Stationary phases used in CEC

Table 5. Properties of commercial stationary phases used in CEC [53]... 

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

Table 3 Some Examples of Chiral Stationary Phases Used in CEC Applications, Resulting Resolution, and Efficiency... 

The particle size of typical HPLC. stationary phases used in CEC is not thought to affect the EOF. Knox and Grant [4] predicted from theory and later proved experi-... 

The nature of the stationary phase used in CEC influences the magnitude of the EOF and also the type of separation that can be performed. The degree of ionisation of the surface silanol groups on the phase particles plays an important role in the amount of EOF generated. The majority of early work published on CEC was performed using standard LC stationary phases. Most of these phases are not very suitable to use for CEC. especially when the pH of the mobile phase is less than 6. The silanol groups on the surface of a typical phase are not ionised at low pHs and so EOF is reduced in the packed capillary. 

CEC is a miniaturized separation technique that combines capabilities of both interactive chromatography and CE. In Chapter 17, the theory of CEC and the factors affecting separation, such as the stationary phase and mobile phase, are discussed. The chapter focuses on the preparation of various types of columns used in CEC and describes the progress made in the development of open-tubular, particle-packed, and monolithic columns. The detection techniques in CEC, such as traditional UV detection and improvements made by coupling with more sensitive detectors like mass spectrometry (MS), are also described. Furthermore, some of the applications of CEC in the analysis of pharmaceuticals and biotechnology products are provided. 

Nowadays, porous monoliths have found an extensive use in CEC of organic compounds, which represents a powerful separation tool, complementary to HPLC. CEC is a hybrid method in which the separation is performed through the phase distribution mechanisms of traditional HPLC (reversed-phase, ion exchange, etc.), while the flow of the mobile phase through column packing is affected by electroosmotic forces, as in electrophoresis. The coexistence of a stationary phase and an electric field permits separation not only of ions but also of neutral compounds, due to their different electrophoretic mobhity and different distribution between the mobile and the stationary phases. 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

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