Post-acceleration detector

The mass range requirement invariably means that FAB is used in conjunction with a magnetic sector instrument. Conventional detectors, such as the electron multiplier, are not efficient for the detection of large ions and the necessary sensitivity is often only obtained when devices such as the post-acceleration detector or array detector are used. Instruments capable of carrying out high-mass investigations on a routine basis are therefore costly and beyond the reach of many laboratories. 

Discrimination of slower ions as compared to faster ions is observed with SEMs, [243] CEMs, and MCPs as well. [239,244,245] This means a reduction in sensitivity upon reduction of the acceleration voltage of a mass spectrometer, and of course, with increasing ion mass (Eq. 4.3). In order to reduce such effects and especially to improve sensitivity for high-mass ions, post-acceleration detectors have been developed. [236,245] In post-acceleration detectors the ions are accelerated immediatly in front of the detector by a voltage of 10-30 kV before they hit the first dynode or the first MCP. 

Wang, G.H. Aberth, W. Falick, A.M. Evidence Concerning the Identity of Secondary Particles in Post-Acceleration Detectors. Int. J. Mass Spectrom. Ion Proc. 1986, 69, 233-237. 

Figure 1. Schematic illustration of the post-acceleration detector.

Instrumentation. HPLC isolations were performed on a Beckman 421A system using a Vydac column (C-18, 4.6 x 250 mm). Liquid secondary ion mass spectra (LSIMS) were recorded in the positive ion mode on a Kratos (Manchester, UK) MS-50S mass spectrometer equipped with a 23 kG magnet and post-acceleration detector. The LSIMS ion source has been described elsewhere (28). A Cs+ ion beam of energy 10 keV was used as the primary beam (21). Spectra were recorded (300 sec per decade) with a Gould ES-1000 electrostatic recorder. Tandem MS experiments were performed on a Kratos Concept IIHH (Manchester, UK) four sector instrument of EBEB... 

A variant of the EM is the Daly detector (Daly, 1960) in which ions are accelerated by 30 KV (this is called post-acceleration because it occurs after mass analysis) into a conversion dynode, which generates a significant number ( 10) of secondary electrons. These electrons are accelerated into a scintillator and converted into light, which is detected with a photomultiplier. The Daly detector offers high gain, low noise, and excellent stability. Other variants of the post-acceleration detector exist a simple configuration uses a metal plate to convert the ions into electrons for detection with an EM. Another variant of the EM is the microchannel plate (Coplan et al 1984 Odom et al 1990 Wiza, 1979). MicroChannel plate EM detectors have excellent sensitivity but poor gain stability. When operated in... 

Surface that is held at high potential so that ions striking the surface produce electrons that are subsequently detected, Post-acceleration detector. 

Post-acceleration detector Post-source decay (PSD)... 

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