Determination of Fe, Cr and Cu in used lubricating oils by ET AAS using a microemulsion process for sample preparation

Abstract

The analysis of metals in used lubricating oils has been widely reviewed and published in the literature, especially because they provide valuable information about the lubricated system. The determination of metals such as Fe, Cr and Cu may indicate the presence of contaminants in the oil or wear of parts of motors. In this work, a method for the determination of Fe, Cr and Cu by atomic absorption spectrometry with electrothermal atomization (ET AAS) using a microemulsion process for sample preparation has been developed. A pseudo-ternary phase diagram was built in order to investigate the region where a homogeneous and stable system (microemulsion) could be achieved. The optimized conditions for the microemulsion formation were: 2.0% (w/w) of lubricating oil, 31% (w/w) of aviation kerosene, 5% (w/w) of nitric acid 14 mol L⁻¹, 5% (w/w) of Triton X-100 and 57% (w/w) of n-propanol. The developed method is simple and fast, involving few steps and avoiding the use of carcinogenic solvents. Moreover, the increased stability of the microemulsion and of the analytes in a microemulsified medium and the use of aqueous standards for calibration were achieved. The values of the detection limits for Fe, Cr and Cu were 1.9 mg kg⁻¹, 0.04 mg kg⁻¹ and 0.2 mg kg⁻¹, respectively. Good accuracy was attained since the results obtained were not significantly different within 95% level of confidence (Student's t-test) from those of an acid-digestion method and the certified value of the NIST certified reference material wear-metals in lubricating oil (SRM 1084a). Therefore, the method developed in this work becomes an efficient alternative for the determination of Fe, Cr and Cu in used lubricating oil.