Thermal desorption behavior of hemiacetal, acetal, ether, and ester oligomers

Thermal desorption methods are widely used in the aerosol community to obtain information on particle volatility, and are often coupled with mass spectrometry to separate chemical components prior to analysis. One of the challenges of using these methods is that it is not yet clear how different components respond to temperature, in particular whether they desorb intact or decompose by reversible or irreversible reactions prior to desorption. In this study, we analyzed the thermal desorption behavior of four major classes of oligomers: hemiacetals, acetals, ethers, and esters, which are potentially present in secondary organic aerosol (SOA) and are primarily formed through particle-phase accretion reactions. The results show that when all four of these oligomers are desorbed in our thermal desorption particle beam mass spectrometer at ∼160 °C on millisecond timescales (real-time analysis) they reach the ionization region as the intact oligomer. This is also true for acetal, ether, and ester oligomers desorbed at much lower temperatures on timescales of tens of minutes, whereas hemiacetal oligomers decompose reversibly to the original alcohol and aldehyde monomers. A key factor that influences the desorption behavior of oligomers appears to be whether reversible decomposition occurs by unimolecular rearrangement or whether it involves hydrolysis, and thus requires water that may be lost from particles during heating and thus not available for reversible decomposition prior to desorption. The results should aid others in interpreting thermal desorption analyses, and in extracting information about the linkages that bind oligomers and the types of accretion reactions by which they were formed.

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