A high accuracy primary ratio method for the determination of iodine in complex matrices by double isotope dilution using MC-ICPMS and 129I spike

Abstract

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 ¹²⁹I 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⁻¹ (61 ng g⁻¹¹²⁹I) 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® (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 ¹²⁹I spike to 0.5 g of sample and heating to 100 °C 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).