Extra Column Dispersion - Dispersion in Tubular Conduits > Page 26

It is seen in figure 9A that, as would be expected, the variance increases linearly with the tube length. These variance values must be added to that of the column and to other extra column dispersion variances to obtain the final variance of the peak. Under extreme circumstances, it might be possible to reduce the tube diameter to 0.008 cm (0.003 in) I.D., and so decrease the tube variance by about a factor of eight. However, this would increase the chance of tube blockage very significantly.

The curve in figure 9B is more informative from a practical point of view. Although standard deviations are not additive, they do give an idea of the actual band width caused by a tube alone. Thus, a tube 10 cm long and O.012 cm I.D. can result in a peak with a standard deviation of 4 ml. This would be equivalent to a peak with a base width of 16 ml and, as it will be seen later, many short columns packed with particles, 3 mm (or less) in diameter will produce peaks of commensurate size. Thus, if high efficiency columns are to be used, then connecting tubes must either be eliminated altogether, or reduced to the absolute minimum in length.

In practice, it can be extremely difficult to use short lengths of a connecting tube, particularly for column detector connections. This is because detectors are often designed such that the sensor cell is situated some distance from the exterior union that connects it to the column.