Electrospray Ionization Spectra of Large Molecules

With sufficient resolution the charge state of an ion can readily be determined from the spacing of the isotopes. For instance, spacing for a singly charged ion is 1 Da while for a quadruply charged ion it is 0.25 Da. 

FIGURE 3.11 Electrospray mass spectrum of apo-myoglobin and the deconvoluted data giving the molecular mass of the protein. 

Accurate calibration of the mass scale is especially important when determining the masses of biopolymers because errors are magnifled during deconvolution, e.g., a 0.1 Da error for a 10 charge state ion becomes a 1 Da error when the observed mk value is multiplied by 10 to obtain the mass of the analyte. With good mass calibration, the experimental error in the molecular mass for a small protein, such as myoglobin, is 1 Da. 

The envelopes become narrower as the charge state of the ions increases hecanse the spacing between isotopes is divided by the nnmber of charges on the ion. 

FIGURE 3.12 Effect of charge state on the width of the isotope envelope. 

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