Interference in the PTR-MS measurement of acetonitrile at m/z 42 in polluted urban air—A study using switchable reagent ion PTR-MS

Abstract

In Proton Transfer Reaction Mass Spectrometer (PTR-MS) measurements of the atmosphere, the signal at m/z 42 is commonly regarded as a unique measure of acetonitrile. However, two other ions potentially contribute to the signal at m/z 42. These are 13C isotopologues of C3H5+ and the product ion C3H6+ produced by reaction of NO+ and O2+ (present in trace amounts in the H3O+ reagent gas), with a number of volatile organic compounds. Thus, there is the possibility of interference in the measurement of acetonitrile at m/z 42 by PTR-MS.Interference in the measurement of acetonitrile at m/z 42 was quantified in urban air over 17 days in Sydney, Australia, in summer. A PTR-MS with Switchable Reagent Ion capability was used for measurements at m/z 41 and 42 in three different primary reagent ion modes, O2+, NO+ and H3O+, to quantify the contribution of non-acetonitrile compounds to the signal at m/z 42 when the PTR-MS was operating in H3O+ reagent ion mode. Acetonitrile dominated the ion signal at m/z 42; however non-acetonitrile ions contributed 5–41% of the total ion signal at m/z 42. The average corrected and uncorrected acetonitrile concentrations were 120pptv and 148pptv respectively.The interference in the m/z 42 signal was calculated from known or interpolated concentrations of compounds identified as potential interferrants. Overall the isotopologue correction is due to alkenes including isoprene with probable contributions from other compounds not measured in this study. The other component of the interference, produced by reactions of O2+, is due to alkanes and alkenes.Levoglucosan, a biomass burning tracer in atmospheric particulate matter was more highly correlated with the corrected acetonitrile signal than the uncorrected acetonitrile signal.Measurements of acetonitrile by PTR-MS at m/z 42 in urban air will frequently require correction because of the non-trivial concentrations of alkanes and alkenes commonly observed in urban air.