Application of an argon-cooled inductively coupled plasma to the analysis of metals in lubricating oils

Master Thesis


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University of Cape Town

A sequential atomic emission spectrometer, using a low power (1,7 kW) argon-cooled inductively coupled plasma (ICP) system, was used for the determination of wear metals and additives in lubricating oils with xylene as solvent. The modified Simplex method was applied to establish the optimum experimental parameters for 21 individual spectral lines. The optimization was applied to single element analysis on the basis of net signal-to-background ratio (SBR) using the standard cross flow nebulizer and then repeated using the high solids Babington-type nebulizer. Weighted compromise conditions were calculated resulting in a maximum loss of 50% in the SBRs of the individual elements. These conditions result in an analytically useful plasma that gives low detection limits and high sensitivities for the analysis of metals in lubricating oils. Analytical parameters such as detection limit and analytical range for the analysis of 17 elements present in lubricating oil as wear metals or contaminants and 4 other elements present in the most common lubricating oil additives were determined. The detection limits reported here for oil in xylene are of the same order as the published limits for aqueous and for oil-in-MIBK solutions. However, the most valuable property of the ICP-AES was found to be the astounding analytical range which allows determinations to be made over a large concentration range without the need for serial dilutions. The effect of sample preparation was investigated by employing ultrasonic agitation prior to withdrawing the used oil from the sample vial. An upward trend was found in the element concentrations from manual shaking of the oil to ultrasonic agitation, indicating a particle distribution effect. Finally, the technique was tested successfully by analysing an NBS reference standard and by participating in several "round robin" used oil analyses. ICP-atomic emission spectrometry can be successfully applied to the analysis of metals in lubricating oils with a speed, accuracy and precision which at least equals, and even improves upon any other of the established techniques.

Bibliography: pages 46-50.