Liquid Chromatography Detectors - Dispersion in Detector Sensors > Dispersion in the Detector Sensor Volume > Apparent Dispersion from Detector Sensor Volume > Page 13

Apparent Dispersion from Detector Sensor Volume

The detector can only respond to the average value of the solute concentration throughout the sensor cell. At the extreme, if the sensor cell volume was large enough to contain two closely eluted peaks the response would appear as a single peak, albeit very distorted in shape. This extreme condition rarely occurs, but serious peak distortion and loss of resolution can quite often happen. This will be evident when the sensor volume is of the same order of magnitude as the peak volume. The problem can be particularly severe when columns of small diameter are being used. The situation is depicted in figure 7.


Figure 7 Effect of Sensor Volume on Detector Output

Consider the elution profile of a peak eluted from a column 3 cm long, 3 mm I.D. packed with particles 3 m in diameter as shown in figure 7. If the peak is eluted at a (k') of 2, from figure 7 it is seen that the peak width at the base is about 14 ml wide. The sensor cell volume is 2.5 ml and the portion of the peak in the cell is depicted in the figure. The detector will obviously respond to the mean concentration of the slice contained in the 2.5 ml sensor volume. It is also clear that, if the sensor volume is increased, a larger part of the peak will be contained in the cell. As a consequence, the  output will be an average value of an even larger portion of the peak which will produce serious peak distortion. The effect of a finite sensor volume can be easily simulated with a relatively simple computer program and the output from such a program is shown in figure 8.