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Evaluation of a New Vocus Reagent-Ion Source and Focusing Ion-Molecule Reactor for use in Proton-Transfer-Reaction Mass Spectrometry

submitted on 13.06.2018 and posted on 13.06.2018 by Jordan Krechmer, Felipe D. Lopez-Hilfiker, Abigail R. Koss, Manuel A. Hutterli, Carsten Stoermer, Benjamin L. Deming, Joel R. Kimmel, Carsten Warneke, Rupert Holzinger, John T. Jayne, Douglas R. Worsnop, Katrin Fuhrer, Marc Gonin, Joost de Gouw
We evaluate the performance of a new chemical ionization source called Vocus, consisting of a discharge reagent-ion source and focusing ion-molecule reactor (FIMR) for use in protontransfer-reaction time-of-flight mass spectrometry (PTR-TOF) measurements of volatile organic compounds (VOCs) in air. The reagent ion source uses a low-pressure discharge. The FIMR consists of a glass tube with a resistive coating, mounted inside a radio-frequency (RF) quadrupole. The axial electric field is used to enhance ion collision energies and limit cluster ion formation. The RF field focuses ions to the central axis of the reactor and improves the detection efficiency of product ions. Ion trajectory calculations demonstrate the mass-dependent focusing of ions and enhancement of the ion collision energy by the RF field, in particular for the lighter ions. Product ion signals are increased by a factor of 10 when the RF field is applied (5,000-18,000 cps ppbv-1), improving measurement precision and detection limits while operating at very similar reaction conditions as traditional PTR instruments. Due to the high water mixing ratio in the FIMR, we observe no dependence of the sensitivity on ambient sample humidity. In this work, the Vocus is interfaced to a TOF mass analyzer with a mass resolving power up to 14,000, which allows clear separation of isobaric ions, observed at nearly every nominal mass when measuring ambient air. Measurement response times are determined for a range of ketones with saturation vapor concentrations down to 5×104 μg m-3 and compare favorably with previously published results for a PTR-MS instrument.


This manuscript is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 654109.


Email Address of Submitting Author

Email Address(es) for Other Author(s)


Aerodyne Research, Inc.



ORCID For Submitting Author


Declaration of Conflict of Interest

Felipe Lopez-Hilfiker, Manuel Hutterli, Carsten Stoermer, Joel Kimmel, Katrin Fuhrer and Marc Gonin are employees of TOFWERK, which has developed and commercialized the Vocus PTR-TOF. Jordan Krechmer, John Jayne, Joel Kimmel and Douglas Worsnop are employees of Aerodyne Research Inc., which has partnered with TOFWERK to develop and commercialize atmospheric chemistry research applications of the Vocus PTR-TOF. Joost de Gouw worked as a part-time consultant for Aerodyne Research Inc. during this project.