Raman quantification of (R)-(+)-limonene in mineral oil: An integrated PCA/PLS and DFT-PED approach

A Raman spectroscopy-based method was developed for the quantification of (R)-(+)-limonene in mineral oil using spectral pretreatment, PCA/PLS chemometric analysis, and DFT-assisted vibrational interpretation. Seven concentration levels between 1 and 50% w/w were prepared gravimetrically and analyzed using five independent measurements per level to evaluate spectral repeatability. The experimental spectra showed that the intense aliphatic CH stretching envelope is not the most selective region for limonene quantification because of the strong contribution of the hydrocarbon matrix. Instead, the 1700-1600 cm^-1 region, associated with CC stretching vibrations of the cyclohexenic ring and vinyl group, provided higher analytical specificity. DFT calculations in the gas phase and in an implicit heptane medium, together with PED analysis, supported the assignment of the main Raman bands. HOMO, LUMO, and MEP analyses further showed that the unsaturated region is the principal electronically differentiated domain of the molecule under both conditions. PCA showed concentration-dependent spectral organization with low intragroup dispersion, while the PLS model constructed from the full pretreated spectra provided satisfactory internal predictive performance using three latent variables. The model achieved R² = 0.990, RMSEC = 1.70% w/w, an average recovery of 104.8%, and a mean relative error of 4.8%. The proposed method is therefore suitable for formulation-level quantification of limonene in hydrocarbon-rich matrices within the evaluated range, although further independent validation would be required to assess external robustness and lower-concentration applicability.