Desorption kinetics with PTR-MS: Isothermal differential desorption kinetics from a heterogeneous inlet surface at ambient pressure and a new concept for compound identification

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• 作者：Erna Schuhfried^a ; Eugenio Aprea^b ; Luca Cappellin^a ; ^b ; Christos Soukoulis^b ; Roberto Viola^b ; Tilmann D. Mä ; rk^a ; Flavia Gasperi^b ; Franco Biasioli^b ; ^{franco.biasioli@iasma.it}
• 关键词：Differential desorption kinetics ; Heterogeneous surface ; Isotherm ; Ambient pressure ; Memory effect ; High-temp-proton transfer reaction-mass spectrometry
• 刊名：International Journal of Mass Spectrometry
• 出版年：2012
• 期刊代码：43_13873806
• 类别：ch
• 出版时间：15 March, 2012
• 卷：314
• 期：Complete
• 页码：33-41
• 文件大小：617 K

摘要

Proton transfer reaction-mass spectrometry (PTR-MS) is a soft ionization mass spectrometric technique for monitoring volatile organic compounds (VOCs) with a very low limit of detection (LOD) (parts per trillion by volume) and excellent time resolution (split seconds). This makes PTR-MS a particularly interesting instrument for investigating surface desorption kinetics of volatile organic compounds (VOCs) under realistic conditions, i.e., at ambient pressure from a heterogeneous surface. Here, we report on the investigation of heterogeneous inlet surface kinetics with PTR-MS and based thereon, develop concepts to assist compound identification in PTR-MS. First, we studied differential isothermal desorption kinetics using heterogeneous inlet surface data measured by Mikoviny et al. with their newly developed high-temp-PTR-MS. The best fit to their data is obtained with bimodal pseudo-first order kinetics. In addition, we explored the normalization of the data and calculated data points of the desorption isotherms. We found evidence that the interesting part of the isotherm can be linearized in a double log plot. Then we investigated the idea to use memory effects of the inlet system to assist compound identification. At the moment, the main problem is the dependence of the kinetics on the initial equilibrium gas phase adsorption concentration, and thus, the surface coverage. As a solution, we suggest an empirical, quasi-concentration independent, yet compound specific parameter: the normalized desorption time t_nd describing the decline of the signal to 1/e² of the initial concentration, normalized to an initial concentration of 10,000 counts per second (cps). Furthermore, we investigated property-property and structure-property relationships of this new parameter. Further possible improvements are discussed as well.