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, with each peak being separated from its neighbor by 4a. The last peak of chromatogram is rather a 

Peak capacity can be very effectively improved by using temperature programming in GC or gradient elution in LC. However, if the mixture is very complex with a large number of individual solutes, then the same problem will often arise even under programming conditions. These difficulties arise as a direct result of the limited peak capacity of the column. It follows that it would be useful to derive an equation that 

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From the Plate Theory, the peak width at the base is given by 

Consider the point where the last two peaks merge. At this point the retention volume of the last peak, minus half the peak width at the base, will equal the retention volume of the penultimate peak, plus half its peak width at the base. 

Under isocratic development, if the early peaks of the mixture are adequately separated, then the late peaks are often broad, and consequently, at concentrations so low that they are hardly detectable. Conversely, if the late peaks are eluted at a sufficiently low k values to improve detection limits, the early peaks become bunched together and are not resolved. This problem is obviated considerably by employing gradient elution, but if there are a large number of individual solutes present in the sample,then the same problems will arise. These difficulties are caused by the column having a limited peak capacity and it is, therefore, important to determine how to calculate peak capacity and how to control it. From the Plate Theory, the peak width at the base is given by - 

Consider the chromatogram In figure (5), which dlagrammatlcally represents (r) resolved peaks, each peak being separated from Its neighbor by (4o). 

If the number of peaks that can be fitted Into the chromatogram between the dead time and the time for the complete elution of the last peak fs(r), then, 

Graph of Peak Capacity against Capacity Factor 

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The curves show that the peak capacity increases with the column efficiency, which is much as one would expect, however the major factor that influences peak capacity is clearly the capacity ratio of the last eluted peak. It follows that any aspect of the chromatographic system that might limit the value of (k ) for the last peak will also limit the peak capacity. Davis and Giddings [15] have pointed out that the theoretical peak capacity is an exaggerated value of the true peak capacity. They claim that the individual (k ) values for each solute in a realistic multi-component mixture will have a statistically irregular distribution. As they very adroitly point out, the solutes in a real sample do not array themselves conveniently along the chromatogram four standard deviations apart to provide the maximum peak capacity. 

The peak capacity, n, of a single-column chromatographic system generating N theoretical plates is given by ... 

A practical method for enhancing the peak capacity, and thus the resolution of analytes in multicomponent complex mixtures, can be achieved by changing the mode of the separation during the chromatographic analysis, employing a column switching system in order to optimize a separation. 

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