Vapor Pressures of Substituted and Unsubstituted Monocarboxylic and Dicarboxylic Acids Measured Using an Improved Thermal Desorption Particle Beam Mass Spectrometry Method

Abstract

A temperature programmed thermal desorption method we developed previously for measuring vapor pressures and enthalpies of sublimation of low-volatility organic aerosol compounds has been improved and applied to a suite of unsubstituted and substituted carboxylic acids. The method employs a thermal desorption particle beam mass spectrometer to monitor the evaporation of a submonolayer of monodisperse, submicron particles in vacuum. The desorption profile is then analyzed using evaporation rate theory to obtain a Clausius-Clapeyron equation. Improvements to the original instrument design include a redesigned vaporizer that provides uniform temperatures (< 1°C spatial variation) and the addition of a low-stick vaporizer coating that significantly reduces adsorption of molecules to the vaporizer surface. Modifications have also been made to the theory previously used to extract vapor pressures from evaporation measurements. The modified instrument and theory were evaluated by analyzing homologous series of monocarboxylic (C13–C22) and dicarboxylic (C4–C12) acids and comparing with results obtained using other methods. In addition, hydroxymonocarboxylic and oxodicarboxylic acids were analyzed to investigate the effect of a hydroxyl or carbonyl group, its location, and the carbon chain length on carboxylic acid volatility. Measured vapor pressures and enthalpies of sublimation were also compared with values calculated using SPARC, a group contribution computational method.