Detectors dead time

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

Also important is the effect of detector dead time. When ions are detected using a pulse counting (PC) detector, the resultant electronic pulses are approximately 10 ns long. During and after each pulse there is a period of time during which the detector is effectively dead (i.e. it cannot detect any ions). The dead time is made up of the time for each pulse and recovery time for the detector and associated electronics. Typical dead times vary between 20 and 100 ns. If dead time is not taken into account there will be an apparent reduction in the number of pulses at high count rates, which would cause an inaccuracy in the measurement of isotope ratios when abundances differ markedly. However, a correction can be applied as follows ... 

Isotope ratio analysis, 131-134 abundance, 131 correction factor, 131 detector dead time, 132-133 mass bias, 131. 132(f) precision of, 133... 

Detector dead time. This is the time required for the detection and electronic handling of an ion pulse. If another ion strikes the detector surface within the time required for handling the first ion pulse, the second ion will not be detected and, hence, the observed count rate will be lower than the actual value. If this is not corrected for, inaccurate isotope ratio results will be reported. In ICP-MS, several mathematical methods should be applied for its evaluation and correction. 

Errors in timing. For the determination of elements yielding short half-life indicator radionuclides (such as in the determination of oxygen via 7.3 sec16N), accurate timing is extremely important. For these cases electronic scaler timers are to be preferred over electromechanical types of timers. Errors due to variable detector dead-time must also be considered when the gross activities of the sample and the standard differ appreciably and the indicator radionuclide is short-lived. 

Detector coincidence loss (detector dead time). 

An implicit assumption in Eqs. (17) to (19) is that the probability D(E ) that a neutron of energy E is detected, is independent of t, which is not true if detector dead time effects are significant. 

Since the results obtained are independent of sample geometry and scattering power, sample attenuation, multiple scattering and detector dead time effects can all be eliminated as a possible cause of the observed anomalies. 

Corrections need to be made for instrumental effects, such as mass bias and detector dead-time (see Section 3.10). Using the signal matching procedures outlined in this guide this can be achieved by running an alternating sequence of standards and samples. 

Isotopic discrimination (mass fractionation, detector dead-time, mass bias)... 

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. 

The problem of the detector dead time is avoided by the Hanbury-Brown-Twiss setup [215]. This setup is the basis of almost all TCSPC photon correlation experiments. 

The principle is shown in Fig. 5.100. The investigated light signal is split by a 1 1 beam splitter, and the two light signals are fed into separate detectors. One detector delivers the start pulses, the other the stop pulses of a TCSPC device. The stop pulses are delayed by a few ns to place the coincidence point in the centre of the recorded time interval. The setup delivers a histogram of the time differences between the photons at both detectors. Because separate detectors are used for start and stop, there is no problem with detector dead time. 

In order to reduce systematic errors, such as ion counting errors due to detector dead time, it is better to fix the isotope amount ratio as close to unity as possible. In order to do this it is necessary to know the approximate concentration of the analyte in the sample prior to spiking. An exact 1 1 isotope amount ratio can be achieved by using an iterative matching... 

Detector dead time Also important is the effect of... 

For a measurement of continuous beam current (i.e. for rates of ion arrival that are too large for ion counting detection as a result of detector dead time, see below), it is appropriate to replace the ion counting time window with a more suitable parameter the appropriate parameter is the time constant (t ) of the detection chain (usually dominated by the RC time constant of the current-to-voltage converter, see Section 7.4). If the 4 value is instantaneously increased to a new value, the 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. 

In case of pulsed lasers, the probability of ion generation approaches unity per pulse, but detector dead time can result in errors if too many ions arrive at the detector in a short time. However, pulsed-laser approach is ideal in detecting extremely small numbers of atoms. 

This sample preparation method is not instrument specific but was developed using a Thermo Fisher Element 2 MS-ICP-MS. Whatever instrument is used, the instrument should be mass calibrated. Although it is preferable for the sake of accuracy that all uranium isotopes be acquired in the same detector mode, cross-calibration should be up to date for single collector instruments, as should detector dead time. Since the Thermo Fisher magnetic sector instrument with a low-resolution ion slit in good condition produces data with flat top peaks, and automatically updates detector crosscalibration after acquisition of peak data with sufficient intensity, frequent recalibrations will not be necessary on this instrument or on the Thermo Fisher multicollector instruments. Detector dead time is relatively stable on this instrument but should be up to date. 

Although it is preferable for the sake of accuracy that all uranium isotopes be acquired in the same detector mode, cross-calibration should be up to date, as should detector dead time. Mass calibration should be performed every 2-3 days for accurate isotope ratio determination. 

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