Extra Column Dispersion - Dispersion in the Detector Sensor Volume > Page 38


Now, if the sensor has a finite volume (which is necessary for a concentration sensitive device) the volume of the sensor can cause dispersion and contribute to the peak variance in two ways. Firstly, as a result of Newtonian flow through the sensor, there will be dispersion resulting from the parabolic velocity profile of the fluid through the cell sensor volume, and this will generate a variance similar in form to that from cylindrical connecting tubes.

Secondly, there will be a peak spreading which is not dependent on Newtonian flow, but is a direct result of the sensor having a finite volume. This is a quite different type of dispersion and needs to be considered in some detail

If the sensor has a significantvolume, it will not measure the instantaneous concentration at each point on the elution curve, but it will measure the average concentration of a slice of the peak equivalent to the sensor volume. As a consequence, the true profile of the peak will not be monitored. The net effect will first (if the sensor volumes is still significantly smaller than the peak volume) give the peak an apparent dispersion. Then (as the sensor volume is increased and becomes commensurate to that of the peak volume) the peak profile will be distorted and eventually become sufficiently severe to impair resolution. In the extreme case, the situation could arise that two peaks could coexist in the sensor at one time and only a single concentration profile would be represented. This apparent peak spreading is not strictly a dispersion process but, in practice, will have a similar effect on chromatographic resolution.