Liquid Chromatography Detectors - Dispersion in Detector Sensors > Dispersion in the Detector Sensor Volume > Dispersion in Detector Sensors Resulting from Newtonian Flow > Page 10

Dispersion in the Detector Sensor Volume

The finite nature of the detector sensor volume can cause peak dispersion and contribute to the peak variance by two processes. Firstly there will be dispersion resulting from the Newtonian flow of fluid through the cell in much the same manner as the flow of a viscous fluid through an open tube. This will furnish a variance similar in form to that predicted by Golay but, as the tube length is small and the tube length to radius ratio much larger than that from a connecting tube, a different equation is necessary to describe the dispersion effect.

Secondly, there will be a peak spreading which results from the finite volume of the sensor. If the sensor has a significant volume, the concentration measured will not be that entering the detector cell but the average concentration throughout the cell. Thus, the true profile of the peak can not be monitored. If the  sensor volume is significantly smaller than the peak volume the effect will merely give the peak an apparent dispersion. However, if the sensor volume becomes of the same order of magnitude as the peak volume, then the peak profile will be distorted and resolution will be lost.  In the extreme case two peaks could coexist in the sensor at one time and only a single peak will be represented.

 The effect of viscous flow on dispersion will first be considered.

Dispersion in Detector Sensors Resulting from Newtonian Flow

Most sensor volumes are cylindrical in shape, are relatively short in length, and have a relatively small length-to-diameter ratio. The small length-to-diameter ratio is in conflict with the premises assumed in the development of the Golay equation for dispersion in an open tube. Atwood and Golay (11) extended the theory of dispersion in open tubes to tubes having small length-to-diameter ratio. The theory is complex and not relevant here as, if appropriate cell design is employed, the dispersion from viscous sources will be negligible. Nevertheless, the effect on solute profiles is shown in figure 5.