Ordinary mass spectra tell us the molecular weight (MW) of the molecule: we could easily see, for example, that the bee pheromone on p. 48 had MW 114 even without knowing its structure. When we revealed it was C7H14O, we had to use other information to infer this, because 114 could also be many other things, such as C8H18 or C6H10O2 or C6H14N2. These different atomic compositions for the same molecular weight can nonetheless be distinguished if we know the exact molecular weight, since individual isotopes have non-integral masses (except 12C by defi nition). The table below gives these masses to five decimal places, which is the sort of accuracy you need for meaningful results. Such accurate mass measurements are obtained by a technique called high-resolution mass spectrometry.

For the bee pheromone on p. 48, the accurate mass turns out to be 114.1039. The table below compares possible atomic compositions for an approximate MW 114, and the result is conclusive. The exact masses to three places of decimals fi t the observed exact mass only for the composition C7H14O. You may not think the fi t is very good when you look at the two numbers, but notice the difference in the error expressed as parts per million. One answer stands out from the rest. Note that even two places of decimals would be enough to distinguish these four compositions.

●In the rest of the book, whenever we state that a molecule has a certain atomic composition you can assume that it has been determined by high resolution mass spectrometry on the molecular ion.

One thing you may have noticed in the table above is that there are no entries with just one nitrogen atom. Two nitrogen atoms, yes; one nitrogen no! This is because any complete molecule with C, H, O, S, and just one nitrogen in it has an odd molecular weight. This is because C, O, S, and N all have even atomic weights—only H has an odd atomic weight. Nitrogen is the only element from C, O, S, and N that can form an odd number of bonds (3). Molecules with one nitrogen atom must have an odd number of hydrogen atoms and hence an odd molecular weight.