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8.1.1 What are Isotope Peaks?

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.

John S. Riley, DSB Scientific Consulting