Determination of selenium in biological materials by isotope dilution analysis with an octapole reaction system ICP-MS

An inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) with an octapole reaction system (ORS) was used for the determination of selenium by isotope dilution analysis (IDA) in biological reference materials and serum samples from healthy subjects. The potentially interfering argon dimers ((ArAr+)-Ar-40-Ar-38 and Ar-40(2)+) at the selenium masses 78 and 80 were almost eliminated by the use of hydrogen as reaction gas. Thus, the detection limits were improved five times for Se-78 and the measurement of Se-80 (the main selenium isotope with an abundance of 49.61%) is now possible. An enriched Se-77 solution was prepared and characterized by reverse IDA and isotope ratios measured were Se-78/Se-77 and Se-80/Se-77. Instrumental parameters were optimised in order to obtain optimal precision and accuracy in the isotope ratio measurement of Se by ORS-ICPQMS. The precision achieved for the isotope ratio measurements reached 0.2% (RSD for n = 5) for both ratios. Systematic errors, including detector dead time (47 +/- 2 ns), mass bias effects (about 3%) and spectroscopic interferences due to the presence of bromine in the samples, were corrected. The accuracy of the measured selenium isotope ratios was improved by correcting the intensity signals of the selenium isotopes for SeH+ formation (about 3%). The proposed IDA method has been applied to the determination of Se in biological reference materials (serum, urine and tissues) and the results showed good agreement with the certified values.