Ion/ion reactions for oligopeptide mixture analysis: application to mixtures comprised of 0.5-100 kDa components.

Oligopeptide mixtures have been subjected to electrospray ionization, accumulated within a quadrupole ion trap, and subjected to ion/ion proton transfer reactions with anions derived from perfluoro-1,3-dimethylcyclohexane. Various mixtures were studied with approximate molecular weight ranges of 0.5-8.5, 12-30, 45-100, and 0.5-100 kDa. Mixtures of known composition were studied to evaluate the mixture complexity amenable to electrospray combined with ion/ion reactions to reduce spectral complexity associated with multiple charging. Mixture analysis with at least 40 components of low and medium molecular weight and roughly comparable solution concentrations appears to be straightforward. No matrix effects upon ionization were implicated in the data for the low and medium molecular weight mixtures but bovine albumin appeared to inhibit signals from bovine transferrin and chicken conalbumin in the high molecular weight mix. Furthermore, the presence of abundant low mass-to-charge ions appeared to inhibit signals from high molecular weight proteins (> 40 kDa) in the 0.5-100 kDa mix. Such an observation is consistent with dynamic range limitations that can arise from discrimination based on ion space charge effects, although an ionization matrix effect could not be precluded from the data reported here. The results reported here indicate that the limitation to mixture complexity amenable to electrospray mass spectrometry imposed by spectral congestion associated with multiple charging can be significantly reduced via ion/ion reactions. The use of ion/ion reactions can therefore facilitate the study of other factors that can impose limitations to mixture analysis, such as matrix effects upon ionization and differences in ion transmission, accumulation, storage, and detection efficiencies.