nbsp;   The Peak Capacity of a Chromatographic Column The peak capacity of a column has been defined as the number of peaks that can be fitted into a chromatogram between the dead point and the 'last peak', each peak being separated from its neighbor by 4s. The 'last peak' of  chromatogram is vague term because it depends somewhat on a number of unrelated factors such as the detector sensitivity and the column efficiency. As a result, the 'last peak' can be either arbitrarily specified or defined by the properties of the column and/or the chromatograph with which it is used. Limited peak capacity can be a serious problem in the analysis of multi-component mixtures if the capacity of the chromatogram is insufficient to contain all the peaks discretely. Isocratic development results in the early peaks being adequately separated and the late peaks very broad and eluted at concentrations is so low that they can hardly be detected.

the peak capacity. Davis and Giddings (28) have pointed out that the theoretical peak capacity is an exaggerated value of the true peak capacity. The individual (k') values for each solute in a realistic multi-component mixture will have a statistically irregular distribution. They very adroitly point out, that the solutes in real samples do not array themselves conveniently along the chromatogram four standard deviations apart to provide the maximum peak capacity. Nevertheless, the theoretical peak capacity values given by equation (77) can be used as a reasonable practical guide for comparing different columns although, in practice the theoretical values for peak capacity may never be realized. Courtesy of the Journal of Chromatographic Science   Figure 25. Graph of Peak Capacity against Capacity Ratio It is clear from Figure 25 that any property of the chromatographic system that limits the magnitude of (k') must also limit the peak capacity. One such property, that

k') Retention Time (min.) Peak Capacity 10-6 (g/ml) 220 73.6 134 10-7 (g/ml) 2,200 736.0 177 10-8 (g/ml) 22,000 7360.0 194 Table 1 shows that increasing the minimum detectable concentration by an order of magnitude (e.g., from 10-7 g/ml to 10-8 g/ml) the maximum capacity ratio is also increased by an order of magnitude. However, this only produces an increase in peak capacity of 10%. In addition, the small increase in peak capacity is realized at the expense of a retention time that is increased from twelve hours to about five days. Attempting to increase the peak capacity of a chromatographic system by using higher detector sensitivity is very costly in time. Increased peak capacities are best achieved by constructing columns of high intrinsic efficiency

provides the optimum selectivity. Mass overload also produces asymmetric peaks with a sharp front and a sloping tail which can be advantageous for the production of high purity fractions by peak cutting. This advantage is illustrated in the last example given. The first peak can be collected as a fraction up to a point just before the sharp front of the second peak starts. This will produce a very pure fraction of the first peak The second peak is re-run in an identical manner and as the first peak is now an impurity (and therefore present at a low concentration) it is not overloaded and, therefore, will be eluted as a symmetrical peak in front of the second main peak. If required, the second peak can be collected just after the trace of the first peak is eluted and will also be extremely pure. The first fraction of the second separation, still containing a mixture of both peaks, although containing a very small percentage of the total mixture can be recycled if considered appropriate

nbsp;                          (76) Rearranging to provide an expression for (r )                          (77) It is clear from equation (77) that the peak capacity is controlled by the column efficiency and the capacity ratio of the last eluted peak.  The peak capacities of a series of columns having different efficiencies were calculated for a range of peak capacity ratios employing equation (77). The results are shown as curves relating peak capacity to capacity ratio in Figure 25

;        (55) Equation (55), developed by Purnell (13) in 1959 can be used to calculate the efficiency needed to separate a given solute pair from the capacity factor of the first eluted peak and their separation ratio. It is very useful in column design. This derivation utilizes, (kA), the capacity ratio of the first eluted peak but A similar equation which takes a slightly different form, can be derived using (kB), the capacity ratio of the second peak. Both equations give the same numerical result.   Figure 18. Graph of Log Efficiency against Capacity Factor for Solute Pairs Having Different Separation Ratios