Cold vapour atomic fluorescence studies on the behaviour of mercury(II) and mercury(II)-thiol complexes. An alternative route for characterization of -SH binding groups

The behaviour of Hg-II and Hg-II-thiol complexes (RSH = L-cysteine, DL-penicillamine, propane-2-thiol, glutathione, thiosalicylic acid) following their reduction with alkaline sodium tetrahydroborate to give Hg-O has been studied by using a continuous flow reaction system coupled with atomic fluorescence spectrometric (AFS) detection. The quantitative reduction of Hg-II to Hg-O takes place with a specific amount of sodium tetrahydroborate according to the stoichiometric reaction of mercury with alkaline NaBH4. The complete reduction of Hg-II-thiol complexes to Hg-O requires a molar excess of NaBH4, of up to six orders of magnitude, depending on the type of complex. Under an appropriate excess of reductant, Hg-II and its thiol complexes are not distinguishable giving the same AF molar response. The method allows the discrimination of Hg-II from Hg-II-thiol complexes without any preliminary separation. Applications to the indirect titration of thiols and to the determination of the number of accessible-SH groups in pure ovalbumin samples are reported.