on the pK(a) of the thiolic group. Under an appropriate excess of reductant, Hg(II) and its thiol complexes are not distinguishable giving the same response. These properties allow the discrimination of Hg(II) from Hg(II)-thiol complexes without any preliminary separation and the quantification of thiol groups. Instrumental detection limits are as low as 2.5 pg, permitting sample dilution, therefore, minimizing the risk of possible interferences occurring with complex real matrices. The method has been applied to quantification of thiol groups in wine samples. Comparison with results obtained by HPLC coupled to atomic fluorescence detection confirmed the promising potentialities of the method. (C) 2007 Elsevier B.V. All rights reserved.