Open tubular columns band broadening

For an understanding of band broadening in chromatographic systems, the linear velocity of the mobile phase is more important than the column volumetric flow rate. The mobile phase velocity and flow rate in an open tubular column are simply related by... 

Fig. 10.6 Band broadening factors in open tubular columns. Liquid...

In packed columns, all three terms contribute to band broadening. For open tubular columns, the multiple path term, A, is 0, so bandwidth decreases and resolution increases. In capillary electrophoresis (Chapter 26), both A and C go to 0, thereby reducing plate height to submicron values and providing extraordinary separation powers. 

The miniaturization of detector cells is also extremely difficult and the technological problems have not yet been solved because the detection limit should also be decreased or, at least, kept constant [2-6]. Some progress in the design of very small cells for UVD and FD has been reported [3-7], but the miniaturization of RD and RID seems much more difficult in spite of some suggestions [8]. Similarly, the development of open tubular columns is plagued by the lack of a suitable detector with a small contribution to band broadening. 

In 1957 Marcel Golay published a paper entitled Vapor Phase Chromatography and the Telegrapher s Equation [Anal. Chem., 29 (1957) 928]. His equation predicted increased number of plates in a narrow open-tubular column with the stationary phase supported on the inner wall. Band broadening due to multiple paths (eddy diffusion) would be eliminated. And in narrow columns, the rate of mass transfer is increased since molecules have small distances to diffuse. Higher flow... 

Beginning with the most favorable case, band broadening in open tubular columns is satisfactorily described by the Golay equation, extended to situations of appreciable pressure drop by Giddings, Eq. (1.22)... 

According to Scott the average linear velocity can be replaced by (4uq / [P -i- 1]) in Eq. (1.23) to permit evaluation entirely in terms of the outlet velocity [130]. If X = 0, y = 1, and dp = rc is substituted into Eq. (1.23) then this equation can be used as an alternative to Eq. (1.22) to account for band broadening in evaluating open tubular columns [121,130]. For gas-solid chromatography the stationary phase mass transfer... 

The separation impedance represents the difficulty of achieving a certain performance and should be minimized for optimum performance. The highest performance is achieved by a column, which combines low flow resistance and produces minimal band broadening. For open tubular columns the column internal diameter, dc, replaces dp in Eqs. (1.27), (1.28) and (1.30). 

The multipath term (A) This term applies to columns packed with support particles. It becomes zero for open tubular columns when the mobile phase velocity is slow enough for the flow to be laminar (i.e., without mrbulent eddies). In a packed column, the paths of individual analyte molecules will differ as they take different routes through the spaces between the particles. Thus they will travel varying distances before they exit the column, and the difference between these distances contributes to band broadening. The relative magnitude of the multipath term depends on the particle and column dimensions. If Fig. 11.3 depicted a packed column, A would be a constant value for all values of u, and would appear as a horizontal line raising the curve by a constant amount. The multipath process is illustrated in Fig. 11.4. 

Open tubular columns. The band broadening obtained in an open tubular column can be expressed by the Golay equation (Golay, 1958). This summarizes the contribution to band broadening from longitudinal diffusion. 

Capillary electrophoresis can provide extremely narrow bands. Three mechanisms of band broadening in chromatography are longitudinal diffusion (B in the van Deemter equation 21-7), the finite rate of mass transfer between the stationary and mobile phases (C in the van Deemter equation), and multiple flow paths around particles (A in the van Deemter equation). An open tubular column in chromatography or electrophoresis reduces band broadening (relative to that of a packed column) by eliminating multiple flow paths (the A term). Capillary electrophoresis further... 

This is a technological challenge and only recently have such columns started to appear. A disadvantage is that the loading capacity of open tubular columns is very low. Advantages are the increased sensitivity with concentration-sensitive detection, such as ESI-MS, for samples of limited availability, less adsorption problems with macromolecules such as proteins, and the absence of frits. All coimections must be with true zero dead volume connectors in order to avoid band broadening. 

The process of band broadening (Figure 2.1) is measured by the column efficiency or the number of theoretical plates N, equation (2.24)), which is equal to the square of the ratio of the retention time to the standard deviation of the peak. In theory, the value of N for packed columns has only a small dependency on k and may be considered to be a constant for a particular column. Column efficiency in open-tubular systems decreases markedly with increased retention. For this reason open-tubular liquid chromatography systems must be operated at relatively low kf values (see section 2.5.S.2). 

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