Pressurized liquid extraction to determine Mg, Al, Ti, Cu, Sn and Pb in lubricating oils by inductively coupled plasma mass spectrometry

A method to determine metals in lubricating oils was developed using pressurized solvent extraction (PLE) with a semi-automatic extractor ASE 200 (Dionex, USA) by inductively coupled plasma mass spectrometry (ICP-MS). The special nature of the sample made it necessary to adsorb it on a “drying” agent, which allowed us to handle it as a solid, and to use a cleaning agent in the extraction cell. A Plackett–Burman experimental design was used to study the variables determining the extraction conditions of Mg, Al, Ti, Cu, Ag, Sn and Pb: pH of the aqueous phase used as the extractant, mass of the dispersing agent, mass of the cleaner agent, temperature, time of the extraction cycle and number of cycles. The pH was revealed as the critical variable (optimum pH = 0.5, adjusted with concentrated HCl). Good recoveries (80–118%) were obtained when a certified lubricating oil (SRM 1084a) was analysed, but only low recoveries were obtained for Ag. The procedure was applied to analyse crankcase oils. The method proposed here has important advantages over the conventional procedures employed to analyse lubricating oils as it provides aqueous sample extracts which reduce instrumental interferences, enable the use of aqueous standards and, hence, avoid the use of organic solvents. Thus, it is environmentally friendly, following the green analytical chemistry guidelines.