The composition that was adopted in the preparation of the three-component solution for later analysis was 0.1 g sample, 2.5 g aviation kerosene, 0.1 g of water with nitric acid (1.4 mol L−1) and the volume completed to 10mL with n-propanol. After the addition of all components, the system was shaken manually for a few seconds until the formation of a clear and stable solution. The standards for calibration were prepared in the same way, replacing the sample by 0.1 g of mineral oil to simulate the lubricating oil and adding the inorganic standards into the aqueous phase. Both three-component solution of standard and samples remained stable, without separation of phases, for at least 10 months.