Surfactants and Cyclodextrins

Cyclodextrins (CD) are the most popular chiral selectors for chiral separations by MEKC. The separation mechanism is based on differential partitioning of solutes between the micellar and CD aqueous phase.

Table 3 Selected mixed micellar systems used in MEKC

Mixed micellar system

Surfactants in the mixture11

Anionic-nonionic surfactants

Anionic-anionic surfactants

Anionic-cationic surfactants

Anionic-zwitterionic surfactants Nonionic-nonionic surfactants

Cationic-cationic surfactants

SDS and Brij-35

SDS and Tween 60

SDBS abd Brij-35

SDS and Tween 20

Bile salts and polyoxyethylene-

4-dodecyl ether SDS and sodium cholate SDS and sodium octyl sulfate SDS and bile salts Two different bile salts LiPFOS (fluorocarbon) and

LiDS (hydrocarbon) Fluorosurfactants FC 128 and

FC 134 SDS and SB-12 Tween 20 and Tween 80 Triton X-100 and Brij-35 TTAC and OTAC TTAB and DTAB

aSDS = sodium dodecylsulfate; Brij-35 = polyoxyethylene (23) dodecanol; Tween 20 = polyoxyethylene (20) sorbitane monolau-rate; Tween 60 = polyoxyethylene (20) sorbitane monostearate; SDBS =sodium dodecyl benzenesulfonate; LiPFOS : lithium perfluorooctane sulfonate; LiDS = lithium dodecyl sulfate; SB-12 = N-dodecyl-N,N'-dimethyl-3-ammonio-1-propanesulfonate; TTAC = tetradecyltrimethylammonium chloride; OTAC = octyl-trimethylammonium chloride; DTAB = dodecyltrimethylam-monium bromide.

The sensitivity of mass spectrometry (MS), and the possibility of obtaining molecular information on compounds, make the on-line coupling of MEKC with MS highly attractive. Electrospray ionization (ESI) has been one of the most popular ionization techniques in coupled CE-MS. Although MEKC is a convenient separation technique for neutral analytes, problems are encountered in the on-line MEKC-ESI-MS interface connection because the micelles in the electrolyte solution are nonvolatile and tend to contaminate the MS. A number of approaches have been developed to overcome the problems of separating neutral compounds, while at the same time preventing micelles from entering the mass spectrometer. These include use of the heart-cut technique, high molecular mass surfactants, a semipermeable membrane interface, anodically migrating micelles, and the partial filling technique. An electro-spray-chemical ionization interface is a possibility for certain types of online MEKC-MS applications.

Most of the surfactants used in separations have been anionic.

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