In a 2-phase extraction, the membrane separates the donor phase
(aqueous sample) from the acceptor phase which is a water-immiscible
organic solvent. The pores of the hydrophobic membrane are filled with the
acceptor phase. This 2-phase system resembles a ‘traditional’ liquid/liquid
extraction because the acceptor phase is in direct contact (as immobilized in
the membrane pores) with the aqueous sample: analyte molecules can
diffuse directly into the acceptor phase as shown in Figure 1. The pores of
the membrane can be cleaned (if necessary) by flushing with donor and
acceptor phases while maintaining slightly higher pressure on the acceptor
side. This type of 2-phase system may be used to extract neutral organic
analytes from aqueous samples.