Page 48 - Tandem Techniques

Page 30

It is seen that the standard deviation of a peak is directly related to the column volume and the capacity
ratio of the respective solute. The peak volume can be taken as equivalent to four volume standard
deviations, and thus the peak volume is also related to column volume and hence its dimensions and the
capacity ratio of the eluted peak. It follows that the sensing volume of any associated spectroscopic
instrument, i.e. the sample cell, can be much larger for a packed column than a capillary column before
it is in danger of holding more than one solute peak. However, for an extremely complex mixture, the
capillary column must be employed in GC to realize the necessary resolution. The peak volume
increases as the retention time increases for all types of column. Consequently, as each peak must be
ascribed equal importance, the interface system and sensor must be designed to accommodate the peaks
of smallest volume, i.e. the first peaks eluted. This can place even more stringent demands on the
sensor-interface system when a capillary column is employed.

There is a choice of two distinctly different types of GC columns, as already discussed, and advantage
should be taken of this choice when designing tandem systems. Capillary columns, despite being nearly
thirty years old, have the image of being the 'state of the art' in column technology. This has arisen
partly because they are easier to fabricate commercially in an optimum form, and partly because of the
glamour of the exceptionally high efficiencies that are available from them. However, they can
sometimes place almost impossible demands on any associated technique and, in many cases, the very
high efficiencies may not be required. There is no 'Philosopher's Stone' for the analyst, the column
chosen should always be on the basis of efficacy and not popularity and, if used in conjunction with a
tandem instrument, it must be in accordance with the unique requirements of the associated equipment.

Peak Dispersion

The second process that occurs in the column is the progressive dispersion of each solute band during
its passage through the column. In contrast to solute retention or solute selectivity, peak dispersion is
strongly dependent on the particle size of the packing, the physical properties of the solute and