so far discussed, the peak can appear to be further dispersed by the combined time constant of the sensor and its associated electronics. It must be emphasized that the time constant of the system can not actually disperse an eluted peak, but its effect of it on the sensor measurement can produce an apparent peak dispersion. Thus the term appear is used as the solvent profile itself is not changed, only the profile as presented on the recorder or printer.  The effect of the detector time constant can be calculated and the results from such a calculation are shown in figure 9.   Figure 9. Peak Profiles Demonstrating Distortion Resulting from Detector Time Constant

Detector Time Constant Peak dispersion resulting from the time constant of the sensor and its associated electronics can be significant in both GC and LC, particularly when filter circuits are introduced to remove inherent detector noise. The effect of the detector time constant can be theoretically examined (see Extra Column Dispersion ) and calculated and the results from such calculations are shown in figure 6. The undistorted peak, that would be monitored by a detector with a zero time constant, is about 4 seconds wide. Thus, for a GC packed column operating at 20 ml/min. this would represent a peak having a volume of about 1.3 ml. It is important to note that the dispersion is only apparent. The term apparent is used as the solute concentration profile, itself, is not actually changed, only the profile as presented on the recorder or printer.    Figure 6. Peak Profiles Demonstrating Distortion Resulting from Detector Time Constant

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.

nbsp; (3) It is seen that at high values of (g), the retention time approaches a constant value. The relationship between and (g) is depicted in figure 4. Figure 4 Graph of against (g) Figure 4 shows that there is little advantage in employing inlet/outlet pressure ratios much above 5 as values in excess of this do not reduce elution time significantly. If the column is very long, and consequently has a high flow impedance, higher inlet pressures may be necessary to obtain the optimum flow rate but this may not significantly reduce the elution time. In figure 5, the log of the retention time is plotted against (g) for both compressible and incompressible mobile phases. It is seen that for a compressible mobile phase the retention time falls to a constant level when (g) is about 5 or 6. In contrast, for

nbsp; The curve shows very significant a.c. noise, which was smoothed by a 5-point smoothing routine available in the recorder software and the smoothed curve is included in figure 22. The smoothed curve is also shown expanded to full scale together with the logarithm of the expanded curve. From the slope of the linear portion of the logarithmic curve the time constant of the sensor could be calculated and was found to be about 2.5 s. A time constant of this magnitude is completely unacceptable for high speed liquid chromatography as a complete separation can be achieved in a period commensurate with the time constant of the cadmium sulfide detector. Solid state photo sensors are much faster than the cadmium sulfide photo resistor but the fastest sensor appears to be the photo multiplier, such as the IP-28 or similar type. The same experiment was carried out using the photo multiplier and the decay curves obtained are

The performance of the photomultiplier (representing a sensor with a fast response) is shown in the lower curves of figure 10. The time constant, determined from the slope of the log curve, was only 40 milliseconds.  A response time of 40 milliseconds is acceptable for most LC separations. Nevertheless in fast LC separations, solutes can be eluted in less than 100 milliseconds in which case an even faster response might be necessary.   Contemporary sensors and electronic systems use fast solid state sensors and solid state electronic components. Thus, most commercial detector systems are sufficiently fast for the vast majority of chromatography applications. As a general rule, the overall time constant of an