The Importance of a Correct Dead Time Setting in Isotope Ratio Mass Spectrometry: Implementation of an Electronically Determined Dead Time to reduce Measurement Uncertainty

Dead time determintions on a mass spectromety system with ion counting detection can either be done using an isotope ratio measurement approach or via electronic examination of individual components, e.g., the pulse amplifier. Depending on the dead time of each component in the signal chain, the electronically determined result may not represent the true value for the total system, i.e., there might be a series of hardware related dead times. However, a hardware set artificial dead time in the pulse counting sytem that is long enough might yield a system dead time that can be represented by this set dead time solely. It is shown in this work that a carefully chosen artificially introduced set dead time yields a system that may be characterized to a much lower uncertainty than is possible using ratio-based measurement approaches. Ultimately, the impact of the dead time on the combined uncertainty of an isotope abundance ratio measurement may be negligible even at very high count rates, i.e., > 1 Mcps. In other words, incorporating the dead time contribution in a combined uncertainty calculation would no longer be necessary.JRC.D.4-Isotope measurement