nbsp; Figure 53. The Electron Impact Ion Source Electron Impact Ionization   Electron impact ionization source has already been briefly discussed but will now be dealt with in more detail. Electron impact ionization is a relatively hard ionizing process, and, as a consequence, most of the parent molecule is often broken up producing a variety of fragments with a relatively small amount of the parent ion. In some circumstances, if the molecule is sufficiently labile, no parent ion may be produced at all.   Figure 53. The Electron Impact Ion Source     A diagram of an electron impact ion source is shown in figure 53. Electrons, formed by thermal emission from a heated

figure 56. Spectrum (A) was produced using methane as the reagent gas and shows significant fragmentation together with the protonated parent ion. Spectrum (B), however, obtained with butane gas exhibits the protonated molecular ion only.   The chemical ionization source and simple ion impact source are very similar and most electron impact sources can also work as chemical ionization sources. When used in the electron impact mode dual-action sources are not as efficient as the dedicated electron impact sources, but the ionization efficiency is not reduced by more than 50%. Continuous use of a chemical ionization source causes significant contamination which results in the build-up of residues from the ionization process As a result, the source requires baking-out fairly frequently.   A diagram of a typical gas inlet system for chemical ionization is shown in figure 57,         Figure 57. A Gas Inlet System for Chemical Ionization  

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 &

. Thus although parts of the secondary structure of the molecule are disclosed, the lack of a parent ion would again make structure elucidation difficult. MidÐelectron energies (ca 20 eV) still provides numerous low molecular weight fragments but together with a clear, unambiguous, parent ion thus providing ample information for structural identification. It is clear that electron energy is an important variable and must be optimized, to obtain suitable structure elucidation.   Chemical Ionization   Chemical ionization is a far more gentle form of ionization and is frequently used to accurately determine molecular weights. Firstly a reagent gas such as methane is ionized in a simple electron impact ion source. The partial pressure of the reagent gas is adjusted so that it is about two orders of magnitude greater than that of the sample. The reagent ions collide with the sample molecules and produce ions. As the energy of the reagent ions never exceeds 5 eV, the ionization

applications. The author used a IGC gas chromatograph fitted with a katherometer detector and a packed stainless steel column, 13.2 m long, 3.17 mm I.D., containing Chromosorb PAW 45/60 mesh, coated with 28% of DC 200 stationary phase. Helium was used as the carrier gas at a flow rate of 20 ml/min. The column eluent was split through a low-dead-volume splitter directly to the mass spectrometer. The mass spectrometer was a Riber AQX 156 quadrapole filter having a mass range of 0-300. Electron impact ionization was employed with energies of 70 eV, and the mass spectrometer ion source was maintained at about 100ûC. An example of the total ion current chromatogram of a crude silane sample is shown in figure 62

with their much smaller flow rates eliminated the need for concentrators. In addition, the mass spectrometer pumping system could easily dispense with such gas flows and peak concentrations were high enough for direct injection into the mass spectrometer.   Ion Generation   Before a molecule can be deflected electrostatically or magnetically, it must be ionized and/or broken up into charged fragments. There are two common methods of ion production used in GC/MS systems, namely, electron impact ionization (EI) and chemical ionization (CI). More recently, a third method has become popular and that is inductively coupled plasma ionization (ICPI). This ionizing method was largely employed for liquid or solid samples but has now been applied to GC/MS systems for certain types of inorganic analysis