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Proton-transfer-reaction mass spectrometry (PTR–MS) has emerged as a useful tool to study the atmospheric chemistry of volatile organic compounds (VOCs), which are implicated in the formation of ozone and aerosols in polluted air. In PTR–MS, ambient air is continuously pumped through a drift-tube reactor and the VOCs in the sample are ionized using proton-transfer reactions with H₃O⁺ ions. The H₃O⁺ and product ions are detected with a quadrupole mass spectrometer. The technique combines a fast response time (1s) with a low detection limit (10–100 parts-per-trillion), and allows atmospheric measurements of many important VOCs and their oxidation products in a variety of field experiments. Here, the sensitivity of PTR–MS with respect to a number of VOCs is characterized. The measured sensitivity, obtained using calibrated mixtures of VOCs in air, is compared with a calculated sensitivity and a …