Peak Capacity of a Column

Peak capacity can be effectively improved by using temperature programming in GC (Section 4.4.3b) or 

This estimate for Cp is a lower limit since the last detected peak will he broader than the earlier ones, so many more peaks can he fitted in than suggested by this simple expression. 

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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... 

However, with practical samples the way the (k ) values of the individual components for any given complex solute mixture are distributed is not predictable, and will vary very significantly from mixture to mixture, depending on the nature of the sample. Nevertheless, although the values for the theoretical peak capacity of a column given by equation (26) can be used as a reasonable practical guide for comparing different columns, the theoretical values that are obtained will always be in excess of the peak capacities that are actually realized in practice. 

The above-mentioned equation for n in fact is only valid for isocratic separations and if the peaks are symmetric the peak capacity is larger with gradient separations. Tailing decreases the peak capacity of a column. In real separations the theoretical plate number is not constant over the full k range. However, it is even more important to realize that a hypothetical parameter is discussed here. It is necessary to deal with peaks that are statistically distributed over the accessible time range. The theory of probabilities allows us to proceed from ideal to near-real separations. Unfortunately, the results are discouraging. 

In isocratic mode, the peak capacity of a column, P, is the theoretical number of peaks that can be separated with a given resolution within a given analysis time. In isocratic chromatography, it is given by the following relationship (40,41) ... 

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... 

One final quality parameter that is sometimes specified in test methods is the peak capacity of the column set. In the literature of the OECD, this peak capacity is required to have a value of 6.0 or greater. This peak capacity is defined by Eq. (2) ... 

Another approach to defining the separation capacity of a column is by its peak capacity (the number of peaks than can be resolved at any specific resolution, usually R, i, in a given separation time). For SEC the pe2dc capacity, PCgc, is given approximately by... 

The peak capacity of a given column system is given by equation (15) in chapter (5) on page (69) and is reiterated here,... 

The capacity or retention factor (k) is a measure of retention of a sample component. It should not be confused with the loading capacity of a column, which is expressed as milligrams of sample bound per milliliter of gel and represented by the area under a peak. The capacity factor may be calculated for any individual peak in a chromatogram. For example, the capacity factor for peak 2 in Figure B4.2.4 is derived from Equation B4.2.3, where Vjq is the elution volume of peak 2 and Vm is the volume of the mobile phase (i.e., the total bed volume). 

Resolution, R, is dependent not only on the selectivity and the capacity of a column but also on the efficiency of the system. Typically, an R value greater than 0.8 is required for accurate quantitation of two peaks. The most effective way to alter resolution is to change either the selectivity or the capacity of the column. 

Since there is no selectivity (a) involved in GPC, the ability to separate a pair of compounds depends upon the calibration curve and the efficiency of the column(s). One measure of the ability of the GPC column(s) to separate is the peak capacity of the column, which is defined as the number of resolvable peaks per chromatogram, n. The peak capacity (16) for a GPC column used in the analysis of small molecules is related to the number of theoretical plates (N) according to the equation... 

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