Quantitative analysis of entacapone isomers using surface-enhanced Raman spectroscopy and partial least squares regression.

Surface-enhanced Raman spectroscopy (SERS) and partial least squares (PLS) regression have been applied for the quantification of entacapone isomers E and Z in solution. Nine mixtures of isomers Z and E in ethanol ranging from 0% to 100% w/w were analyzed, for a total entacapone concentration of 1 × 10(-3) mol L(-1). Upon deposition onto commercially available Klarite® gold plates, highly intense and reproducible SERS spectra were obtained from the entacapone isomers. Based on the spectral measurements, a two-component PLS model for correlation of predicted and real content of the isomers mixtures was developed. Root-mean-square error of the predicted composition was found to be 8% of isomer Z in the isomers mixture, corresponding to the absolute concentration of 8 × 10(-5) mol L(-1) of isomer Z in solution.