Electronic photoplate

The microchannel plate detector can, however, also work with a metal anode that gathers the stream of secondary electrons at every channel exit. To avoid any confusion, the term array detector is preferably used to describe a microchannel plate where every microchannel remains as an individual ion-detecting element. This array detector acts as electronic photoplates. Indeed, it resembles that of a photographic plate ions with different m/z ratios reach different spots and may be counted at the same time during the analysis. The advantage of array detectors is that analyser scanning is not necessary and therefore sensitivity is improved because simultaneous detection of ions implies that more ions are collected, and this greater efficiency leads to lower limits of detection than for other detectors. 

Another electro-optical ion detector, which is called the electro-optical array detector, allows the simultaneous measurement of ions spatially separated along the focal plane of the mass spectrometer. It combines the microchannel plate and Daly detector. The ions are converted in a microchannel plate into electrons that are amplified. The released secondary electrons finally strike a phosphorescent screen that emits photons. These photons are then detected with a photodiode array or CCD detector. This array detector acts as electronic photoplates. 

Figure 2.21. Schematic of (a) a photoplate detector (b) a Faraday cup (c) a discrete-dynode electron multiplier (EM) of Venetian blind type and (d) a continuous dynode EM. Parts (c) and (d) reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

Focal plane detectors are used primarily to detect ions separated in space by, for example, magnetic sector analyzers (see Section 2.2.2). The objective of an ideal focal plane detector is to simultaneously record the location of every ion in the spectrum. In many ways the photoplate (see Section 2.3.1) is the original focal plane detector, but it has today been more or less replaced with designs that rely on EM detectors (see Section 2.3.3). A common arrangement is to allow the spatially disperse ion beams simultaneously to impinge on an MCP (see Section 2.3.3.2). The secondary electrons generated by the ion impacts then strike a one- or two-dimensional array of metal strips and the current from the individual electrodes is recorded. A tutorial on the fundamentals of focal plane detectors is found in Reference 283. Reference 284 provides a relatively recent review of MS detector-array technology. 

Modern devices use photodiodes or CCD arrays (Sect. 4.5.2) instead of photoplates. With a diode width of 25 p.m, the peak of a symmetric line profile extending over 3—5 diodes can be determined by a least-squares fit to a model profile within 1—5 xm, depending on the S/N ratio. When the array is placed behind a spectrometer with a dispersion of 1 mm/nm, the center of the line can be determined within 10 nm. Since the signals are read electronically, there are no moving parts in the device and any mechanical error source (backlash) is eliminated. 

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