Operational optimisation of ICP - octopole collision/reaction cell - MS for applications to ultratrace selenium total and speciation determination

This paper describes the optimisation of inductively coupled plasma-mass spectrometry (ICPMS) equipped with collision/reaction cell (C/RC) technology for selenium analysis at ultratrace level. Gas flow rates for helium and hydrogen were carefully optimised using experimental design methodology in order to efficiently remove interferences (ArAr+ dimer) while keeping maximum sensitivity on Se-80. A mixture of both gases with 0.5 ml min(-1) of He and 3.8 ml min(-1) of H-2 was selected as the best compromise. Voltage of the different ionic lenses disposed around the C/RC, allowing a better focalisation of the ion beam and kinetic energy discrimination, was also investigated and retained values were: - 14 V for quadrupole bias, - 13 V for octopole bias, - 20 V for cell entrance, - 20 V for cell exit and - 45 V for plate bias. Detection limits in total analysis are close to 30 ng (Se) l(-1) which represents a three times improvement to the ones obtained without C/RC. These conditions were tested with HPLC coupling for speciation of selenite (SeIV), selenate (SeVI), selenomethionine ( SeMet) and selenocystine (SeCyst). Depending on selenium species, the detection limits achieved in C/RC mode monitoring Se-80 are in the range 70 - 180 ng (Se) l(-1) which represents a 2 times improvement in comparison with Se-82 monitoring in standard mode. Accuracy of the method was tested by analysing spiked water and certified reference material (Rain water, TM-Rain95, National water research institute, Canada).