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the cold screen, by virtue of its high pumping speed (approximately 10 1/s.cm ) provides for extremely low organic background signals and fast recovery of the system after each analysis.

Figure 20 shows a completely automated Curie-point Py-MS system. This automated system was built at the F.O.M. Institute for Atomic and Molecular Physics in Amsterdam (ref. 43) and has been in operation since 1976. Manual versions based on the principles shown in Figure 18 or following a somewhat different approach are commercially available (ref. 120). Removal of the expansion chamber and pre-heating of the reaction tube walls requires mass ranges up to m/z 800 or higher if full use is to be made of the information available from large evaporated or pyrolysed molecular species, e.g. lipids. The increased scanning time per spectrum caused by this large mass range has to be compensated for by slower heating rates of the sample or longer residence times of the pyrolysate in the expansion chamber, if used.

4.6. ION DETECTION

In principle, ion counting has considerable advantages over analogue ion current monitoring. The large bandwidth of pulse amplifiers used in ion counting systems allows the recording of extremely small signals at maximum scanning speed of the quadrupole mass spectrometer. Current amplifiers, in contrast, have lower bandwidths at high amplifications, thus limiting the scanning speed or spoiling peak resolution. Moreover, the use of pulse amplifiers greatly facilitates the automation of Py-MS systems since samples producing unexpectedly small or large signals do not require a change in amplification setting by the operator. Further advantages of pulse counting systems are the direct insight into the signal sampling statistics for each peak, the lack of immediate signal reduction with decreases in multiplier gain (provided that the gain remains above a certain minimum level), and the absence of signal fluctuations caused by mechanical vibrations in multi-stage dynode multipliers (e.g. caused by mechanical pumps). A serious disadvantage of ion counting, however, is the limited counting rate of commercially available systems. Typical maximum counting rates are of the order of 10 ions/s. Assuming a scan rate of 10 amu/s, the maximum number of ions accurately counted for any given mass will be less

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