10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Utilizing Bromide Chemical Ionization Technique in Detecting Oxidized Organic Compounds
Xucheng He, YEE JUN THAM, Siddharth Iyer, Mikko Sipilä, Matti Rissanen, University of Helsinki
Abstract Number: 719 Working Group: Instrumentation
Abstract Highly oxidized multifunctional (HOM) compounds have been found to play a crucial role in the atmospheric aerosol formation[1] and yet, direct measurement of many of these oxidized organics in the ambient air has remained challenging. Mass spectrometric methods using nitrate ionization[1], iodide ionization[2], and protonated water clusters[3] have been recently applied for direct detection of highly oxidized organic molecules in laboratory and field studies.
In this work, we explore the possibility of using a bromide chemical ionization technique to detect oxidized organic compounds. A Chemical Ionization Atmospheric Pressure interface Time-Of-Flight (CI-APi-ToF) was optimized to operate using bromide as the reagent ion. A series of test runs and calibrations for the instrument were conducted in our laboratory and scientific experiments were carried out in the CLOUD chamber in CERN. The results show that the bromide chemical ionization technique can detect a broad range of oxidation products; in form of CxHyOz- and/or (CxHyOz)Br- from oxidation of alpha-pinene (by O3 and OH) under various temperature conditions. An interesting observation is that the hydrogen numbers in most of the deprotonated/dehydroxylated organic ions are odd, indicating that the bromide ionization process has stripped off one hydrogen atom (deprotonation) or potentially together with an oxygen atom (dehydroxylation) from the neutral molecules, which were not widely observed with the nitrate ionization method. The detected oxidized organic ions from bromide CIMS were compared with HOM species detected by a nitrate CIMS to examine if they present the same molecular structure or not under different experimental conditions (i.e. with or without NOx). Another question that arose from our observations is whether the deprotonated/dehydroxylated ions and the organic-bromide cluster ions represent the same oxidation products, but are charged by a different mechanism (i.e., clustering vs charge transfer reaction), or are they compounds with same chemical formula but with a different chemical structure, i.e., are they isomeric products.
Overall, our study shows that the bromide ionization technique can detect a wider range of oxidized organic species, especially lower-oxidized species, compared to the nitrate CIMS. The comparison between the two ionization methods also helped shed light on the molecular structure of the detected oxidized organic species.
References:
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