Electron-multiplier tubes

As with mass analyzers, many types of mass detectors equipped with an electron multiplier are available. Most common mass detectors are the chan-neltron, Daly detector, electron multiplier tubes, and the Faraday cup. All generate a current when charged analytes generated in the source and separated in the analyzer impinge on them. This current is recorded as a function of the masses selected by the electrical-field settings. 

Detectors include (1) Geiger-Mueller tube, (2) ionization chambers, (3) scintillation counters, (4) proportional counter, (5) electron-multiplier tubes, and (G) nondispersive detectors using cooled lithium-drifted Si detectors, See Fig. 3... 

Electron multiplier tubes are similar in design to photomultiplier tubes. They consist of a primary cathode and a series of biased dynodes that eject secondary electrons. Therefore, any incident charged particle induces a multiplied electron current. A channeltron is a hom-shaped continuous dynode stmcture that is coated on the inside with an electron-emissive material. Any charged particle, but also high-energy U Vor X-ray photons, striking the channeltron creates secondary electrons that have an avalanche effect to create the final current. 

The photoelectrons can be detected by a hemispherical analyzer or an electron multiplier tube or a multichannel detector such as a microchan-nel plate. The photoelectron detector usually has energy resolution of 0.3 -0.4 eV, acceptance solid angle of 6% of solid angle 27t. The energy of the... 

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. 

Each element of an array detector is essentially a small electron multiplier, as with the point ion collector, but much smaller and often shaped either as a narrow linear tube or as somewhat like a snail shell. 

Figure 1. Fast-flow reaction apparatus. Ions or ion clusters are introduced into the flow tube from various sources and reactions proceed after they encounter the reactants added through a ring injector located at a selected position in the flow tube. The disappearance of the reactant ions and formation of products is monitored with the quadrupole mass spectrometer/electron multiplier shown. Taken with permission from ref. 19.

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Figure 6. Diagram of our 1-atm ion mobility spectrometer (IMS) apparatus (a) stainless steel source gas dilution volume, (b) septum inlet, (c) needle valve, (d) Nj source gas supply, (e) source and drift gas exhaust, (f) flow meter, (g) pressure transducer, (h) insulated box, (i) drift tube, (j) ion source, (k) Bradbury-Nielson gate, (I) Faraday plate/MS aperture, (m) drift gas inlet, (n) universal joint, (o) electrostatic lens element, (p) quadrupole mass filter, (q) 6"-diffusion pump, (r) first vacuum envelope, (s) channeltron electron multiplier, (t) second vacuum envelope, (u) 3"-dif-fusion pump, (v) Nj drift gas, (w) leak valve, (x) on/off valves, (y) fused silica capillary, (z) 4-liter stainless steel dilution volume, (aa) Nj gas supply.

At the electron multiplier detector, each arriving ion starts a cascade of electrons, just as a photon starts a cascade of electrons in a photomultiplier tube (Figure 20-12). A series of dynodes multiplies the number of electrons by 105 before they reach the anode where current is measured. The mass spectrum shows detector current as a function of mlz selected by the magnetic field. 

The experiments of Kistiakowsky and Kydd [1] were done by single-pulse photolysis with a 500-J flashlamp, the reaction vessel contents being sampled via a pinhole leak into the electron ionization source of a Bendix time-of-flight (TOF) mass spectrometer. Mass spectra were obtained by pulsed extraction of ions from the ion source at 50-fis intervals after the flash. The signal from the electron multiplier detector was displayed on a cathode ray tube, which was photographed with a rotating drum camera. 

An XPS spectrometer contains an X-ray source - usually Mg Ka (1253.6 eV) or A1 Ka (1486.3 eV) - and an analyzer which, in most commercial spectrometers, is hemispherical in design. In the entrance tube, the electrons are retarded or accelerated to a value called the pass energy , at which they travel through the hemispherical filter. The lower the pass energy, the smaller the number of electrons that reaches the detector, but the more precisely is their energy determined. Behind the energy filter is the actual detector, which consists of an electron multiplier or a channeltron, which amplifies the incoming photoelectrons to measurable currents. Advanced hemispherical analyzers contain up to five multipliers. For further details of these instruments the interested reader should refer to other textbooks [20, 21]. 

This last process can be accomplished by the time of flight of the ions down an evacuated tube. Detection is usually by electron multipliers. The most common application of the MS process is in gas plants, where many gases can be detected simultaneously, or multiple streams can be examined sequentially. 

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