Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

306 DETECTION AND ESTIMATION OF INTERMEDIATES3.2 IONIC SPECIESIt is natural that mass spectrometric techniques should have been applied tospecies that are already charged, and a number of studies have been made of bothnegative and positive ion formation in chemical reactions. One of the earliest investigationsby mass spectrometry of ions produced outside the spectrometer wasthat of Brasefield'" who produced ions in an electric discharge. Although suchsystems are of great importance in the study of ion-molecule reactions, they do notreally fall within the scope of this chapter, and most of the results to be discussedconcern chemi- or photo-ionization.The problems of sampling, already discussed for uncharged species, are augmentedin the case of ions. Boundary potentials are often set up at the wall containingthe sampling orifice, and an ion sheath may be formed at the wall (thesheath is a positive space charge, since the species of highest mobility are electrons).Except under particularly unfavourable conditions, the boundary potential willnot result in appreciable acceleration of positive ions (or retardation of negativeions). However, some electrostatic lens effect may operate at the orifice, thus falsifyingthe sample. Again, the loss of positive ions to the surface may be enhancedduring passage through the orifice. Pahl'" has considered in detail the possiblemisfortunes which may befall ions during passage through a sampling orifice.IFocusing /and accelerating {=I = -)--- 7slits 0.2 x 1.5 crn - -Split slit--'-for focusing/ OVOltS- 200ElliDtical 20CYlindricat 1 41 I0d'I- 205150Entrance orifice4W &Fig. 10. Device for sampling ions. Typical operating voltages are given.