6.1.2 Molecular Mass Determination for Total Unknowns

For analysis of a total unknown (as opposed to an identity confirmation), the molecular mass should be carefully determined. The presence or absence of the MI signal in the 70 eV EI mass spectrum is an important clue to structure. Molecules with some functional groups decompose sufficiently rapidly that the absence of a molecular ion signal may rule them out.

• As mentioned in Section 6.1.1, the MI is not generally the largest mass peak in the spectrum.
• The assignment of a signal as the MI requires examination of the totality of data available, both mass spectrometric and that collected from other techniques. Useful non-mass spectrometric data includes GC retention data (from which broad inferences about molecular mass may be made in some cases), IR spectra, chemical tests, etc. For example, if one has strong evidence that a compound is an alcohol, it may be reasonably assumed that the largest mass peak is not the MI, but may be [M-H$_{\textrm{2}}$O] $^{\textrm{+.}}$.
• Functional classes give certain patterns in the mass spectrum; if such a pattern suggests a functional group known to commonly give an MI signal, one can put some emphasis on the largest mass cluster of signals as candidates for the MI. So the mass spectrometric data as a whole can be used to make inferences about the MI signal expected.
• Chemical ionization is a powerful technique for molecular mass determination.
• Classical techniques of molecular mass determination should not be forgotten. The mass spectrometer, like all instrumental techniques, is not a substitute for wet chemistry for total unknowns.