Sources of systematic errors in mercury speciation using Grignard reagents and capillary gas chromatography coupled to atomic spectrometry

This paper describes spectral and chemical interferences that occurred when using Grignard reagents for derivatisation of mercury species and microwave-induced plasma atomic emission (MIP-AES) or atomic absorption spectrometry for detection. When butylmagnesium chloride, dissolved in THF, was used to convert methyl and inorganic mercury to non-polar butyl derivatives it was established that reaction by-products, eluted near to the methylbutylmercury peak, caused changes in the background emission or absorption. The use of other Grignard reagents, pentyhnagnesium bromide and hexylmagnesium bromide, was investigated. These showed poor derivatisation efficiency and species stability. Iodide and bromide at concentrations of the order of 1 mg 1−1 were found to de-alkylate organic mercury species when present in an organic matrix. In addition, if these halides were not completely removed from the GC column under a normal chromatographic run, they reacted with derivatised mercury species in subsequently injected samples. Organomercury iodide or bromide compounds were formed and these were identified by GC-MS. This reaction reduced the signal magnitude for dibutylmercury, corresponding to inorganic mercury. Replacement of indigenous methyl groups with butyl groups was demonstrated on prolonged contact of samples containing methyl and dimethylmercury with butylmagnesium chloride. The degree of methyl group replacement appeared to be proportional to the reaction time.