Introducing the Extended Volatility Range Proton-Transfer-Reaction Mass Spectrometer (EVR PTR-MS)

Abstract. Proton-transfer-reaction mass spectrometry (PTR-MS) is widely used in atmospheric sciences for measuring volatile organic compounds in real time. In the most widely used type of PTR-MS instruments, air is directly introduced into a chemical ionization reactor via an inlet capillary system. The reactor has a volumetric exchange time of ~ 0.1 s enabling PTR-MS analyzers to measure at a frequency of 10 Hz. The time response does, however, deteriorate if low-volatility analytes interact with surfaces in the inlet or in the instrument. Herein, we present the “Extended Volatility Range” (EVR) PTR-MS instrument which mitigates this issue. In the EVR configuration, inlet capillaries are made of passivated stainless steel and all wetted metal parts in the chemical ionization reactor are surface-passivated with a functionalized hydrogenated amorphous silicon coating. Heating the entire set-up to 120 °C further improves the time-response performance.

We carried out time-response performance tests on a set of 29 analytes having saturation mass concentrations C⁰ in the range between 10⁻³ and 10⁵ µg m⁻³. 1/e-signal decay times after instant removal of the analyte from the sampling flow were between 0.2 and 90 s for gaseous analytes. We also tested the EVR PTR-MS instrument in combination with the CHARON particle inlet, and 1/e-signal decay times were in the range between 5 and 35 s for particulate analytes. We show on a set of exemplary compounds that the time-response performance of the EVR PTR-MS instrument is comparable to that of fastest flow tube chemical ionization mass spectrometers that are currently in use. The fast time response can be used for rapid (~ 1 min equilibration time) switching between gas and particle measurements. The CHARON EVR PTR-MS instrument can thus be used for real-time monitoring of both gaseous and particulate organics in the atmosphere. Finally, we show that the CHARON EVR PTR-MS instrument is capable of detecting highly oxygenated species (with up to eight oxygen atoms) in particles formed by limonene ozonolysis.