Analytical Chemistry

Table 4.20 Diffusion coefficients of species of different sizes

Species RMM (MW) D (cm^2 s–^1 ) 105

HCl 36 3.05

NaCl 58 1.48

β-alanine 89 0.93

• 1,2-aminobenzoic acid 137 0.84
  glucose 180 0.67

citrate 192 0.66

cytochrome C 13 370 0.11

β-lactoglobulin 37 100 0.075

catalase 247 500 0.041

myocin 480 000 0.011

tobacco mosaic virus 40 590 000 0.0046

Electro-osmosis (p. 172) plays a very significant role in HPCE because the interior surface of a quartz
capillary develops a negative charge when in contact with aqueous solutions due to the ionization of
surface silanol groups (Si—OH) above pH 4 and the adsorption of anions. As a result, a layer of cations
from the bulk solution builds up close to the wall to maintain a charge balance by forming an electrical
'double-layer'. The high fields employed cause a pronounced electro-osmotic flow (EOF) as the highly
solvated cations are drawn towards the cathode (Figure 4.53). All analyte species, whether cationic,
anionic or neutral, are carried towards the cathode by the EOF and hence through a detector cell
positioned close to the cathodic end of the tube. As the EOF originates at the wall of the capillary, an
essentially flat flow-profile is produced across the tube thereby minimizing band-spreading and
resulting in very high efficiencies.

A schematic diagram of an HPCE system is shown in Figure 4.54. The fused-quartz capillary is
generally 50–75 cm long with an i.d. of 25– 100 μm

Figure 4.53

Differential solute migration superimposed on electro-osmotic flow in capillary

zone electrophoresis.