Measuring acetic and formic acid by proton-transfer-reaction mass spectrometry: sensitivity, humidity dependence, and quantifying interferences

Abstract

Abstract. We present a detailed investigation of the factors governing the quantification of formic acid (FA), acetic acid (AA), and their relevant mass analogues by proton-transfer-reaction mass spectrometry (PTR-MS), assess the underlying fragmentation pathways and humidity dependencies, and present a new method for separating FA and AA from their main isobaric interferences. PTR-MS sensitivities towards glycolaldehyde, ethyl acetate, and peroxyacetic acid at m/z 61 are comparable to that for AA; when present, these species will interfere with ambient AA measurements by PTR-MS. Likewise, when it is present, dimethyl ether can interfere with FA measurements. For a reduced electric field (E/N) of 125 Townsend (Td), the PTR-MS sensitivity towards ethanol at m/z 47 is 5–20 times lower than for FA; ethanol will then only be an important interference when present in much higher abundance than FA. Sensitivity towards 2-propanol is <1% of that for AA, so that propanols will not in general represent a significant interference for AA. Hydrated product ions of AA, glycolaldehyde, and propanols occur at m/z 79, which is also commonly used to measure benzene. However, the resulting interference for benzene is only significant when E/N is low (≲100 Td). Addition of water vapor affects the PTR-MS response to a given compound by (i) changing the yield for fragmentation reactions and (ii) increasing the importance of ligand switching reactions. In the case of AA, sensitivity to the molecular ion increases with humidity at low E/N but decreases with humidity at high E/N due to water-driven fragmentation. Sensitivity towards FA decreases with humidity throughout the full range of E/N. For glycolaldehyde and the alcohols, the sensitivity increases with humidity due to ligand switching reactions (at low E/N) and reduced fragmentation in the presence of water (at high E/N). Their role as interferences will typically be greatest at high humidity. For compounds such as AA where the humidity effect depends strongly on the collisional energy in the drift tube, simple humidity correction factors (XR) will only be relevant for a specific instrumental configuration. We recommend E/N ~ 125 Td as an effective condition for AA and FA measurements by PTR-MS, as it optimizes between the competing E/N-dependent mechanisms controlling their sensitivities and those of the interfering species. Finally, we present the design and evaluation of an online acid trap for separating AA and FA from their interfering species at m/z 61 and 47, and we demonstrate its performance during a field deployment to St. Louis, USA, during August–September of 2013.

Temporal variation of VOC fluxes measured with PTR-TOF above a boreal forest

“…Higher fragmentation in the PTR-TOF (61.0284 Da), when compared to the PTR-Quad (60.5 to 61.5 Da), could account for a part of the lower acetic acid fluxes with the EC method. However, even when the acetic acid main fragment C 2 H 3 O + (Baasandorj et al, 2015) is taken into account, in addition to the signal from the parent mass, the EC fluxes are still lower than the SLP results. Another uncertainty comes from the lack of acetic acid in the calibration standard.…”

“…Kajos et al (2015) reported similar discrepancies in toluene + p-cymene concentration measurements with the PTR-Quad at the site. Also, acetic acid fragments when measured with the PTR method (Baasandorj et al, 2015). Higher fragmentation in the PTR-TOF (61.0284 Da), when compared to the PTR-Quad (60.5 to 61.5 Da), could account for a part of the lower acetic acid fluxes with the EC method.…”

Characterization of total ecosystem-scale biogenic VOC exchange at a Mediterranean oak–hornbeam forest

“…In this case the fragmentation pattern is not accounted for and all fragments have to be added up to arrive at the total flux, excluding those that could be associated with calibrated compounds. Two exceptions were made: the sensitivity for acetic acid was halved (9.55 ncps ppb −1 ) as it was not calibrated and its major fragment (C 2 H 3 O + ; Baasandorj et al, 2015) was disregarded. For ethanol (C 2 H 7 O + 1 ) the methanol sensitivity was used.…”

Measurement of formic acid, acetic acid and hydroxyacetaldehyde, hydrogen peroxide, and methyl peroxide in air by chemical ionization mass spectrometry: airborne method development

“…Table S1 provides a summary of literature surface and aircraft measurements for GA in urban, biomass burning, biogenic, and mixed environments. GA's primary loss is by HO oxidation and wet deposition (Bacher et al, 2001). The effective Henry's Law constant for GA (70 M hPa −1 ) is surprisingly large (Betterton and Hoffmann, 1988) and an order of magnitude larger than that for HAc (7.8 M hPa −1 ) at a temperature of 288 K (Johnson et al, 1996;results below).…”