Linearity Testing and Dead-Time Determination for Ion Counters of the Neptune MC-ICP-MS using the IRMM-072/-073/-074 Series of Uranium Isotope Reference Materials

The Environmental Sample Laboratory of the International Atomic Energy Agency (IAEA) routinely analyses environmental swipe samples for their bulk U and Pu isotope TM amounts and ratios using a Neptune Plus 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 on 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 materials series for the investigation of non-linearity or dead time effects of ion counters. For instance, a new series of gravimetrically prepared uranium isotope reference materials, the so-called IRMM-074 series, with the 235 238 U/ U isotope ratio (Thermo Fisher Scientific) multi collector – 233 235 U/ U isotope ratios varying from 1.0 to 10 -6 held constant at unity and the prepared and certified. This series is suited for calibration of secondary electron multipliers used widely in isotope mass spectrometry, in particular for TIMS (thermal ionization mass spectrometry), ICP-MS and AMS (accelerator mass spectrometry). 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. For the procedures for most of the ion counters only statically measured ratios are used, and no dynamic (peak-jumping) ratios are needed, which makes these procedures independent on plasma instabilities and therefore ideal for MC-ICP-MS instruments. For example, for the ion counter CDD IC5, the result for the dead-time was t = (66.3 ± 1.9) ns (k=2), which is traceable to the SI due to the use of the IRMM-072 reference materials. Based on the uncertainty of this dead-time, for any isotope ratio measurements with count rates at the level of 500,000 cps, the uncertainty arising from the dead time correction will be below 0.1%.