PTR-MS stands for Proton Transfer Reaction Mass Spectrometry, which is an analytical technique used to measure the concentrations of volatile organic compounds (VOCs). It is a highly sensitive technique that can detect compounds at concentrations of parts per trillion (ppt) or lower. PTR-MS works by ionizing compounds in the sample using a proton beam, which then results in the formation of charged ions that are then analyzed by a mass spectrometer. This enables the detection and quantification of a wide range of VOCs and other trace compounds.