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Open Tubular Columns   The majority of contemporary open tubular columns are made from fused quartz although, even today, in some applications (mainly hydrocarbon mixtures) stainless steel columns are still used providing equivalent performance. Nevertheless, fused quartz columns are perceived as "state of the art" columns. The internal surface of an open tubular column normally requires some deactivation and/or cleaning before it can be coated with stationary phase. Deactivation procedures (although usually very simple) are usually considered as highly proprietary. Under certain circumstances samples may need the column to be specially treated including particular column deactivation procedures, but most samples, can be successfully analyzed on columns that are deactivated by relatively simple clean-up procedures an example of which is the following.   A simple clean-up

Again the retention time of all the peaks are reduced, but not to the same extent as with benzene overload. This is probably due to the distribution coefficient of naphthalene being much larger than benzene and, although the deactivation of the silica gel will be greater, there will be much less solute in the mobile phase to increase the elution rate. This might imply hat that the greater change in retention from benzene over load arises more from the increased polarity of the mobile phase than from the deactivation of the stationary phase. A sample of more polar solutes (viz. anisole, benzyl acetate and acetophenone) were examined, employing a 5% v/v of diethyl ether in n-heptane as the mobile phase. The

produced by the same technique at much lower temperatures (6) but the tubes are not as mechanically strong or as inert as quartz capillaries. Surface treatment is still necessary with fused quartz columns to reduce adsorption and catalytic activity and also make the surface sufficiently wettable to coat with the selected stationary phase. The treatment may involve washing with acid, silanization and other types of chemical treatment, including the use of surfactants.   Deactivation procedures used for commercial columns are kept highly proprietary. However, a deactivation program for silica and soft glass columns that is suitable for most applications would first entail an acid wash. The column is filled with 10% w/w hydrochloric acid, the ends sealed and the column is then heated to 100˚C for 1 hour. It is then washed free of acid with distilled water and dried. This procedure is believed to remove traces of heavy metal ions that can cause

the plate. To improve the air space saturation with solvent, the walls, or part of the walls, of the chamber are sometimes covered with filter paper to act as a wick that soaks up the solvent and provides a greater surface area for evaporation. The use of a paper wick is depicted on the right-hand side of figure 29. The saturated solvent vapor in the chamber not only prevents solvent evaporating from the plate surface but partly controls the retention mechanism by surface deactivation. The solvents are selectively adsorbed on the surface of the stationary phase causing the solutes to interact, not with the native silica surface, but with the silica surface covered with the most strongly interacting solvent. It should be emphasized, however, that the equilibrium between the solvent vapor and the plate will not be the same as the equilibrium between the solvent and the plate. For example, for a binary mixture of solvents having concentrations of solvent in

the column, carried by the mobile phase, until it contacts sufficient stationary phase surface to allow it to be held on the surface under equilibrium conditions. This will result in a spreading process similar to sample volume overload and, if this were the sole contribution to mass overload, could be treated in a similar manner. The peaks would be square topped and similar in shape to those shown in figure 5. However, superimposed on this band dispersion process, is that arising from the deactivation of the adsorbing surface and the change in polarity of the mobile phase due to the presence of the solute. If the charge is substantial, the sample will occupy a significant slice of the column immediately after injection and the adsorbent (stationary phase) will become partially deactivated causing all the solutes in the mixture to be accelerated through the column with consequent reduced retention times. The increased migration rate is further aggravated by the higher polarity of