Fig. 1. Schematic diagram of the SERS detection test platform

Fig. 2. The result of Preprocessing for Raman signal data

Fig. 3. Key variables selection by CARS
(a): Variation trend of variable; (b): Variation trend of RMSECV in cross-validation; (c): Regression coefficient path

Fig. 4. Best variables selected by CARS

Fig. 5. The Voigt resolution of Raman spectra

Fig. 6. The result of Voigt analysis
(a): Peak area; (b): Peak width at half height; (c): Gaussian coefficient; (d): Lorentzian coefficient

Fig. 7. Classification result of oil-paper samples

Fig. 8. Cumulative variance of the first eight principal components

Fig. 9. Loading plot for PC1 and the average value of the principle component for each sample

Fig. 10. Scatter plots of the PC scores for four classes of oil samples, with the PC scores derived from the Raman spectra

Fig. 11. Relationship between ageing time and WPEE of Raman spectra