IDENTIFYING CHARGE STATES OF PEPTIDES IN LIQUID-CHROMATOGRAPHY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

The addition of submillimolar levels of sodium acetate to the mobile p hase during an LC/ESI-MS experiment encourages the formation of sodium replacement ions in addition to the normally observed protonated spec ies. The spacing of these sodiated species allows unambiguous determin ation of the charge state of the ions and, hence, their actual mass. S uch information is very useful when unknown or modified peptides are b eing analyzed. At the levels reported here (250 muM), the sodium salt seems to cause no deterioration in chromatographic performance and doe s not unduly foul the mass spectrometer.