Plate height peak capacity

Resolution, like the separation factor, differs for each specific component pair and therefore fails as a global criterion of separation. For analytical separations, more universal criteria have evolved, such as plate height, number of plates, rate of generation of plates, and peak capacity (Chapter 5). While these indices differ somewhat from one component to another, they effectively establish a ballpark figure of merit for different systems and different conditions of operation. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Therefore, a 4a separation (R = 1), in which peak retention times differ by four times the width at half-height, corresponds to a 2% area overlap between peaks.1 The maximum number of peaks that could be separated in a given time period assuming a given value of R, is defined as the peak capacity.1 The peak capacity must be greater — usually much greater — than the number of components in the mixture for a separation to succeed. The resolution of two compounds can also be written in terms of the number of plates of a column, N, the selectivity, a, and the capacity factors, k, and k j, as12... 

It follows, that if the detector was to be effective and produce the true Gaussian form of the eluted peak then, (Ca), must at all times be unity and consequently the detector must have the same plate capacity as that of the column. This means that the detector must employ the same absorbant, have the same geometry and be packed to give the same plate height as the column. It is obvious to accomplish this, the column must also be the detecting cell and the temperature sensing element must be placed in the column packing itself. 

Choose a peak with a capacity factor greater than 5 when you measure plate height for a column. 

Although the component zones are Gaussian, plate height expressions are meaningless because the zones (and the resulting separation structure) do not evolve continuously as a result of steady migration. However, resolution and peak capacity are still valid indices of separation, as discussed below. 

Partition coefficient, 9, 10 Partition ratio, 11 time optimization of, 57-58 Peak, definition of, 69 Peak capacity, 18, 19 Pellicular supports, 157 Permeability, 63-64 Phase selection diagrams, 218-219 Phase volume ratio, 11 Pinkerton (ISRP) columns, 225-226 Plate height, 17 Plate number, 14-16 Plate theory, 3, 28 Polarity index, 210, 211 Pore size of LC supports, 157 Porosity, 27 Precision, 99-100 Preparative scale ... 

As is evident from the preceding discussion, the retention behavior of a polypeptide or protein P- expressed in terms of the capacity factor k is governed by thermodynamic considerations. Peak dispersion, on the other hand, arises from time-dependent kinetic phenomena, which are most conveniently expressed in terms of the reduced plate height he, . When no secondary effects, i.e., when no temperature effects, conformational changes, slow chemical equilibrium, pH effects, etc. occur as part of the chromatographic distribution process, then the resolution Rs, that can be achieved between adjacent components separated under these equilibrium or nearequilibrium conditions can be expressed as... 

Resolution, Theoretical Plate Heights and Peak Capacity. 86... 

A chromatographic peak should be narrow and Gaussian in nature. In practice, the peaks are often broad and non-Gaussian the more time the solute spends in a column, the broader the peak. The performance of a chromatographic system is described in terms of a number of parameters, including capacity factor, selectivity factor, plate height, plate number, and resolution. 

Also, peak height is inversely proportional to peak width, so, as the particle size decreases to increase plate numbers and subsequently the resolution, an increase in sensitivity is obtained, since narrower peaks are taller peaks. Narrower peaks also mean more peak capacity per unit time. Efficiency is proportional to column length and inversely proportional to the particle size [45,48]. Therefore, the column can be shortened by the same factor as the particle size without loss of resolution. 

B=l, C= 6. Using the known relationship between plate count and theoretical plate height, we obtain the peak capacity ... 

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