Plasma pre-treatment for peptide fragmentation in ESI
When a nanoelectrospray (nanoESI) emitter was first exposed to a helium plasma (see illustration of experimental setup), ESI, a widely accepted “soft” ionization technique, caused significant and useful peptide fragmentation. The appearance of abundant peptide-fragment ions in the mass spectrum was ascribed to electrolyte release into the solution. Labile phosphate groups on the peptide were preserved during fragmentation.
Mechanistic studies suggest that the role of the plasma is to release electrolytes from the glass emitter so that they can be taken up into the spray solution to cause a high local electrolyte concentration. The fragmentation itself is related to ion formation under these special ESI conditions. These conditions, together with the relatively low spray voltage applied, enable the direct ejection of solvated ions with relatively high speeds in the early stages of the spray. The subsequent gas-phase collisional activation of the solvated ions produces the fragments observed in the mass spectra. A higher degree of solvation around the charged groups may
account for the preservation of phosphate groups during activation.