DESORPTION IONIZATION AND IN-FLIGHT FRAGMENTATION OF MONOALKALI-GLYCINE ADDUCTS - PLASMA-DESORPTION MASS-SPECTROMETRY AND ABINITIO CALCULATIONS

Interpretation of Cf-252 plasma-desorption mass spectra of glycine and alkali salt mixtures is correlated to thermochemical values. The interpretation of direct spectra is based on a model considering competitive fragmentations among the ionic bonds involved in the crystalline structure; the desorption yield of Li, Na, K, Rb and Cs alkali-glycine adduct ions is shown to depend upon the relative lattice energies of the different salts bound in the solid mixture. Ab initio calculations were performed at the 6-31G*//3-21G level to evaluate the dissociation energies associated with the in-flight decomplexation reactions GlyCat+ --> Gly + Cat+ (1) for Cat+ = Li+, Na+ and K+ whether we consider that the GlyCat+ desorbed species enter the field-free area in their 'preformed geometry' or that they rearrange during the acceleration step. The experimental metastable transition rates associated with these reactions were determined to be in good agreement with the theoretical values for both GlyCat+ geometries under consideration. We calculated the cation affinities of glycine to be 271 kJ/mol, 188 kJ/mol and 127 kJ/mol for Li+, Na+ and K+, respectively. The fragmentation of GlyCat+ species into immonium ions, GlyCat+ --> [H2N-CH2]+ + CO + HOCat (2), is also experimentally and theoretically investigated. The competition between reactions (1) and (2) is favourable to (2) for Cat+ = H+ and to (1) for Cat+ = Li+, Na+ and K+. From both competitive reactions, the comparison between theoretical values and experimental observations allows one to estimate the average internal energy of the GlyCat+ desorbed species at about 300 kj/mol in the field-free area of the mass spectrometer.