Peak broadening axial diffusion

The major source of band-broadening in CZE is longitudinal diffusion. Longitudinal diffusion refers to the axial diffusive spreading of the solute from the solute zone into the bulk solution as it travels down the capillary. The variance in peak width contributed by longitudinal diffusion is given by... 

A better way to look at chromatographic efficiency is to consider the dynamic processes that contribute to peak broadening. Recall that chromatographic peaks are Gaussian and that the width of a peak at its base is approximately four standard deviations w = 4(7. The factors that contribute to peak broadening are additive provided the variance (a ) is used instead of the standard deviation. The total variance (ff tot) is the sum of variances due to multipaths (cT mp). axial diffusion (u dif). resistance to mass transfer (ff mt) and extra-column ([Pg.82]

FIGURE 4.4 Schematic distributions of axial ion coordinates in flow-driven FAIMS after some time upon injection into the gap (from the left) at fast (a), slow (b), and zero (c) flows. The peak broadening is due to diffusion and charge density fluctuations. 

The first term describes the longitudinal diffusion in the axial direction. The second term is called the Taylor diffusion coefficient and describes band broadening due to the parabolic flow profile and therefore radial diffusion. The height equivalent to a theoretical plate, H, is a measure of the relative peak broadening and is defined as... 

But an overly slow flow rate results in long retention times and increases peak broadening resulting from axial diffusion. 

Flow rate. A normal flow rate is often considered to be 1.0-1.5 mb min . A somewhat faster eluent flow rate may be used to obtain a faster separation, but at a cost of diminished chromatographic efficiency. Very low flow rates are best avoided as they increase peak broadening due to axial diffusions. 

When there is a size distribution in the particles packed in a chromatographic column, the distribution does not change the first moment of the elution peak while it may affect peak broadening through the contribution not only of intraparticle diffusion but also of other transport processes such as axial dispersion. The evaluation of the latter effect is not clear but it is possible to make a prediction of the effect of particle size distribution on the intraparticle diffusion contribution, 8i. This was done by Chihara, Suzuki and Kawazoe (1977) for several typical distribution functions. The effeevt of the particle size distribution can be accounted for by introducing a correction factor, F, into the expression of 6d. 

Obviously, the aim is for a chromatographic separation in which peak width is narrow relative to the time of elution (wi/Vr is minimized), i.e., the number of theoretical plates is maximized. There are three main factors that give rise to band broadening (1) multiple path effect (eddy diffusion), (2) axial (longitudinal) diffusion, and (3) mass transfer—slow transfer/equilibration between mobile and stationary zones. 

In the present problem (illustrated in Figure 6.2.2(a)), we will also find that the solute pulse introduced at z = 0 win show up (on a radially averaged basis) as a concentration peak with a broadened base as z becomes large. However, this broadening of the solute profile in the z-direction is not due to the molecular diffusion coefficient Djs of species i in the solvent. Rather, it arises primarily due to the radially nonuniform axial velocity profile (6.1.1b, c) of flow in a tube. It is identified as an axial dispersion or convective dispersion. This phenomenon was first studied by Taylor (1953, 1954), and is often described also as Taylor dispersion. 

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