MONITORING CARBOXYLIC-ACID FORMATION IN ENGINE OILS BY LIQUID-CHROMATOGRAPHY WITH FLUORESCENCE DETECTION

Straight chain aliphatic carboxylic acids (C-6-C-22) were selectively derivatized in oxidised engine oils with the fluorescent label 9-anthracenemethanol after dialysis of the oils to remove polymeric additives, organometallic oxidation products and solid debris. The ester derivatives were separated by reversed-phase liquid chromatography and quantified using fluorescence detection (lambda(ex) = 251 nm; lambda(em) = 412 nm). Calibrations over the range 1-4 mg ml-1 were linear (0.9987 less than or equal to r(2) less than or equal to 1.0000). The limit of detection (S/N = 3) for octadecanoic acid was 85 mu g ml(-1) in n-heptane (4.3 ng on-column). Results showed that carboxylic acids are formed during the oxidation of oils in car engines and that their concentrations are directly related to the degree of oil degradation.