Solvent extraction of solid samples, which is commonly known as solid–liquid extraction, but which should be referred to, in a more correct use of the physicochemical terminology, as leaching or lixiviation, is one of the oldest ways of solid sample pretreatment. Among the techniques used for implementation of this step, Soxhlet has been the leaching technique mostly used for a long time. This assertion is supported by the fact that Soxhlet has been a standard technique during more than one century and, at present, it is the main reference to which the performance of other leaching methods is compared.
Counter-current extraction