Process of Mass Spectroscopy:

The mass of a molecular ion have to be even unless the molecule consists of an odd number of nitrogen atoms. This is since nitrogen is the just only 'organic' element with an even mass number and an odd valency. Hence, an odd numbered mass for a molecular ion is an indication of the existence of at least one nitrogen atom. Occasionally, the molecular ion is not observed in the spectrum. This is since electron ionization needs compounds to be vaporized at high temperature and the molecular ion might fragment before it can be detected. In cases such as this, it is essential to carry out the ionization under milder conditions such that the molecular ion is less likely to fragment (that is by chemical ionization or by fast atom bombardment). You might ask why these milder conditions are not employed routinely. The reason is that fragmentation can provide helpful information about the structure of the molecule.

The molecular ion peak is generally strong for aromatic amines, nitriles, ?uorides and chlorides. Aromatic and heteroaromatic hydrocarbons will as well provide intense peaks if there are no alkyl side chains present greater as compared to a methyl group. Though, the peaks for molecular ions can be not present for long chain hydrocarbons, highly branched molecules, and alcohols.