Mathematical Charge Deconvolution

For the charge states of neighboring peaks belonging to a pair at m/zi (higher value) and m/zi (lower value) we have  

Assuming all charges from protonation and using Wh for the mass of a proton, m/zi is determined by 

Example For the ease of calculation we use nominal mass (Chap. 3.1.5) and assume the charges from protonation. Consider the first peak at m/z 1001, the second at m/z 501. Now rii is obtained according to Eq. 12.9 by calculating it from 

Note As each pair of signals delivers an independent mass value for the hypothetical singly charged ion, mass accuracy can greatly be enhanced in ESI by multiple determination of this value and subsequent calculation of the average. 

In case of cationization instead of protonation, amh has to be replaced by the corresponding mass of the cationizing agent, e.g., NH/, Na, or K . (The recognition of cationized species is addressed in Chap. 8.5.7). 

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] 

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