Three Ionization Modes
EI (Electron Ionization), PCI (Positive Chemical Ionization) and NCI (Negative Chemical Ionization) are generally used in GCMS analysis. It is important to understand the features of each ionization method in order to select the most suitable one. PCI is a soft ionization method, which produces less fragment ions, and is an effective way for determining molecular weight.
This common method obtains ions from sample molecules. Electrons from a filament are accelerated to 70eV and directed into an ionization chamber, where they impact with the sample molecules present. Because this is a "hard" ionization method, the molecules are cleaved and fragments are produced. Structural information can be obtained from the fragments, and a database of spectra makes library searches possible. This is a highly reproducible and reliable method for qualitative and quantitative analysis.