Micellar electrokinetic capillary separation process

The incorporation of micelles in the mobile phase in capillary zone electroporesis permits the efficient separation of a variety of neutral compounds. Efficiencies in excess of 100,000 plates/m are routinely attained. The mass transport processes which are important in micellar electrokinetic capillary chromatography are described, along with the technique. The technique is particularly useful for biological separations. Preliminary data and discussion related to column selectivity and efficiency are presented. 

Micellar Electrokinetic Capillary Chromatography. Surfactants that form micelles in solution are added to the buffer in the capillary. When the solute is injected, it partitions itself between the buffer and the micelle. Migration of the solute depends on the amount of time it spends in the micelle versus the time it spends in the buffer. Therefore, the separation of analytes occurs due to differences in the partition coefficient between the two phases, much like in a chromatographic process. 

The phenomena just described are quite similar to what occurs in a liquid partition chromatographic column except that the stationary phase is moving along the length of the column at a much slower rate than the mobile phase. The mechanism of separations is identical in the two cases and depends on differences in distribution constants for analytes between the mobile aqueous phase the hydrocarbon pseudostationary phase. The process is thus true chromatography hence, the name micellar electrokinetic capillary chromatography. Figure 33-15 illustrates two typical separations by MECC. 

Desiderio et al. (1998) reported a quantitative method of analyzing dyes in lipstick using micellar electrokinetic capillary chromatography (MEKC) with diode array UV detection. This electrophoretic method was optimized for the separation of seven cosmetic dyes Eosin Y (Cl 45380), certifiable as D C Red No. 22 erythrosine (Cl 45430) cyanosine (Cl 45410), certifiable as D C Red No. 28 Rhodamine B (Cl 45170) Orange II (Cl 15510), certifiable as D C Orange No. 4 Chromotrope FB (Cl 14720) and tartrazine (Cl 19140), certifiable as FD C Yellow No. 5. The process was fast (3 min per separation). 

Capillary electrochromatography (CEC) is a rapidly emerging technique that adds a new dimension to current separation science. The major "news" in this method is that the hydrodynamic flow of the eluting liquid, which is typical of HPLC, is replaced by a flow driven by electro-endoosmosis. This increases considerably the selection of available separation mechanisms. For example, combinations of traditional processes such as reversed-phase- or ion-exchange- separations with electromigration techniques are now possible. Also, CEC is opening new horizons in the separation of non-polar compounds, and thus represents an alternative to the widely used micellar electrokinetic chromatography. 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

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