With a well-tuned and calibrated, high resolution MS instrument, a molecular formula(e) can be devised from the m/z for an ion peak. In cases where more than one molecular formula fits, knowing the accuracy of the MS instrument can help in narrowing down the choices.
The first step is to identify the molecular ion peak and its ionization, e.g. [M+H]+, [M+Na]+, etc. so as to take into account any additional adduct information in fitting a molecular formula. Secondly, use common elements with likely valences such as C, H, O, N (III, IV, V) and S (II, IV, VI) to try to fit a molecular formula.
An ESI+ mass spectrum for an unknown organic compound is shown below. The cationized peak at m/z 216.1749 corresponds to an [M+H]+ and is accurate up to four significant decimal places. The following table illustrates how a mass tolerance of 0.001 Da can assist in narrowing down the list of molecular formulae.
# MF Mass (Da) Difference
1 C15H21N1 215.1674 0.0003
2 C12H25N1S1 215.1708 0.0037
3 C10H21N3O2 215.1634 -0.0037