Gas Chromatography - Tandem Techniques - The Quadrapole Mass Spectrometer > Page 36


Electrons are generated by a heated filament which then pass across the ion source to the anode trap. The sample vapor is introduced in the center of the source and the molecules drift, by diffusion, into the path of the electron beam. Collision with the electrons produce a molecular ion together with ionized molecular fragments, the size of which is determined by the energy of the electrons. The electron energy is controlled by the accelerating potential applied between the filament and the anode trap. The ions that are produced are deflected by the potential applied to the ion-repeller electrode into the accelerating region of the mass spectrometer.


Chemical Ionization

If the sample vapor is mixed with a large excess of a reagent gas as it passes into the electron beam, an entirely different type of ionization can take place. As the reagent gas is in considerable excess with respect to the sample, the reagent molecules are preferentially ionized relative to those of the sample. When the reagent ions collide with the sample molecules by the usual gas kinetic processes, they produce sample + reagent ions or in some cases protonated ions. This type of ionization is called chemical ionization and is a very gentle form of ionization. Very little fragmentation takes place and parent ions + a proton or a molecule of the reagent gas are produced. As a consequence, the molecular weight of the parent ion can easily be obtained. To achieve this type of ionization little modification to the normal electron impact source is required and a conduit for supplying the reagent gas is all that is necessary.



The Quadrapole Mass Spectrometer


The most popular mass spectrometer that is used in combined gas chromatography/mass spectrometry systems is the quadrapole mass spectrometer, either as a single quadrapole or as a triple quadrapole. Theses types of mass spectrometer can also easily provide MS/MS spectra (a technique that will be discussed later).


A diagram of a single quadrupole mass spectrometer is shown in figure 25. The operation of the quadrupole mass spectrometer is quite different from that of the sector instrument. The quadrupole spectrometer consists of four rods which must be precisely straight and parallel and so arranged that the beam of ions is directed axially between them. Ideally the rods should be hyperbolic in cross section but in practice less expensive cylindrical rods are nearly as satisfactory. A voltage comprising a DC component (U) and a radio frequency component (Vcoswt) is applied between adjacent rods, opposite rods being electrically connected, as shown in figure 25. Ions are accelerated into the center, between the rods, with a relatively small potential ranging from 10 to 20 volts. The theory of the quadrapole is somewhat complex and will not be given in detail here. Readers interested in a more comprehensive treatment of the quadrapole mass spectrometer are referred to an appropriate book on mass spectrometry.