278 Section E – Spectrometric techniques
● the identification of unknowns and of structural features from fragmentation
patterns, sometimes with the aid of computerized library searches;
● quantitation by selected ion monitoring(SIM) for both molecules and
elements.
The interpretation of molecular mass spectra is accomplished by comparisons
with the spectra of known compounds and the application of a set of empirical
rules. The following are particularly useful.(i) The nitrogen rulestates that compounds with an even-numbered RMM
must contain zero or an even number of nitrogen atoms, and those with an
odd-numbered RMM must contain an odd number of nitrogen atoms.
(ii) The unsaturated sites ruleprovides a means of calculating the number of
double-bond equivalents in a molecule from the formula
No. of C atoms +^1 ⁄ 2 (no. of N atoms) -^1 ⁄ 2 (no. of H +halogen atoms) + 1
For example, for C 7 H 7 ON, the formula gives 7 +0.5 -3.5 + 1 = 5 double
bond equivalents. This corresponds to benzamide, C 6 H 5 CONH 2 , the
aromatic ring being counted as three double bonds plus one for the ring.
(iii) The intensity of the molecular ion peak decreaseswith increasing chain
length in the spectra of a homologous series of compounds and with
increased branching of the chain.
(iv) Double bonds and cyclic structures tend to stabilize the molecular ion,
saturated rings losing side chains at the a-position.
(v) Alkyl-substituted aromatic rings cleave at the b-bond to the ring giving a
prominent peak at m/z91, which corresponds to the tropylium ion, C 7 H 7 +.
(vi) Small neutral molecules, such as CO, C 2 H 4 , C 2 H 2 , H 2 O and NH 3 are often
lost during fragmentation.
(vii) The C-C bond adjacent to a heteroatom (N,O,S) is frequently cleaved
leaving the charge on the fragment containing the heteroatom, whose
nonbonding electrons provide resonance stabilization, e.g.- CH 3 ●
CH 3 - CH 2 - Y+●-R Æ CH 2 =Y+-R ́ +CH 2 - Y-R
McLafferty rearrangementsin carbonyl compounds are common, e.g.
HCH 2 - CH 2 - CH 2 - CO+●-OR ́ C 2 H 4 +CH 2 =C(O+●H)-ORA neutral molecule of ethene is eliminated in the process (Rule (vi)).
Some examples of fragmentation patterns in EI mass spectra are shown in
Figures 7- 15. The relative intensities of the peaks within a spectrum are a reflec-
tion of the statistical probability of particular bonds in the molecule being
cleaved and are closely related to the bond energies.Spectra of saturated straight chain compounds are characterized by clusters
of peaks 14 mass units (CH 2 groups) apart, as successive C-C bonds along the
chain are cleaved in different molecules. Octane(Fig. 7) has a base peak at
m/z43 due to the CH 3 CH 2 CH 2 +fragment ion, and a small molecular ion peak
at m/z114 (Rule (iii)).
Branching of the chain alters the relative intensities of the clusters, as shown
by the spectrum of the isomeric 2,2,4-trimethylpentane(Fig. 8), which has a
base peak at m/z57 due to the (CH 3 ) 3 C+fragment ion, and no significant m/z71,
85 or molecular ion peaks (Rule (iii)).