Organic trace analysis using high resolution mass spectrometry for the characterization of ancient, present and simulated atmospheric systems
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Abstract
Atmospheric aerosol particles are strongly affecting air quality, climate, and human health. Particularly, atmospheric aerosols can have major effects on the formation, properties and lifetime of clouds and significantly affect the human respiratory and cardiovascular system. Despite extensive research over the last decades, large uncertainties still remain regarding the organic fraction of such airborne particles, which is often accounting for the majority of the particulate mass. Providing high sensitivity and selectivity towards single molecules, mass spectrometry (MS) is a well-suited technique for the chemical analysis of atmospheric aerosol particles. Although MS has been proven to be a versatile and powerful tool in the field of aerosol research, the identification and trace analysis of single organic molecules still demands improvements due to insufficient mass resolution. The aim of this work was to develop analytical methods for the analysis of single organic compounds in highly-complex atmospheric matrices and to establish ultra-high-resolution mass spectrometry (UHRMS) as a versatile tool in atmospheric aerosol research. In particular, ultra-high-performance liquid chromatography (UHPLC) coupled to electrospray ionization ultra-high-resolution mass spectrometry (ESI-UHRMS) was used for the quantification of tracer molecules in ice cores and molecular characterization of laboratory generated aerosols. Furthermore, an atmospheric pressure chemical ionization Orbitrap mass spectrometry (APCI-Orbitrap-MS) technique was applied for the real-time determination of single molecules in ambient air using negative and positive ionization mode. The first part of this study is focused on the development of a single, comprehensive trace analytical method for the sensitive quantification of new and more appropriate low-volatile marker compounds in snow and ice samples. Solid phase extraction (SPE) using anion exchange functionalities was used for extraction and enrichment of the compounds from the molten sample matrix. As a proof-of-principle, the optimized method was applied for the analysis of ice core samples from the Belukha glacier in the Altai mountain range. Several organic trace components were determined for the first time in an ice core from the Belukha glacier and quantified in the low ng/g-range within a single analytical method. In the second part, UHPLC coupled to UHRMS was applied for the comprehensive molecular characterization of submicron-particles generated in the Cosmics Leaving Outdoor Droplets (CLOUD) laboratory chamber at the European Organization for Nuclear Research (CERN), Geneva. Besides attributing the identified molecules to certain molecular classes, a special focus was on the identification of highly oxidized multifunctional organic compounds (HOMs). It was shown that, varying mixing-ratios of SO2 led to a different distribution of organic mono-/di-nitrates, indicating an SO2-dependant formation pathway. In conclusion, a unique compound-list of identified SOA molecules including the exact molecular mass and the retention time of detected isomers was obtained. Finally, inspired by the participation in the CLOUD 10 campaign at CERN, the need for real-time ultra-high-resolution mass spectrometry was noticed and led to the development and characterization of APCI-Orbitrap-MS for the real-time measurement of atmospheric aerosol particles. Optimization and characterization of the APCI-Orbitrap-MS was performed by laboratory-generated model aerosol exhibiting a high time resolution, a linear response over three orders of magnitude for sub 100 nm particles and detection limits in the low ng/m³ range. As a proof of principle, the ambient aerosol composition was analyzed by sampling PM2.5 particles from the outside of the laboratory building in alternating ionization mode. Due to the soft ionization procedure, molecular ions are preserved and the deprotonated or protonated ions represent the main signal. A subsequent non-target screening, as well as single organic compound detection and quantification in aerosols, was performed under ambient atmospheric conditions without preconcentration or filter sampling steps. Particularly, in the non-target screening, the molecular composition of ambient organic aerosol during night- and daytime was examined both in negative and positive ionization mode. With the presented mass spectrometric system it was possible to detect highly oxidized organic nitrates, organic di-nitrates and nitrooxy organosulfates with a high time resolution in the ambient particle phase. In conclusion, it has been shown that the Orbitrap technique can be used as a versatile tool offering a number of advantages for the analysis of organic aerosols in offline as well as online mode, which will help to shed light on the atmospheric aerosol composition and formation mechanisms.