A new selenized yeast reference material (SELM-1) produced by the Institute for National Measurement Standards, National Research Council of Canada (INMS, NRC) certified for total selenium (2,059+/-64 mg kg(-1)), methionine (Met, 5,758+/-277 mg kg(-1)) and selenomethionine (SeMet, 3,431+/-157 mg kg(-1)) content is described. The +/-value represents an expanded uncertainty with a coverage factor of 2. SeMet and Met amount contents were established following a methanesulfonic acid digestion of the yeast using GC-MS and LC-MS quantitation. Isotope dilution (ID) calibration was used for both compounds, using 13C-labelled SeMet and Met. Total Se was determined after complete microwave acid digestion based on ID ICP-MS using a 82Se spike or ICP-OES spectrometry using external calibration. An international intercomparison exercise was piloted by NRC to assess the state-of-the-art of measurement of selenomethione in SELM-1. Determination of total Se and methionine was also attempted. Seven laboratories submitted results (2 National Metrology Institutes (NMIs) and 5 university/government laboratories). For SeMet, ten independent mean values were generated. Various acid digestion and enzymatic procedures followed by LC ICP-MS, LC AFS or GC-MS quantitation were used. Four values were based on species-specific ID calibration, one on non-species-specific ID with the remainder using standard addition (SA) or external calibration (EC). For total selenium, laboratories employed various acid digestion procedures followed by ICP-MS, AFS or GC-MS quantitation. Four laboratories employed ID calibration, the remaining used SA or EC. A total of seven independent results were submitted. Results for methionine were reported by only three laboratories, all of which used various acid digestion protocols combined with determination by GC-MS and LC UV. The majority of participants submitted values within the certified range for SeMet and total Se, whereas the intercomparison was judged unsuccessful for Met because only two external laboratories provided values, both of which were outside the certified range.

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This study describes the analysis of arsenicals in three marine based certified reference materials (CRMs), DORM-2, DOLT-2 and TORT-2, all produced by the National Research Council of Canada. Extraction protocols involving accelerated solvent extraction (ASE) and sonication were investigated and arsenicals were determined on-line by anion-exchange and cation-exchange high-performance liquid chromatography (HPLC). The major species in all three tissues was found to be arsenobetaine (AsB), corresponding to 90%, 67% and 73% of the water-soluble arsenic in DORM-2, DOLT-2 and TORT-2, respectively. Other species quantified were arsenocholine (AsC 1 ), tetramethylarsonium ion (TMAs 1 ), trimethylarsonioproprionate (TMAP), dimethylarsenic acid (DMA), monomethylarsenic acid (MMA), arsenosugar D (As-sug D), arsenate (As V ) and arsenosugar A (As-sug A). The major species detected in all three materials were AsB and DMA, which together accounted for 97.4% (DORM-2), 95.4% (DOLT-2) and 91.7% (TORT-2) of the sum of the water soluble As species. The agreement for the determination of AsB by the different methods was good for all three materials, whereas some discrepancy was evident in the results for DMA. When an ASE approach with 50% acetic acid in methanol was used for the extraction, between 22% (DOLT-2) and 58% (TORT-2) more DMA was determined compared with extraction by sonication with water. Some discrepancy was evident between the sum of the species extracted and both the total As determined in the extracts and the certified As content based on complete digestion of the materials. The sum of the species recovered by sonication corresponded to 84% (DOLT-2) to 94% (DORM-2) of the total As determined in the aqueous extract. In comparison with the certified As content of the three materials, the sums of the extracted species range from 46% (DOLT-2) to 76% (TORT-2) and 102% (DORM-2).

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