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High performance capillary electrophoresis - T.E.A.M.

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Modes<br />

the stationary phase in LC. Surfactants can be anionic,<br />

cationic, non-ionic, zwitterionic, or mixtures of each (table<br />

14). In each category, alkyl chain length or structure can<br />

also be varied. In addition, MEKC can be performed using<br />

bile salts or microemulsions. In all cases, variations in<br />

buffer concentration, pH, temperature, or use of additives<br />

such as urea, metal ions, or chiral selectors can also be used<br />

to affect selectivity.<br />

Biological detergents CMC Aggregation<br />

(mM) number<br />

Anionic SDS 8.2 62<br />

Cationic DTAB 14 50<br />

CTAB 1.3 78<br />

Non Ionic Octylglucoside — —<br />

n-Dodecyl-b-D-maltoside 0.16 —<br />

Triton X-100 0.24 140<br />

Zwitterionic CHAPS 8 10<br />

CHAPSO 8 11<br />

Table 14<br />

Surfactants<br />

Bile Salt Cholic acid 14 2– 4<br />

Deoxycholic acid 5 4 –10<br />

Taurocholic acid 10 –15 4<br />

As in chromatography, organic modifiers can be added to<br />

manipulate solute-micelle interaction. Modifiers such as<br />

methanol, acetonitrile, and 2-propanol have all been used<br />

successfully. Added to the running buffer in concentrations<br />

from a few percent up to 50 % (v/v) can lessen hydrophobic<br />

interactions between the solute and micelle. They can also<br />

decrease the hydrophobic interactions which maintain<br />

micellar structure, allowing more rapid chromatographic<br />

kinetics.<br />

Surfactants used for MEKC can also interact with the<br />

<strong>capillary</strong> wall and have dramatic effects on the EOF as well<br />

as solute-wall interactions. The direction of solute and<br />

66

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