Direct coupling of solid phase microextraction and quartz tube-atomic absorption spectrometry for selective and sensitive determination of methylmercury in seafood: an assessment of chloride and hydride generation

Abstract

The direct coupling between headspace solid phase microextraction and quartz tube-atomic absorption spectrometry detection has been evaluated for speciation of methylmercury in seafood after volatilization of its hydride or chloride derivative. Following variable optimisation in both procedures, a critical influence of the fibre sorbent material was observed. Liquid-phase coatings such as polydimethylsiloxane (PDMS) and ‘solid’ phase coatings such as polydimethylsiloxane-divinylbenzene (PDMS/DVB) or fused silica were compared. Best LODs were provided by the silica fibre for CH₃HgH (0.4 ng mL⁻¹) and by the PDMS/DVB fibre for CH₃HgCl (0.06 ng mL⁻¹). In general, repeatability was about 6% for the preconcentration of both Hg derivatives in the different fibre coatings. The affinity to sorption of CH₃HgH was slightly improved when using silica fibres loaded with L-cysteine or palladium nitrate. Sorption/desorption studies with fused silica fibres allowed us to hypothesise that CH₃HgH formed upon reaction of MeHg⁺ with NaBH₄–NaOH suffered from decomposition onto the fibre to yield Hg⁰ like direct reaction between Hg²⁺ and NaBH₄–NaOH. Generation of the chloride derivative showed an enhanced selectivity as compared with generation of the hydride. Nevertheless, experimental evidence of decomposition of the chloride derivative upon heating is also given. For methylmercury determination in seafood, hydride or chloride generation and headspace SPME were applied after ultrasound assisted extraction of methylmercury in 2 mol dm⁻³ HCl. Validation was carried out against CRM DORM-2 (dogfish muscle) and CRM 464 (Tuna fish), demonstrating good trueness of the methods proposed. A better LOD of MeHg⁺ is obtained as compared with other SPME couplings described in the literature.