Ion Mobility Spectrometer (IMS) coupled with a time-of-flight Mass Spectrometer (tofMS) for high-resolution ambient gas and aerosol analysis

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http://urn.fi/URN:NBN:fi-fe201804208705
Title: Ion Mobility Spectrometer (IMS) coupled with a time-of-flight Mass Spectrometer (tofMS) for high-resolution ambient gas and aerosol analysis
Author: Chellapermal, Robert
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
URI: http://urn.fi/URN:NBN:fi-fe201804208705
http://hdl.handle.net/10138/299766
Thesis level: master's thesis
Discipline: Atmosphere-Biosphere Studies
Abstract: It has proven to be challenging to detect and analyse the tens of thousands of precursors for secondary organic aerosol (SOA) species in ambient air. Models have shown that SOA in particular are still underestimated by an order of magnitude or more. Currently, instrumentation has evolved in the field of atmospheric-pressure mass spectrometers to be able to continuously identify the molecular species in ambient air at high resolution. This includes also neutral clusters if an ionisation mechanism is implemented. Yet, molecular-level information is still missing. The key questions include: What is the bulk density of atmospheric particles? What is the molecular composition of isomeric and isobaric compounds? Most high-resolution mass spectrometers are unable to answer these research questions, as the main output is merely a mass-to-charge (m/z) ratio. By measuring the electrical mobility of these particles, as well as the molecular composition, these questions can be answered. This motivated the atmospheric science community to combine mass spectrometry (MS) instruments with mobility instruments for measuring ambient aerosol and gas phases. An extensively well-defined and studied method, ion mobility spectrometry (IMS), has gained popularity in recent years due to its useful application in the fields of explosives detection and pharmaceutical compound analysis. By coupling an ion mobility spectrometer (IMS) with a time-of-flight mass spectrometer at ambient pressure (APiTOF), we aim at providing additional molecular-level information. With electrospray-ionisation (ESI) and a custom X-ray ion source, neutral clusters can also be detected. For the first time, we have successfully run a 2-month measurement campaign and analysed the data with this setup from a rural boreal forest site in Finland. Before this campaign, calibration of the effective true length of the drift region was performed based on known mobility chemicals (tetra-alkyl-ammonium halides) in the laboratory. The resulting effective length (~19 cm) was utilised to calculate reduced mobility, K0, for all detected masses. Knowing electrical mobility and mass, and assuming spherical particles, density was also computed. All measured compounds in the range of m/z=50 to 500, had lighter bulk density than water calculated. An additional tool linking particles with the same mobility was investigated to show the various applications of the new dataset.Cluster-level analysis of new particle formation (NPF) events and non-events could be further researched and utilised to obtain novel information about molecular clusters.


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