Application of mid infrared spectroscopy and iPLS for the quantification of contaminants in lubricating oil

The feasibility of using a narrow spectral range in the mid infrared region for a rapid and conclusive determination of contaminants (gasoline, ethylene glycol and water) in lubricating oil was investigated. A graphically-oriented local multivariate calibration modeling procedure called interval partial least-squares (iPLS) was applied to select a spectral range that provides the lowest prediction error, in comparison to the full-spectrum model. The region from 650 to 784 cm−1, attributed to aromatics, was selected for gasoline determination. The region from 918 to 1052 cm−1, for C−O stretching from glycol, was selected for ethylene glycol determination. Finally, the region from 3598 to 3732 cm−1, absorption from hydrogen bonded OH groups, was chosen for water determination. In the determinations, a root mean square error of prediction (RMSEP) of 0.34% for gasoline, 0.037% for ethylene glycol and 0.023% for water, were obtained after interval selection.