10.2 Support media
The pioneering work on protein electrophoresis by Arne Tiselius (for which he
received the Nobel Prize in Chemistry in 1948) was performed in free solution.
However, it was soon realised that many of the problems associated with this
approach, particularly the adverse effects of diffusion and convection currents, could
be minimised by stabilising the medium. This was achieved by carrying out electro-
phoresis on a porous mechanical support, which was wetted in electrophoresis buffer
and in which electrophoresis of buffer ions and samples could occur. The support
medium cuts down convection currents and diffusion so that the separated compon-
ents remain as sharp zones. The earliest supports used were filter paper or cellulose
acetate strips, wetted in electrophoresis buffer. Nowadays these media are infrequently
used, although cellulose acetate still has its uses (see Section 10.3.6). In particular, for
many years small molecules such as amino acids, peptides and carbohydrates were
routinely separated and analysed by electrophoresis on supports such as paper or thin-
layer plates of cellulose, silica or alumina. Although occasionally still used nowadays,
such molecules are now more likely to be analysed by more modern and sensitive
techniques such as high-performance liquid chromatography (Section 11.3). While
paper or thin-layer supports are fine for resolving small molecules, the separation of
macromolecules such as proteins and nucleic acids on such supports is poor.
However, the introduction of the use of gels as a support medium led to a rapid
improvement in methods for analysing macromolecules. The earliest gel system to
be used was the starch gel and, although this still has some uses, the vast majority
Capillary wall
–––––– –––– –
––
++++++
+ +
+
+ +
+ + +
+
+ +
+
+
++
+ + + + + +
+ +
+ +
+
+
+ +
+ +
Acidic silanol groups impart negative charge on wall
Counter ions migrate toward cathode, dragging solvent along
Fig. 10.3Electroosmotic flow through a glass capillary. Electrolyte cations are attracted to the capillary
walls, forming an electrical double layer. When a voltage is applied, the net movement of electrolyte
solution towards the cathode is known as electroendosmotic flow.
403 10.2 Support media
