Comparison of near-infrared and Raman spectroscopy for the determination of chemical and physical properties of naphtha

Near-infrared (NIR) and Fourier transform (FT) Raman spectroscopy have been compared and evaluated for the quantitative analysis of naphtha with the use of partial least-squares (PLS) regression. Naphtha has been chosen for this purpose since it is a complex mixture of similar hydrocarbons; therefore, both spectroscopic methods can be evaluated with a complex sample of different concentration ranges and also a physical property. Six different chemical compositions [total paraffin, total naphthene (cycloalkane), total aromatic, C6 paraffin, benzene, and cyclopentane] and specific gravity (as a physical parameter) have been selected to evaluate both spectroscopic methods. PLS calibration models for each property have been developed by using both NIR and Raman spectra without spectral pretreatment. Both methods showed good correlation with the corresponding reference methods, but NIR provided the better calibration performance over Raman. The superior signal-to-noise ratio as well as spectral reproducibility of NIR led to the improved calibration performance even though Raman spectroscopy provided richer spectral information. The signal-to-noise ratio, reproducibility of measurement, and richness of spectral information should be simultaneously considered for proper selection of a spectroscopic method, especially for quantitative analysis.