Capillary electrophoresis neutral molecule separations

The type of electrophoresis we have been discussing so far is called capillary zone electrophoresis. Separation is based on differences in electrophoretic mobility. If the capillary wall is negative, electroosmotic flow is toward the cathode (Figure 26-20) and the order of elution is cations before neutrals before anions. If the capillary wall charge is reversed by coating it with a cationic surfactant (Figure 26-24) and the instrument polarity is reversed, then the order of elution is anions before neutrals before cations. Neither scheme separates neutral molecules from one another. 

The buffer system is a combination of buffer for electrophoresis and eluent for the particular chromatographic mode being employed. Figure 5.14 shows the separation of a group of neutral molecules using a capillary packed with a reversed-phase material.38 The buffer was a mixture of 4 mM sodium tetraborate (pH 9.1) and acetonitrile (20 80, v/v). The separation was compared with a micro-HPLC separation in which the same capillary was used but the eluent was pressure driven. As can be seen in Figure 5.14, sharper peaks were obtained with the EOF-driven system. 

CZE, also referred to as free solution capillary electrophoresis (FSCE), or open tubular capillary electrophoresis (OT CE), is the format originally described, in which the capillary is filled with an electrolyte buffer solution. In CZE, molecules are separated directly according to their charge, and inversely according to their solution drag force. Neutral molecules are moved through the capillary by the EOF. There are many additives which can be used to either dynamically deactivate the fused capillary waU, and prevent undesirable solute sticking, or to enhance solute selectivity, or both. 

Capillary electrochromatography (CEC) is a method of chromatographically separating neutral analytes in an electroosmotically-generated flow. As with CE, the liquid matrix is propelled by an imposed electric field. Unlike capillary electrophoresis, however, solute molecules are not separated by differences in charge/mass ratio because they are neutral - thus a stationary phase is needed... 

Capillary electrophoresis has a wide applicability. High molecular weight compounds such as proteins, nucleic acids and oligosaccharides can be separated as well as smaller biomolecules such as peptides and amino acids. CE is not restricted to charged analytes. Neutral molecules can be separated from each other by employing a variation of CE called micellar electrokinetic chromatography (MEKC). This is frequently used for the separation of chiral drugs in pharmaceutical research. 

Capillary electrophoresis is primarily limited to small molecules that are water solul le because of their compatibility with the run buffer. Other similar techniques such as isoelectric focusing and capillary zone elec-trophoresis ° have aided in the separation of proteins by allowing for the separation of larger proteins. In addition these techniques can separate isoforms of proteins and peptides by using an extraordinarily low pH range. However, capillary electrophoresis cannot separate neutral compounds, and... 

Capillary electrophoresis Recently, this technique has revealed its highest capability in separation of bulk sweeteners. Since bulk sweeteners lack both a charge and a strong UV chromophore, several derivatization reactions have been proposed (e.g., interaction with oxoacid or metal ions). An alternative for bulk sweeteners is their separation in a fused-silica capillary using indirect fluorescence detection or indirect UV detection. Electrokinetic micellar chromatography has extended the applicability of electrophoretic techniques even to neutral molecules such as carbohydrates. 

Owing to its outstanding high-performance separation potential also capillary electrophoresis has been employed in different modes and with different detection methods for separation and determination of MC. Both capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), which employs a surfactant above the critical micelle concentration to separate neutral and uncharged molecules based on electromigration principle, have been employed for determination of... 

This mouthful of words describes a form of capillary electrophoresis that separates neutral molecules as well as ions (Figure 23-21). The key modification in micellar electrokinetic capillary chromatography is the use of micelles in the capillary solution. Micelles are described in Box 23-2, which you should read now. 

Figure 23-21 Separation of neutral molecules by micellar electrokinetic capillary electrophoresis. The average plate count in this experiment is 250 000 in 50 cm of capillary length. [J. T. Smith. W. Nashabeh, and Z. E. Rassi, Anal. Chem. 1994,66,1119.]...

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