Electron multiplier dead time

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

The detector dead time is the time taken for a detection system to recover from an ion pulse. If a second ion hits the detector before it has recovered it will not be recorded. This will bias the count rate, which will appear lower than it really is. The dead time for the usual electron multiplier detectors favoured in MS instrumentation is the order of 15 to 100 ns. The detector dead time can be determined quite easily, but the IDMS methodologies described below (Section 5) can compensate for this effect and so it is necessary only to establish this value periodically and not for each determination. 

Poisson counting statistics ultimately limit signal-to-noise ratios in conventional ICP-MS instruments. The improvement achieved by increased intensity is directly proportional to the square root of the counts s . The temptation is, therefore, to work with ever-larger counts. However, the detectors (usually electron multipliers) will have a limit to their linearity and will ultimately saturate. Even over the normal linearity range, typically up to 10 counts s , there will be an influence from the detector s dead time . There is no need to correct for dead time if a matching procedure is being adopted, but in other cases, for very accurate measurements, the multiplier will need to be characterised for dead time. 

EM Dead Time As mentioned in Section 4.2.3.3.2, all Electron Multipliers exhibit an effect referred to as dead time. This describes the interval of time after an ion impact has occurred over which the detector is not able to record any signal, i.e. appears dead. This stems from the fact that each electron cascade generated from the respective ion impact on the respective detector spans some length of time. 

Measurement of Intense Signals When the signal count rate (frequency) approaches the dead time of the Electron Multiplier, there are several options that can be implemented to avoid the introduction of dead time effects. Such count rates are commonly noted when examining multiple secondary ion signals of very different intensities. 

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