Reaction monitoring using Raman spectroscopy and chemometrics

The synthesis and hydrolysis of ethyl acetate was monitored with a Raman spectrometer equipped with a 785-nm laser, confocal optics, an optical fiber, a holographic transmission grating, and a charge coupled device (CCD) detector. In the Raman spectra, the signals were seen to vary in proportion to the concentrations of ethanol, acetic acid and ethyl acetate. First-order reaction, rate constants were determined for the hydrolysis reaction, under different experimental conditions according to a full factorial experimental design. The rate constants were determined either by using the peak height at one single wavelength, or by using score values from principal components analysis (PCA) or partial least squares (PLS). The rate constants determined agree well with literature values. It is shown that the single wavelength and the PCA/PLS approaches give similar reaction profiles, where the standard normal variate (SNV) transformation was used for spectral pretreatment. The use of Raman spectroscopy for reaction monitoring in opalescent mixtures is also shown and discussed.