Abstract

One of the key advantages of ICP-MS instrumentation is its wide dynamic range covering the low ng l⁻¹ up to the hundreds of µg ml⁻¹ level, some eight orders of magnitude. For the PerkinElmer SCIEX ELAN 6000, this is achieved using a dual stage, discrete dynode detector enabling relatively small ion signals to be measured in pulse counting mode whilst higher ion fluxes are quantified using an analogue capability. To ensure linearity of calibration, the detector response in its two modes of operation must be established for all analytes of interest for a particular experiment. Although excellent linear correlation data for the dual calibration (r = 0.9999) has been routinely obtained in our laboratory, it has been established that the slopes for the pulse and analogue regions of the dual response differ significantly. Consequently, when the instrumental response for an analyte is established with standards measured in the pulse mode, significant bias in the determination of elemental concentrations can result for samples giving rise to analogue signals. The difference in response and the resulting bias can be illustrated when performing isotope ratio measurements, where the minor isotope is measured in pulse counting mode and the major isotope in the analogue mode. The consequences of this non-linear response for the isotopic abundance measurement of uranium in terms of the apparent mass discrimination factors and their relative stabilities is presented for the determination of ²³⁸U and ²³⁵U.