Monitoring tropospheric composition using time of flight chemical ionisation mass spectrometric techniques
2014-12-15T10:36:03Z (GMT) by
The use of proton transfer reactions (PTR) to quantify volatile organic compounds (VOCs) occurring in a variety of environments has increased in recent years. Here a 44.4 MBq 241Am source is used to ionise water vapour generating H3O+ protonated water molecules. This thesis describes how proton transfer reaction products fed into a time-of-flight mass spectrometer (TOF-MS), can produce simultaneous mass spectra for the entire mass range of reacting species in an experiment and produce a mass resolution m/Deltam greater than 1000.;The scope of application of the investigation has been expanded to include the use of alternative chemical ionisation reagents with proton transfer reactions to form a broader based technique called chemical ionisation reaction mass spectrometry or CIRMS.;Measurements of atmospheric VOC loadings at the University of Leicester show pronounced weekday and subdued weekend cyclic variations in atmospheric VOC loading and demonstrate the value of the equipment in monitoring Leicester's growing urban pollution.;The PTR-TOF-MS is shown to support chemical ionisation reactions using the reagents, NO+, O2+, as in earlier selected ion flow tube (SIFT) work. In experiments using a selection of commonly occurring VOCs as analytes, NO+ and, to a lesser extent, O 2+ were found to be useful alternatives reagents to H 3O+. The results obtained were similar to the SIFT work with the same reagents, but they show the influence of the increased collision energy introduced by the potential gradient applied to the drift-tube and demonstrate the advantages of the broader based CIRMS approach.;The value of switching to NO+ or O2 + as alternative chemical ionisation reagents has been demonstrated for use in the time-of-flight mass spectrometry environment.