Protonated 1,4-difluorobenzene C6H5F2+: A promising precursor for proton-transfer chemical ionization

[Display omitted] •Protonated 1,4-difluorobenzene C6H5F2+ is a soft PTR reagent for oxygenated VOCs.•Protonated 1,4-difluorobenzene induces less fragmentation than H3O+.•Trace analysis in air using C6H5F2+ has been performed on standard VOC mixture. The reactivity of protonated 1,4-difluorobenzene C6H5F2+ as a Proton Transfer Reaction (PTR) precursor in a compact Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer has been evaluated in comparison with H3O+ precursor. The C6H5F2+ ion reacts with unsaturated and oxygenated VOCs by proton transfer. Kinetic studies have shown that this precursor induces less fragmentation than H3O+, so that the protonated molecule is detected in cases where H3O+ leads to total fragmentation following proton transfer. For the analytes considered the reaction rate constants are ca 80% of the capture rate constants. Use of C6H5F2+ in trace analysis conditions has been tested on a standard mixture of VOCs diluted in air. The protonation rates measured in this way using both precursors depend linearly on concentration, allowing quantitative analysis. Their ratio to theoretical capture rates is generally slightly higher with C6H5F2+ than with H3O+.