Wear metal determination in lubricating oils by reversed-phase dispersive liquid–liquid microextraction and microwave induced plasma optical emission spectrometry†

A periodic study of engine oil can allow one to anticipate possible breakdowns that a vehicle could have. Due to the complexity of the matrix of this type of sample, a sample treatment prior to analysis is necessary. Analytical chemistry is constantly searching for simpler, more sensitive and environmentally friendly methods. The reversed phase dispersive liquid–liquid microextraction fulfills all the characteristics for the analysis in this type of sample by microwave induced plasma optical emission spectrometry (MIP OES). In this work, the extractant solvent used for the microextraction procedure is a diluted acid solution (i.e., 3 M HCl), which is a less-hazardous solvent, in comparison with other solvents used in microextraction procedures. Besides, it is perfectly compatible with MIP OES. The main experimental factors affecting the extraction of Cr, Cu, Mn, Mo, and Ni (i.e., amount of sample, extractant type, acid concentration, extractant volume, extractant time, and centrifugation time and speed) are optimized using a multivariate analysis consisting of two steps: a Plackett–Burman design followed by a circumscribed central composite design. Under optimum conditions (i.e., amount of sample: 5.9 g; extractant volume: 60 μL; extractant type: HCl; acid concentration: 3 M; extraction time: 3 min; centrifugation time: 3 min; centrifugation speed: 2000 rpm), the proposed analytical method is validated and employed to analyze different samples (i.e., used and unused engine oils). Two calibration methods have been evaluated since matrix effects have been observed in the used engine oil sample. These effects have been eliminated using standard addition calibration obtaining RSD and recovery values in the range of 4–12% and 94–106%, respectively, for samples spiked with 1 μg g⁻¹ of Cr, Cu, Mn, Mo and Ni. Finally, the greenness of this method has been assessed by the Eco-Scale metrics.