Knowledge Bank

ICP-MS measurement of selenium, an essential trace element and potential cancer preventative, is complicated by argon-containing molecular ions at each of the selenium isotopes. SeH+ complicates measurement of tracers based on isotope ratios when methane is used in a dynamic reaction cell to remove the argon-containing molecular ions. Nitrogen and oxygen were tested as alternative reaction gases Methane, nitrogen and oxygen all reacted with ArAr+, ArxH+, Ar2H2+ and ArCl+ Methane, nitrogen and oxygen reduced the ArAr+ signals by 7, 5, and 2.6 orders of magnitude, respectively, with minimal loss of Se+ signal (less than an order of magnitude). The Ar2H+ and Ar2H2+ also react rapidly with each of the reaction gases; however, these reaction rates could not be accurately determined due to the low background equivalent concentration that occurs as a result of their signal. The ArCl+ signal can be reduced by up to 5 orders of magnitude by ion-molecule reaction with methane and up to 4 orders of magnitude with nitrogen. The SeH+ signals increase to about 7% of the corresponding Se+ signal when methane is used as the reaction gas. The SeH+ signals are less than 1% of the corresponding Se+ signal when N2 or O2 reaction gases are used. Isotope ratio precision and accuracy were also assessed in order to determine how small the concentration of a 77Se spike concentration could be accurately measured in the presence of naturally occurring Se in blood plasma and other human biological samples.