Alkaline earth metal salts of CaCO3, BaCO3, and SrCO3 as matrix for tryptophan SIMS analysis

Our experiment is focused on an increase of the molecular secondary ion yield, observed using primary ion beam analysis of biomolecular solids. D-tryptophan, C11H12N2O2, is determined by SIMS under impact of 25 keV Bi+ beam. Matrix-embedded samples with the alkaline earth metal salts of CaCO3, BaCO3, SrCO3, are prepared in forms of pellets and films. The experiment with a CaCO3 pellet shows that D-tryptophan parent-like ions are cationized with alkali metal ions and small functional groups in monomer mass region. Signals of Ca ions are observed with very low intensity. D-tryptophan dimers and trimers are also observed. In contrast, in CaCO3, BaCO3, SrCO3 pellet, D-tryptophan parent-like ion emission is suppressed by strong fragmentation of cellulose, added in pellets. Results from CaCO3 film show ionization of D-tryptophan through proton donation. These comparative experiments can shed some light on details of the cationization phenomena in secondary ion mass spectrometry of biomolecules. Copyright © 2010 John Wiley & Sons, Ltd.