Biomedical Engineering – From Theory to Applications

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Biomedical Engineering – From Theory to Applications
Fig. 9. Schematic diagram of the on-line SEC–SPE–CE system with the Tee-split interface.
The on-line SEC–SPE–CE system was built in three distinct parts: a SEC, a SPE and a CE
part. The SEC part consisted of a pump (pump 1), a valve (valve 1) for introduction of
sample, a SEC column, and a UV detector (detector 1). The SPE part comprised a pump
(pump 2), a micro valve (valve 3) for introduction of acetonitrile, and a SPE column. Valve 2
functioned as a selection valve to direct a fraction of solvent A towards the SPE column or to
detector 1. The CE part of the complete system is framed. Lengths of capillaries are shown in
italics (cm). The CE part consisted of a CE system with a build-in photodiode array detector.
The CE and SPE parts were connected by a micro Tee with a void volume of 29 nL. The SEC
part was filled with solvent A, whereas in the SPE and CE parts BGE was used. Reprinted
from ref. (Tempels et al., 2006), with permission.
Although LC-CE coupling is technically much more difficult than CE-CE, because it has to
be accompanied by collection, evaporation and reconstitution of fraction isolated by LC,
some of these actions can be eliminated implementing an advanced CE stage (with a
concentration capability) into LC-CE. Micro-column liquid chromatography (MLC) can be
used on-line with an advanced (stacking) CE for sample purification and concentration
allowing injection of microliter volumes into the electrophoresis capillary (Bushey &
Jorgenson, 1990; Pálmarsdóttir & Edholm, 1995). By using the double stacking procedure
with assistance of the backpressure almost complete filling of the electrophoresis capillary is
possible without significant loss of CZE separation performance. The combined system has
a much greater resolving power and peak capacity than either of the two systems used
independently of each other.
3.1.2.4 Dialysis-CE
Microdialysis is a widely accepted sampling and infusion technique frequently used to
sample small molecules from complex, often biological, matrices (Adell & Artigas, 1998;
Chaurasia, 1999). In the microdialysis, small molecules are able to diffuse across the dialysis
membrane into the probe, while large molecules, such as proteins and cell fragments, are
excluded. This is the sample cleanup provided by the microdialysis.
On-line microdialysis-CE assays for neurotransmitters to date have been most successful for
easily resolved analytes such as glutamate and aspartate (Thompson et al., 1999; Lada et al.,