Charge deconvolution

On the contrary, it may be argued that the electric field strength locally necessary to evaporate ions from a droplet cannot be attained because of the prior fission of the droplet due to crossing the Rayleigh limit. [23,90] 

More recent work revealed the importance of gas phase proton transfer reactions. [91-94] This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas phase ion-ion reactions (Chap. 11.4.4). The proton exchange is driven by the difference in proton affinities (PA, Chap. 2.11) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

Note There is a continuing debate about ion formation in ESI. [79,87,95] In summary, it may be assumed that CRM holds valid for large molecules [9] while the formation of smaller ions is better described by lEM. [79,95] 

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The above algorithm works well for pure compounds and simple mixtures, but it becomes increasingly difficult to assign all peaks properly when complex mixtures are to be addressed. Additional problems arise from the simultaneous presence of peaks due to protonation and alkali ion attachment etc. Therefore, numerous refined procedures have been developed to cope with these requirements. [102] Modem ESI instrumentation is normally equipped with elaborate software for charge deconvolution. 

Example The ESI mass spectrum and the charge-deconvoluted molecular weights (inset) of bovine serum albumine (BSA) as obtained from a quadrupole ion trap instrument are compared below (Fig. 11.19). Ion series A belongs to the noncovalent BSA dimer, series B results from the monomer. [24]... 

Fig. 11.19. Partial ESI mass spectrum of BSA and molecular weights after charge deconvolution (inset). Charge states are assigned to both series of peaks. Reproduced from Ref. [24] by permission. John Wiley Sons, 2000.

Figure 18 Nano-ESI MS/MS analysis of a triply protonated Man9GlcNAc2 A-linked gly-copeptide. (A) The mass spectrum with peptide y ions indicated and (B) the singly charged deconvoluted mass spectrum of panel A where the sugar-loss ion series is indicated (61).

Fig. 12.25. Charge deconvolution of LR (upper part) and HR (lower part) positive-ion ESI spectra of an artificial protein mixture. The zero charge peak of myoglobin is also shown in expanded view to reveal the delineation of the isotopic pattern. Reproduced from Ref. [122] by permission. Elsevier, 1998.

Note The output of computerized charge deconvolution can often be customized to either display the mass spectrum as it would appear with singly charged ions or to deliver the molecular weights of the corresponding neutrals (as in the above example). The output of neutral represents the only case where the abscissa has to be labeled mass [u] , while mass spectra strictly require m/z on thex-axis ... 

Fig. 12.35. ESI-FT-ICR IRMPD spectrum of negative ions of the phosphorothioate deoxyoligonucleotide 5 -GCCCAAGCTGGCATCCGTCA-3. (a) The mass spectrum as obtained and (b) after charge deconvolution. Only the

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