Behaviour of methyl red under fast atom bombardment conditions

Abstract

AbstractThe influence of the pH, the nature of the matrix and the presence of a surfactant on the positive‐ and negative‐ion abundances in the molecular mass region in the fast atom bombardment (FAB) mass spectra of methyl red was investigated. A small but significant pH effect was observed which was attributed to the non‐ or at the most low surface‐active character of the intact methyl red molecule. As expected, the more basic the solution, the less protonated molecules with respect to M+˙ are observed and in the negative‐ion mode less [M + H]− and more [M – H]− ions with respect to M−˙ were found. In contrast to neutral solutions, both acidic and basic solutions give a long‐lasting stable response of all methyl red ions. For dyes with a moderately negative redox potential such as methyl red, beam‐induced redox reactions seem to play a role in the ionization process, the neutral medium offering the best conditions for reduction processes. The ion intensities in the molecular mass region depend on the nature of the matrix. Protonation of the molecule has been found to be more effective in glycerol than in 3‐nitrobenzyl alcohol; the former also appears to offer the best conditions for reduction processes. Anionic and cationic surfactants effectively suppress the contribution of ions from glycerol in both positive‐ and negative‐ion spectra and generally promote the formation of analyte ions at the surface. The most important effect of the surfactant in a neutral medium seems to be the promotion of a regular transport of ions and molecules to the surface, which permits the creation of stable ion currents, instead of an unstable ion beam if the surfactant is absent. Moreover, when the surfactant is present an increase of the sample ion abundances is observed. Redox reactions involving molecules and molecular ions and also molecules and preformed ions in the solution, brought to the surface by micelles, have been proposed to give some contribution for the small but significant enhanced abundance of [M + nH]+ (n > 1) ions with respect to [M + H]+ ions, in the presence of a surfactant. The results have been rationalized in terms of the surface phenomena while the important role of surfactants for obtaining better FAB mass spectra is emphasized.