Studies in cluster ion formation. Part I. Fast atom bombardment mass spectrometry of tetraalkylammonium halides: Factors influencing cluster ion size and stability

Abstract Positive and negative ion fast atom bombardment mass spectra of tetraalkylammonium halide salts (NR 4 X, where X = Cl, Br, I and NR 4 = NMe 4 , NEt 4 ) have been studied and intense cluster ion formation has been observed. The cluster ion intensity distributions were found to show enhancements at certain cluster numbers (n). The negative cluster ions of NMe 4 X salts showed anomalous ion intensity regions, which differed from both the positive cluster ions of all NR 4 X salts and also the corresponding negative clusters of NEt 4 X salts. The influence of anion and cation size on cluster ion formation and abundances has been studied and it has been established that smaller anion and cation size favours the formation of larger cluster ions. The possible structures of the cluster ions exhibiting relative increased stabilities are discussed.