Finally, another type of separation is using semipermeable membranes (SPMs).
This application does not need a phase separation of sample material and extractant,
because the membrane itself acts as separator. The enrichment of analytes is
typically performed with a liquid sample or a slightly dissolved or moistened solid
sample. For executing the extraction process, the material is enclosed in a bag
consisting of the semipermeable membrane and this bag is placed into the extractant,
where the analytes start to move through the membrane. A sufficient enrichment of
analytes in the solvent needs some time, typically minutes to a few hours. SPMs are
used for extraction of organic substances from food, biota, sediments and soils but
also to clean-up raw extracts derived from other extraction techniques (Fig. 3.12).
General Note
Extraction method are optimized either by an enlarged extraction time that
allows to reach the partition equilibrium or by fastening the dynamic partition
process via external energy supply. Further on, the extraction process can be
accelerated by a high contact surface e.g. by dispersing extractants forming
very small droplets with overall high surfaces.
Semipermeable membrane SPM
Transmit ted components
Retained components
SPM extract (left) and
raw extract (right) for
comparison
Clean-up of a raw extract
by SPM
Fig. 3.12 Scheme of applying semipermeable membranes for the enrichment of analytes from
sample material or for clean-up of raw extracts
28
3 Sample Treatment
This application does not need a phase separation of sample material and extractant,
because the membrane itself acts as separator. The enrichment of analytes is
typically performed with a liquid sample or a slightly dissolved or moistened solid
sample. For executing the extraction process, the material is enclosed in a bag
consisting of the semipermeable membrane and this bag is placed into the extractant,
where the analytes start to move through the membrane. A sufficient enrichment of
analytes in the solvent needs some time, typically minutes to a few hours. SPMs are
used for extraction of organic substances from food, biota, sediments and soils but
also to clean-up raw extracts derived from other extraction techniques (Fig. 3.12).
General Note
Extraction method are optimized either by an enlarged extraction time that
allows to reach the partition equilibrium or by fastening the dynamic partition
process via external energy supply. Further on, the extraction process can be
accelerated by a high contact surface e.g. by dispersing extractants forming
very small droplets with overall high surfaces.
Semipermeable membrane SPM
Transmit ted components
Retained components
SPM extract (left) and
raw extract (right) for
comparison
Clean-up of a raw extract
by SPM
Fig. 3.12 Scheme of applying semipermeable membranes for the enrichment of analytes from
sample material or for clean-up of raw extracts
28
3 Sample Treatment