Characterization of the Vocus PTR-TOF: Analytical Chemistry

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

This work characterizes the performance of the Vocus PTR-TOF through simulations and lab experiments, including direct comparison to a similar PTR-TOF having a conventional drift tube PTR reactor cell.

J. Krechmer, F. Lopez-Hilfiker, et al.
Analytical Chemistry
DOI: 10.1021/acs.analchem.8b02641

Proton Transfer Reaction Mass Spectrometry (PTR-MS) is the most widely used method for real-time monitoring of trace volatile organic compounds (VOCs) in air.  Modern PTR mass spectrometers offer parts-per-trillion by volume (pptv) or better limits of detection with response times that are less than 1 second.  PTR-MS performance has incrementally improved over the past two decades owing to upgrades in mass analyzers and in the ion optics that transfer ions from the reactor cell to the mass analyzer.  The design of PTR-MS reactor cells have changed very little during this time, with most instruments using a conventional drift tube reactor that applies a homogeneous electric field along the axial drift axis to control ion energy, prevent cluster formation, and direct ions through the reactor toward the mass analyzer. 

The Vocus reactor breaks from this traditional design to further improve the achievable sensitivity of PTR-MS.   The most significant change in this focusing ion molecule reactor (FIMR) is the application of a quadrupole RF field in the reactor, in addition to the drift field, to collimate ions on the central axis and increase transmission from the reactor to the mass analyzer.    In further contrast to the traditional drift cell, the reactor cell consists of a resistive glass tube, rather than parallel ring electrodes, to produce a more homogenous field. 

This work represents the first published description and characterization of the Vocus PTR-TOF.  The performance of the system is evaluated through simulation and lab experiment, including direct comparison to a similar instrument having a traditional drift tube reactor.  It is demonstrated that application of the RF field in the reactor improves sensitivity by a factor of 10, leading to increased measurement precision and lower achievable limits of detection.   

The Vocus 2R PTR-TOF used in this work has a mass resolving power (M/ΔM) of up to 12000, which is the highest of any commercially available system.    Measurements of ambient air are used to demonstrate that this high mass resolving power enables more confident identification of unknown peaks and allows more precise quantification of small signals. 

Other key experiments in this work characterize the humidity dependence of the Vocus PTR-TOF and the instrument response time.  In contrast to conventional PTR sources, the sensitivity of the Vocus is shown to have no dependence on the humidity of the sampled air.    Instrument response times, which were measured for several ketones, compare favorably with previously published PTR-MS data.