The Proton-Transfer-Reaction Mass Spectrometer provides a real-time method of identifying and measuring volatile organic compounds in air, at mixing ratios as low as a few parts per trillion by volume (pptv).

An advantage of this method is its high time resolution and that it can measure most of the relevant VOC molecules in the atmosphere with no or minor fragmentation of the VOC molecules, as sometimes occurs using other measurement techniques. The measurement technique involves the generation of H3O+ ions by a hollow-cathode discharge. These ions are mixed with the sample gas in a drift tube, and collisions between the H3O+ and the VOC lead to the transfer of a proton to the VOC molecule. The method is based on the fact that most common VOCs have higher proton affinities than H3O+. Ions are then passed through an ion analyzer system, where the protonated VOC molecules are detected based on their mass to charge ratio. The PTR-MS technique is described in more detail by Lindinger et al (1998).

The instruments maintained are a quadropole PTR-MS, and the high mass resolution version PTR-ToF-MS 8000 both produced by Ionicon.
On-line monitoring of volatile organic compounds at pptv levels by means of proton-transfer-reaction mass spectrometry (PTR-MS) medical applications, food control and environmental research.
W. Lindinger, A. Hansel, A. Jordan, 1998
International Journal of Mass Spectrometry and Ion Processes, 173, 3, 1998, 191-241.
doi: 10.1016/S0168-1176(97)00281-4.