Open tubular columns immobilization

Virtually all current research in SFC utilizes either small bore packed columns with particles of 5-10 micrometers in diameter optimized for use in liquid chromatography or narrow bore, fused silica open tubular columns with Immobilized phases similar to those used in gas chromatography. In the latter case columns of saaller internal diameter, 10-100 micrometers, shorter lengths (generally less than 20 m with 1-10 m being the most common length), and more firmly crosslinked stationary phases are used by coaparison with standard columns for gas chromatography. In all... 

Tlie first chiral separation with open-tubular columns in SFC was published by Roder et al. in 1987 [39]. Schurig and co-workers [40] linked permethylated fi-CyD via an octamethylene spacer to polydimethylsiloxane forming a chiral polymer Chirasil-Dex. The polymer was immobilized on the inner surface of fused-silica capillaries and the capillaries were used for so-called unified chromatography including GC, LC, SFC, and capillary electrochromatography (CEC). 

Figure 7.10 Separations of enantiomers by open-tubular SFC on a 5-m x 50-/xm ID fused silica column coated with a 0.25- m film of immobilized Chirasil-Dex. (Reprinted from Ref. 7 with kind permission of Dr. Alfred Huthig Verlag GmbH, Heidelberg.)...

There are two basic disadvantages to the coated capillary column. First, the limited solute retention that results from the small quantity of stationary phase in the column. Second, if a thick film is coated on the column to compensate for this low retention, the film becomes unstable resulting in rapid column deterioration. Initially, attempts were made to increase the stationary-phase loading by increasing the internal surface area of the column. Attempts were first made to etch the internal column surface, which produced very little increase in surface area and very scant improvement. Attempts were then made to coat the internal surface with di-atomaceous earth, to form a hybrid between a packed column and coated capillary. None of the techniques were particularly successful and the work was suddenly eclipsed by the production of immobilize films firmly attached to the tube walls. This solved both the problem of loading, because thick films could be immobilized on the tube surface, and that of phase stability. As a consequence, porous-layer open-tubular (PLOT) columns are not extensively used. The PLOT column, however, has been found to be an attractive alternative to the packed column for gas-solid chromatography (GSC) and effective methods for depositing adsorbents on the tube surface have been developed. 

The increased popularity of CEC is the result of its advantages that include the vastly increased column efficiency compared to the conventional pressure-driven chromatography. CEC was also adopted for the separations of enantiomers, employing two major modes similar to those used in CE. In the simplest mode, a chiral selector is added to the mobile phase. At present, a less erroneous and more elegant technique uses an open tubular conventional packing or preferably a monolithic column containing an immobilized chiral selector that discriminates the enantiomers. 

In gais chromatography (GC), open-tubular caplllaury columns cam produce much more theoretical plate numbers tham conventional packed columns. Open-tubular capillary columns have been examined am separation tools also in liquid chromatography(LC)(8-12). Coated columns, ca. 20 m X 10-30 vim l.d., produced theoretical plate numbers in excess of 100,000 but long-term stability of these columns was scxnewhat poor even if the mobile phame is saturated with the mobile phase. Accordingly chemically bonded or immobilized stationaury phatses should be preferred. 

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