Plate Theory and Extensions - The Peak Capacity of a Chromatographic Column > Page 91



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. Conversely, if the chromatographic conditions are changed so that the late peaks are eluted at lower (k') values to improve detection limits, the early peaks then merge together and are not resolved.

Peak capacity can be very effectively improved by using temperature programming in GC or gradient elution in LC. Nevertheless, if the mixture is complex containing many solutes, then the same problem may arise even under programming conditions. These difficulties are caused by the limited peak capacity of the column. If an equation is derived that describes peak capacity in terms of column and solute properties, it would also disclose how the peak capacity might be controlled.

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