Online Trace Compound Monitoring with Proton-Transfer-Reaction – Mass Spectrometry (PTR-MS)

Proton-Transfer-Reaction - Mass Spectrometry (PTR-MS) is a versatile tool for sensitive real-time quantification of trace compounds in numerous fields of application (environmental research, process monitoring, food and flavor science, threat compound detection, etc.). In short, reagent ions can be chosen from H3O+, NO+, O2+, Xe+ and Kr+, respectively, and are introduced together with air containing the analytes into a reaction region ("drift tube"), i.e. no sample preparation is required. Reagent and product ions are subsequently analyzed and detected in a mass spectrometer with a response time typically in the 100 ms regime. Recently it has been discovered that the transfer region between the reaction region is one of the most crucial parts for the instrument's overall sensitivity. By employing Radio Frequency (RF) multipole ion guides and ion funnels in this region ion losses can efficiently be suppressed and the sensitivity can be improved by orders of magnitude. Here, we present data obtained with different ion focusing devices and compare not only the effects on sensitivity, Limit-of-Detection (LoD) and mass resolution, but also on the reduced electric field strength (E/N) in the reaction region. The latter often is a weak point for RF devices, as the E/N is no longer well-defined and thus measurement results become difficult to interpret. 

Furthermore, we installed an ion funnel and hexapole ion guide in series, which results in a sensitivity of close to 2000 cps/ppbv and a LoD of about 10 pptv for 1 s and <1 pptv for 60 s integration time, respectively (measured using a certified gas standard). By switching the ion funnel on and off, we compare branching ratios at various E/N and demonstrate that the RF device has negligible influence on the ion chemistry. Additionally, this switching procedure enhances the dynamic range of the instrument to six orders of magnitude. 

On the application side we present the real-time monitoring of trace gases in syngas in a Fischer-Tropsch process using a multiplexing unit, so that a series of sampling streams can be analyzed with one PTR-MS instrument. Stimulated by the observed effects of E/N on the ion chemistry we have developed a new software tool, "Automated Measurement and Evaluation" (AME), which performs measurements at various ionization conditions and subsequently processes the data with real-time output. We demonstrate this process, e.g., by using isopropyl alcohol and acetic acid, two isobaric compounds, that are both detected at m/z 61 (protonated parent) and m/z 43 (fragment), which hinders identification and quantification. Using a substance library containing the fragmentation ratios of both compounds at different E/N levels the AME software separates and quantifies the two isobars independently. Additionally, AME also averages the data, to reduce noise, exports and displays the processed data.

We gratefully acknowledge that FP receives support by the EU via the H2020-ITN project "IMPACT" (674911).