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, an ion source, a drift tube, an ion transfer region, and a time-of-flight mass spectrometer, are in detail characterized by a number of laboratory experiments. The optimized instrumental configuration enables us to gain high intensities of hydronium (H3O+) ions, typically ∼7 × 105 counts for 1-min integration at a drift tube pressure of ∼5 Torr. It also suppresses background signals, and interferences from sample air (NO+ and O2+), which undergo fast reactions with volatile organic compounds, to ∼0.5% of those of H3O+ ions. We find that the use of the custom-built discharge source show higher overall sensitivities than of a commercially available radioactive source. Potentials to detect oxygenated VOCs (aldehydes, ketones, and alcohols), halocarbons, and amines are also suggested. The detection limits for acetaldehyde, acetone, isoprene, benzene, toluene, and p-xylene were determined to be at the sub-ppbv levels for a 1-min integration time. A good linear response at trace levels is certified, but slight sensitivity dependency on water vapor contents is revealed. We finally demonstrate that the instrument can be used for on-line monitoring to detect large variations from emission sources in real-time.