10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Characterizing the Thermal Desorption Behavior of Hemiacetal and Acetal Oligomers
MEGAN CLAFLIN, Paul Ziemann, University of Colorado
Abstract Number: 1471 Working Group: Aerosol Chemistry
Abstract The presence of oligomers in atmospheric aerosols has been observed both from field measurements and for particles formed in laboratory experiments. The formation of these compounds has been attributed to particle-phase accretion reactions that occur on timescales that range from seconds to a few hours. Common types of oligomers that are formed in aerosols include hemiacetals, acetals, and peroxyhemiacetals. While the existence of these compounds in aerosols has been verified, their detection using preferred methods by the atmospheric community, such as online mass spectrometers that utilize thermal desorption, can be difficult due to decomposition or fragmentation. Thermal desorption methods, like the FIGAERO-CIMS, offer the opportunity to detect compounds separated by their volatility. Yet how oligomers, especially those that form via reversible reactions, respond to thermal desorption is unclear. There has been evidence that under some conditions, the thermal ramping can cause the oligomers to revert to their monomer building blocks, and thus when detected they would not reflect the true aerosol composition.
To study the effects of thermal desorption on hemiacetals and acetals, both synthesized standards and the SOA formed from the reaction of β-pinene with NO3 radicals were investigated. Previously, we have shown that >90% of the β-pinene SOA is made up of hemiacetal and acetal oligomers. Therefore, this system provides an opportunity to look at the thermal desorption properties of real SOA that has been carefully characterized and is made almost entirely of oligomers.
The thermal desorption behavior was studied using two types of mass spectrometers. The first was a Temperature Programmed Thermal Desorption Particle Beam Mass Spectrometer (TPTD-PBMS) which uses electron ionization and a thermal desorption ramp rate of 2 ⁰C min-1. This instrument was used to study how these classes of compounds are ionized in real time, by instantaneous vaporization, versus how they behave when slowly desorbed by heating. The second was a Chemical Ionization Ion Trap Mass Spectrometer (CI-ITMS) equipped with a Direct Insertion Probe (DIP) that uses a thermal desorption ramp of 10 ⁰C min-1, a ramp rate that is comparable to that used with the FIGAERO-CIMS.
The results of our experiments show that when hemiacetals are analyzed in real time they are ionized as the intact oligomer. However, when they are slowly heated using a thermal desorption method the hemiacetals decompose and are instead ionized as their monomers. This effect was not seen for acetals, which were ionized as the intact oligomer for both real time and thermal desorption analyses.
Our intention of characterizing how these types of oligomers respond to thermal desorption detection methods is to give insight on the interpretation of data for aerosol systems that are likely to contain these classes of compounds. Without accounting for decomposition of oligomers, the consequence is likely misassigning the volatility of an aerosol and thus misrepresenting the physical properties of the particle.