Detectors microchannel plates

Figure Bl.23.10. Schematic diagram of a scattering and recoiling imaging spectrometer (SARIS). A large-area (95 X 75 nnn ), time-resolving, position-sensitive microchannel plate (MCP) detector captures a large...

For either the in-line or hybrid analyzers, the ions injected into the TOF section must all begin their flight down the TOF tube at the same instant if arrival times of ions at a detector are to be used to measure m/z values (see Chapter 26, TOF Ion Optics ). For the hybrid TOF instruments, the ion detector is usually a microchannel plate ion counter (see Chapter 30, Comparison of Multipoint Collectors (Detectors) of Ions Arrays and MicroChannel Plates ). 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

Comparison of Multipoint Collectors (Detectors) of Ions Arrays and MicroChannel Plates... 

Another form of array is called a microchannel plate detector. A time-of-flight (TOP) mass spectrometer collects ions sequentially in time and can use a point detector, but increasingly, the TOP instrument uses a microchannel plate, most particularly in an orthogonal TOP mode. Because the arrays and microchannel plates are both essentially arrays or assemblies of small electron multipliers, there may be confusion over their roles. This chapter illustrates the differences between the two arrays. 

Consider again two detection elements, and suppose an ion beam has been dispersed in time such that ions of m/z 100 arrive at each of several elements (Figure 30.5). In this TOF mode, the next ion of m/z 101 has not yet arrived, and the ion of m/z 99 has arrived previously. Although the m/z ions are dispersed in time over a region of space and strike different elements of the detector, they are collected and monitored simultaneously because all of the microchannels are electronically connected. The operation of the microchannel plate is much easier than that of the array because all the elements are monitored as one at the plate, while each element must be monitored separately in the array. The microchannel plate detector is tremendously useful for those cases in which ions... 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

Fhght times are extremely short (microseconds) for all of the ions, and therefore the scanning of the total mass spectrum from m/z 1 to about m/z 2000-3000 appears to be instantaneous on a human time scale. The arrival of ions at the finishing point is determined by a time-to-digital (TDC) microchannel plate collector (detector). 

The total trajectory of the ions is approximately V-shaped, the top of one leg of the V being the position of the pusher electrode and the top of the other being the position of the ion collector (a microchannel plate detector). 

Alternatively, the ions in a mass spectrometer can also arrive at a multipoint collector as a temporally dispersed beam. Therefore, at any point in time, all ions of the same m/z value arrive simultaneously, and different m/z values arrive at other times. Ail elements of this collector detect the arrival of ions of one m/z value at any one instant of time. This type of detector, which is also an array, is called a microchannel plate collector of ions. 

To differentiate tteir functions and modes of operation, the array collector of spatially dispersed m/z values is still called an array collector for historical reasons, but the other multipoint detector of a temporally dispersed range of m/z values is called a microchannel plate (typically used in time-of-flight instruments). 

LEIS the energies of the ions are too low for PIPS detectors and would lead to a sig-nal-to-noise ratio less than unity. A stack of microchannel plates is, therefore, used to detect the ions in LEIS. The detection efficiency of the microchannel plates is included in the experimental transmission factor Tin Eq. (3.34). 

Figure 2. Schematic of apparatus for ZEKE-PFI spectroscopy, including magnetically shielded electron flight path and microchannel plate detector. Not to scale flight path is 50 cm long interaction zone is 1 cm long.

Figure 2. (a) Reflection TOF mass spectrometer, (b) Depicts the electrostatic potentials. With a judicious selection of potential, the daughter ions arising from metastable decay arrive at the detector prior to the parent ions which have higher kinetic energy. MCP denotes a microchannel plate charged particle detector, (a) Taken with permission from ref. 22 (b) Taken with permission from ref. 19. 

J. Wiza. MicroChannel Plate Detectors. Nucl. Instr. Meth., 162(1979) 587-601. 

A. Westman, G. Brinkmalm, and D. F. Barofsky. MALDI Induced Saturation Effects in Chevron MicroChannel Plate Detectors. Int. J. Mass Spectrom. Ion Proc., 169/170(1997) 79-87. 

The time resolution of a phase fluorometer using the harmonic content of a pulsed laser and a microchannel plate photomultiplier is comparable to that of a single-photon counting instrument using the same kind of laser and detector. 

A much better time resolution, together with space resolution, can be obtained by new imaging detectors consisting of a microchannel plate photomultiplier (MCP) in which the disk anode is replaced by a coded anode (Kemnitz, 2001). Using a Ti-sapphire laser as excitation source and the single-photon timing method of detection, the time resolution is <10 ps. The space resolution is 100 pm (250 x 250 channels). 

In this final section, we summarize the operation and characteristics of the principal vacuum tube and solid state detectors that are available for red/near-IR fluorescence studies. These include conventional photomultipliers, microchannel plate versions, streak cameras, and various types of photodiodes. Detector applicability to both steady-state and time-resolved studies will be considered. However, emphasis will be placed on photon counting capabilities as this provides the ultimate sensitivity in steady-state fluorescence measurements as well as permitting lifetime studies. 

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