A Proton-Transfer-Reaction Mass Spectrometer (PTR-MS), which employs fast mass spectrometry to detect Volatile Organic Compounds (VOC). Owing its low detection limits for many compounds and its high frequency of acquisition, it is particularly useful instrument in measurements of eddy-covariance fluxes and monitoring of atmospheric concentrations. The PTR-MS technique has been reviewed by R. S. Blake, P. S. Monks, A. M. Ellis: Proton-Transfer Reaction Mass Spectrometry. Chem. Rev., 109 (3) (2009), 861-896.