The accuracy and precision of the advanced Poisson dead‐time correction and its importance for multivariate analysis of high mass resolution ToF‐SIMS data

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) data collected in single ion counting mode suffers from dead‐time effects that lead to potentially confusing artefacts when common multivariate analysis (MVA) methods are applied to the data. These artefacts can be eliminated by applying an advanced Poisson dead‐time correction that accounts for the signal intensity in the dead‐time window preceding each time channel. Because this correction is nonlinear, it changes the noise distribution in the data. In this work, the accuracy of this dead‐time correction and the noise characteristics of the corrected data have been analysed for spectra with small numbers of primary ion pulses. A simple but accurate equation for estimating the standard deviation in the advanced dead‐time corrected data has been developed. Based on these results, a scaling procedure to enable successful MVA of advanced dead‐time corrected ToF‐SIMS data has been developed. The improvements made possible by using the advanced dead‐time correction and our recommended scaling are presented for principal components analysis of a ToF‐SIMS image of aerosol particles on polytetrafluoroethylene. Recommendations are made for using the advanced dead time correction and scaling ToF‐SIMS data in order optimize the outcomes of MVA. Copyright © 2014 John Wiley & Sons, Ltd.     

The statistics of ToF‐SIMS data revisited and introduction of the empirical Poisson correction

Generation of time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) data involves two overarching processes: secondary ion production and secondary ion detection. The interpretation of ToF‐SIMS data is facilitated if the intensities of the as‐measured mass spectra are proportional to the abundances of the species under investigation. While secondary ion yield is normally taken to be a linear process, ion detection is not owing to detector dead‐time effects. Consequently, methods have been devised that attempt to linearize, or correct, data that are affected by the dead time. In this article, we review the statistics of ToF‐SIMS data generation and confirm a report in the literature that abundance estimates from so‐called Poisson corrections are biased. We show that these corrections are only unbiased asymptotically and that a rigorous probabilistic analysis can quantitatively account for the observed bias. Two sources of bias are identified, one having a statistical basis and one due to the form of the correction equation at high ion detection rates. Based on insights gained from this analysis, we propose a new correction equation, the empirical Poisson correction, which largely eliminates the statistical bias. The performance of the proposed correction is illustrated by reanalyzing 14 experimentally measured datasets that suffer from varying levels of dead‐time effects. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of a new dead‐time correction method and conventional models for SIMS analysis

Recently, secondary ion mass spectrometry (SIMS) has been used in the analysis of not only impurities but also matrix elements, thus requiring a wide dynamic range for SIMS analysis. However, SIMS detectors, which are mostly used in pulse counting systems, have difficulties with detector saturation. In this paper, we investigate whether a dead‐time model that was developed for X‐ray measurement is applicable for SIMS analysis. We then compare a new correction method with conventional correction methods for detector saturation in SIMS analysis. We report that the new method can better correct the intensity in regions of higher intensity than that achieved by conventional methods. Copyright © 2012 John Wiley & Sons, Ltd.     

A software counting loss correction method for neutron activation analysis with short-lived radionuclides

A method has been developed for the correction of counting losses in NAA for the case of a mixture of short-lived radionuclides. It is applicable to systems with Ge detectors and Wilkinson or successive approximation ADC's and will correct losses from pulse pileup and ADC dead time up to 90%. The losses are modeled as a constant plus time-dependent terms expressed as a fourth order polynomial function of the count rates of the short-lived radionuclides. The correction factors are calculated iteratively using the peak areas of the short-lived radionuclides in the spectrum and the average losses as given by the difference between the live time and true time clocks of the MCA. To calibrate the system a measurement is performed for each short-lived nuclide. In a test where the dead time varied from 70% at the start of the measurement to 13% at the end, the measured activities were corrected with an accuracy of 1%.     

Comparison of mathemical methods for the calculation of retention indices at high temperature in gas chromatography

The retention indices of three homologous series (2-alkanones, 1-alkanols, cycloalksanones) have been determined at high temperature by the application of two new adaptation methods: A multiparametric least-squares regressions iterative method based on the dertermination of the adjusted retention times and a cubic interpolation directly using the uncorrected retention times without dead time correction. The two methods were applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87 stationary phases), while the second includes five glass capillary columns (four methyl-silicons with different film thicknesses and Apolane-87 stationary phases). The retention indices obtained with a multiparametric and a cubic interpolation methods were compared with each other and with those calculated by Grobler's, Guardino's, Kaiser's and Kovàts' methods. The influence of coating, film thickness, and temperature on them was investigated.     

