Near-IR Reflectance Spectroscopy for the Determination of Motor Oil Contamination in Sandy Loam

We have investigated the application of near-IR reflectance spectroscopy to the determination of motor oil contamination in sandy loam. Although the present work is concerned with a specific case of contamination, we discuss the possibility of applying the method to other organic contaminants and other types of soil. The spectral region considered was 1600–1900 nm, which contains the first overtone of the CH stretch. Using a commercial Fourier transform spectrometer together with cross-validated partial least-squares data analysis, the one-sigma precision for the determination of motor oil in sandy loam was 0.17 wt % (0.13 to 0.26 wt % at the 95% confidence level). The largest contribution to the precision of the determination was sampling error, or inhomogeneity in each sample. Given the precision limit imposed by the sampling error, we found that the performance of the spectrometer could be lowered without affecting the overall precision. In a modeling exercise, adequate performance was obtained with a spectrometer having only seven spectral channels with a spectral resolution of 10 nm and a spectral noise level of 10⁻³ absorbance units. A design for an inexpensive miniature instrument is presented.