Linearity testing and dead-time determination for MC-ICP-MS ion counters using the IRMM-072 series of uranium isotope reference materials

The Environmental Sample Laboratory (ESL) of the International Atomic Energy Agency (IAEA) routinely analyses environmental swipe samples for their bulk U and Pu isotope amounts and ratios using a Neptune Plus (TM) (Thermo Fisher Scientific) multi collector-inductively coupled plasma mass spectrometer (MC-ICP-MS). The instrument is equipped with the so-called L5 ion counter package, which comprises ten Faraday cups, three classical discrete dynode secondary electron multipliers (SEM), and two compact discrete dynode (CDD) electron multipliers. In contrast to classical SEMs, CDDs only have about twice the width of a Faraday cup and are mounted in-line with the Faraday cups within the multi collector array. The Institute of Reference Materials (IRMM) of the European Commission's Joint Research Centre (JRC) has developed several dedicated uranium isotope reference material series for the investigation of non-linearity and dead time effects of ion counting systems. For instance, a new series of gravimetrically prepared uranium isotope reference materials, the so-called IRMM-074 series, with the U-235/U-238 isotope ratio held constant at unity and the U-233/U-235 isotope ratios varying from 1.0 to 10(-6) has been prepared and certified. This series is suited for calibration of secondary electron multipliers used widely in isotope mass spectrometry, in particular for thermal ionization mass spectrometry (TIMS), ICP-MS and accelerator mass spectrometry (AMS). The new IRMM-074 series was prepared as a replacement for the already exhausted IRMM-072 predecessor series, which is still on stock at the IAEA. In collaboration between the IAEA and the IRMM, dedicated new procedures have been developed for taking advantage of the IRMM reference materials for linearity testing and dead time determination for the various ion counting detectors of two Neptune Plus (TM) MC-ICP-MS instruments at the IAEA. Only statically measured ratios were used, and dynamic (peak-jumping) ratios were avoided, which makes these procedures independent on plasma instabilities and therefore ideal for MC-ICP-MS instruments. The dead times of the ion counting systems were found to depend not on the detectors themselves but only on the pulse amplifiers, which allowed to easier investigate them by connecting each pulse amplifier to any detector of the same type within the detector configuration. The new procedures might well be applicable for similarly designed MC-ICP-MS instruments which have already been installed at other laboratories working in the nuclear safeguards and environmental fields.