Isotope dilution (ID) method has been widely applied to studies of elemental speciation and certification of the concentrations of geological reference materials.One of the key factors restricting the application of the ID method is the difficulty in identifying the optimal ratios of the isotope tracer (spike) to sample (S/N) and in estimating the error propagation.Here,using Se isotope as an example and employing a Monte Carlo method,we found that the optimal choice of spike and S/N are 77Sespike and 0.7,respectively.The S/N in the range from 0.1 to 4 can produce sufficiently low errors (＜ 0.4 ％).Extending this method to other elements such as Cr,Fe,Ni,Sr,Cd and etc.,their optimal spike and S/N were also presented.According to the optimized parameters of Se,we determined the Se concentrations of geological reference materials (GRMs) by thiol cotton fiber (TCF)-hydride generation ID-MC-ICP-MS.The relative standard deviation is ＜ 5 ％,and the detection limit is reduced to 0.1 ng/g.Most of the measured concentrations are consistent with those recommended by IGGE or reported previously.For some GRMs,discrepancies (e.g.6.90 ％ for GSS-5) exist between our measurements and previous ones,which can be explained by the inhomogeneity of GRMs and/or the different digestion,purification and measurement methods among laboratories.