Improving PTR-ToF-MS: implementation of a radio frequency ion funnel and an investigation into buffer-gas doping

Volatile organic compounds (VOCs) are ubiquitous in the Earth’s atmosphere. VOCs are produced from biogenic sources such as forests, or anthropogenic sources such as fossil fuel combustion. Many areas of research involve measuring VOCs, from atmospheric science, to medical science and homeland security with implications for health, the environment, and safety. It is crucial that VOCs are detected quickly and with a high sensitivity: proton transfer reaction - mass spectrometry (PTR-MS) offers a solution and potential enhancements of the PTR-MS technique are discussed here. A drift tube capable of simultaneously functioning as an ion funnel is demonstrated in PTR-MS for the first time enabling a much higher proportion of ions to exit the drift tube and enter the mass spectrometer than would otherwise be the case. An increase in the detection sensitivity for VOCs of up to two orders of magnitude and an increase in Limit of Detection of one order of magnitude is delivered, allowing lower concentrations of VOCs to be detected. An alternate way to change how the drift tube behaves is to alter the buffer gas. The collision energy within the drift tube is investigated in order to ascertain what advantages changing the buffer gas from nitrogen to argon yields with respect to sensitivity and fragmentation of analytes. For several compounds, the sensitivity is increased and fragmentation reduced. If sensitivity can be increased and/or fragmentation reduced within a complex mixture of analyte ions, then analysis of these mixtures will be simplified. Finally, a standard PTR-MS instrument is compared with the ion funnel equipped PTR-MS instrument in an urban, megacity (ClearfLo campaign, London, UK) in order to test the instrument and enhanced sensitivity for field measurements.