Interferences during mercury speciation determination by volatilization, cryofocusing, gas chromatography and atomic absorption spectroscopy: comparative study between hydride generation and ethylation techniques

The interfering effects of sodium chloride and a mixture of metal ions on the speciation determination of mercury by derivatization (hydride generation or ethylation), preconcentration by cryotrapping, gas chromatographic separation and detection by quartz furnace atomic absorption spectroscopy were investigated. The potential of the masking agent EDTA to overcome the interference was also studied. The experiments were arranged according to a 2n + 1 factorial experimental design, so the possibility of synergistic effects among the variables considered was also taken into account. NaCl interferes in the determination of methylmercury (MeHg+) by ethylation, but causes fewer problems in the case of inorganic mercury (Hg2+) or if the analysis is carried out by hydride generation. The presence of metal ions strongly decreases the sensitivity in the determination of both compounds by hydride generation, but has no effect if ethylation is used as the derivatization technique. EDTA only partially eliminates the interfering effect of metals in the determination of MeHg+ by hydride generation. It also avoids decomposition of MeHg+ to Hg0, promoted by NaCl, after derivatization of the compounds by hydride generation. The results obtained are discussed in terms of a comparison between the two derivatization techniques used, i.e., hydride generation and ethylation. Potential chemical pathways and reaction mechanisms for the observed interferences are also given.