Extra Column Dispersion - Sources of Extra Column Dispersion > Dispersion in Connecting Conduits > Page 2

 

The different sources of dispersion may be summarized as follows;

Dispersion in the Sample Valve

There are two quite different processes associated with the sample valve that can cause dispersion. Firstly there is a peak broadening effect, inherent in the sampling system itself and which can not be avoided. This dispersion arises from the finite nature of the sample volume and is a direct function of it and, as a consequence, is quite independent of the sample valve design Secondly, there is dispersion that results from the Newtonian flow of sample contained in the mobile phase as it flows through the sample cell. This dispersion effect, however, depends strongly on the configuration of the sample valve and can be almost eliminated (or at least reduced to a very small level) by proper design.

Dispersion in Connecting Conduits

Dispersion that occurs in connecting tubes, or conduits, is exclusively due to the parabolic velocity profile of the mobile phase passing through it. This always occurs when the mobile phase velocity is less than the critical velocity (the velocity at the onset of turbulent flow). As the critical velocity of gases and liquids is well above those used in practical gas and liquid chromatography, this type of dispersion is always present unless the parabolic flow pattern is disrupted. The conditions where a parabolic velocity profile exists across a conduit is termed Newtonian Flow and any connecting tube between the sample valve and the column, the column and the detector, or any other cylindrical conduit will contribute to peak dispersion.