Sensitivity and specificity of atmospheric trace gas detection by proton-transfer-reaction mass spectrometry

Abstract

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 reasonable agreement is obtained. It is shown how the sensitivity depends on the pressure in the drift tube and on the humidity of the sample air. In PTR–MS, only the mass of the ionized trace gases is determined, which is a useful but not a unique indicator of the trace gas identity. A combination of gas chromatography and PTR–MS (GC–PTR–MS) has been developed to investigate which compounds contribute to the signal at a certain mass. Air samples collected in the city of Utrecht in The Netherlands and at the remote Sonnblick Observatory in Austria were analyzed by GC–PTR–MS. The results clearly indicate that PTR–MS measurements of VOCs such as methanol, acetonitrile, acetaldehyde, benzene and toluene are free from interference by other compounds. For other VOCs, such as acetone and methyl ethyl ketone, interference cannot always be ruled out entirely. For some masses like 69amu, a large number of biogenic VOCs produce the same signal, demonstrating the need for GC–PTR–MS methods. It is also shown that PTR–MS measurements can likely be used to determine the sum of the concentration of C2-benzenes, despite the fact that these compounds cannot be independently measured with PTR–MS.