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.
In general, although both positive and negative ions are formed in the ion source, only negative ions are electrically extracted from the source for NCI analysis. Negative ions are produced mainly by an electron capture process, which provides us with a very high-sensitivity tool for analyzing high electron affinity samples such as halogenated compounds.
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