Vortex-assisted reverse-phase dispersive liquid–liquid microextraction for on-site determination of Ti, Cr, Mn, Ni, and Cu in lubricating oil using total reflection X-ray fluorescence spectroscopy

Abstract

Critical equipment in transportation vehicles, such as aircraft engines, requires on-site spectral analysis. Total reflection X-ray fluorescence (TXRF) coupled with vortex-assisted reverse phase dispersive liquid–liquid microextraction (RP-DLLME) is a powerful tool with great potential to achieve low detection limits in the field detection of wear elements in lubricating oil. However, the traditional RP-DLLME method is cumbersome and not suitable for on-site applications. This study proposes a simplified dilution and RP-DLLME procedure that eliminates the centrifugation step and optimizes the extractant collection procedure. The optimized RP-DLLME procedure validated by Plackett–Burman and central composite design methods is as follows: the lubricating oil sample was diluted 20-fold with petroleum ether. Next, 2.9 mL of the diluted sample was added into a 5 mL conical centrifuge tube, followed by 79 µL of 0.1 M HNO₃ as the extractant. After vortexing for 3 minutes, let the mixture stand for several seconds to complete phase separation. About 60 µL of the extractant was then taken directly from underneath the organic phase with a pipette. Subsequently, 40 µL of this extractant was added to 10 µL of the standard solution to complete the process of standard addition. Finally, measurements were carried out according to the TXRF aqueous solution measurement protocol. By employing the standard addition method to correct for matrix effects, the relative responses of the analyte elements to the internal standard (Ga) were effectively re-established for the sample matrix, leading to satisfactory quantitative results with the proposed method. The RSD for five replicate measurements for Cr, Mn, Fe, Ni and Cu was between 4% and 12% and recovery rates were between 88% and 112%.