Multiple path effects

It is also possible for solute species to diffuse laterally (in a radial direction) across the column, and thus to move from one flow path into another. This effect reduces the amount of dispersion produced by the multiple path effect as it tends to equalise the speed of the solute species in the column. The longer the time a solute species spends in the column, the more lateral diffusion will occur, so that flow dispersion is reduced by using low flow rates of mobile phase. 

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. 

Fig. 47. "Multiple-path effect" (eddy diffusion) as the cause of broadening of a peak A B) = Resultant substance profile...

As a general rule it may be said that low values of lead to narrow peaks. The multiple-path effect or eddy diffusion... 

Also referred to as the multiple-path effect, eddy diffusion (2 dp) is minimized by using small particles of support materials. A support of 100/120 mesh produces a more efficient column than do 60/80-mesh particles and should be used whenever possible. A support of lower mesh, such as 80/100 or 60/80, should be selected to avoid a high pressure drop within a long column. This term is also independent of linear velocity or flowrate. 

Rg. 2. Illustration of the three principal causes of band broadening (a) multiple-path effect (b) longitudinal diffusion effect (c) mass-transfer (non-equillbrium) effect. Reproduced from A. Braithwaite F.J. Smith, Chromatographic Methods, 5th edn, 1996, first published by Blackie Academic Professional. 

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