*2.2.1 Electron impact ionization*

A hot-cathode discharges electrons through the EI, resulting in an electron beam-forming at the ionization chamber between the glowing cathode and the capture anode (**Figure 4**). Once the molecules go through the electron beam, an electron is bumped out of the molecule's surface, which gives a radical cation. The obtained ions at an electron energy of 70 eV are unstable and deteriorate rapidly, generating characteristic fragments that are automatically identified through the spectrum libraries. The GC–MS allows easy and reliable identification as well as the quantification of the molecules existing in the user database. Currently, NSIT 20 Mass Spectral Library has 350.643 carefully evaluated spectra.

### *2.2.2 Chemical ionization*

The CI is similar to the EI, except that the reactant gas molecules are ionized and not the analyte molecules (**Figure 5**). Ammonia, methane, or isobutane may be used as a reactant gas. The charge transfer due to the deprotonation (negative ion mode) or protonation (positive ion mode) occurs between the analyte molecules and the ionized reactant gas. The negative CI is particularly very sensitive. The detection of a quantity of octafluoronaphthalene corresponding to 200,000 molecules was successfully reached in 1992 when McLafferty and Michnowicz used negative CI [7]. The CI generates fewer fragment ions contrary to EI.

#### *2.2.3 Field ionization*

The FI almost does not generate any fragments. A high voltage is applied to a carbon-activated metal fiber in the source chamber. The excavating of separate

**Figure 4.** *Schematic representation of EI source.*

*Mass Spectrometry Coupled with Chromatography toward Separation and Identification… DOI: http://dx.doi.org/10.5772/intechopen.100517*

electrons from the analyte molecules occurs due to the high field strengths that form at the tips of the branches of the carbon dendrites [8, 9]. The FI is a less sensitive ionization technique compared to the EI and CI (**Figure 6**).
