In the example above, involving benzamide (C7H7NO), the molecular ion (M+) peak had a mass
to charge ratio (m/e) of 121. This value is calculated using the most abundant isotopes of the elements present
in the molecule:
| 7 | * | 12C | = | 84 |
| 7 | * | 1H | = | 7 |
| 1 | * | 14N | = | 14 |
| 1 | * | 16O | = | 16 |
| 121 |
This type of calculation is called the unit mass molecular ion. Please refer to the spectrum for benzamide.
The observed abundance of the suspected molecular ion peak must correspond to expectations based on the assumed molecular structure. For example, highly branched substances undergo fragmentation readily; therefore, these types of substances will not have a very intense molecular ion peak. The intensity of molecular ion peaks will usually correspond to the following sequence:
aromatic compounds > conjugated alkenes > alicyclic compounds > sulfides > unbranched hydrocarbons > mercaptans > ketones > amines > esters > ethers > carboxylic acids > branched hydrocarbons > alcohols
For instance, if an unknown compound is suspected to be an alcohol, then the molecular ion peak will be quite small or completely absent.
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