High accuracy measurements of Fe isotopes using hexapole collision cell MC-ICP-MS and isotope dilution for certification of reference materials

Abstract

An ICP-MS equipped with collision cell, sector field and multi-collector technology has been applied to developing analytical procedures for Fe in terms of isotope ratio measurements and isotope dilution analysis. Therefore a detailed study on the reduction of molecular interferences by a variety of collision gases (He, Ar, Kr, H₂, D₂ and N₂) was performed. The efficiency of the reduction has been checked by high resolution mass scans. The argon based interferences disturbing the determination of ⁵⁴Fe and ⁵⁶Fe could be completely removed when using Faraday detectors. The interferences on mass 57 could be reduced to a level which enables accurate measurements. Also, the ⁴⁰Ar¹²C interference, which disturbs the chromium correction, could be completely removed. Reproducibilities of 0.01% could be reached for ⁵⁴Fe/⁵⁶Fe isotope ratios and 0.02% for ⁵⁷Fe/⁵⁶Fe isotope ratios. Under these measurement conditions a sensitivity of 1.2 V per 50 µg kg⁻¹ Fe is available. This corresponds to 1.5 × 10⁹ cps per mg kg⁻¹. Based on these results highly accurate procedures for Fe isotope ratio determinations were set up, reaching a performance level superior to that of other ICP-MS instruments. Consequently, IDMS procedures were developed on this basis in order to perform certification measurements at the highest metrological level. The first application was the contribution to the Fe reference value by IMEP-12, an interlaboratory comparison with approximately 350 participants worldwide. The analysed Fe mass content is 0.2150 mg kg⁻¹ with its expanded uncertainty (k = 2) of 0.0012 mg kg⁻¹ (0.56%), which is ten times less than the uncertainty of the second certification laboratory. Another application was the certification campaign of the two reference materials Tea Leaves and Polish Herbs. The results obtained with the ICP-MS procedure, 542 ± 11 mg kg⁻¹ and 543 ± 12 mg kg⁻¹, were compared with the results analysed by TIMS, 541 ± 12 mg kg⁻¹ and 541 ± 13 mg kg⁻¹, and agree very well within the stated uncertainties (k = 2). The results presented demonstrate quite well the suitability of the developed IDMS procedures for certification or reference measurements.