Development of a PTR-TOFMS instrument for real-time measurements of volatile organic compounds in air

A proton transfer reaction time-of-flight mass spectrometer (PTR-TOFMS) has been developed for real-time measurements of volatile organic compounds in air. The instrument is designed to be operated with a hollow cathode discharge ion source and an ion drift tube at relatively high pressures. Each component of the system, the ion source, the drift tube, the ion transfer region, and the time-of-flight mass spectrometer were characterized in detail by a number of laboratory experiments. The optimized instrumental configuration enables us to gain high intensities of hydronium (H3O+) ions, typically 7E+5 counts for 1-min integration at a drift tube pressure of about 5 Torr. It also suppresses background signals up to 0.5% of the intensity of H3O+ ions, as well as interferences from sample air (NO+ and O2+), which undergo reactions with volatile organic compounds. The detection limits for propene, acetaldehyde, acetone, isoprene, benzene, toluene, and p- xylene are estimated to be in the range of 10-100 pptv for a 1-min integration time. A good linear response at trace levels was confirmed, but a slight sensitivity dependence on water vapor content was revealed.