5.2.1.2 Ionization Techniques for Volatile Analytes Entering the MS from a GC
5.2.1.3 Repulsion and Accelerator Plates, Slits, and Electronic Focusing Lens:
Ions, regardless of the way they are generated, need to be accelerated into the mass filter/analyzer in order to separate ions of different masses. Since the majority of the ionization sources produce positively charge species, the most common way of accelerating ions is to place a positively charged plate on the “upstream” side of the system. This plate repels the cations toward the mass filter/analyzer. Most systems require ions to have a minimum velocity, so negatively charged plates are placed on the “downstream” side of the instrument, just prior to the mass filter, to accelerate the ion in that direction (shown earlier in the EI and CI animations). The accelerator plates also act as slits since a relatively small hole is present in the middle of the plates that allow some of the ions to pass through the plate/slit and into the mass filter.
Accelerator plates/slits can also act as “gates” to the mass filter. This is accomplished by placing a positive charge on the slit that will repel the entry of an ion fragment or packet of ions to the system. Gates are used to hold up the entry of new ions to the mass filter until all of the ions have passed through to the detector. After this, the polarity on the gate is returned to negative and a new set of ion fragments is allowed to enter the mass filter. This type of gating system is important in the time-of-flight mass filters discussed in Section 5.5.4.
Some systems, especially the quadrupole mass filter require the stream of ions to be focused into a narrow point in order to allow successful mass to charge separation. One such electrical lens is the Einzel lens that is analogous to a focusing lens in an optical spectrophotometer. Figure 5.5 illustrates how an Einzel lens works. Six plates are in parallel, three on each side, and are exposed to the potentials shown below. These potentials set up a set of electrical field lines that act to bend the ions near the outside of the plates toward the center. Ions are focused to a small point for entry into the mass filter. The series of lenses stretch the length of a given beam of ions since ions on the outside (near the plates) have to travel a longer distance to reach the focal point.
Figure 5.5. An Einzel Lens (Electronically Focusing Lens).
The Einzel lens above is shown and explained as six horizontal plates. In practice, Einzel lens are vertical plates with a hole in each plate. Thus, the applied electrical potential creates three-dimensional field lines that focuses the ion beam to a point where the entrance slit/hole to the next component is located.
Electrostatic, magnetic, and time-of-flight instruments have only repulsion and accelerator plates. In addition to these plates, quadrupole instruments have a focusing lens to help introduce the ions towards the center of the mass filter/analyzer.
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