Open tubular column surface preparation

Open tubular columns are prepared with positively or negatively charged surface coatings. The best efficiency is obtained with <25 pm ID capillaries. 

Porous-layer- open tubular (PLOT) and support-coated open tubular (SCOT) columns are prepared by extending the inner surface area of the capillary tube. A layer of particles can be deposited on the surface or the column wall can be chemically treated to create a porous adsorbent layer. Obviously some of the wall-modified open tubular columns discussed in section 2.3.3 could be... 

There are surprisingly few studies of the retention mechanism for open tubular columns but the theory presented for packed columns should be equally applicable. For normal film thicknesses open tubular columns have a large surface area/volume ratio and the contribution of interfacial adsorption to retention should be significant for those solutes that exhibit adsorption tendencies. Interfacial adsorption has been shown to affect the reproducibility of retention for columns prepared with nonpolar phases of different film thicknesses [322-324]. The poor reproducibility of retention index values for columns prepared from polar phases was demonstrated to be c(ue to interfacial... 

Dhanesar, S.C., Coddens, M.E., and Poole, C.R, Surface roughening by sodium chloride deposition for the preparation of organic molten salt open tubular columns, /. Chromatogr. Sci., 23,320-324,1985. 

Solutions to the above problem are required if efficient open tubular columns are to be prepared. The energy of the smooth glass surface can be increased by chemical... 

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. 

Solutions to the above problea are required if efficient open tubular colunns are to be prepared. The energy of the saooth glass surface can Sse Increased by roughening or chemical Modification, or the surface tension of the stationary phase can be lowered by the addition of a surfactant. Roughening and/or cheMical modification etre the most widely used techniques for column preparation the addition of a surfactant, although effective, modifies the separation properties of the stationary phase and may also limit the thermal sted>ility of columns prepared with high temperature stable phases. 

Another approach is similar to that used in for the preparation of polymer-layer open tubular GC columns (PLOT). Horvath s group prepared capillaries with a porous polymer layer as shown in Fig. 13 by in situ polymerization of vinylbenzylchloride and divinylbenzene [183]. The reaction of the N,N-di-methyldodecylamine with chloromethyl groups at the surface simultaneously afforded strong positively charged quaternary ammonium functionalities and attachment of C12 alkyl chains to the surface. The unreacted chloromethyl groups... 

For wall-coated open tubular (WCOT) columns, the stationary phase covers the inside surface of the column. The film thickness of the stationary phase can vary from 0.05 to 5 pm. It can be simply deposited on the surface, can originate from the reticulation of a polymer on the silica surface or can be bound to the silica through covalent bonds. The surface of the silica is treated before the stationary phase is deposited to avoid problems of wetability, desorption and stability over time. This treatment can involve attack by HC1 at 350 °C or the deposition of a fine coat of alumina particles. Afterward, the stationary phase is either deposited or prepared in situ by polymerisation at the inner surface of the column. Covalent bonding via Si-O—Si-C allows organic compounds to be bound to the silica surface. In the latter case, the columns are particularly stable and can be rinsed periodically allowing them to recover their initial performance. The efficiency of these columns can reach 150000 theoretical plates. 

Several types of surface modification have been employed [37,38] such as etching or deposition of very fine inert supports before application of the phase. The latter are referred to as support coated open tubular (SCOT) or by others as porous layer open tubular (PLOT) columns. They have a higher sample capacity than liquid coated columns due to the greater amount of phase per unit length. Several recent papers have described modified methods of column preparation [39-41] using Silanox 101 as the support. 

Column-liquid chromatography (CLC) can be conveniently divided into those systems which use packed columns and those which use open tubes (Figure 3.1). Capillary tubes (<4 < 350 pm) are used in open-tubular chromatography and the stationary phase is coated on the internal surface. Packed-column systems can be sub-divided arbitrarily into capillary columns, microbore columns, analytical columns and preparative columns according to the internal diameter of the column (Figure 3.1). 

Capillary Columns. Capillary columns have no packing the liquid phase is simply applied directly to the walls of the column. These columns are referred to as waU-coated, open-tubular (WCOT) columns. The reduction in surface area (compared to packed columns) is compensated for by tiny column diameters (perhaps 0.1 mm) and impressive lengths (100 m is not uncommon). Capillary columns are the most powerful columns used for analytical separations. Mixtures of several hundred compounds can be completely resolved on a capillary GC column. These columns require a more sophisticated and expensive chromatography instrument. Capillary columns, because of their tiny diameters, can accommodate only very small samples, perhaps 0.1 jxL or less of a dilute solution. Capillary columns cannot be used for preparative separations. 

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