KINETIC-STUDY OF THE DEGRADATION OF LUBRICATING MOTOR OIL BY LIQUID-CHROMATOGRAPHY AND PHOTOACOUSTIC SPECTROMETRY

Lubricating motor oil samples were collected throughout one oil change period of 5000 km and analyzed for saturate, aromatic, polynuclear ar omatic hydrocarbon (PAH) and polar species using normal-phase high-per formance liquid chromatography (HPLC) with ultraviolet light absorptio n 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 determi ning aromatic concentration. Moreover, the PAS method offers shorter a nalysis time, comparable reproducibility, and improvement in detection sensitivity over the HPLC method. The analyses data obtained were use d to study the degradation kinetics of the motor oil against mileage. The aromatic species were found to increase in concentration with pseu do zeroth-order kinetics, the PAH species rose from zero to some highe r amount, whereas the saturate and the polar species remained essentia lly constant over the mileage period.