March 3, 2008
On an MS dataset, the x-axis is a measure of the mass to charge (m/z) ratio. Although it is easy to classify a signal as belonging to a singly charged ion (+1 or -1), one needs to be capable of recognizing a multiply charged (+2, -2, +3, etc.) ion as not to misinterpret the MS data. Multiply charged ions are commonly seen in MS data on peptides, proteins, diamines and diacids.
Ions of a multiply charged isotope cluster will appear at a fractional m/z, i.e. much less than one mass unit apart. The A+1 peak of a doubly-charged ion will appear at an m/z 0.5 above the monoisotopic peak.
Illustrated below is a case of an ion cluster from a centroid ES+ spectrum for a compound with a nominal mass of 400 Da. A doubly charged ion, [M+2H]+2, with a nominal mass at 402 Da, will have a signal appear at mass/charge = 402/2 = 201. If the monoisotopic peak, for the compound with all 12C, is present at 201.1, then the signal from the 12C+1 form is seen at 201.6.