ANALYSIS OF POLYPEPTIDES BY HIGH RESOLUTION FIELD DESORPTION MASS SPECTROMETRY

An equimolar mixture of silver methanesulfonate and silver trifluoromethanesulfonate is proposed as a calibrating material for exact mass determination in field desorption mass spectrometry. The use of a mass scale established by double exposure of a sample, and a 1:1 mixture of silvermethanesulfonate and silver trifluoromethanesulfonate on a photographic plate was investigated using a variety of peptides of known composition. The accuracy of the method was tested by recording the mass spectra of the peptides at a resolution of about 4,000. Ions that range in mass from 294 for a fragment ion of pentaphenylalanine, to mass 1147 for the {2M+H}('+) ion of methionine enkephalin were recorded. The measured mass was compared with the expected mass calculated from the known atomic composition of the ion. The precision was determined by repetitive measurements of the exact masses of some of the peptides. The results indicate that the mass scale established is useful for the determination of the exact masses of field desorbed ions over the mass range of 294 to 1147, and gives excellent accuracy (about 10 ppm) and precision (about 9 ppm) in the mass range examined.