Determination of insolubles in diesel lubricating oil by FIA-visible spectrometry

Insolubles determination is one of the parameters usually recommended to evaluate the residual life of oil because their presence at elevated levels in diesel lubricating oil changes the viscosity, prematurely clogs filters and is one of the major factors in causing abrasive engine wear. The proposed method employs visible spectrophotometric detection in association with flow injection analysis. The results obtained by this method were compared with the ones obtained by Fourier transform infrared spectrometry (FT-IR) since this is the most employed method for insolubles determination. The proposed method presented a linear response from 0 to 3% (w/w) of insolubles in pentane (ASTM D-893). The sampling frequency was about 30 samplesh(-1), with a relative standard deviation (n=5) of 2.4% or better. Accuracy was evaluated analysing 98 real samples and the results obtained with the FIA-spectrophotometric method were plotted against those obtained by the FT-IR method by means of linear regression. Slope and intercept of the straight line obtained were compared with the theoretical values of 1 and 0 by means of the joint-confidence ellipse F-test. At the confidence level of 95% no evidence of a difference was found between both methods.