Dependence of detector dead time on analyte mass number in inductively coupled plasma mass spectrometry

ICP mass spectrometry is being increasingly used for isotopic analysis and for element determination by means of isotope dilution. For accurate isotope ratio determination, both the mass discrimination and the detector dead time have to be appropriately corrected for. For a Finnigan-MAT Element and a Perkin-Elmer SCIEX Elan 5000 ICP mass spectrometer, the constancy of the detector dead time across the mass range was systematically investigated. For the Element, which is equipped with a conversion dynode and a secondary electron multiplier with discrete dynodes, the dead time was observed to be independent of the analyte mass number. For the Elan 5000, however, which is equipped with a Channeltron—continuous dynode—electron multiplier, the detector dead time was observed to be strongly mass-dependent. The decreasing gain of Channeltron electron multipliers with increasing ion mass may be at the origin of the behaviour established. This observation has to be taken into account when highly accurate isotope ratio measurements are required and hence the detector dead time has to be properly determined and corrected for.