A Proton Transfer Reaction-Quadrupole interface Time-Of-Flight Mass Spectrometer (PTR-QiTOF): High speed due to extreme sensitivity

Abstract

Here we introduce a new prototype of a Proton Transfer Reaction-Time-Of-Flight Mass Spectrometry (PTR-TOFMS) instrument. In contrast to commercially available PTR-TOFMS devices so far, which utilize a transfer lens system, the novel prototype is equipped with a quadrupole ion guide for the highly effective transfer of ions from the drift tube to the mass spectrometer; hence we call it PTR-QiTOF, whereas “Qi” stands for “Quadrupole interface”. This new interface greatly improves the TOF mass resolution because of favorable injection conditions. Depending on whether we optimize the PTR-QiTOF to maximum sensitivity or maximum mass resolution, we get about 6900 and 10,400 m/Δm mass resolution, respectively, already at m/z 149 (increasing with ascending masses). Furthermore, we increase the pressure in the drift tube from typically 2.2mbar to 3.8mbar and the drift tube voltage from 600V to 1000V. We directly compare the sensitivities of a commercial state-of-the-art PTR-TOFMS instrument to this “high pressure” PTR-QiTOF prototype and find that these modifications lead to a gain on average by a factor of 25 in terms of sensitivity with a maximum of about 4700cps/ppbv for dichlorobenzene at m/z 147 for the PTR-QiTOF. This is (to our knowledge) the highest sensitivity ever reported for a PTR-MS instrument, regardless of the employed mass spectrometer. The increased sensitivity also has a very positive effect on the detection limit, which lies now at about 20pptv with 100ms and 750ppqv after 1min integration time. We provide data on the linearity of the instrumental response over a concentration range of five orders of magnitude and evaluate the prototype's performance in a real-life test by analyzing the dynamic headspace of a minute amount of trinitrotoluene using only 2s integration time.