ACCURATE MONOISOTOPIC MASS MEASUREMENTS OF PEPTIDES - POSSIBILITIES AND LIMITATIONS OF HIGH-RESOLUTION TIME-OF-FLIGHT PARTICLE DESORPTION MASS-SPECTROMETRY

The possibilities and limitations of time-of-flight particle-desorptio n mass spectrometry are analysed from the standpoint of accuracy of mo noisotopic mass determination. For an instrument with mass reflector, typical mass accuracy was found to be ca. 20 ppm (standard deviation). The main limitation was found to be the mechanism of secondary ion pr oduction itself. A fraction of biological ions are formed in the gas p hase and therefore exhibit both time delay and energy deficit. This le ads to a limited resolving power and non-symmetric ion peaks. A peak s hape model, taking into account gas-phase ion formation, is built and analysed, and a simple correction procedure proposed. The application of the correction has increased the mass accuracy to 12 ppm (standard deviation). It is shown that further progress cannot be achieved by a correction procedure without instrumental improvements.