ANALYSIS OF AMINO-ACID MIXTURES BY PLASMA DESORPTION MASS-SPECTROMETRY

Plasma desorption mass spectrometry (PDMS) was investigated as a means of analysing mixtures of three, four and five amino acids in both pos itive- and negative-ion modes. Fifteen mixtures were tested; each mixt ure contained equimolar amounts of selected amino acids. The PD mass s pectra exhibited MH+ and [M - H]- molecular ions for all the aminoacid s with different desorption-ionization yields. The spectra were more e asily interpreted in the negative- than the positive-ion mode. The des orption order of the amino acids was progressively established by comp aring the molecular ion desorption-ionization yields for each mixture. This desorption order was well correlated in both the positive- and n egation-ion modes with the acid-base thermodynamic data for the amino acids in the gas phase. This observation gives some insight into the d esorption-ionization mechanisms under PDMS conditions.