Manipulation of protein charge states through continuous flow-extractive desorption electrospray ionization: a new ambient ionization technique.

Manipulation of protein charge states in electrospray ionization-mass spectrometry (ESI-MS) has implications for the study of intact proteins, protein-protein interactions, post-translational modifications, and protein sequencing. Control of these protein charge states is often difficult to achieve with conventional methods of analysis. A novel ambient ionization configuration, continuous flow-extractive desorption electrospray ionization (CF-EDESI), is presented as a means to control the charge state distribution of proteins. A key feature of the CF-EDESI technique is the continuous flow needle, which is a hypodermic needle presented orthogonal to the electrospray source and delivers a solvent flow containing analytes for extractive desorption ionization. With this source design, the successful manipulation of cytochrome c and lysozyme charge states with the use of different additives, such as acetic acid and sulfolane, was demonstrated. Results were compared to data obtained with conventional electrospray ionization. Good agreement with previously reported studies of cytochrome c unfolding/folding studies, performed by conventional ESI-MS, is evident. In addition to the protein analysis presented, the CF-EDESI-MS technique should be applicable for analyzing atypical analyte and solvent systems by mass spectrometry while maintaining optimal electrospray source conditions.