Electric field electron spectrometers

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

In a typical mass spectrometer, an organic compound under high vacuum is bombarded with electrons (of about 70 eV energy). Loss of an electron from the molecule followed by various fission processes gives rise to ions and neutral fragments. The positive ions are expelled from the ionisation chamber and resolved by means of a magnetic or an electric field. 

Figure 4.3 Two examples of a 27t-spectrometer collecting electrons within a plane, (a) Suggested set-up with full rotational symmetry around the symmetry axis z. The electric field is supplied by conical bodies, and the detector collects simultaneously all electrons emitted from the source Q into the plane perpendicular to the z-axis (for directional information of electron emission using a discretised detector, see below). From [Kuy68]. (b) Spatial view of a toroidal analyser. The outer and inner toroids which again possess axial symmetry provide the electric field. The detector is a position-sensitive detector (see Section 4.3.2) which records directional information about the electron emission. Hence, the angle dependence of electron emission from the source Q into the plane perpendicular to the symmetry axis is preserved. See also [EBM81, Hue93]. Part (b) reprinted from Nucl. Instr. Meth. B12, Toffoletto et al, 282 (1985) with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1085 KV Amsterdam, The Netherlands.

---