Open tubular CEC

The main bottleneck in the further development of CEC is related with the state of the art of the column manufacturing processes and the robustness of the columns/instrumentation. Moreover, evidence to demonstrate reproducibility of separations from column to column still has to be established. The formation of bubbles in the capillaries due to the Joule heating and variations in EOF velocity on passing from the stationary phase through the frit and into the open tube is still very challenging in packed column CEC. A way to overcome this problem is to use monolithic columns or apply open tubular CEC [108]. Currently, many efforts are placed in improving column technology and in the development of chip-CEC [115] as an attractive option for lab-on-a-chip separations. 

Another way to improve the performance of open-tubular columns was suggested by Sawada and Jinno [83]. They first vinylized the inner surface of a 25 pm i.d. capillary and then performed in situ copolymerization of f-butylacryl-amide and 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPS) to create a layer of polymeric stationary phase. This process does not currently allow good control over the homogeneity of the layer and the column efficiencies achieved in CEC separations of hydrocarbons were relatively low. These authors also recently thoroughly reviewed all the aspects of the open tubular CEC technologies [84]. 

An open-tubular column is a capillary bonded with a wall-supported stationary phase that can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The inner diameters of open-tubular CEC columns should be less than 25 pm that is less than the inner diameters of packed columns. The surface area of fused silica tubing is much less than that of porous packing materials. As a result, the phase ratio and, hence, the sample capacity for open-tubular columns are much less than those for packed columns. The small sample capacity makes it difficult to detect trace analytes. 

Open tubular CEC-MS (OT-CEC-MS) bearing positively charged alkylamino silyl monomers... 

Capillary Electrochromatography. Capillary electrochromatography (CEC) is a hybrid technique that works on the basic principles of capillary electrophoresis and chromatography [41], This mode of chromatography is used on either packed or tubular capillaries/columns. The packed column approach was introduced by Pretorius et al. [60] in 1974, while open tubular CEC was presented by Tsuda et al. [61] a decade later. In 1984 Terabe et al. [62] introduced another modification in liquid chromatography, micellar electrokinetic capillary... 

Cytochrome c Capillary etched with liquid crystals (cholesteryl or cyanopentoxy modified) 60 mM Citric acid and 50 mM P-alanine, pH 3.00 700 or 500 mm x 50 pm i.d 450 or 250 mm effective length, open tubular CEC... 

Open tubular CEC, 176 comparison with packed capillary CEC... 

Recently, Breadmore et al. developed a theoretical model that describes analyte behavior in IE CEC [77]. The model includes the contribution of the CE component through the electrophoretic mobility of the analyte and the ionic strength of the buffer, as well as the ion-exchange component which acts through changes in the composition of background electrolyte. Excellent agreement (r2 > 0.98) between experimental and theoretical data was reported for both packed and open-tubular CEC systems. 

In a study presented by Jinno et al. [124], packed column capillary electrochromatography, open-tubular CEC, and microcolumn liquid chromatography using a cholesteryl silica bonded phase have been studied to compare the retention behavior for benzodiazepines. The results indicated that CEC was a promising method, as it yielded better resolution and faster analysis than microcolumn LC for benzodiazepines. Similar selectivity to HPLC was noted, except for a few solutes that were charged under the separation conditions. Columns packed with the ODS and cholesteryl phases were compared and showed totally different migration orders of the analytes. The retention on the cholesteryl silica sta-... 

The stationary phase can he filled into the capillary, bonded as a film to the capillary walls (open tubular CEC) or introduced into the capillary as a suspension or solution of the mobile phase (pseudo-stationary phase). The separation is, in the case of neutrally charged analytes, based on partitioning between the stationary phase and the mobile phase or, in the case of charged analytes, based on both partitioning and electrophoretic mobility (charge to friction coefficient ratio). 

Mayer et al. reported open-tubular CEC-ESI-MS analysis of enantiomers of the chiral sedative-hypnotic drug hexobarbital by coupling a short Chirasil-Dex-coated capillary (15 and 25 cm X 50 pm i.d.) via a sheath liquid interface to a quadrupole mass spectrometer [11]. The mobile phase was 10 itiM ammonium acetate buffer at pH 7, and the composition of the sheath... 

We recently reported an open-tubular CEC coupling with a sheathless ESI-MS for analysis of some amino acids and peptides [10]. We employed Lupamin, a high molecular weight linear polyvinylamine polymer, as stationary phase coated on the inner wall of an open-tubular capillary column. This column has several merits including easy preparation, low cost, and good reliability. Positively charged amino groups in polyvinylamine in acidic conditions play three important roles ... 

The preparation of open tubular CEC sol-gel columns first involves the pretreatment of the capillary and the preparation of the sol. The gel step for open tubular columns is allowed to proceed for only a short period of time (i.e., 20 min). This short time allows the gel to begin to form typically silanol-containing groups will begin to react with other silanols in the immediate vicinity. This includes the sol reacting with the sUanol groups at the surface of the silica capillary. When the non-reacted sol is rinsed out of the capillary, all of the sol-gel that is not covalently attached to the wall of the capillary will be rinsed out. This will leave a layer of the sol-gel on the interior of the capillary. 

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