The effects of electron and chemical ionization modes on the MS profiling of whole bacteria

Free fatty acid profiling of whole bacteria [Francisella tularensis, Brucel la melitensis, Yersinia pestis, Bacillus anthracis (vegetative and sporulat ed), and Bacillus cereus] was carried out with direct probe mass spectromet ry under 70-eV electron ionization (EI) and isobutane chemical ionization i n both the positive (CI+) and negative modes (CI-). Electron ionization pro duced spectra that contained molecular ions and fragment ions from various free fatty acids. Spectra acquired with isobutane chemical ionization in th e positive mode yielded molecular ions of free fatty acids as well as ions from other bacterial compounds not observed under EI conditions. Spectra ob tained with negative chemical ionization did not contain as much taxonomic information as EI or CI+; however, some taxonomically significant compounds such as dipicolinic acid and poly(3-hydroxybutyrate) did produce negative ions. All ionization modes yielded spectra that could separate the bacteria by Gram-type when observed with principle components analysis (PCA). Chemi cal ionization in the positive ion mode produced the greatest amount of dif ferentiation between the four genera of bacteria when the spectra where exa mined by PCA. (C) 1999 American Society for Mass Spectrometry.