Determination of Molar Mass of the Analyte:

An accurate molar mass determination from mass spectrometry essentially means the correct identification of the molecular ion peak. Depending on the method of ionisation we get the molecular ion (M ) peak or (M+1)⁺  and (M - 1)⁺ ion peaks. You would also recall that the molecular ion is quite unstable and quickly undergo fragmentation. In fact, in about 20% of the cases the molecular ion peak is not observed at all. However, in a large number of cases the cluster of peaks towards the highest m/z values includes the molecular ion peak along with the corresponding isotopic peaks and can be easily identified.

In cases when the molecular ion peak is absent or too weak we may resort to running spectrum with a higher concentration of the analyte or at higher sensitivity. If the nature of the molecule is known then sometimes the molecular ion peak can be reconstructed from the fragments. Various a times, a significant rule called Nitrogen rule comes handy in deciding about the molecular ion. According to this rule a molecule having an even number molar mass must have none or an even number of nitrogen atoms in it; similarly a molecule with an odd number molar mass must have one or an odd number of nitrogen atoms within it. Here again a fragmentation pattern as well as the nature of the molecule helps in sorting out the issue.

In cases of doubt, further spectra acquired through alternative ionisation methods are useful.