Vaporized analyse when carried through the GC column with the help of heated carrier gas theseparation occurs in the column. Then the separated components of the mixture will enter intothe MS through an interphase. This is followed by ionization, mass analysis and detection ofmass-to-charge ratios of ions generated (m/z) from each analyse by the mass spectrometer. Theinterface works as a bridge connecting the GC and MS. Together with the vacuum pumpingsystem, they lower the pressure to meet the high vacuum requirement of MS. The interface isgas-tight and maintained at low and consistent pressure by pumps. Carrier gases can beremoved at this step. With a heater, the interface temperature is kept high (e.g. 50 –350°C) toensure that the eluted compounds from the GC stay in the gaseous phase and only volatilecompounds enter the MS. In GC-MS, 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 modes