Enhancing IR Detection Limits for Trace Polar Organics in Aqueous Solutions with Surface-Modified Sol-Gel-Coated ATR Sensors

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Abstract
An experimental investigation into the use of surface-modified sol-gel-coated Si attenuated total reflectance (ATR) mid-infrared sensors was completed to determine the detection limits for trace amounts of polar organic molecules in aqueous solutions. The surfaces of porous sol-gel films were modified to be mostly hydrophobic in order to concentrate the polar organic analytes in the film while largely excluding the water solvent. This modification of the Si ATR sensor coupled with multivariate partial least-squares calibration methods allowed approximately three orders of magnitude improvement in detection limits for acetone when compared to detection limits from univariate analysis of spectra obtained on uncoated Si ATR sensors. The comparable improvement for isopropanol in aqueous solution was over two orders of magnitude. 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 (CVSEP) of 0.5 and 8.7 ppm, respectively. Using three times the CVSEP as an estimate of the detection limit translates to detection limits of 1.5 and 26 ppm for acetone and isopropanol, respectively.