A

tm to Inj.

Figure 3.8 Schematic representation of MEKC (Veo, velocity of EOF; Vmic, velocity of micelles).

surfactants, added to the buffer, decrease the hydrophobic interactions of the analytes with the micelles and reduce the retention times.

When ion-pairing agents (surfactants, tetraalkylammonium salts, organic amines) are introduced into the system, their effects resemble those observed with ion-pairing chromatography. Urea (2-6 M) is known to increase the solubility in water of hydrophobic molecules; in MEKC separation by very lipophilic compounds was reported improved by highly concentrated urea.

Also used in MEKC are cyclodextrins (CDs), oligosaccharides with a hydrophilic external surface and a hydrophobic cavity, which attract other com-

TABLE 3.1 Micelle-Forming Additives in MEKC

Critical Micelle Additive Concentration (mM)

anionic

Sodium decyl sulfate 33.0

Sodium dodecyl sulfate (SDS) 8.2

Sodium cholate 13.0

Sodium dehydrocholate 10.0

Sodium deoxycholate 5.0

Sodium taurocholate 15.0

cationic

CTAB 14.0

DTAB 1.3

zwitterionic

CHAPS 8.0 Nonionic

Triton X-100 0.3

pounds by hydrophobic interaction. CDs are neutral, highly hydrophilic particles that migrate at the same velocity as the EOF. When added to micellar solutions, CDs decrease the interactions of hydrophobic compounds with the micelles, reducing their capacity factors. Because of their chiral structure and the stereospecific nature of their interactions, CDs are effective in enanti-omer separation.

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