Automatic determination of insolubles in lubricating oils by flow injection analysis employing an LED-photometer detector

A flow injection system is presented for the determination of the insolubles content in used lubricating oil samples. The system is based on the injection of an aliquot of the sample in a stream of organic solvent where it is dispersed, and measurement of the scattered radiation (measured as apparent absorbance) in the visible range (λ = 640 nm). An LED-based photometer was used for this purpose. The whole system including sample injection and data acquisition was controlled by a personal computer. Calibration curves exhibited good linearity (h = 0.415 ± 0.016C + 0.00 ± 0.03, r2 = 0.9995, confidence level of 95%) in the range up to 2.68% (insolubles in pentane). Detection and quantification limits were respectively 0.07% and 0.16% (w/w). The method was validated by analysis of 25 real samples by the proposed method and the FTIR method finding high correlation. Waste generation and reactive consumption is much less than in the official method (ASTM D-893). The proposed method employs 25 mL of kerosene per sample while the official method employs 200 mL of pentane.