The importance of a correct dead time setting in isotope ratio mass spectrometry: Implementation of an electronically determined dead time to reduce measurement uncertainty

Abstract

Dead time determinations on a mass spectrometry 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 system 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.

Consequences of and potential reasons for inadequate dead time measurements in isotope ratio mass spectrometry

“…1a and b show that changes in SEM voltage and/or electronic components of the detection system may cause secondary effects that make a previously determined value of too large. Furthermore, the results presented in our previous study [4] show that the use of an electronically determined value of may actually result in correction with a too small value since the dead times of other components are not included. In that study, deviations as large as 4-5 ns were found between the electronic determined dead time of the amplifier and the dead time determined via ratio measurements.…”

“…On the amplifier of the Element2, three different settings can be chosen; 10, 20 and 70 ns of which the 10 ns seems to be the default value set by the factory. In a previous study, the different settings were examined and agreement between dead times determined via electronic measurements of the amplifier output and ratio measurements were only obtained when the largest setting, 70 ns, were used [4]. These new measurements showing a pulse width of 16 ns from the SEM provides an explanation to this, i.e., the pulse width of the multiplier being large enough to affect the overall dead time of the system when the introduced dead time of the amplifier is relatively short (10 and 20 ns) [3,13].…”

“…These new measurements showing a pulse width of 16 ns from the SEM provides an explanation to this, i.e., the pulse width of the multiplier being large enough to affect the overall dead time of the system when the introduced dead time of the amplifier is relatively short (10 and 20 ns) [3,13]. In fact, for the lowest setting of 10 ns, the systems showed a dead time that corresponded to the width of the SEM pulse [4].…”

“…This is advantageous since the implemented dead time can be relatively well known and stable. In a previous study, the dead time of the pulse counting system of an Element2 was examined using different settings of an introduced dead time on the pulse amplifier [4]. Both electronic measurements of of the pulse amplifier and determinations of via isotope ratio measurements were made, and correspondence between electronic and ratio determined dead times was found only for the largest setting of the introduced dead time (70 ns) which suggests that other components than the amplifier affects the apparent dead time of the system.…”

Principles and Applications of LC-MS/MS for the Quantitative Bioanalysis of Analytes in Various Biological Samples

“…m/z 50 to 500 in 0.1 sec) probably will not provide the accuracy desired (better than ±0.1 %) in isotope ratio measurements. Such isotope ratio measurements are usually made with the Faraday cup detector (Nygren et al, 2006;Harris et al, 1984;Mckinney et al, 1950). Ideally, a statement of sensitivity should explicity define the ion current received by detector for a specified rate of sample consumption.…”