Simple Near-Infrared Spectrometric Sorption-Based Vapor Sensor

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Abstract
A new class of near-IR sensors has been devised which relies on spectroscopic near-IR detection of analytes sorbed on a preconcentrating layer. The new sensors furnish both qualitative and quantitative information by detection of molecular vibrational overtone and combination absorption bands and by measurement of changes in the scattering of the silica-gel medium. A silica-gel thin-layer chromatography (TLC) plate was used as the adsorbent medium. This method is responsive to vapors ranging from water to carbon tetrachloride. Ethyl- and octylderivatized TLC plates were also tested and generally exhibited a weaker response and different spectral features than the original plate. The optical nature of these sensors opens up the possibility of remote analysis by coupling the technique with fiber optics and/or near-IR light-emitting diodes and lasers. Detection limits ranged from 4.1 to 210 ppm by volume. Polar compounds able to hydrogen bond to the surface exhibited lower detection limits than nonpolar compounds.