3.2.4. Effect of applied voltage
The effect of the applied voltage on concentration efficiency was investigated in the range from 12 to 24 kV. As shown in Fig. 3D, peak widths progressively became narrower as the applied voltage decreased until 15 kV. For sweeping, analytes with a high affinity for the micellar carrier (i.e., very hydrophobic) spend a greater amount of time complexed with the micelles and hence have a velocity lower than EOF, while those with less affinity (i.e., less hydrophobic) have velocity more dependent on EOF. Most analytes fall between these extremes, with their velocity a function of affin-ity for the micelles, micelle velocity, and EOF velocity (Palmer, Munro, & Landers, 1999). Sweeping was also found to work better under suppressed or low EOF compared to high EOF conditions (Quirino & Terabe, 1999). Decreasing voltage can slow down the EOF. As a result, high sweeping efficiency was obtained. At lower applied voltage, the peaks broaden gradually and the resolution decreased. Thus, 15 kV was selected as the optimal applied voltage.