Counting statistics dead time correction

These saturation effects, resulting from the loss of ions due to the TDC dead time, can be statistically corrected by applying a correction factor [4], However, there is no effective correction when the quantity of ions increases and several ions arrive at the detector simultaneously. In conclusion, detection systems that operate in pulse counting mode are well suited to detect small quantities of ions by accumulation for a long period of time and when detection of individual ion events is important in order to obtain a good signal-to-noise ratio. 

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. 

The upper limit for ion beam currents that can be measured using ion counting detection (for trace quantitative analysis) is determined by the dead time of the ion detector, since ions arriving during a detector dead time do not give rise to an output pulse and are thus not counted. The time distribution of individual ions arriving at the detector is essentially random but is well described by Poisson statistics this can be exploited in application of a correction factor applied to ion counts to extend the linear range by up to a factor of 10 above the limit imposed by the detector dead time. 

The application of isotope dilution analysis to ICP-MS-based hyphenated techniques, for example HPLC-ICP-MS, can be divided into two diflerent modes the spedes-unspecific mode and species-specific mode. The two modes are illustrated in Figure 4.6. Because IDA-ICP-MS relies on the measurement of isotope ratios by mass spectrometry, the traditional interference to the accuracy and precision of IDA should all be considered and some biases need further correction in this case, such as spectral interferences, dead time of detector, mass discrimination, and statistic and stability of ion counting. 

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