Development of a chemical ionization mass spectrometry technique for the direct measurement of HO₂

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SANCHEZ-THESIS-2016.pdf (821.34 KB)
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Sanchez, Javier
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School of Chemical and Biomolecular Engineering
School established in 1901 as the School of Chemical Engineering; in 2003, renamed School of Chemical and Biomolecular Engineering
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http://hdl.handle.net/1853/58613
Abstract
Hydroperoxy radicals (HO₂) play important roles in tropospheric photochemistry. Despite significant efforts in constraining HO₂ abundance, notable and variable discrepancies exist between atmospheric models and observations. Disagreements may be explained by uncharacterized chemical pathways or deficiencies in current measurement techniques, likely attributable to the indirect nature of such techniques. A direct method for measuring HO₂ would aid in accurately constraining HO₂ chemistry in the atmosphere. In this work, the feasibility of several reagent ions for the direct detection of HO₂ using chemical ionization mass spectrometry (CIMS) was explored. A direct HO₂ detection scheme using bromide as a reagent ion is proposed. Laboratory characterizations suggest that the method is selective and applicable to laboratory and ambient measurements. Ambient observations were made with a high resolution time-of-flight chemical ionization mass spectrometer in Atlanta over the month of June 2015 to demonstrate the capabilities of this direct measurement technique. Observations displayed expected diurnal profiles, reaching daytime median values of ~5 ppt between 2 p.m. and 3 p.m. local time, and displayed no obvious artifacts. Measurement sensitivities of approximately 5.1±1.0 cps/ppt for a bromide ion (79Br−) count rate of 106 cps were observed. The relatively low instrument background allowed for a 3ϭ lower detection limit of 0.7 ppt for 1 minute time resolution. Mass spectra of ambient measurements further support the selectivity of the Br− ionization technique. High resolution mass spectra showed that the 79Br−(HO₂) peak was the major component of the signal at nominal mass-to-charge 112, suggesting high selectivity for HO₂ at this mass-to-charge. Important measurement considerations and future improvements are discussed.
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2016-07-27
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