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Iodine is an essential element and yet is poorly represented in matrix reference materials. In this paper a novel development of the double IDMS equation using 129 I spike is presented which is ideally suited for the provision of primary measurements of iodine in complex matrices. Tolerances of the double IDMS method for iodine determination using a multi-collector ICPMS have been explored. The results indicate that even early approximates of the iodine concentration in the sample will give accurate results. The method is therefore potentially applicable to accurate one-off measurements where an iterative matching of samples and standards is not necessary. In addition, by limiting the procedure to only using standards that do not exceed at any point a maximum activity of 0.4 Bq g À1 (61 ng g À1 129 I) then the samples and standards can be handled outside of radiological control. The new methodology has been applied to the measurement of iodine in established reference materials BCR 063R, NIST SRM1846, NIST SRM1549, a FAPAS s (Food Analysis Performance Assessment Scheme) material C300 6994 and a candidate reference material in manufacture at LGC (LGC7163). The digestion procedure involved the addition of TMAH 5% and 129 I spike to 0.5 g of sample and heating to 100 1C in an oven for 2 hours. Data obtained for the reference materials showed excellent agreement with the reference values for iodine. Expanded relative uncertainties at the 95% confidence level in the different materials for iodine recoveries relative to gravimetric standards varied from 1.4 to 5.7% (k = 2).

This study describes a direct comparison of GC and HPLC hyphenated to ICP-MS determination of tributyltin (TBT) in sediment by species-specific isotope dilution analysis (SS-IDMS). The certified reference sediment PACS-2 (NRC, Canada) and a candidate reference sediment (P-18/HIPA-1) were extracted using an accelerated solvent extraction (ASE) procedure. For comparison of GC and LC methods an older bottle of PACS-2 was used, whilst a fresh bottle was taken for demonstration of the accuracy of the methods. The data obtained show good agreement between both methods for both the PACS-2 sediment (LC-ICP-IDMS 828+/-87 ng g(-1) TBT as Sn, GC-ICP-IDMS 848+/-39 ng g(-1) TBT as Sn) and the P-18/ HIPA-1 sediment (LC-ICP-IDMS 78.0+/-9.7 ng g(-1) TBT as Sn, GC-ICP-IDMS 79.2+/-3.8 ng g(-1) TBT as Sn). The analysis by GC-ICP-IDMS offers a greater signal-to-noise ratio and hence a superior detection limit of 0.03 pg TBT as Sn, in the sediment extracts compared to HPLC-ICP-IDMS (3 pg TBT as Sn). A comparison of the uncertainties associated with both methods indicates superior precision of the GC approach. This is related to the better reproducibility of the peak integration, which affects the isotope ratio measurements used for IDMS. The accuracy of the ASE method combined with HPLC-ICP-IDMS was demonstrated during the international interlaboratory comparison P-18 organised by the Comité Consultatif pour la Quantité de Matière (CCQM). The results obtained by GC-ICP-IDMS for a newly opened bottle of PACS-2 were 1087+/-77 ng g(-1) Sn for DBT and 876+/-51 ng g(-1) Sn for TBT (expanded uncertainties with a coverage factor of 2), which are in good agreement with the certified values of 1090+/-150 ng g(-1) Sn and 980+/-130 ng g(-1) Sn, respectively.

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