A gravimetric standard addition method combined with internal standard calibration has been successfully developed for the accurate analysis of total arsenic in a laver candidate reference material. A model equation for the gravimetric standard addition approach using an internal standard was derived to determine arsenic content in samples. Handlings of samples, As standard and internal standard were carried out gravimetrically to avoid larger uncertainty and variability involved in the

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... ic preparation. Germanium was selected as the internal standard because of its close mass to the arsenic to minimize mass-dependent bias in mass spectrometer. The ion signal ratios of 75 As + to 72 Ge + (or 73 Ge + ) were measured in high resolution mode (R≥10,000) to separate potential isobaric interferences by high resolution ICP/MS. For method validation, the developed method was applied to the analysis of arsenic content in the NMIJ 7402-a codfish certified reference material (CRM) and the result was 37.07 mg·kg -1 ± 0.45 mg·kg -1 which is in good agreement with the certified value, 36.7 mg·kg -1 ± 1.8 mg·kg -1 . Finally, the certified value of the total arsenic in the candidate laver CRM was determined to be 47.15 mg·kg -1 ± 1.32 mg·kg -1 (k = 2.8 for 95% confidence level) which is an excellent result for arsenic measurement with only 2.8 % of relative expanded uncertainty.