Application of isotope dilution to the determination of methylmercury in fish tissue by solid-phase microextraction gas chromatography–mass spectrometry

Species-specific isotope dilution (ID) calibration using solid-phase microextraction (SPME) in combination with gas chromatography–mass spectrometry (GC–MS) for separation and detection of methylmercury (MeHg) in fish tissue is described. Samples were digested with methanolic potassium hydroxide. Analytes were propylated and headspace sampled with a polydimethylsiloxane-coated SPME fused-silica fiber. ID analysis was performed using a laboratory-synthesized 198Hg-enriched methylmercury (Me198Hg) spike. Using selective ion monitoring (SIM) mode, the intensities of Me202HgPr+ at m/z 260 and Me198HgPr+ at m/z 256 were used to calculate the m/z ratio at 260/256, which was used to quantify MeHg in NRCC CRM DORM-2 fish tissue. A MeHg concentration of 4.336±0.091 μg g−1 (one standard deviation, n=4) as Hg was obtained in DORM-2, in good agreement with the certified value of 4.47±0.32 μg g−1 (95% confidence interval). A concentration of 4.58±0.31 μg g−1 was determined by standard additions calibration using ethylmercury (EtHg) as an internal standard. The three-fold improvement in the precision of measured MeHg concentrations using ID highlights its superiority in providing more precise results compared to the method of standard additions. A method detection limit (3 S.D.) of 0.037 μg g−1 was estimated based on a 0.25 g subsample of DORM-2.