Page 39 - Tandem Techniques

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The separation was carried out on a column 2.5 cm long, 2.6 mm I.D., packed with silica gel particles, 3
µm in diameter. It is seen that the first pair of solutes are separated and eluted in less than 1 sec. This
type of separation is not practical for most tandem techniques, except possibly for LC/MS, as the
response of most spectrometers and associated instruments would not be fast enough to obtain the
necessary data during the elution of each peak. Nevertheless, it demonstrates that extremely fast
separations can be obtained from contemporary columns if so desired. In contrast, very high efficiencies
and consequently high resolution, are achieved by using very long columns of very small diameter
packed with particles of relatively large diameter [14]. As the columns must be packed, the minimum
diameter that appears to be practical is about 0.5 mm. An example of a chromatogram from a high
resolving power column is shown in Figure 1.6.

Figure 1.6
The Separation of a Coal Extract on a High-Efficiency Column

The column was 2 m long, 1 mm I.D., and packed with a reverse phase having a particle diameter of 10
µm. The development was isocratic, at a flow-rate of 20 µl/ min. It should be noted that the
chromatogram takes over 50 hours to develop. Unfortunately, the 'cost' of a chromatographic separation
is counted in units of column inlet pressure and time. Once the