15 μl) thus minimizing solvent consumption in the rinsing and elution steps, improving selectivity and
facilitating high solvent flow rates when large volumes of sample are to be processed. In addition,
channelling, which can limit extraction efficiency in packed beds, is eliminated. Discs can be used
singly or stacked in syringe barrels to increase sample capacity or to allow 'mixed-mode' extractions,
i.e. combinations of discs with different sorbents selected to retain a wider range of compounds for
specific types of sample.
Plastic pipette-tips incorporating small sorbent beds are designed for processing very small volumes of
sample and solvents rapidly, and have the advantage of allowing flow in both directions if required.
Automated Solid Phase Extraction
Solid phase extraction can be semi- or fully automated to increase sample throughput and to improve
both precision and accuracy. The degree of automation ranges from the parallel off-line processing of
batches of up to about ten samples using a vacuum manifold to provide suction, to on-line
autosamplers, xyz liquid handlers and robotic workstations. One of the most recent developments is the
utilisation of well plates containing upwards of 96 individual miniature sample-containers in a
rectangular array. Well plates are used in xyz liquid handlers for processing large numbers of samples
prior to the transfer of aliquots to analytical instruments, particularly gas and liquid chromatographs and
mass spectrometers. A set of miniature SPE cartridges, fitted with discs or short packed-beds, can be
placed on top of a well plate, and some commercial systems offer the alternative of using disposable
pipette-tips with a liquid handler.
Solid Phase Microextraction
An important variation of SPE, known as SPME, allows trace and ultratrace levels of analytes in liquid
or gaseous samples to be determined by concentrating them onto a fused-silica optical fibre coated with
a layer of a polymeric substance such as polydimethylsiloxane (PDMS) or Carbowax, both of which are
commonly used as gas chromatographic stationary phases. The 1 cm long fibre is normally attached to a
modified microsyringe in such a way that after concentration of the required sample components into
the coating by exposing it to the gas or liquid to be analysed, it can be inserted directly into the injection
port of a gas or liquid chromatograph (Figure 4.10). To protect the fibre after sampling and to enable it
to be introduced into a gas chromatographic injection port, it is attached to a hollow stainless steel
needle into which it can be retracted. Sorbed analytes are thermally desorbed into the carrier gas stream
after piercing the septum of the injection port and exposing the fibre. Sample introduction into a liquid
chromatograph is via a modified multiport valve and desorption into the flowing mobile phase. The
polymeric coatings used on SPME fibres range in thickness between 5 and 100 μm. They require
conditioning before use but can be used repeatedly.