# Plate Theory and Extensions - Resolving Power of a Column > Page 62

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Curves
relating (n) and (k'_{A}) for
solute pairs having separation ratios of 1.02, 1.03, 1.05 and 1.07, calculated
using equation (55) are shown in Figure 18. As the separation becomes more
difficult (*i.e.,* the separation ratio (a_{A/B}) becomes smaller),
the necessary efficiency (n) for resolution increases rapidly. This is due to
the fact that as (a_{A/B}) becomes smaller,
the peaks become closer, consequently dispersion must be more constrained to
reduce their width. To do this, the column must be made more efficient.
Nevertheless, the dramatic increase in (n) when the capacity factor becomes
small is not so obvious. This makes achieving short analysis times very
difficult, as fast elution is achieved with small (k') values. However, the
greater efficiencies needed at lower (k') values will require longer columns
which will *extend* the analysis times. To resolve a solute pair with a
separation ratio of 1.02, an efficiency of 360,000 theoretical would be
required if the (k') value was 0.5. GC Capillary columns can provide such
efficiencies but, in LC, such efficiencies would be extremely difficult and
costly to produce. It follows that the phase system should be chosen so that
the closest eluted solutes are not eluted at low (k') values. Less efficiency
will be needed and, thus, shorter columns and consequently, shorter *analysis
times *will be achieved. At (k') values that exceed 10, the required
efficiency changes little as the capacity ratio increases. Thus, for fast
analyses, the phase system provide a large separation ratio, but the first peak
should elute at a (k') of 10 or more. The phase system should have high
selectivity and retentive capacity so that minimum efficiency is required and
the column can be as short as possible.