Gas Chromatography Detectors - Peak Dispersion from the Overall Detector Time Constant > Page 16

System Dispersion and Sensor Dispersion

One problem common to all detectors is the peak dispersion that takes place in the mobile phase conduits and sensor volumes of the detector. Dispersion of this type is particularly serious in LC where solute diffusivities are 4 to 5 orders of magnitude smaller than those in gasses. In GC however, due to the much higher diffusion rates detector dispersion is minimal and does not significantly effect chromatographic performance. Consequently detector dispersion in GC detectors will not be discussed in this book, but dispersion in LC detectors will be considered in detail in Liquid Chromatography Detectors .

Peak Dispersion from the Overall 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