Answer 12.4

a) The charged residue model rationalizes the occurrence of multiply charged ions in ESI. Calculate the theoretical m/z values of singly to quadruply protonated ions of bovine insulin, C254H377N65O75S6, provided the isotopic peaks are not resolved.

Here, one has to use relative atomic masses (atomic weights). Thus we have:

[M+H]+, C254H378N65O75S6, m/z 5734.53

[M+2H]2+, C254H379N65O75S6, m/z 5735.54/2 = 1867.77

[M+3H]3+, C254H380N65O75S6, m/z 5736.54/3 = 1912.18

[M+4H]4+, C254H381N65O75S6, m/z 5737.55/4 = 1434.39

b) ESI obviously possesses impressing high-mass capabilities. Nonetheless, ESI applications to polymers are rare. Why?

Due to multiple charging, each component of an oligomer or more important polymer gives rise to several peaks in an ESI mass spectrum. As polymer samples consist of several ten to several hundred components (depending on their average molecular weight and on their polydispersity) the charge distributions of all components will be superimposed in the resulting spectrum. This usually prevents the recognition of single peaks and even of molecular weight distributions.

c) What is “charge deconvolution”?

The term charge deconvolution summarizes mathematical algorithms developed to achieve spectra that only exhibit the (hypothetical) singly charged species. This provides an useful means of data reduction for simplified spectral interpretation.