Abstract
Abstract. Further development and optimisation of a previously described ion trap aerosol mass spectrometer (IT-AMS) are presented, which resulted in more reproducible and robust operation and allowed for the instrument's first field deployment. Results from this 11-day-long measurement indicate that the instrument is capable of providing quantitative information on organics, nitrate, and sulfate mass concentrations with reasonable detection limits (0.5–1.4 µg m−3 for 1 h averages) and that results obtained with the IT-AMS can directly be related to those from Aerodyne aerosol mass spectrometers. The capability of the IT-AMS to elucidate the structure of fragment ions is demonstrated via an MS4 study on tryptophan. Detection limits are demonstrated to be sufficiently low to allow for MS2 studies not only in laboratory but also in field measurements under favourable conditions or with the use of an aerosol concentrator. In laboratory studies the capability of the IT-AMS to differentiate [C4Hy]+ and [C3HyO]+ fragments at the nominal m∕z 55 and 57 via their characteristic fragmentation patterns in MS2 experiments is demonstrated. Furthermore, with the IT-AMS it is possible to distinguish between fragments of the same elemental composition ([C2H4O2]+ at m∕z 60 and [C3H5O2]+ at m∕z 73) originating from different compound classes (carboxylic acids and sugars) due to their different molecular structure. These findings constitute a proof of concept and could provide a new means of distinguishing between these two compound classes in ambient organic aerosol.
Highlights
Despite the fact that atmospheric aerosol particles have an important influence on air quality, public health, and the climate system, the knowledge on the influence of individual aerosol particle chemical components remains limited (Fuzzi et al, 2015)
Apart from the potential influence of lower ion transmission or lower trapping efficiencies for low m/z ions in the ion trap aerosol mass spectrometer (IT-AMS), this is probably mostly due to the strong influence of charge-transfer reactions in the ion trap during the trapping phase in this m/z range, e.g. involving N+, O+, N+2, or O+2 (Dotan et al, 1997; Hierl et al, 1997). m/z 28 (N+2 ), 32 (O+2 ), and 40 (Ar+), and m/z 44 (CO+2 ), are depleted in the IT-AMS compared to the ToF-AMS mass spectrum, likely due to formation of more stable ions by charge-transfer reactions in the ion trap (Ottens et al, 2005)
The results show that the IT-AMS is capable of providing quantitative information on the major nonrefractory submicron species organics, nitrate, and sulfate, with detection limits between 0.5 and 1.4 μg m−3 for 1 h averages
Summary
Despite the fact that atmospheric aerosol particles have an important influence on air quality, public health, and the climate system, the knowledge on the influence of individual aerosol particle chemical components remains limited (Fuzzi et al, 2015). While information on elemental composition of the fragment ions can be obtained with the ToF-AMS, it does not allow for the differentiation between fragment ions of the same elemental composition but with different structural formulas (i.e. isomeric ions) Such information can be obtained using ion trap mass spectrometers (March, 1997), which allow for measuring a “classic” mass spectrum (MS) and enable MSn studies. Kürten et al (2007) introduced an instrument which represents a synthesis of an AMS ionisation chamber (thermal desorption/electron impact ionisation) with a quadrupole ion trap mass spectrometer; a similar instrument was developed by Harris et al (2007) With these systems, a strong reduction in complexity of ambient organic aerosol mass spectra (which consist of a large number of different organic molecules) is achieved compared to “soft” ionisation techniques, while still some additional information, e.g. on molecular functionality, can be obtained which is not accessible from ToF-AMS measurements. Since sugars and carboxylic acids can be associated with different aerosol sources (sugars originate e.g. from biomass burning or primary biological material, while carboxylic acids originate e.g. from photooxidation of organic precursors; Graham et al, 2003), this would help in further improving the differentiation of various organic aerosol components and in source apportionment of atmospheric organic aerosol
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