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 Hg-198-enriched methylmercury ((MeHg)-Hg-198) spike. Using selective ion monitoring (SIM) mode, the intensities of (MeHgPr+)-Hg-202 at m/z 260 and (MeHgPr+)-Hg-198 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 mug 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 mug g(-1) (95% confidence interval). A concentration of 4.58+/-0.31 mug 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 mug g(-1) was estimated based on a 0.25 g subsample of DORM-2. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.