Zone Broadening

Rate theory attempts to explain the kinetic contribution to zone broadening in column chromatography as the sum of three main contributions: flow ani-sotropy (eddy diffusion), axial diffusion (longitudinal diffusion), and resistance to mass transfer. Flow anisotropy is illustrated in Figure 8. When a sample band migrates through a packed bed, the individual flow paths must diverge to navigate around the particles such that individual flow streams are of unequal lengths. These variations in flow direction and rate lead to zone broadening that should depend only on the particle size and homogeneity of the column packing. Flow anisotropy can be

Open Tubular Column Chromatography

Figure 8

umn.

Representation of flow anisotropy in a packed col-

minimized by using particles of small diameter with a narrow particle size distribution in columns with a high and homogeneous packing density. For open-tubular columns, flow anisotropy is not a contributing factor since the streamlines have no obstacles in their way to cause disruption of the sample profile.

Axial diffusion is the natural tendency of solute molecules in the mobile phase to redistribute themselves by diffusion from a region of high concentration to one of lower concentration. Its contribution to zone broadening depends on the solute diffusion coefficient in the mobile phase and the column residence time. Diffusion of solute molecules occurs in all directions but only the components in the plane of mobile-phase migration contributes to the peak profile observed in the chromatogram.

Resistance to mass transfer in either the stationary or mobile phases is a consequence of the fact that mass transfer in the chromatographic system is not instantaneous and equilibrium may not be achieved under normal separation conditions. Consequently, the solute concentration profile in the stationary phase is always slightly behind the equilibrium position and the mobile-phase profile is similarly slightly in advance of the equilibrium position (Figure 9). The

Figure 9 Representation of resistance to mass transfer in the mobile and stationary phases. The dashed line represents the equilibrium position and the solid line the actual position of the solute zones.

Figure 8

umn.

Representation of flow anisotropy in a packed col-

Figure 9 Representation of resistance to mass transfer in the mobile and stationary phases. The dashed line represents the equilibrium position and the solid line the actual position of the solute zones.

Column Flow Efficiency Deemter

Mobile-phase velocity (u)

Figure 10 van Deemter plot of the column plate height as a function of the mobile-phase velocity. The solid line represents the experimental results and the broken lines the theoretical contribution from flow anisotropy (A), axial diffusion (B/u) and resistance to mass transfer (Cu).

Mobile-phase velocity (u)

Figure 10 van Deemter plot of the column plate height as a function of the mobile-phase velocity. The solid line represents the experimental results and the broken lines the theoretical contribution from flow anisotropy (A), axial diffusion (B/u) and resistance to mass transfer (Cu).

resultant peak observed at the column exit is broadened about its zone centre, which is located where it would have been for instantaneous equilibrium, provided that the degree of nonequilibrium is small. Contributions from resistance to mass transfer are rather complicated but depend on the column residence time, mobile-phase velocity, stationary-phase film thickness, the particle size for packed columns, the solute diffusion coefficients in the mobile and stationary phases, and the column internal diameter.

The relationship between zone broadening (column plate height) and the mobile-phase velocity is given by the hyperbolic plot known as a van Deemter curve (Figure 10). The solid line represents the experimentally observed results and the dotted lines the contributions from flow anisotropy (A term), axial diffusion (B/u) and resistance to mass transfer (Cu). In this generic plot we see that there is an optimum velocity at which a particular chromato-graphic system provides maximum efficiency (a minimum column plate height). The position of this optimum velocity and the general curvature of the plot strongly depend on the characteristics of the chromatographic system, as shown by the values given in Table 1.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment