The ion trap aerosol mass spectrometer: field intercomparison with the ToF-AMS and the capability of differentiating organic compound classes via MS-MS

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 [C₄H_y]⁺ and [C₃H_yO]⁺ 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.