Chromatography Forum

Hi, it is my first post here, and i need some enlightenment regarding my study, so, hopefully i could get more information here.. thanks before..

my question is, is extra column volume same as extra column dispersion? and if i have high extra column volume, is it always lead to high extra column dispersion? what factors affecting the extra column dispersion except for the diameter and length of the tubing?

much regards,
Zee

Definition: Extra-column volume From LCResources website
The extra-column volume of an HPLC system includes the entire volume of liquid in the system between the point where the sample is introduced and the column inlet plus the volume between the column outlet and the outlet of the detector cell.
And please look here: http://d-scholarship.pitt.edu/7338/1/to ... td2011.pdf
Or google for Extra column volume and you will get a lot of valuable literature, or look for the articles of John Dolan at LCGC.
Good luck

They are related, but not identical. Think of extra-column volume as the case and dispersion as the effect.

All extra-column volume is not equivalent. The effect of pre-column volume, for example, may be mitigated by on-column concentration if the diluent is substantially weaker than the mobile phase, and a poorly assembled fitting may make a dispropotionately large contribution to dispersion because of tthe "mixing chamber" effect of the change in diameter.

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Extra-column volume is the physical volume from the injector to the end of the detector whereas extra-column dispersion is a process. Unfortunately, the term dispersion is quite loosely used and rarely defined in the chromatography literature. Engineers have a better version of this term. They define dispersion as process of mass transport which includes diffusion and convection. Assume no chemical interactions of the solute with the stationary phase surfaces.

In chromatography, you know that convection is due to fluid flow (the mobile phase) and there is nothing which can stop the molecule from walking here and there (diffusion). Loosely, for our purposes, dispersion refers to spreading of the injected band just by fluid flow pattern in a packed bed or in an empty tube. One can sort of measure or quantify dispersion by the variance of the peak. The wider the peak the more dispersion it has experienced assuming it had no chemical interaction with the stationary phase.

Regarding the interesting part, is extra-column volume related to extra-column dispersion? The answer is yes and no. If you have a really long tube, but coiled or knitted in a proper manner, the peak will not spread as much as it should if it were simply a long tube.