Elimination of the concentration dependence in mass isotopomer abundance mass spectrometry of methyl palmitate using metastable atom bombardment

An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concent ration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and met astable atom bombardment (MAB). The latter generates a beam of metastable s pecies capable of ionizing analyte molecules by Penning ionization. We obse rved that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of rela tive isotopomer abundance ratios on sample concentration. Ionization using the P-3(2) and P-3(0) states of metastable krypton (9.92 and 10.56 eV, resp ectively) resulted in almost no molecular ion fragmentation, and the isotop omer abundances quantified were essentially independent of sample concentra tion. ionization using the P-3(2) and P-3(0) states of metastable argon (11 .55 and 11.72 eV, respectively) showed molecular ion fragmentation intermed iate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence o f isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer f rom a fragment cation to a neutral molecule is the gas phase reaction mecha nism responsible for the concentration dependence. Alternative explanations , e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements o f methyl palmitate, in particular, and of fatty acids methyl esters, in gen eral. (C) 2001 American Society for Mass Spectrometry.