, the ion source converts gaseous molecules (M(g)) intocharged ions (Mn+) by means of electron bombardment or collision/reaction with a reagent gas.Electron ionization (EI) and Chemical ionization (CI) are the main ionization modes used inGC-MS. The characteristics of each ionization method and the processes that occur in the ionsource chamber are described in depth in this section. It serves as a guide to understand andselect the most suitable ion source for your required analysis. The process of ionisation notonly ionise the molecule but also break the molecule into the positive or negative modes2.