Quantitative in Situ Monitoring of an Elevated Temperature Reaction Using a Water-Cooled Mid-Infrared Fiber-Optic Probe

Abstract

A novel water-cooled mid-infrared fiber-optic probe is described which is heatable to 230 °C. The probe has chalcogenide fibers and a ZnSe internal reflection element and is compact and fully flexible, allowing access to a wide range of standard laboratory reaction vessels and fume cupboard arrangements. Performance is demonstrated via the in situ analysis of an acid-catalyzed esterification reaction in toluene at 110 °C, and the results are compared with those from a conventional extractive sampling loop flow cell arrangement. Particular emphasis is given to the quantitative interpretation of the spectroscopic data, using gas chromatographic reference data. Calibration data are presented for univariate and partial least squares models, with an emphasis on procedures for improving the quality of interpreparation calibration and prediction through the use of focused reference analysis regimes. Subset univariate procedures are presented that yield relative errors of <5%, and bias-corrected partial least squares procedures are described that result in relative errors of interpreparation calibration and prediction consistently <3%. This paper demonstrates the considerable power of fiber-optic mid-IR spectroscopy combined with bias correction partial least squares procedures for the efficient in situ quantitative analysis of laboratory scale reactions.