This work concerns a proton transfer reaction time-of-flight mass spectrometer, PTR-TOF-MS, a bespoke model manufactured by Kore Technology Ltd. for Smiths Detection. This instrument achieves 'soft' ionization of volatile organic compounds (VOCs) by proton transfer from protonated water vapour in a reaction chamber at 1 mbar (= 100 Pa). The resulting ions are separated by mass in a field-free time-of-flight tube prior to detection by a multi-channel plate. The instrument was modified to facilitate direct determination of the electric field in the reaction chamber.
Sensitivity measurements determined a value of 4-6 counts per second per parts per billion by volume (ncps ppbv-1) normalised to 106 H3O+. The calibration gas mixture used in this investigation consisted of 14 compounds, (alkylbenzenes and chlorobenzenes) spanning an m/z range of 78 to 180. Each of these was separately investigated over E/N = 90 to 245 Td to establish fragmentation behaviour and possible interfering contributions. For example, several of the alkylbenzenes fragmented to product ions occurring at m/z 79, the same value as that of protonated benzene. Most of this occurred at the higher E/N values with ethylbenzene a notable exception. The isobaric compounds ethyl benzene and the xylenes exhibit very different fragmentation patterns so enabling differentiation of these two compounds. However, it is not possible to distinguish the individual xylene isomers using this method.
Benchmarking was continued using the hexenol compounds cis-3-, cis-2-, trans-3- and trans-2-hexen-1-ols. This work demonstrated that the same four product ions are seen for all of the hexenol isomers at m/z 39 (C3H3+), 41 (C3H5+), 55 (C4H7+) and 83 (C6H11+) when reacted with H3O+ in a PTR-TOF-MS. A characteristic peak at m/z 99 was seen in trans-2-hexen-1-ol and cis-2-hexen-1-ol at low E/N values (m/z 101, is absent. In trans-3-hexen-1-ol and cis-3-hexen-1-ol the MH+ ion at m/z 101 is seen at these lower E/N values but there is no product ion at m/z 99. This suggests a possible method for distinguishing between the 2- and 3-hexenols. It may also be possible to further identify the individual isomers from the differences in the percentage yield of these product ions
