Charge state and mass assignment

Whereas a raising charge state compresses the m/z of an ion into a mass scale conveniently accessibly by most quadrupole and sector instruments, another problem remains: mass assignment can only be made if the charge state of an ion is known.

One approach to overcome the problem is resolution. The simulation below shows the theoretical peak shape of a 1+ charged ion of 15.300 amu at mass resolutions of R = 740, 2000, and 20000, respectively (Ref.) If the ion was doubly charged instead, the signal would be observed at m/z 7650 with the peaks of the isotopic pattern at 0.5 u distance and so on. Consequently, the resolving power needed to resolve the isotopic peaks is independent from charge state and it is equal to the mass of the analyte. On the other hand, if the isotopic pattern is resolved, you can easily calculate charge state and thus mass of the analyte.
Example below: signal at m/z 1434, distance of isotopic peaks 0.25 amu, R >> 6000: M = 5734 amu, i.e. insulin.

The limit of resolution under ESI conditions is around R = 20.000 for most sector instruments. This is demonstrated by the HR-ESI spectrum of the 11+ peak of lysozyme (left) and the 13+ peak of myoglobin (right) (Ref. h).

Charge deconvolution

ESI spectra showing series of multiply charged ions may be charge deconvoluted following a simple algorithm. This is based on the knowledge that ESI does not "jump" over charge states, i.e. the lower m/z neighbor must have a charge state that is greater by exactly 1, e.g. low mass neighbor of a 3+ ion must be a 4+ ion and so on. There are nice programs for charge deconvolution available, however, in practice the procedure may still become rather complicated if several series from mixtures are to be deconvoluted or if the analyte is forming not only [M+H]+ but also [M+Na]+ and [M+K]+ quasimolecular ions.

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06 May 2009
The Little Encyclopedia of Mass Spectrometry - a compact source explaining terms and acronyms in MS, e.g., EI, CI, DCI, DEI, FAB, FD, FI, FT-ICR, MALDI, ESI, tandem MS, GC-MS, LC-MS, mass analyzer, electron impact, ionization, chemical ionization, electrospray, electron ionization, peptide sequencing, field desorption, fast atom bombardment, LT-FAB, matrix, matrix-assisted laser desorption, nanoESI, ion trap, TOF, Q-TOF, triple quadrupole, magnetic sector, SIM, PD-MS, sample introduction, ...