Single-point electron multipliers

An assemblage (array) of single-point electron multipliers in a microchannel plate is designed to detect all ions of any single m/z value as they arrive separated in time. Thus, it is not necessary for each element of the array to be monitored individually for the arrival of ions. Instead, all of... 

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. 

In modem mass spectrometry, ion collectors (detectors) are generally based on the electron multiplier and can be separated into two classes those that detect the arrival of all ions sequentially at a point (a single-point ion collector) and those that detect the arrival of all ions simultaneously (an array or multipoint collector). This chapter compares the uses of single- and multipoint ion collectors. For more detailed discussions of their construction and operation, see Chapter 28, Point Ion Collectors (Detectors), and Chapter 29, Array Collectors (Detectors). In some forms of mass spectrometry, other methods of ion detection can be used, as with ion cyclotron instmments, but these are not considered here. 

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. 

An electron multiplier can be thought of as a point detector in that a single conversion dynode and multiplier are configured to detect the ion signal. Ions to be detected must first be maneuvered to a single precise position. An alternative possibility, in which numerous electron multipliers are configured together to provide an array [73] requires miniaturization and juxtaposition of the individual electron multipliers into a continuous detector. 

Renard and Deloche [261] examined the surface diffusion of physi-sorbed tritium on a single crystal Ni lll] surface. The gas was deposited as a patch with the crystal held at 4K and the concentration profile across the surface was determined by collection of the radiation emitted from tritium in a channeltron electron multiplier. For the diffusion experiments, the collector was positioned so as to collect radiation from a point well outside the original patch area and the sample was then heated to temperatures in the range 13—20 K. Desorption was also appreciable from, the physisorbed layer and so they derived the coverage-time relation (at fixed temperature)... 

Energy-optimized, single-Slater values for the electron subshells of isolated atoms have been calculated by Clementi and Raimondi (1963). For the electron density functions, such values are to be multiplied by a factor of 2. Values for a number of common atoms are listed in Table 3.4, together with averages over electron shells, which are suitable as starting points in a least-squares refinement in which the exponents are subsequently adjusted by variation of k. A full list of the single values of Clementi and Raimondi can be found in appendix F. 

The reaction sequence shown above illustrates three important aspects of chemistry that will be shown to be very important in the discussion of atmospheric chemistry in Section 2.8. The first of these is that a reaction may be initiated by a photochemical process in which a photon of light (electromagnetic radiation) energy produces a reactive species, in this case the Cl- atom. The second point illustrated is the high chemical reactivity of free radical species with unpaired electrons and incomplete octets of valence electrons. The third point illustrated is that of chain reactions, which can multiply manyfold the effects of a single reaction-initiating event, such as the photochemical dissociation of Cl2. 

The result is multiplied by JV, the total number of electrons, in the definition of an atomic property. The reader is reminded that the mode of integration indicated by N dx [l/ ijy as used in this definition of an atomic average is the same as that employed in the definition of the electronic charge density, p r) (eqns (1.3) and (1.4)). From this point on the subscript T will be dropped from the coordinates of the electron whose coordinates are integrated only over 2 and all single-particle, unlabelled coordinates and operators will refer to this electron. 

At this point we may introduce the spin of the electrons into the wave function (in the same manner as for helium) by multiplying each single-electron orbital function by either < ( ) or /3(w). For convenience we shall include these spin factors in the functions u (l), etc., so that hereafter a, j3, y, represent four quantum numbers n, l, mi, and m, for each electron and 1, 2, represent four coordinates n, d,-, electron case, treatment of this degenerate energy level by perturbation theory (the electron interactions being the perturbation) leads to certain combinations... 

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