100% humidity independent PTR-MS: Novel method and proof-of-concept^*

Abstract

It has been well-known and studied that the quantification of various compounds with Proton-Transfer-Reaction – Mass Spectrometry (PTR-MS) is dependent on the sample gas humidity. For example, if the formaldehyde concentration in outdoor air remains constant, but the weather and thus the humidity changes, the measured signal intensity for protonated formaldehyde will change. Established countermeasures against this effect include labor-intensive calibrations at different humidity levels and subsequent data correction, the use of filters or membranes to dry the sample air, which will also trap analytes of interest, and the introduction of excessive amounts of humidity into the PTR reaction region to reduce the relative but not the absolute humidity changes caused by the sample gas, which alters the well-defined ion chemistry and again limits quantification accuracy. 
Here, we present a novel solution to the humidity dependence problem that comes virtually without any drawbacks, does not require any additional labor by the user, and does not diminish analytes of condensable nature. A target humidity for the PTR reaction region is chosen before the actual measurement. Via the controlled injection of water vapor into the reaction region, the humidity is kept constant throughout the analysis, independent of the sample gas constitution. Therefore, it can be excluded that any changes in the acquired ion signal intensities are artifacts caused by changing sample humidity, and, if required, calibration measurements only have to be performed at one humidity level. 
We introduce the instrumental adaption necessary for using the novel method and subsequently present proof-of-concept results of measurements in a laboratory, as well as in a "real-life" environment, utilizing two different PTR-MS instruments.