全二维气相色谱第二维死时间的测定

建立了两种恒压模式下全二维气相色谱第二维死时间的测定方法。一种方法是利用不同压力下的相对保留时间差规律,计算非同步调制的全二维气相色谱第二维的保留时间,再利用正构烷烃同系物的保留规律线性拟合计算第二维的死时间;测定的第二维的死时间与温度的线性相关系数大于0.997。另一种方法是在已知化合物保留因子和温度关系的条件下,在一次程序升温中测定此化合物的3个以上不同流出温度条件下的表观保留时间,再根据该表观保留时间计算出死时间与温度的关系。实验结果表明,两种方法对死时间测定的偏差小于0.05 s。这两种方法适合于各种类型的全二维气相色谱,无论其调制方式是同步还是非同步。     

On the statistical control of 'loss-free counting' and 'zero dead time' spectrometry

The assessment of the statistical counting uncertainty is discussed for two pulse loss correction methods in nuclear spectrometry: the 'loss-free counting' technique based on the virtual pulse generator method and the 'zero dead time' technique with 'variance spectrum'.     

Another method of dead time correction

A new method of the correction of counting losses caused by a non-extended dead time of pulse detection systems is presented. The approach is based on the distribution of time intervals between pulses at the output of the system.     

Application of short-lived isotopes spectrometry coupled with k0 standardization to the neutron activation analysis of geological materials

The described procedure is purely instrumental. The aim is to determine efficiently the elemental composition of geological materials by neutron activation, using short-lived isotopes. Our procedure requires the use of a gamma-spectrometric system fitted with a real time correction module for the counting losses and the quasi absolute k₀-method. Consequently, the two constraints inherent in the analysis of short-lived isotopes, i.e. decreasing dead time counting and relative standardization were overcome.     

Linearity of the instrumental intensity scale in TOF‐SIMS—a VAMAS interlaboratory study

A VAMAS interlaboratory study involving 21 time‐of‐flight SIMS instruments from nine countries has been conducted to evaluate the linearity of the instrumental intensity scale and procedures for intensity correction. Analysts were supplied, by National Physical Laboratory (NPL), with a protocol for analysis (closely aligned with ISO 23830) together with a reference material of polytetrafluoroethylene (PTFE) tape. The primary ion beam current is varied to provide secondary ion intensities that span the linear to nonlinear regime. The natural carbon isotope ratios ¹²C_xF_y⁺/¹³C¹²C_x?1F_y⁺ for five peaks are used to evaluate the linearity, without a need to measure the ion beam current. A method is given for determining the linearity as a function of secondary ion intensity, with and without dead time correction. It is found that single ion counting statistics is closely obeyed, and the linearity achievable is generally excellent with careful application of dead time correction. Three quarters of instruments in the study achieved better than 95% linearity at a count rate of 0.8 measured counts per pulse, equivalent to 1.6 secondary ions impinging the detector per primary ion pulse. We discuss factors affecting linearity and the precise application of dead time correction and give guidance for practical analysis. This includes suboptimal detector efficiency, inhomogeneous intensities across the rastered area, inadequate charge compensation, and the choice of peak integration limits. The interlaboratory study shows that the method to determine linearity is generally applicable, robust and provides an excellent basis for the development of an ISO standard. © Crown copyright 2011. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Correction for residual dead-time losses in INAA based on short-lived radionuclides

A correction procedure is presented for the residual bias due to dead time losses in thermal INAA based on short-lived radionuclides. It is based on a linear least-squares fitting, starting from three pre-chosen (average) half-lives.     

Quadrupolar echo deuteron magnetic resonance spectroscopy in ordered hydrocarbon chains

The quadrupolar spin echo from deuterons in ordered hydrocarbon systems is shown to provide a much more reliable spectrum than the conventional free induction decay Fourier transform. Spectrometer dead time is eliminated, phase correction uncertainties removed and signal/noise enhanced.     

Extending the dynamic range of proton transfer reaction time-of-flight mass spectrometers by a novel dead time correction

Proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) allows for very fast simultaneous monitoring of volatile organic compounds (VOCs) in complex environments. In several applications, food science and food technology in particular, peaks with very different intensities are present in a single spectrum. For VOCs, the concentrations range from the sub-ppt all the way up to the ppm level. Thus, a large dynamic range is necessary. In particular, high intensity peaks are a problem because for them the linear dependency of the detector signal on VOC concentration is distorted. In this paper we present, test with real data, and discuss a novel method which extends the linearity of PTR-TOF-MS for high intensity peaks far beyond the limit allowed by the usual analytical correction methods such as the so-called Poisson correction. Usually, raw data can be used directly without corrections with an intensity of up to about 0.1 ions/pulse, and the Poisson correction allows the use of peaks with intensities of a few ions/pulse. Our method further extends the linear range by at least one order of magnitude. Although this work originated from the necessity to extend the dynamic range of PTR-TOF-MS instruments in agro-industrial applications, it is by no means limited to this area, and can be implemented wherever dead time corrections are an issue.     

