In-line monitoring of esterification using a miniaturised mid-infrared spectrometer

A miniaturised mid-infrared (MIR) spectrometer attached directly to an in-line immersion probe with an attenuated total reflectance crystal has been used to monitor the progression of the batch reaction of crotonic acid and 2-butanol in toluene. Univariate calibration, using the signal at 1188 cm(-1) in the 2nd derivative spectrum, gave accurate (average error, 1.6%) and precise (average relative standard deviation, 5.2%) estimation of 2-butyl crotonate concentrations in the range 0.08-0.49 mol dm(-3). Calibration by partial least squares was of no additional benefit in this application. The performance of in-line MIR spectrometry was comparable to that of an off-line reference gas chromatography method and superior to that of other in-line process analysis techniques (near-infrared, Raman or UV-visible spectrometries).