Post-detector electronics

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. 

The fluorescence detector is a specific and concentration-sensitive detector. It is based on the emission of photons by electronically excited molecules. Fluorescence is especially observed for analytes with large conjugated ring systems, e.g., polynuclear aromatic hydrocarbons and their derivatives. In order to extend its applicability range, pre-column or post-column derivatization strategies have been developed [9]. 

Fig. 3.5. A schematic diagram of the McMullan PEELS acquisition system currently in operation (courtesy of Dr C. Walsh). Simple switching of the post-quadrupole deflection coil can direct the transmitted electrons to separate detectors to form a bright field image as well as a two-dimensional micro-diffraction image respectively.

The Biology Department beam line includes a station for protein crystallography (with Supper oscillation camera and FAST TV diffractometer) and a station for small angle diffraction (with a three-circle goniostat and MWPC electronic area detector). The latter station may be available for optimised anomalous dispersion crystallographic studies. The optical design for each consists of a bent pre-mirror, double crystal monochromator and bent post-mirror the mirrors have rhodium coatings (Wise and Schoenborn 1982). 

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... 

IEEE 1996. Institute of Electrical and Electronics Engineers. Report No. 325-1996. IEEE Standard Test Procedures for Germanium Gamma-Ray Detectors. Posted online at iee-explore.ieee.org in 2002. 

Figure 2.1 Schematic diagram of an electron microscope with X-ray detector and post column energy filter. [Reproduced from Hawkes (2004)]...

In addition, the spatial distribution of the inelastically scattered electrons carries a strong forward-bias such that the majority of the signal can be collected by the on-axis post-specimen EELS spectrometer commonly employed in the AEM. This makes EELS a far more efficient technique in terms of signal collection than XEDS, where the detector samples only a tiny fraction (< 1%) of the emitted X-ray photons. An energy resolution of less than 1 eV in the EELS spectrum is readily achievable in most commonly available instrumentation a value that is far superior to the 150 eV spectral resolution of XEDS. Finally, and perhaps most uniquely, with careful analysis, the EELS spectrum can also yield information related to the electronic structure and inter-atomic bonding present in the specimen, both of which are not detectable with the XEDS technique. 

Unlike UV or fluorescence detectors, ED does not exploit a physical property of an analyte, but an induced chemical change that results from an electrochemical reaction. ED must, therefore, be considered to be a type of post-colunm chemical reaction detector. ED differs, however, from other post-colunm reactors used in HPLC in that no reagents (other than electrons) or reaction devices are normally required to effect the chemical change in the analyte. In addition, the reaction kinetics are usually fast leading to minimal extra-colunm effects. 

In Transmission Electron Microscopy (TEM), a very high energy monoenergetic electron beam (100 to 400 keV) passes through a thin specimen (less than 1000 nm) of diameter less than 3 mm (necessary to fit in the electron optics column). A series of post specimen lenses transmits the emerging electrons, with spatial magnification up to 1,000,000, to a detector (fluorescent screen or video camera) viewed in real time. 

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