Mass Spectrometry A Textbook - Department of Mathematics and ...

82 3 Isotopes
The isotopic patterns of sulfur and silicon are by far not as prominent as those
of chlorine and bromine, but as pointed out, their contributions are sufficiently
important (Fig. 3.5).
The relevance of oxygen and sulfur isotopic patterns is nicely demonstrated by
the cluster ion series in fast atom bombardment (FAB) spectra of concentrated sulfuric
acid, where the comparatively large number of sulfur and oxygen atoms
gives rise to distinct isotopic patterns in the mass spectrum (Chap. 9).
Fig. 3.5. Calculated isotopic patterns for combinations of elemental silicon and sulfur. The
peak shown at zero position corresponds to the monoisotopic ion at m/z X. The isotopic
peaks are then located at m/z = X+1, 2, 3, ...
Note: The preferred procedure to reveal the presence of S and Si in a mass
spectrum is to examine the X+2 intensity carefully: this signal's intensity will
be too high to be caused by the contribution of 13 C2 alone, even if the number
of carbons has been obtained from X+1 without prior subtraction of the S or Si
contribution.
Example: The isotopic pattern related to the elemental composition of ethyl
propyl thioether, C5H12S, (Chap. 6.12) is shown below. The contributions of 33 S
and 13 C to the M+1 and of 34 S and 13 C2 to the M+2 signal are indicated (Fig. 3.6).
If the M+1 peak resulted from 13 C alone, it would indicate rather the presence of 6
carbon atoms, which in turn would require an M+2 intensity of only 0.1 % instead
of the observed 4.6 %. The introduction of Si to explain the isotopic pattern would
still fit the M+2 intensity with comparatively low accuracy. For M+1, the situation
would be quite different. As 29 Si alone demands 5.1 % at M+1, there would be no
or 1 carbon maximum allowed to explain the observed M+1 intensity.