Mass spectrometry electron multipliers

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). 

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. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

Richter S, Goldberg SA, Mason PB, Traina AJ, Schwieters JB (2001) Linearity tests for secondary electron multipliers used in isotope ratio mass spectrometry. Inti J Mass Spectrom 206 105-127 Rihs S, Condomines M, Sigmarsson O (2000) U, Ra, and Ba incorporation dining precipitation of hydrothermal carbonates imphcations for Ra-Ba dating of impure travertines. Geochim Cosmochim Acta 64 661-671... 

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 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.

M. G. Electron Multipliers in Mass Spectrometry Effect of Molecular Structure. Rev. Sci. lustrum. 1956, 27, 109. 

Electron impact (El) ionization is one of the most classic ionization techniques used in mass spectrometry. A glowing filament produces electrons, which are then accelerated to an energy of 70 eV. The sample is vaporized into the vacuum where gas phase molecules are bombarded with electrons. One or more electrons are removed from the molecules to form odd electron ions (M+ ) or multiply charged ions. Solids, liquids and gases can be analyzed by El, if they endure vaporization without decomposition. Therefore the range of compounds which can be analyzed by El is somewhat limited to thermally stable and volatile compounds. The coupling with gas chromatography has been well established for... 

McLafferty, E.W., Horn, D.M., Breuker, K, Ge, Y., Lewis, M.A., Cerda, B., Zubarev, R.A. and Carpenter, B.K. (2001) Electron capture dissociation of gaseous multiply charged ions by Eourier-transform ion cyclotron resonance. Journal of the American Society for Mass Spectrometry,... 

Figure 1.2 shows the basic instrumentation for atomic mass spectrometry. The component where the ions are produced and sampled from is the ion source. Unlike optical spectroscopy, the ion sampling interface is in intimate contact with the ion source because the ions must be extracted into the vacuum conditions of the mass spectrometer. The ions are separated with respect to mass by the mass analyser, usually a quadrupole, and literally counted by means of an electron multiplier detector. The ion signal for each... 

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

Significant Mass Spectrometry operational parameters were as follows ionization voltage, 70 eV ionization current, 200ifA source temperature, 200°C electron multiplier voltage, 1.9 kV integration time, 1 ms/a.m.u.. For optimum sample transfer, a transfer temperature of 200°C was employed. 

Figure 1 Schematic diagram of a typical commercial inductively coupled plasma mass spectrometry (ICP-MS) instrument (A) liquid sample, (B) peristaltic pump, (C) nebulizer, (D) spray chamber, (E) argon gas inlets, (F) load coil, (G) sampler cone, (H) skimmer cone, (I) ion lenses, (J) quadrupole, (K) electron multiplier detector, (L) computer.

Significant Mass Spectrometry operational parameters follows ionization voltage, 70 eV ionization current, source temperature, 200 C electron multiplier tension. 

U-series nuclide activities can be measured directly by detection of their emitted nuclear particles, e.g., alpha particle counting by solid-state detectors (Ivanovich and Harmon, 1992). In contrast, measurements by mass-spectrometry do not require waiting for Nature to take its course. Atoms of the sample are ionized and accelerated so that charged particles of the nuclides themselves can be measured by Faraday cups or electron multipliers (see Goldstein and Stirling, 2003). Mass-spectrometry is hence a more rapid technique. Typically mass-spectrometry measurements take tens of minutes to hours, while counting methods require days to weeks. 

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