By now you may have noticed that there is ``always'' an [M+1]
ion (where M is the molecular mass) that is much smaller than the
molecular ion signal. This [M+1] ion does not go away for averaged
spectra, background subtracted spectra, when the signal-to-noise ratio
is large, etc. Is it a mistake, like some instrumental problem?
The answer is that these are isotope peaks. In fact, not just the molecular ion has them; every peak in the spectrum has them. This section explores the origin of these peaks in more detail.