Reverse EOF

ILs were tested as additives to the phosphate-acetate buffer for the separation of chlorophenoxy and benzoic herbicide acids [47]. Again, it was found that in the 40 mM phosphate-acetate BGE containing 10% ACN and having pH 4.5, the addition of 10 mM l-butyI-3-methyIimidazoium could reverse EOF. The shoulder-merged peaks of two herbicide acids, 2,4-dichIorobenzoic... 

The detection limits and the analyte-to-matrix ratios for inorganic and organic anion impurities in boric acid, obtained using hydrostatic or electro-kinetic injection and with sample stacking (the capillary is filled with the sample up to the detector, a voltage is applied to preconcentrate the sample anions at the sample-buffer interface, a reversed EOF is used to remove the matrix components and... 

A new type of mixed-mode stationary phase which incorporates both strong anion-exchange (triehtylammonium methylstyrene) moieties and C18 (octadecylacrylate) ligands was used by Scherer et al. to separate acetylsalicylic acid and three of its metabolites in less than 2 min [42], A rapid reversed EOF (1.8 mm/s) was generated over a wide range of pH, from 3 to 9. It was found that EOF velocity shows variation with pH, the magnitude of which is a function of the content of SAX versus C18 moieties. 

The ultimate challenge for any chromatographic technique is to separate neutral, ionic, acidic, and basic compounds found in real samples in the same run. Toward this goal, a series of papers published simultaneous separations of acidic, neutral, and basic compounds. Klamfl et al. used a mixed-mode (C6/ SAX) silica-based packing to combine reverse EOF, hydrophobic interactions, as well as electrophoretic migration for the separation of a lab-made sample... 

Figure 13.34 Use of cationic surfactant to reverse electroosmotic flow (a) normal EOF toward cathode (no surfactant) and (b) reversed EOF toward anode (cationic surfactant bilayer). (Katz et al., used with permission.)...

Cationic surfactants strongly bind to the soil particles. As the surfactant concentration increases, hemimicelle formation on the sorbed surface cause charge reversal, which may eventually result in a reverse EOF (Li and Gale, 1996 Kaneta, Tanaka, and Taga, 1993). The problem of using cationic surfactants in electrokinetic... 

Performing injections proved to be problematic for this system. While the positively charged monolith and reversed EOF improved separation performance, they also significantly enhanced electrophoretic discrimination during injection. In addition, despite the tuned chemistry, the analytes interacted with the amine functionalities and neutralized them, completely suppressing the EOF in the injection arms after multiple injections. This prevented long-term use of the device. Use of disposable, perhaps plastic, chips appears to be a simple solution to this problem. 

Provide a strong reversed EOF for driving mobile phase and analytes from the inlet end of the separation capillary to the electrospray tip... 

Negatively charged species EM of the anionic species is close to or greater than EOF mobility Reversal EOF mode by acidic pH or by addition of cationic surfactants at concentrations below the CMC that form a positively charged layer on the inner wall of the capillary... 

Cationic sm-factants such as tetradecyltrimethyl-ammonium bromide (TTAB), cetyltrimethylammoni-um bromide (CTAB), and cetyltrimethylammonium chloride (CTAC) have also been useful for MEKC analysis. Most cationic sm-factants have an alkyltri-methylammonium group, and their counterions are halides. The addition of cationic surfactants to the backgroimd electrolytes (BGE) caused the reversal of electroosmotic flow (EOF) owing to a positively charged capillary wall on account of the adsorption of cationic sm-factants. As a result of the reversed EOF, the polarity of the electrodes has to be reversed in order to detect the analytes. 

The magnitude and direction of electroosmotic flow can be changed by various treatments of the silica inner wall. For example, addition of a cationic surfactant to the background electrolyte results in a positive surface that gives a reversed EOF, i.e., toward the anode. 

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