Liquid Chromatography Detectors - Dispersion in Detector Sensors > Dispersion Resulting from the Detector Time Constant > Page 16

The undistorted peak, monitored by a detecting system with a zero time constant, is about 4 seconds wide. An LC column operating at a flow rate of 1 ml/min. and having a peak base-width of 4 seconds would represent a peak with a volume of about 67 ml. It follow, that the peaks depicted would represent those eluted fairly late in the chromatogram. However, despite the late elution, the distortion is still quite severe. To avoid distortion of the early peaks the time constant would need to be at least an order of magnitude less. Scott et al. (12) measured the time constants of two photocells and their results are shown in figure 10.

Figure 10. The Response Curves of Two Photocells

The output each photocell to fast transient changes in incident light intensity was monitored with a high speed recorder. The curves for the cadmium sulfide photocell, figure 10 (chosen as an old type, sensor with a very slow response) is shown at the top of the figure. From the slope of the log curve, the time constant was calculated to be about 2.5 seconds. Such an extremely slow response would be impractical for modern chromatographic systems (i.e., two or more peaks could elute within the period of the time constant). The result of the slow response of the cadmium sulfide sensor, would be to cause the peaks merge into a single distorted peak.