An aid to identification is the ability to determine the relative molecular mass (RMM) with a high
degree of accuracy and to establish the empirical molecular formula. The former depends on
recognition of the parent or molecular ion peak M produced when an electron is ejected from the
molecule
Because of the high electron beam energies used, fragmentation processes predominate in many
compounds and the parent peak is often weak or missing from the spectrum. It can sometimes be
identified by re-scanning the spectrum at a much lower electron beam energy when its intensity should
grow at the expense of all other peaks. Alternatively, increasing the sample size induces bimolecular
collisions resulting in proton transfer and a larger M + 1 peak. However, the use of CI or FI is being
increasingly preferred as much larger M or (M + 1) peaks are obtained. A comparison of EI, CI and FI
spectra for the same compound is shown in Figure 9.54.
The empirical formula of a compound can be established by evaluating the intensities of isotope peaks
M + 1, M + 2, etc. relative to the parent peak. Isotope peaks arise because of the natural abundance of
certain heavier isotopes in some molecules of the sample. Thus a compound containing a single carbon
atom will produce an M + 1 peak 1.08% of the intensity of the parent peak because 1.08% of the
molecules contain a^13 C atom. If the molecule contains two carbon atoms the M + 1 peak will be 2.16%
of the parent peak and an M + 2 peak due to molecules containing two^13 C atoms will be produced of
relative intensity 0.01%. Table 9.10 lists the principal stable isotopes and their relative abundances for a
number of elements. The selection of likely molecular formulae which correlate with particular parent
masses and their isotope peak ratios is facilitated by a set of tables constructed by Beynon. 3 All
possible molecular formulae for compounds containing carbon, hydrogen, oxygen and nitrogen up to a
molecular weight of 500, together with the calculated isotope peak ratios, are listed. The presence of
chlorine, bromine or sulphur can be inferred from an abnormally large M + 2 peak whilst an abnormally
small M + 1 peak suggests the presence of the monoisotopic elements iodine, fluorine or phosphorus.
Beynon's tables also include calculations of the relative intensities of M + 2, M + 4, M + 6, etc. peaks
for compounds containing one or more atoms of chlorine or bromine. Extracts from the tables are
illustrated in Tables 9.11 and 9.12. Some of the formulae listed can be automatically eliminated on the
basis of the nitrogen rule. This states that a molecule of even-numbered molecular weight must contain
no nitrogen or an even-number of nitrogen atoms.
Additional information that can be derived from the molecular formula is the number of unsaturated
sites. The total number in a molecule includes