Ion Sources and Methods of Ionization

Ion sources have two main functions to generate ions that represent the components of a sample and to transfer all ions into the mass analyzer. 

There have been numerous ionization techniques developed over the years, and their relative importance has varied as new ones have been commercialized. In all ionization sources energy is added to the neutral molecules present, including those from the analyte and from other species, e.g., co-extracted components and residual background. Agents used to produce ions include electrons, ions, electric fields, and photons. 

Ions can be formed within the vacuum chamber of the mass spectrometer or outside the instrument at atmospheric pressure. Examples of in vacuo ionization are electron ionization (El), chemical ionization (Cl), and matrix-assisted laser desorption/ion-ization (MALDI). Ionization techniques carried out outside the vacuum system are collectively termed atmospheric pressure ionization (API). The most important API methods are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Among other API techniques are atmospheric pressure photo-ionization 

As stated already, the amount of energy imparted to the neutral analyte molecules varies with the type of ionization and, combined with the structure of the 

FIGURE 2.6 The choice of ionization method is often determined by the polarity of the analyte. 

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