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| Titel |
Measuring acetic and formic acid by proton-transfer-reaction mass spectrometry: sensitivity, humidity dependence, and quantifying interferences |
| VerfasserIn |
M. Baasandorj, D. B. Millet, L. Hu, D. Mitroo, B. J. Williams |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 3 ; Nr. 8, no. 3 (2015-03-18), S.1303-1321 |
| Datensatznummer |
250116218
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| Publikation (Nr.) |
 copernicus.org/amt-8-1303-2015.pdf |
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| Zusammenfassung |
| 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. |
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