Surface-modified sol-gel coated IR/ATR waveguide sensors

ttenuated total reflectance sampling, or waveguide sampling, is commonly used to obtain infrared spectra of aqueous solutions. The detection limits achieved for the determination of organic analytes in aqueous solutions using waveguide sampling are limited by the strong water absorption features and relatively small effective sample thickness due to the short penetration depth of the evanescent wave into the sample. An experimental investigation into the use of surface-modified sol-gel-coated Si ATR mid-infrared sensors was carried out to improve the detection limits for trace amounts of organic molecules in aqueous solutions.;Sol-gel processing has been used to produce a porous silica film on the surface of an infrared waveguide. The surface of the sol-gel film was modified with a silanizing reagent to produce a hydrophobic coating. The resulting waveguides were used to obtain infrared spectra from samples of benzonitrile and benzene in water. Hydrophobic molecules such as benzonitrile and benzene readily extract from an aqueous sample into the hydrophobic film, while water (the major interference), is repelled from the film lowering its concentration in the region probed by the evanescent wave. The detection limits achieved for benzonitrile and benzene in water are 0.4 and 2.5 ppm, respectively. The benzonitrile spectral features obtained with the modified waveguide are enhanced by more than a factor of 10;The IR performance of such a film to polar organic molecuies was also investigated. This surface modified sol-gel coated Si ATR sensor coupled with multivariate partial least squares calibration methods allowed approximately three orders of magnitude improvement in detection limits when compared to univariate analysis of spectra obtained on uncoated Si ATR sensors. Actual infrared detection limits for acetone, isopropanol and phenol in aqueous solutions were determined to be 2, 26 and 3 ppm, respectively. Studies performed on mixtures of acetone and isopropanol in water confirmed that quantitative analyses could be performed on these mixture systems with cross-validated standard errors of prediction of 0.5 and 8.7 ppm, respectively.;A porous film modified to produce a hydrophilic surface environment was also produced and characterized. The surface modification used to produce this film resulted in...