chirality of the compounds, hydrophobicity, and size. The theory of operation is
simple. Because the capillary is composed of silica, silanol groups are exposed in
the internal surface, which can become ionized as the pH of the eluting buffer is
increased. The ionization attracts cations to the silica surface, and when current
is applied, these cations migrate toward the cathode, which causes a fl uid migration
through the capillary. This fl ow can be adjusted by changing the dielectric strength
of the buffer, altering the pH, adjusting the voltage, or changing the viscosity.
Under these conditions, both anions and cations are separated in a single separation,
with cations eluting fi rst. Neutral molecules (e.g., pesticides) can be separated
by adding a detergent (e.g., sodium dodecyl sulfate) to the buffer, forming micelles
into which neutral molecules will partition based on their hydrophobicity. Because
the micelles are attracted to the anode, they move toward the cathode at a slower
rate than does the remainder of fl uid in the capillary, thus allowing separation. This
process is called micellar electrokinetic capillary chromatography (MEKC) (Figure
24.5 b). Many of these analyses can be carried out in 5 – 10 min with sensitivities in
the low parts - per - billion range. A UV detector is usually used, but greatly sensitivities
can be obtained using laser - induced fl uorescence detectors.