Systematic Steps in Interpreting a Mass Spectrum

The following general steps must be followed to interpret the mass spectrum of an unknown compound [21 Additional Reading 1]  

From the isotope pattern, determine the elemental composition (Section 6.3). Does it fit with the mass of the molecular ion If the molecular ion is of low abundance and fails to yield a reasonable elemental composition, calculate the composition of the next abundant fragment ion and add to it the composition of the neutral lost to arrive at the composition of the target molecule (see Table 6.6 for the formulas of neutral losses). 

Calculate the R + DB value (Section 6.4.4). In addition to ascertaining whether an ion is OE or EE, this information could provide a major clue to the structure (e.g., whether it is a saturated or an unsaturated or contains a ring). 

Make a judgment of the abundance of the M+ ion. It reflects the stability of the molecule. The M+ ion signal increases with increased unsaturation and number of rings but decreases substantially with chain length and chain branching aromatic compounds yield a prominent M+ ion. In contrast, the molecules that have a facile cleavage site produce low abundant M+.  

Pay attention to the general appearance of the spectrum long-chain aliphatic compounds show a characteristic hydrocarbon pattern, whereas aromatic compounds produce a few but highly abundant ions. 

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