CEC has a number of advantages over both CE and HPLC. The flat flow profile arising from the EOF
gives plate heights of about half the particle diameter, and this is several times the efficiency attainable
with microbore HPLC columns. In addition, compared to HPLC, solvent consumption is greatly
reduced which facilitates coupling CEC to mass spectrometry. The stationary phase provides an
additional source of selectivity over and above differences in electrophoretic mobility. Furthermore,
there is no need to use a micelle-forming surfactant additive as in the MEKC mode of CE when
separating neutral solutes. This is also an advantage for CEC-MS because solvents containing high
concentrations of surfactants such as sodium dodecyl sulphate (SDS) can cause difficulties with some
ionization sources.
CEC Columns
CEC capillaries are packed by first creating a porous frit at one end of the tube by sintering a thin plug
of powdered silica, then pumping in a slurry of the stationary phase using an HPLC pump, and finally
creating a second silica frit at the other end of the tube to retain the packing. The columns are then filled
with a buffer solution in which they should always be stored to avoid damaging the packed bed by
drying out. Their operational stability is often not particularly good because of a tendency for gas
bubbles to form in the buffer due to the heating effect of the current flow (Joule heating). This is turn
leads to the development of dry zones that destroy the EOF and ultimately to complete failure of the
separation. Possible remedies to these problems include pressurizing one or both ends of the capillary to
inhibit bubble formation, thorough de-gassing and filtering of the buffer solution and providing a very
efficient cooling system to counteract Joule heating. Unfortunately, the limited availability of
commercially packed capillaries, their cost and fragile nature, especially of the silica frits, poor
reproducibility and high failure rates have all hindered the development of CEC.
Mobile Phase and Sample Injection
Mixtures of methanol or acetonitrile with either phosphate, TRIS or MES (Table 4.23) buffer solutions
are the most frequently used, but many others are being investigated. The higher above 4 the buffer pH
is, the greater the EOF generated and the faster the separation. The composition of the mobile phase can
have dramatic effects on both the EOF and the selectivity of the separation via the sorption processes
with the stationary phase.
Samples of 1–20 nl are usually introduced by electrokinetic rather than hydrodynamic injection (p.
176), but precision, as for CE, is only about 2%.