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Quantitative Chromatographic Analysis
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| where (n) | is the column efficiency in theoretical plates, |
| (vm) | is the volume of mobile phase per plate, |
| (vs) | is the volume of stationary phase per plate, |
| and (KA),(KB) | are the distribution coefficients of solutes (A) and (B) |
Bearing in mind that (a) the separation ratio is defined as,
, then, 
(11)
Figure 27. The Separation of Two Solutes
Now, by definition for adequate separation, DV = 4s
or 
(12)
Equating equations (11) and (12)
Dividing throughout by (vm) and remembering that the separation ratio (k') is given by, 
Then , 
Solving for (n), 
(13)
Equation (13) was developed by Purnell in 1959 [20]. From the separation ratio of the solute pair, the efficiency required to separate them can be calculated. If there is inadequate efficiency, reduce the flow rate to the optimum velocity and if, more efficiency is still required, use a longer column or a column packed with smaller particles (the latter is to be preferred as longer analyses will not arise).
