The electron ionization (EI) is a common ionization method which is used for GC-Ms analysis in the drug testing programs. This is a hard ionization process because it involves a lot of high level energy. The EI source mainly consists of repellers, elements and ion focuser. The gas phase of the sample comes into the MS from GC through a transfer line which connects both of them and enters the EI immediately. A high energy electron beam bombards the gas phase molecule from the element. The electrons under impact tear the electrons of the molecule to create free radicals, which are molecules have unpaired electrons. These radials are referred as molecular ions (M+*) as they have emerged from the original molecule. The free radicals are extremely reactive and unstable and they immediately de-fragment themselves in order to become stable ions. These fragments rearrange to stabilize themselves. A mass analyzer therefore, records the m/z of these fragments while plotting them as bar graph. The fragmentation pattern is thus referred as the mass spectrum of the molecule. EI is so highly energetic that these molecules fragment a lot and the M+* of the molecule is not really observed in the spectrum.
Many EI’s utilize a voltage of about 70eV to ionize them which is more than enough to make sure that the molecules fully fragment themselves in every analysis. The EI’s spectrum produces consistent fragmentation for specific molecules, regardless of who the manufacturer is. Every compound has a unique mass spectrum that causes EI-MS to be quite a powerful tool to identify compounds. Vast, extensive computerized libraries for mass spectrum have also allowed creating an excellent database for locating many drugs and their various metabolites. The search and match software available with the mass spectrum located and identifies thousands of unknown drugs which can be detected in seconds. This is the reason why GC-EI-MS are benchmarks in confirming drugs.