Raman spectroscopy for the identification of differences in the composition of automobile lubricant oils related to SAE specifications and additives

This study employed Raman spectroscopy for the identification of spectral differences in automotive lubricant oils with different SAE specifications. Samples of 18 lubricant oils were obtained from a local auto parts store. Spectra were obtained with a dispersive Raman spectrometer (830 nm) and separated according to the type of base oil in the samples: mineral, semi-synthetic and synthetic. Also, samples were separated according to the SAE specifications regarding the viscosity grade in low and high temperatures. Principal component analysis (PCA) showed that the lubricants with mineral base oils presented peaks referred to aromatic hydrocarbons and additives containing aromatics, sulfites/dissulphites and sulfonates, while semi-synthetic and synthetic base oils presented peaks referred to alkanes. Other peaks referred to alkanes and olefins were also seen in synthetic base oils. The discriminant analysis based on quadratic distance applied to the PCA variables was able to discriminate the lubricant samples according to the type of base oil with 89.7% accuracy and discriminate the SAE viscosity grade for low and high temperatures with 95.8% and 90.0% accuracy, respectively. The use of Raman spectroscopy makes possible nondestructive evaluation of lubricant oils, which may replace expensive and complex traditional tests and methods of quality control.