Reaction monitoring using Raman spectroscopy and chemometrics

The synthesis and hydrolysis of ethyl acetate was monitored with a Raman sp ectrometer equipped with a 785-nm laser, confocal optics, an optical fiber, a holographic transmission grating, and a charge coupled device (CCD) dete ctor. In the Raman spectra, the signals were seen to vary in proportion to the concentrations of ethanol, acetic acid and ethyl acetate. First-order r eaction, rate constants were determined for the hydrolysis reaction, under different experimental conditions according to a full factorial experimenta l design. The rate constants were determined either by using the peak heigh t at one single wavelength, or by using score values from principal compone nts analysis (PCA) or partial least squares (PLS). The rate constants deter mined agree well with literature values. It is shown that the single wavele ngth and the PCA/PLS approaches give similar reaction profiles, where the s tandard normal variate (SNV) transformation was used for spectral pretreatm ent. The use of Raman spectroscopy for reaction monitoring in opalescent mi xtures is also shown and discussed. (C) 1999 Elsevier Science B.V. All righ ts reserved.