Isotope dilution SPME GC/MS for the determination of methylmercury in tuna fish samples

The development of a rapid, precise and accurate analytical method for the determination of methylmercury in tuna fish samples is described. The method is based on the use of isotope dilution GC/MS with electron impact ionization, a widespread technique in routine testing laboratories. A certified spike containing Hg-202-enriched methylmercury was used for the isotope dilution of the samples. After extraction of the methylmercury from the sample, methylmercury was propylated using sodium tetrapropyl borate in SPME vials and the analytes were sampled from the headspace for 15 min. For isotope measurements, the molecular ion (MePrHg+) was used in the SIM mode. Five molecular ions were monitored, corresponding to the Hg-198, Hg-199, Hg-200, Hg-201 and Hg-202 isotopes. The detection at masses corresponding to Hg-198 was used to correct for in + I contributions of C-13 from the organic groups attached to the mercury atom on the Hg-199, Hg-200, Hg-201 and Hg-212 masses with simple mathematical equations, and the concentration of methylmercury was calculated on the basis of the corrected Hg-200/Hg-202 isotope ratio. The Hg-202-enriched methylmercury spike was applied, with satisfactory results, to the determination of methylmercury in the certified reference material BCR 464. The method was successfully applied to the determination of methylmercury in tuna fish samples, and the obtained results were included in the CCQM-P39 interlaboratory exercise, organized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) with excellent agreement between our results and the average obtained by the other participants. Copyright (c) 2005 John Wiley & Sons, Ltd.