Under conditions where the sensor volume is very large, then two closely eluted peaks could be contained in the sensor volume at the same time. In such a situation, the concentration profile of the peaks as monitored would appear as a single peak (although very distorted in shape). However, whatever the volume, the output of the sensor will always represent the average concentration in the sensor cell.
The effect of a finite sensor volume on a peak profile can be readily simulated with a relatively simple computer program and the output from such a program is shown in figure 21.
Although not the most adverse case, the selected example, clearly demonstrates a situation where the volumeofthe sensor seriously deforms the peak profile and, consequently, degrades the resolution. A microbore column is employed for the simulation and, thus, the eluted peaks have relatively small peak volumes. In fact, the peak volumes are commensurate with the volume of the sensing cell. Surprisingly, it is shown that a sensor volume of 1 ml still has a very significant impact on the peak width and obviously, if the maximum resolution is required from the column, then the volume of the sensor cell must certainly be no greater than 2 ml.
It is clear that the results from the use of a 5 ml sensor cell would be virtually useless and, unfortunately, many commercially available detectors do have sensor volumes as great as, if not greater than 5 ml. Consequently, if small bore columns are to be employed, such sensor volumes must be studiously avoided.
For their efficient use, small volume cells (sensors) must be very carefully designed. It should be noted that, reducing the cell diameter will reduces the volume and, as a consequence, increase the noise, thus, reduce the detector sensitivity: Reducing the cell length, in general, will maintains the same noise, but reduces the detector response and, thus, also the sensitivity. It is clear the design of the sensor cell will require some considerable skill and care, a good understanding of cell dispersion, detector response and detector noise.