Liquid-liquid extraction is one of the standard methods for removing hydrophilic byproducts, reactants, or reagents. Several examples of the parallel usage of this technique for purification of combinatorial libraries have been described . This method is only appropriate for very simple separation problems and can be performed in an automated fashion by using a liquid handler. The method totally fails when polar compounds have to be purified because emulsions can appear or products remain in the aqueous phase. Coupled to a solid-phase extraction system, automated liquid-liquid extraction can be performed together with a 96-needle pipetting system such as Quadra 96 . Separation problems, for example removal of amines from a combinatorial library, were solved by applying several extraction cycles. Dichloromethane and dilute HCl were mixed and separated through filtration using solid-phase diatomaceous earth. The phase separation was achieved via vacuum filtration through a 96-well filterplate carrying a hydrophobic membrane which held the aqueous phase back. Liquid-liquid extraction can be easily automated by using pipetting workstations. Reaction mixtures are mixed with buffers and the upper or lower phase, depending on the density, is removed. Efficient mixing can be achieved through redispensing, which can be repeated several times. The use of a capacity sensor represents a more sophisticated approach. This sensor type is available for several liquid handlers (Zinsser, Tecan). A drawback of this method is when phase separation is not complete or undetectable, erratic results are obtained.
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