Surface-enhanced Raman spectroscopy (SERS) and partial least squares (PLS) regression have been applied for the quantification of entacapone isomers <i>E</i> and <i>Z</i> in solution. Nine mixtures of isomers <i>Z</i> and <i>E</i> in ethanol ranging from 0% to 100% w/w were analyzed, for a total entacapone concentration of 1 × 10<sup>−3</sup> mol L<sup>−1</sup>. Upon deposition onto commercially available Klarite<b>®</b> 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 <i>Z</i> in the isomers mixture, corresponding to the absolute concentration of 8 × 10<sup>−5</sup> mol L<sup>−1</sup> of isomer <i>Z</i> in solution.

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