Extra Column Dispersion - Sources of Extra Column Dispersion > Dispersion in the Volume of the Detector Sensor > Page 3

Dispersion from Unions and Frits

Dispersion that takes place in unions (e.g., those between connecting tube and column or between column/connecting tube and detector) and in frits will also result from any Newtonian flow present in the mobile phase conduit system. However, these devices can usually be designed to provide minimum dispersion.

Dispersion in the Volume of the Detector Sensor

In a similar manner to the sample valve, dispersion in the detector sensor also arises from two different sources The first emanates from the Newtonian flow through the cell which occurs at all practical flow rates used in chromatographic separations. It is, in fact, equivalent to the dispersion that takes place in a very short cylindrical tube with a relatively small length-to-radius ratio. The second, results from the sensor having an essential, albeit small, but finite volume, the magnitude of which may be of the same order of magnitude as that of the peak. The latter dispersion may, not only broaden the peak, but also appear to distort the peak particularly if two solutes are eluted close together. As already stated, none of these sources of extra column dispersion can be completely eliminated but, by careful design, they can be reduced to a level where they no longer significantly impair column performance.

Before extra column dispersion can be considered in detail, however, it is necessary to determine the volume of the peaks produced in contemporary high efficiency columns by the normal column dispersive processes. This is necessary in order to place a specific limit on the acceptable level of extra column dispersion. It will be seen that extra column dispersion can be far more serious in liquid chromatography than in gas chromatography and so the two types of column system will be considered separately. Extra column dispersion in LC systems will be considered first.