Collectors microchannel plates

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). 

A fuller description of the microchannel plate is presented in Chapter 30. Briefly, ions traveling down the flight tube of a TOF instrument are separated in time. As each m/z collection of ions arrives at the collector, it may be spread over a small area of space (Figure 27.3). Therefore, so as not to lose ions, rather than have a single-point ion collector, the collector is composed of an array of miniature electron multipliers (microchannels), which are all connected to one electrified plate, so, no matter where an ion of any one m/z value hits the front of the array, its arrival is recorded. The microchannel plate collector could be crudely compared to a satellite TV dish receiver in that radio waves of the same frequency but spread over an area are all collected and recorded at the same time of course, the multichannel plate records the arrival of ions not radio waves. 

Diagram showing a flow of ions of m/z a, b, c, etc. traveling in bunches toward the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves toward the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ion.s of m/z a, b, c, etc. are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to V m/z). 

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... 

For a microchannel plate, the back end of each element is left open, as shown in Figures 30.5 or 30.6, and forms a microchannel. Any electrons emerging from any element are all detected by the one collector plate. 

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. 

Timing of Electrical Pulses Resulting from Ion Arrivals at the MicroChannel Plate Collector... 

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). 

Each element of an array or a microchannel plate ion collector is essentially an electron multiplier, similar in operation to the type used for a point ion collector but very much smaller. 

Unlike the array collector, with a microchannel plate all ions of only one m/z value are detected simultaneously, and instrument resolution does not depend on the number of elements in the micro-channel array or on the separation of one element from another. For a microchannel plate, resolution of m/z values in an ion beam depends on their being separated in time by the analyzer so that their times of arrival at the plate differ. 

In a beam of ions separated in time according to m/z value, the total time taken for ions of different m/z values to arrive at a microchannel plate is so short (about 30 psec) that the spectrum appears to have been obtained instantaneously. Thus, for practical purposes, the array and microchannel plate collectors produce an instantaneous mass spectrum, even though the first detects a spatially dispersed set of m/z values and the second detects a temporally dispersed set. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

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