An o/w emulsion/microemulsion system was proposed for analysis of used lubricating oil by Souza et al. [25], for determination of Cr, Mo, Ti, V and Ni by ICP–OES, and by Aucélio et al. [22], for determination of Sb and Sn by GF AAS. The emulsion was prepared following a specific sequence of addition of sample/HNO3/xylene/Triton X-100 and water, with assistance of an ultrasonic bath, in order to guarantee its homogeneity. The authors reported that using up to 5% (in mass) of sample and 6% (in mass) of surfactant Triton X-100, the emulsion remained stable
for 40 min, which was enough time to perform the analysis [25]. An o/w microemulsion system based on surfactants mixture, lubricating oil and co-surfactant (sec-butanol) was proposed by Burguera et al. [27] for aluminum determination in lubricating oils. A sequential injection (SIA) was applied for online sample preparation. The sonication and repetitive change of the flowing direction improve the stability of the system, allowing attaining the microemulsion.