Extra Column Dispersion - Dispersion in the Detector Sensor Volume > The Effect of Viscous Flow on Dispersion in a Detector Sensor > Page 40

From the data given in table 4 it is clear that both the length and the I.D. of the cell have a strong influence on the control of dispersion. Moreover, the magnitude of the dispersion, in terms of peak variance, can be comparable to the peak variance produced by the column alone. As a consequence, such dispersion would seriously degrade the performance of all high efficiency columns and, in particular render microbore columns virtually useless. Because of the nature of the experiments, the major source of dispersion can not be identified and the effect of Newtonian flow through the cell can not be differentiated from the dispersion due to sensor volume.

The combined effect of the two types of dispersion are shown as elution curves in figure 17. The column used to produce the elution curves in the upper chromatogram was 24 cm long, 4.6 mm I.D. The mobile phase was tetrahydrofuran and the column was operated at a flow rate of 1 ml/min. The solute injected was benzene. The column used to produced the elution curves in the lower chromatogram was 1 m long, 1 mm I.D. and the same solvent was used at a flow rate of 40 ml/min Benzene was also used a the solute. It is seen that the reduction in cell volume has a dramatic effect on both peak width and peak shape. The large 25 ml cell causes significant peak asymmetry as well as excessive peak dispersion A result which is predicted by the work of Atwood and Golay (11) which is discussed below. It is seen that the large sensor cell has a disastrous effect on the band width of the solute eluted from the microbore column. Clearly, even cell volumes of 3 ml are too large for use with 1 mm I.D. columns and relatively few contemporary detectors have cell volumes less than 3 ml.


J. Chromatogr. 169(1979)51

Figure 17. Peak Profiles from Detector Having Different Cell Volumes