No abstract

Adsorption–desorption behaviors of polar and nonpolar volatile organic compounds (VOCs), namely, isopropanol and nonane, on mesoporous silica were studied using optical reflectance spectroscopy. Mesoporous silica was fabricated via electrochemical etching of silicon and subsequent thermal oxidation, resulting in an average pore diameter of 11 nm and a surface area of approximately 493 m2/g. The optical thickness of the porous layer, which is proportional to the number of adsorbed molecules, was measured using visible light reflectance interferometry. In situ adsorption and desorption kinetics were obtained for various mesoporous silica temperatures ranging from 10 to 70 °C. Sorption as a function of temperature was acquired for isopropanol and nonane. Sequential adsorption measurements of isopropanol and nonane were performed and showed that, when one VOC is introduced immediately following another, the second VOC displaces the first one regardless of the VOC’s polarity and the strength of its interaction with the silica surface.

Continuous measurement of the concentrations of ethanol and its metabolites, acetaldehyde and acetic acid, in vivo is important for the alcohol research community. Most studies only measure ethanol because accurate measurement of all three compounds is challenging. Measurement inside tissue/brain is done using a microdialysis technique, followed by off-line analysis using gas chromatography (GC). To realize simultaneous measurement of ethanol and its metabolites, one can take advantage of infrared (IR) spectroscopy as a rapid and reagent-free method. Here we report a feasibility study of using IR spectroscopy to simultaneously measure ethanol, acetaldehyde and acetic acid in aqueous solution. Different concentrations in transmission mode at different optical pathlengths and using attenuated total reflectance (ATR) were measured. In vitro microdialysis was performed on the mixture of the three compounds, and the collected sample was measured using IR to demonstrate the capability of quantifying the concentrations of the three analytes simultaneously. Lastly, to overcome the limitations of the microdialysis technique, direct measurement using evanescent-field IR spectroscopy can be a potential alternative. A hydrophobic polymer coating that adsorbs ethanol and excludes water, could improve sensitivity. Sorption kinetics in polymethyl methacrylate (PMMA) and polydimethylsiloxane (PDMS) coatings on an ATR crystal were measured. Both polymers demonstrate preferential adsorption of ethanol over water.

of gases on solid surfaces. [9] This understanding aided the pioneering development of the first preconcentrator in 1969 by Dave et al. [10] This first preconcentrator utilized a polymer as the material for adsorption and desorption. Later, the first carbonaceous preconcentrator was developed in 1975 by Russell et al. [11] Until 1979, most preconcentration research relied on large instrumentation for analyte detection. However, in 1979, the development of the first micro gas chromatography (μGC) demonstrated that miniature detection methods are also possible. [12] The term "preconcentrator" was not used until 1989 by Thomas et al. [13] Later, the usability of oxide materials for preconcentration was explored; Casalnuovo et al. reported a SiO 2 based preconcentrator in 2001. [14] Ueno et al. introduced a metal-organic framework (MOF) based preconcentrator in 2007. [15] These major advances in preconcentrator development are shown in a timeline in Figure 1. The number of publications containing the term "gas preconcentrator," "gas pre concentrator," "gas pre-concentrator," "VOC preconcentrator" over time since 1989 is shown in Figure 2.In this review, recent developments of gas preconcentrators, preconcentrator materials, and preconcentration techniques are summarized. Different materials, their recent uses, limitations, and future potential are reviewed and discussed.