Determination of methylmercury in fish tissues by isotope dilution SPME-GC-ICP-MS

Abstract

A method is described for the accurate and precise determination of monomethylmercury (MMHg) by species specific isotope dilution (ID) calibration using solid phase microextraction (SPME) in combination with gas chromatographic (GC) separation and inductively coupled plasma mass spectrometric (ICP-MS) detection. Samples were digested with methanolic potassium hydroxide, derivatized in aqueous solution with sodium tetrapropylborate and headspace sampled with a polydimethylsiloxane coated SPME fused silica fiber. The analyte was then directly transferred from the fiber to the head of the GC column for desorption by insertion of the fiber through the heated injection port. Reverse spike ID analysis was performed to determine the accurate concentration of an in-house synthesized ¹⁹⁸Hg-enriched monomethylmercury (MM¹⁹⁸Hg) spike [candidate Certified Reference Material (CRM) EOM-1] using two natural abundance MMHg standards. Concentrations of 0.719 ± 0.015 and 4.484 ± 0.029 µg g⁻¹ (one standard deviation, n = 4) as Hg were obtained for MMHg in NRCC CRMs DOLT-2 and DORM-2, respectively, using the present method. These are in good agreement with the certified values of 0.693 ± 0.053 and 4.47 ± 0.32 µg g⁻¹ (as 95% confidence interval). A MMHg concentration of 4.69 ± 0.12 µg g⁻¹ (one standard deviation, n = 4) as Hg in DORM-2 was subsequently determined by standard additions calibration using ethylmercury (EtHg) as an internal standard. A nearly 4-fold improvement in the precision of determination using ID was obtained, clearly demonstrating its superiority in providing more precise results compared to the method of standard additions. The method was applied to the determination of MMHg in a new biological CRM DOLT-3 and a concentration of 1.540 ± 0.025 µg g⁻¹ (one standard deviation, n = 6) as Hg was obtained. A method detection limit (3σ) of 2.1 ng g⁻¹ was estimated for a 0.25 g of subsample, sufficiently low for the routine determination of MMHg in most biological samples.