Improved performance in germanium detector gamma-spectrometers based on digital signal processing

In an HPGe spectroscopy system, Digital Signal Processing (DSP) replaces the shaping amplifier, correction circuits, and ADC with a single digital system that processes the sampled waveform from the preamplifier with a variety of mathematical algorithms. DSP techniques have been used in the field of HPGe detector gamma-ray spectrometry for some time for improved stability and performance over their analog counterparts. Recent developments in HPGe detector construction and new liquid nitrogen-free cooling methods have resulted in HPGe detectors which are better adapted to the needs of the application. Some of these improvements in utility have degraded the spectroscopy performance. With DSP, it is possible to reduce the changes, in real time, in several aspects of detector performance on a pulse-by-pulse basis, which is not possible in the old analog environment. In the past, in designing for the analog regime, flexibility was limited by issues of component size, number and cost. In the digital domain, the problem translates to the need for a DSP with enough speed and an efficient algorithm to achieve the desired transformation or correction to the digitally determined pulse shape or height, event-by-event. The use of DSP allows the peak processing to be tuned to the preamplifier peak shape from the detector rather than being set to an average value determined from several detectors of the type in question. The selection of the filter can be automatic or manual. The following corrections are now possible: ballistic deficit correction, peak resolution improvement by reducing the impact of microphonic noise, increase throughput by reducing pulse processing time, and loss-free (zero dead time) counting.     

反相毛细管电色谱中两种电渗流测定方法的比较

在毛细管电色谱中电渗流的测定方法有许多种,常用的是示踪剂法。本文将硫脲示踪剂法和迭代分析法测定的电渗流结果加以对比分析,结果表明:在不同有机调节剂体积分数、不同电压、不同电解质浓度下,虽然用两种方法测得的电渗流随诸因素的变化规律符合电渗流变化的一般规律,但强极性硫脲分子存在电迁移现象,使测得的电渗流较真实值有所偏离。偏离的大小随有机调节剂体积分数、电解质浓度等实验条件变化。     

14 MeV neutron activation analysis using short-lived products

Cyclic activation using pneumatic shuttling system and switch off and on the neutron source and detector are described in order to eliminate some uncertainties by the provision of more accurate timing, the measurement of the effective activating neutron flux and the correction for the detection system dead time.     

Maintaining Accuracy in Gamma-Ray Spectrometry at High Count Rates

Gamma-ray spectrometry losses through pulse processing dead time and pile-up are best assayed with an external pulse technique. In this work, the virtual pulse generator technique as implemented commercially with the Westphal loss free counting (LFC) module is set up and tested with four high resolution gamma-ray spectrometers. Dual source calibration and decaying source techniques are used in the evaluation of the accuracy of the correction technique. Results demonstrate the reliability of the LFC with a standardized conventional pulse processing system. The accurate correction during high rate counting, including during rapid decay of short lived activities, has been the basis for highly precise determinations in reference materials studies.     

Migration time correction for the analysis of derivatized amino acids and oligosaccharides by micellar capillary electrochromatography

Migration-time reproducibility is essential in the use of capillary electrophoresis to identify components in mixtures. Two methods based on the migration time of either one or two reference markers are proposed for improving migration time reproducibility. These methods were evaluated to determine the migration time reproducibility for phenylthiohydantoin-amino acids, fluorescein thiohydantoin-amino acids, and tetramethylrhodamine labeled oligosaccharides. In the best case, the relative standard deviation of the migration time was reduced from >3% without correction to <0.04% with the two-marker correction.     

The correction for the counting yield in nuclear spectroscopy

Pulser and live timer are alternate tools. Dead time effects can be expressed in terms of a pulse rate dependent factor of the counting yield. The task of their correction should be shifted from the live timer of the ADC to a central timing unit. A new method is proposed, combining the advantages of the pulser and the live timer, where by each selected and accepted event is adjoined to a clock time interval and each selected but not accepted event to a dead time interval. The length of each interval is determined by the arrival of the next selected event.     

An innovative method for dead time correction in nuclear spectroscopy

All nuclear spectroscopy systems, whether measuring charged particles, X-rays, or gamma-rays, exhibit dead time losses during the counting process due to pulse processing in the electronics. Several techniques have been employed in an effort to reduce the effects of dead time losses on a spectroscopy system including live time clocks and loss-free counting modules. Live time extension techniques give accurate results when measuring samples in which the activity remains roughly constant during the measuring process (i.e., the dead time does not change significantly during a single measurement period). The loss-free counting method of correcting for dead time losses, as introduced by HARMS and improved by WESTPHAL (US Patent No. 4,476,384) give better results than live time extension techniques when the counting rate changes significantly during the measurement. However, loss-free counting methods are limited by the fact that an estimation of the uncertainty associated with the spectral counts can not be easily determined, because the corrected data no longer obeys Poisson statistics. Therefore, accurate analysis of the spectral data including the uncertainty calculations is difficult to achieve. The Ortec® DSPEC ^PLUS implements an improved zero dead time method that accurately predicts the uncertainty from counting statistics and overcomes the limitations of previous loss-free counting methods. The uncertainty in the dead-time corrected spectrum is calculated and stored with the spectral data (Patent Pending). The GammaVision-32® analysis algorithm has been improved to propagate this uncertainty through the activity calculation. Two experiments are set up to verify these innovations. The experiments show that the new method gives the same reported activity and associated uncertainties as the well-proven Gedcke-Hale live time clock. It is thus shown that over a wide range of dead times the new ZDT method tracks the true counting rate as if it had zero dead time, and yields an accurate estimation of the statistical uncertainty in the reported counts.