Open tubular columns phase ratio

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... 

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. 

One way to increase the phase ratio of open-tubular columns is to use a polymeric stationary phase instead of a bonded molecular monolayer (Figure 6). 

Packed capillary columns (Figure 8) have a greater sample capacity than open-tubular columns because of the increased surface area and, hence, greater phase ratio. Greater sample capacities result in increased sensitivity and selectivity. More than 95% of the CEC columns... 

The properties of open tubular columns shown in figures (I) to (6) indicate that the areas where such columns would have practical use is very restricted. At pressures in excess of 10 ps.i., and whatever the nature of the separation, whether simple or difficult, the optimum column diameters are so small that they would be exceedingly difficult to fabricate or coat with stationary phase. The maximum sample volumes and extra column dispersion that couid be tolerated would also be well below that physically possible at this time. At relatively low pressures, that Is at pressures less than 10 p.s.l. the diameter of the optimum column is large enough to fabricate and coat with stationary phase providing the separations required are difficult i.c. the separation ratio of the critical pair must be less than 1.03. However, even under these conditions the sample volume will be extremely small, the extra column dispersion restricted to an almost impossibly low limit and the analysis time would be very long Nevertheless, open tubular columns used for very difficult separations... 

The phase ratio is an important parameter used in the design of capillary (open tubular) column separations.1 This quantity relates the partition coefficient (K) to the partition ratio (k) ... 

This poor performance, relative to the high efficiencies produced, results from the high phase ratio inherent with open-tubular columns. The high phase ratio is due to there being very little stationary phase in the capillary column (the film is very thin). The corrected retention volume of a solute is directly proportional to the amount of stationary phase in the column, and solutes, in general, are eluted from a capillary column at relatively low k values relative to the magnitude of their distribution coefficient. 

Besides the stationary-phase selectivity, the phase ratio (volume of the mobile phase/volume of the stationary phase) is an important consideration in practical chromatography. Thus, within the available arsenal of GC columns, the efficiency increases in the following direction packed column < micropacked column < support-coated open tubular column (or wide-bore capillary column) < conventional... 

The ratio VfA/V is referred to as the phase ratio / in open tubular column GC and describes the retention characteristics of the column where P = Vq/V Vq = volume of carrier gas). 

Because of unfavorable mass transfer properties in liquids, highly efficient separations and short separation times potentially available for open tubular columns can be realized only in columns of small internal diameter (< 25 xm) [309]. These columns have very low phase ratios and serious detection problems arise. Several methods have been proposed to Increase the surface area, and hence the stationary phase capacity, by chemical etching of the interior wall [335] or by adhesion of a thin porous silica or polymer layer to the wall [336-338]. The sol-gel process allows an increase in surface area and formation of a retentive chemically bonded phase in a single step. None of these processes, however, adequately address the problems of low retention, low sample capacity, poor sample detectability, and unfavorable handling characteristics that prevent wider use of open tubular columns in capillary electrochromatography. 

It was demonstrated that many homologous series could be eluted from an open tubular column, which apparently could not be eluted from the packed column with pure carbon dioxide. The inability to elute polar compounds from packed columns was attributed to excess column activity, not to the low polarity of the mobile phase. Today, we recognize that the open tubular column has perhaps 100 times higher void volume per unit surface area than a typical packed column. This difference in phase ratio makes inherent retention 1/100 as great on the open tubular column, compared to a totally porous column packing with small pores. 

Phase ratio. The ratio of mobile to stationary-phase volumes. Thicker stationary-phase films yield longer retention times and higher peak capacities. For open-tubular columns, = VqIV d l df. 

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