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 accur ate isotope ratio determination, both the mass discrimination and the detector dead time have to be appropriately corrected for. For a Finni gan-MAT Element and a Perkin-Elmer SCIEX Elan 5000 ICP mass spectromet er, 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 d ynodes, the dead time was observed to be independent of the analyte ma ss number. For the Elan 5000, however, which is equipped with a Channe ltron-continuous dynode-electron multiplier, the detector dead time wa s observed to be strongly mass-dependent. The decreasing gain of Chann eltron electron multipliers with increasing ion mass may be at the ori gin of the behaviour established. This observation has to be taken int o account when highly accurate isotope ratio measurements are required and hence the detector dead time has to be properly determined and co rrected for.