The interference free determination of selenium in environmental and clinical samples by ICP-MS
The determination of selenium in environmental and clinical samples by ICP-MS is complicated by both poor sensitivity and severe interferences resulting from both the sample matrix and the argon plasma. The purpose of this study was to investigate ways of overcoming these problems thus enabling Se to be determined both accurately and precisely. A novel procedure for the accurate determination of selenium in serum using electrothermal vaporisation inductively coupled plasma mass spectrometry (ETV-ICP-MS) has been developed. A simple 1:20 dilution of the serum with 1% nitric acid negates the need for a lengthy sample digestion procedure. Several of the interferences normally associated with the analysis of selenium by ICP-MS are successfully eliminated. Analytical method characteristics include; detection limits of approximately 0.l ng gˉ¹ for 77Se and 82Se, short and long term reproducibility between 4.7% and 4.9%, and 3.2% and 3.8% (RSD) for 77Se and 82Se respectively, and accuracy of ±1.81% (77Se) and± 1.10% (82Se) for the certified reference material NIST SRM 1598. Further development of the procedure involved the application of isotope dilution analysis with the measurement of the 82Se/77Se isotope ratio, following spiking with selenium enriched in 77Se. Accuracy (±0-2%) and precision (±1-3%) of the method is demonstrated with the analysis of several certified reference materials (TMRAIN-95, LGC 6010, TMDA-54.2 and NIST 1598) where all results fell within the certified limits. A comparison of the new ETV procedures with established ICP-MS methods involving hydride generation and the use of organic solvents with pneumatic nebulisation was performed. Full uncertainty estimates for each of the procedures investigated were calculated. The uncertainty estimates calculated highlight the improvements in accuracy and precision achievable with isotope dilution analysis, demonstrated by a 2.5 fold improvement in the uncertainty compared with the non-IDMS ETV procedure. The addition of nitrogen and helium to the different Ar gas streams of the ETV-ICP-MS system was performed. The introduction of both nitrogen and helium to the argon nebuliser and outer gas streams, resulted in a reduction in the interference from argon polyatomic species. Careful optimisation of plasma parameters such as RF power and gas flow rates using a plasma containing 50% helium in the argon outer gas stream, successfully gave rise to a 2-fold improvement in the detection limits for 76Se and 78Se compared with an argon only plasma. This study has resulted in the successful development of a high accuracy procedure to determine selenium in both environmental and clinical samples. This is of great importance considering the high level of interest regarding selenium and human health matters and the significance of accurate analytical data.