Fast transient signals

Despite these strengths, ICP-MS has also some important drawbacks, many of them related to the spectral isotopic and/or chemical interferences, which affect analyte signal intensities and, therefore, the applicability of the technique. The complexity of the optimisation of the methodological and operating conditions, the differences in the ionisation rates of the various elements, the sequential isotopic measurements and the limited speed of signal acquisition (a serious drawback in multielemental analysis of fast transient signals) are some other problems to be considered. 

The/ocfll plane camera (FPC), still in initial development, consists of an array of 31 Faraday cups, each 145 p.m wide. Up to 15 mjz values can be measured simultaneously. This detector shows improved precision compared with single channel detectors and has the ability to measure fast transient signals such as those from laser ablation. The detector design is described in the references by Barnes et al. and Knight et al. cited in the bibliography. 

Of all atomic spectroscopic methods, ICP-MS is unrivalled concerning its detection power (Bencs et al. 2003). Its capability to process fast transient signals is crucial for the combination with sample preparation methods that generate impulse signals, like e.g. ablation techniques or most on-line preconcentration systems. 

There is no question that TOF ICP-MS, with its rapid, simultaneous mode of measurement, excels at multielement applications that generate fast transient signals. 

The time response of a photoconductive detector is limited by the carrier lifetimes. PbS detectors for instance have time constants between 0.1 - 1 ms while InSb detectors reach a few us. For measurements of fast transient signals photodiodes are more suitable (see Sect.4.5.8). 

The potential of the most commonly used mass spectrometers (quadrupoles and magnetic sector-field single collectors) for elemental analysis has been discussed in detail in Parts 2.1 and 2.2.1 of this chapter. However, some intrinsic limitations still remain with these sequentially scanned systems, particularly when transient or time-dependent signals (such as those produced by laser ablation (LA), electrothermal vaporisation (ETV), flow injection (FI) and chromatography) are used to analyse a large number of isotopes. These scan-based systems can measure only a single m/z at a unit of time. Hence, truly simultaneous determination of multiple isotopes, particularly when fast transient signals are analysed, is not possible without the introduction of spectral skew . ... 

Leach, A. M. and Hieftje, G. M. (2002) Factors affecting the prodnction of fast transient signals in single shot laser ablation indnctively conpled plasma mass spectrometry. Appl Spectrosc., 56, 62. 

Infante, H. Goenaga, Heisterkamp, M., Benkhedda, K., Van Campenhout, K., Blust R., and Adams, F. C. (2001) ICP-TOF-MS for rapid simultaneous multi-element analysis in fast transient signals. Spectra Anal, 220,23. 

As interesting as these few experiments conducted for chromatography with MC-ICP-MS detection are, the results are not conclusive. Most experiments reveal a drift of the analyte isotope ratio during peak elution, and no common rule can be applied to explain this effect. So far, it has not been determined if this drift is of instrumental origin, as a result of the fact that multicollector detection systems are by default not conceived for fast transient signal acquisition. Also, it is not yet clear what effect this drift may have on the accuracy of isotope ratios obtained on transient signals. 

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