Ionisation Chamber or Ion Source
As mentioned earlier, the mass spectrometer works by sorting out the charged particles by using magnetic and/or electric fields. Therefore, a compound/molecule must be charged or ionized to be analysed by a mass spectrometer. In the ionisation chamber (also called ion source) the molecule is ionised by using one of the many methods available for the purpose. The ion sources fall within two types as follows:
- Gas phase sources
- Desorption sources
Gas phase sources: In these sources the sample is first vaporised and then ionised. These are used for low molecular weight (< 1000 Da) samples which are thermally stable i.e., do not decompose on heating and have low boiling points (< 500oC). There are three methods that belong to this category. These are:
i) Electron ionisation (EI)
ii) Chemical ionisation (CI)
iii) Field ionisation (FI)