Open tubular columns retention

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

In general, (Q) and ( ) will be equal, but the general case is assumed, where they are not. Equation (37) gives an explicit and accurate expression for the retention volume of a solute. The importance of each function in the expression will depend on the physical properties of the chromatographic system. At one extreme, using an open tubular column in GC, then... 

In practice, it is more difficult to optimize resolution as a function of the relative retentlvity than to optimize retention. Thus, unless the mixture is very complex or contains components that are particularly difficult to separate it may be possible to optimize a particular separation using the linear equation (1.72) as demonstrated by Bttre [177]. Figure 1.13 illustrates the relative change in peak position for a polarity test mixture with two identical, serially coupled open tubular columns, coated with a poly(dimethylslloxane) and Carbowax 20 M stationary phases, as a function of their relative retentlvity on the second column. The linear relationship predicted by equation (1.72) effectively predicts the relative peak positions and indicates that a nearly... 

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 30.1 m long and has an inner diameter of 0.530 mm. It is coated on the inside wall with a layer of stationary phase that is 3.1 p,m thick. Unretained solute passes through in 2.16 min, whereas a particular solute has a retention time of 17.32 min. 

Figure 24-6 Effect of stationary phase thickness on open tubular column performance. Increasing thickness increases retention time and Increases resolution of early-eluting peaks. Conditions DB-I stationary phase in 15-m-long x 0.32-mm-diameter wall-coated column operated at 40°C with He linear velocity of 38 cm/S. [Courtesy J6W Scientific. Folsom. CA]...

EFFECTIVE NUMBER OF PLATES. Desty et al. (31) introduced the term effective number of plates, N, to characterize open tubular columns. In this relationship adjusted retention volume, VR, in lieu of total retention volume Vp, is used to determine the number of plates. This equation is identical to Purnell s separation factor discussed below. 

Precision of retention data with open tubular columns is better than packed columns and one is able to see changes in the separation factors when a change is made in the carrier gas (e.g., changing from helium to hydrogen). 

Figure 5 compares chromatograms of A8- and A9-THC-TMS and 11-hydroxy-A8-THC-TMS obtained on a glass capillary column and a 6ft. x 2mm I.D. packed column. The retention times of the cannabinoids are comparable, but the resolution and sensitivity achieved on the capillary column is far superior. The capacity of the capillary column is of course far less than that of the packed column. Nevertheless, by using a Grob-type splitless injector (7) we are able to inject several microliters of solution containing up to 50 ng of each cannabinoid without overloading the column. Use of a support-coated open tubular column should increase the column capacity, but at some sacrifice of resolution and sensitivity. 

Once the sample reaches the chromatographic column, the separation process starts. The time necessary for a component injected into the chromatographic column to elute is called the absoiute retention time tR. The separation is based on different retention times of the components of the mixture. These retention times are different because the partition of each analyte between the two phases, the gas phase in motion and the stationary phase, are different. Hydrogen, helium, and nitrogen are common gases used as mobile phase. Two basic types of columns are known packed columns containing solid support particles coated with the stationary phase, and open-tubular columns with the stationary phase as a film on the inner wall (capillary columns). Because the retention time tpfi) of the analyte i is temperature dependent, the chromatographic column of any (GC) is put in an oven with temperature control capability. 

Although the exact mechanism is not very clear, the following factors may contribute to the modifier effect on chromatographic retention. Polar modifiers may cover the active sites of the stationary phase (deactivation) so that solute retention is reduced. This can be explained by the differences in retention change between packed and open-tubular columns when small amounts of modifiers were used. Open-tubular columns normally do not show the drastic changes in retention or efficiency upon the addition of small amounts (<2%) of modifier as most packed columns do. These less drastic differences were caused by the differences in the degree of deactivation of the packed column stationary phase as compared with the open-tubular-column stationary phase. An open-tubular column has fewer active sites present and, thus, fewer active sites are present for the modifier to deactivate. 

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. 

When capillary column temperature is raised, as will be necessary in the determination of enthalpies and entropies of probe/polymer interactions, the retention times of the probes will increase. This might seem odd since it is normal to expect an increase in temperature to result in a decrease in retention time. This behavior is due to the gas viscosity. When the temperature of a gas is increased, its viscosity is also increased (as opposed to liquids where the opposite is true). In a system having a constant pressure drop (as with open tubular columns), an increase in the viscosity results in a simultaneous decrease in the velocity of the carrier gas as shown in the following relationship ... 

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). 

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