Kinetic study of the degradation of lubricating motor oil by liquid chromatography and photoacoustic spectrometry

Abstract

Lubricating motor oil samples were collected throughout one oil change period of 5000 km and analyzed for saturate, aromatic, polynuclear aromatic hydrocarbon (PAH) and polar species using normal-phase high-performance liquid chromatography (HPLC) with ultraviolet light absorption and refractive index detections. The oil samples were also analyzed using a new photoacoustic spectrometry (PAS) method. Fair correlation was established between the HPLC method and the PAS method for determining aromatic concentration. Moreover, the PAS method offers shorter analysis time, comparable reproducibility, and improvement in detection sensitivity over the HPLC method. The analyses data obtained were used to study the degradation kinetics of the motor oil against mileage. The aromatic species were found to increase in concentration with pseudo zeroth-order kinetics, the PAH species rose from zero to some higher amount, whereas the saturate and the polar species remained essentially constant over the mileage period.