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An isotope dilution cold vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) method featuring gaseous introduction of mercury via tin chloride reduction has been developed and applied to the quantification and certification of mercury in various NIST standard reference materials: SRM 966 Toxic Metals in Bovine Blood (30 ng [multiplied by] [mL.sup.-1]); SRM 1641d Mercury in Water (1.6 [micro]g [multiplied by] [mL.sup.-1]); and SRM 1946 Lake Superior Fish Tissue (436 ng [multiplied by] [g.sup.-1]). Complementary mercury data were generated for SRMs and NIST quality control standards using cold vapor atomic absorption spectroscopy (CVAAS). Certification results for the determination of mercury in SRM 1641d using two independent methods (ID-CV-ICPMS and CVAAS) showed a degree of agreement of 0.3% between the methods. Gaseous introduction of mercury into the ICPMS resulted in a single isotope sensitivity of 2 x [10.sup.6] counts [multiplied by] [s.sup.-1]/ng [multiplied by] [g.sup.-1] for [sup.201]Hg and significantly reduced the memory and washout effects traditionally encountered in solution nebulization ICPMS. Figures of merit for isotope ratio accuracy and precision were evaluated at dwell times of 10, 20, 40, 80, and 160 ms using SRM 3133 Mercury Spectrometric Solution. The optimum dwell time of 80 ms yielded a measured [sup.201]Hg/[sup.202]Hg isotope ratio within 0.13% of the theoretical natural value and a measurement precision of 0.34%, on the basis of three replicate injections of SRM 3133.