Isotope amount ratio measurements by ICP-MS: aspects of software induced measurement bias and non-linearity

Abstract

The effect of a post-acquisition software induced bias on the measurement of the ²³⁵U/²³⁸U isotope amount ratio in the reference material IRMM-184 was studied. The total uranium concentration ranged from 7 to 2500 pg g⁻¹ [80 (²³⁵U) to 3500000 (²³⁸U) counts per second], thus covering the entire pulse counting range of the ICP-MS instrument. The bias for the measured ratio increases with decreasing count rate, and it is mostly governed by the count rate of the ²³⁵U isotope. Blocking the data into large subgroups is one way of reducing the effects of the bias; however, a decreased number of blocks is accompanied by an increased scattering of the standard uncertainty. Using a more laborious, manual data evaluation procedure of a less processed raw measurement signal not only solves the problem, it also increases the linear measurement range by at least ten times. At the lowest concentration, 7 pg g⁻¹ total uranium concentration, the combined uncertainty of the observed ratio was less than 5% (k = 2).