4.2—
Solid Phase Extraction
Summary
Principles
Selective transfer of material in sub-microgram to milligram quantities between a solid sorbent and a
liquid phase; separations depend on different relative affinities for the two phases based on adsorption,
size or charge; selectivity achieved by pH control, solvent composition and surface chemistry of the
sorbent.
Apparatus and Instrumentation
Syringe-barrel cartridges, disk-holders, plastic pipette-tips, well plates; vacuum manifolds for semi-
automatic batch processing; fully automated autosamplers, xyz liquid handlers and robot-controlled
work stations.
Applications
Increasingly used for sample clean-up prior to chromatographic analysis and pre-concentration of trace
and ultra-trace levels of analytes. Largely replacing solvent extraction.
Disadvantages
Packed sorbent beds may suffer from poor efficiency due to channelling; solid phase microextraction
difficult to calibrate and replicate, especially with gaseous samples.
Solid phase extraction (SPE) involves the separation of components of samples in solution through their
selective interaction with and retention by a solid, particulate sorbent. SPE depends on differences in
the affinities of the various components of the sample for the sorbent. The mechanisms of the
interactions are virtually identical to the sorption processes that form the basis of liquid
chromatographic separations (p. 80). The choice of solvent, the pH and ionic strength of aqueous
solutions, and the chemical nature of the sorbent surface, especially its polarity, are all of importance in
controlling the selectivity and efficiency of an extraction.
Solid Phase Sorbents
These are generally either silica or chemically modified silica similar to the bonded phases used in
high-performance liquid chromatography (p. 124 et seq) but of larger particle size, typically 40– 60 μm
diameter. Silica with chemically bonded alkyl chains such as C 18 (ODS) or C 8 , phenyl, amino, ion-
exchange groups or unmodified silica are the most widely-used, although polymeric resins
(polystyrene/divinyl benzene copolymers), Florisil (activated magnesium silicate), alumina and
charcoal also find applications. Some SPE sorbents are listed in Table 4.7 along with the predominant
interaction mechanism for each one. All four chromato-