Liquid Chromatography Detectors - Dispersion in Detector Sensors > Dispersion Resulting from the Detector Time Constant > Page 17

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 LC detecting system should be less than 50 milliseconds. For specially very fast separations, a lower value of 15 milliseconds may be necessary. Fast sensors and electronics will respond to high frequency noise so the chromatographic system must be designed to reduce short term noise. This may involve magnetic screening to reduce the effect of stray, low-frequency electromagnetic fields from nearby power supplies and any high energy consuming laboratory equipment.


In general, as the peaks in LC separations can be extremely small all sources of dispersion must be taken into account. It follows that in the design of the chromatograph, careful steps must be taken to minimize the effect from such dispersion sources and to ensure the integrity of the separation is maintained.