PTR-SRI-ToF-MS analysis of aroma compounds: influence of drift tube E/N ratio on sensitivity and fragmentattion

International audienceThe aim of this work was to compare detection and fragmentation patterns of aroma compounds obtained with three different precursor ions and several E/N source parameter values with the proton transfer reaction (PTR-MS) methodology. The reactant ions H3O+, NO+ and O2+. were generated in a Switchable Reagent Ions (SRI) source of a PTR-ToF-MS (Ionicon 8000, Innsbruck, Austria). Precursor ions plasmas were characterized at different E/N ratio in the drift tube. For proton transfer reaction, the hydronium primary ion was the most abundant ion (> 80%) for all E/N ratios above 120 Td, but the sensitivity decreased quickly with the highest E/N values. Even though NO+ was the most abundant primary ion in the ion plasma formed from ambient air, contaminant ions were detected with high (O2+. and NO2+) or medium (H3O+) intensities. For O2+. generated from pure oxygen, O2+. was the main precursor ion, the presence of contaminant ions was limited to a few percent of H3O+ at low E/N ratios and the reactant ion production increased significantly with increasing E/N values. The mass spectra of 12 aroma compounds belonging to various chemical classes were recorded in the three ionization modes at different E/N values. Although H3O+ ionization generally displayed limited fragmentation with increasing E/N ratios (essentially with acids, esters and some alcohols), the sensitivity decreased significantly with increasing E/N values, mainly above 120 Td. For NO+ ionization, expected charge transfer or hydride abstraction reactions were in competition with proton transfer reaction due to the presence of contaminant primary ions. O2+. ionization, the most energetic charge transfer reaction, displayed enhanced fragmentation, not always sensitive to increasing E/N values. For both NO+ and O2+. ionizations, the global sensitivity decreased significantly with increasing E/N ratios