Chromatography zone broadening

Two-dimensional separations can be represented on a flat bed, by analogy with planar chromatography, with components represented by a series of dots . In fact, zone broadening processes in the two dimensions result in elliptically shaped spots centred on each dot . Overlap of the spots is then possible, but Bertsch (30) also showed how the contributors to the overall resolution, R, along the two axes, and Ry contribute to the final resolution according to the following ... 

The separation efficiency in chromatography and the sample throughput in CFA and FIA are better the narrower the test substances zones. The experimental conditions must therefore be selected so that the zone broadening in the detection system is as low as possible. The zone width is conveniently expressed in terms of the time standard deviation, o, or the volume standard deviation, oy, which are related by the expression,... 

In this system the bands (zones) broaden because of diffusion effects and nonequilibrium. This broadening mechanism is fairly symmetrical and the resulting elution bands approach the shape of a Gaussian curve. This system best explains liquid or gas partition chromatography. The system may be viewed in two ways ... 

One may study zone broadening in gas chromatography by observing the shape of the elution peak which is Gaussian in ideal systems. The base width of the Gaussian curve is measured in standard deviation units, therefore... 

FLOW. The rate at which zones migrate down the column is dependent upon equilibrium conditions and mobile phase velocity on the other hand, how the zone broadens depends upon flow conditions in the column, longitudinal diffusion, and the rate of mass transfer. Since there are various types of columns used in gas chromatography, namely, open tubular columns, support coated open tubular columns, packed capillary columns, and analytical packed columns, we should look at the conditions of flow in a gas chromatographic column. Our discussion of flow will be restricted to Newtonian fluids, that is, those in which the viscosity remains constant at a given temperature. 

The simplest picture of chromatographic zone broadening comes from the random walk model. The random walk model for chromatography will be developed at some length in Chapter 11 here we present a synopsis. 

Assuming that zone broadening is caused only by nonequilibrium, calculate the effective diffusion coefficient for a zone in gas chromatography using the typical parameters teq = 1(T2 s, v = 10 cm/s and R = 0.20. After 1 min calculate how far the zone has migrated and its width 4[Pg.248]

Elution, gradient, 161, 195 Elution chromatography, 6 Equations, summary table of, 22 End capping, 168 External standard, 105 Extra-column zone broadening, 203... 

The kinetic contributions to zone broadening are evaluated by fitting data for the column plate height, as a function of the mobile-phase velocity, to a mathematical model describing the relationship between the two parameters. Several models have been used in the above experiment, but those by de Ligny and Remijnsee and Knox and Pryde, and developed by Guiochon and Siouffi are most widely used and, at least for a first approximation, allow for comparison and determination of the differences between TLC and column chromatography... 

The fractogram of a polydisperse sample is a superposition of the separation and of the zone broadening. This is shown in Fig. Ic, where the spread zones of the discrete species are overlapped and the fractogram is, in fact, a convolution of all individual zones. Whenever the zone spreading is important, the accurate MMD or PSD can be calculated from the experimental frac-tograms only by using a correction procedure. An efficient correction method applicable in FFF [4] was derived from well-known correction procedures used in size-exclusion chromatography [5]. 

List the variables that lead to zone broadening in chromatography. 

The analyst should understand the physical basis for zone broadening in gas chromatography in order to properly select the right column system for the job to be done. In the following discussion, the term (jgoi of Equation 22.9 will be expanded into separate factors, each factor adding to H. The broadening factors to be considered are ... 

Electroosmotic flow has emerged as a viable alternative transport mechanism to pressure-driven flow in column chromatography. Benefits include a plug-flow profile (reduced transaxial contributions to zone broadening) and a mobile phase velocity that is independent of the column length and particle size. The electroosmotic-driven flow is governed by the dielectric constant of the mobile phase, the zeta potential at the stationary phase/mobile phase interface, and the applied electric field. The efficiency obtainable is limited by double layer overlap or radial dispersion induced by inefficient heat dissipation. 

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