Chemical ionisation 

Many samples analysed by electron impact do not give a molecular ion, this is because electron impact is an energetic ionisation method and tends to cause fragmentation.

Chemical ionisation is a softer form of ionisation and produces much less fragmentation. The technique is to use an excess of a reagent gas such as Ammonia, Methane etc, which is introduced into the ion source along with the sample to be analysed. The electron beam causes ionisation of the reagent gas, the resulting ions collide with sample molecules to produce pseudo-molecular ions as {M+H]+ or [M+NH4]+ and only limited fragmentation.