Leucine and isoleucine in chemical ionization and plasma desorption mass spectrometry: A comparative study

The NH 3 and CH 4 chemical ionization mass spectra of leucine and isoleucine are compared with their plasma desorption mass spectra. Mass‐analysed ion kinetic energy (MIKE) and collision‐induced dissociation (CID)/MIKE experiments are also presented and the mechanisms of the formation and dissociation of immonium ions [M – CO 2 H 2 ] + are discussed. A comparison between the various ionization techniques reveals that plasma desorption produces pseudo‐molecular ions whose internal energy lies between zero and 300 kJ mol −1 .