MECHANISM OF PRODUCTION OF IONS IN ELECTROSPRAY MASS-SPECTROMETRY

A mechanism for the production of multiply charged molecular ion species in electrospray mass spectrometry (ES-MS) is still required. A concise discussion of a recently published ionic solution equilibrium model offering a partial mechanism is presented. That publication proposed that the ion abundance-charge profile could be fitted by one or a series of superimposed Gaussian functions, in accord with a solution equilibrium model. It is shown that indeed a simulated mass spectrum based on a solution model can compare well with the observed spectrum. However, some new and recently published experimental evidence is presented which shows that the ES mass spectra of many proteins give rise to multiply charged molecular ions which carry higher charges than those calculated by the model. Further, the ion abundance-charge profile is very sensitive to some experimental parameters, e.g. cone voltage; it does not necessarily reflect the solution or gaseous ion populations in the mass spectrometer source. Therefore, the concept that gaseous multi-charged ions originate from equivalently charged solvated ions in electrically neutral solution must be treated with caution.