LEUCINE AND ISOLEUCINE IN CHEMICAL-IONIZATION AND PLASMA DESORPTION MASS-SPECTROMETRY - A COMPARATIVE-STUDY

The NH3 and CH4 chemical ionization mass spectra of leucine and isoleu cine are compared with their plasma desorption mass spectra. Mass-anal ysed ion kinetic energy (MIKE) and collision-induced dissociation (CID )/MIKE experiments are also presented and the mechanisms of the format ion and dissociation of immonium ions [M - CO2H2]+ are discussed. A co mparison between the various ionization techniques reveals that plasma desorption produces pseudo-molecular ions whose internal energy lies between zero and 300 kJ mol-1.