Plate height plates, number

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

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. 

Another example is the confusion between the terms efficiency, efficacy, and selectivity of a separation. It is not always mentioned how these responses are obtained. Usually, the efficiency is either expressed as the number of theoretical plates or plate number, N, or as the height equivalent to a theoretical plate or plate height, HETP or With efficacy, usually efficiency, thus plate count, is meant, although it is not always clearly specified. The selectivity, in general, is found by calculating the selectivity or separation factor, a, which is a measure for the... 

Height equivalent to a theoretical plate. h h (reduced plate height). The number obtained by dividing the column length by the theoretical plate number. 

Using the van Deemter equation, estimate the plate height and number of theoretical plates in a 1-m long column using particles of diameter 0.014 cm and a helium flow velocity of 10 cm/s. Ignore gas compression effects. Previous work with the column has shown that the parameters A and y are 1.0 and 0.70,... 

The genesis of the terms plate height" and "number of thcoreiical plates is a pioneering theoretical study of Marlin and Synge in which they treated a chromatographic column as if it were similar to a distillation column made up of numerous discrete but contiguous narrow layers called iheoretical plates. At each plate, equilibration of the solute between the mobile and stationary phases was assumed to take place. Movement of the solute down the column was then treated as a stepwise transfer of equilibrated mobile phase from one plate to I he next. 

Same as theoretical plate height. A number characterizing the quality of the column filling. It is expressed as the length of a column that would be equivalent to one theoretical plate determined by the plate theoiy. 

Same as height equivalent to a theoretical plate. Theoretical plate number is a number that characterizes the separation efficacy of the column. The higher this number the narrower are the peaks in the chromatogram. 

Band Asymmetry. The peak asymmetry factor AF is often defined as the ratio of peak half-widths at 10% of peak height, that is, the ratio b/a, as shown in Fig. 11.2. When the asymmetry ratio lies outside the range 0.95-1.15 for a peak of k =2, the effective plate number should be calculated from the expression... 

In 1972-1973 Knox et al. [3, 4, 5] examined, in considerable detail, a number of different packing materials with particular reference to the effect of particle size on the reduced plate height of a column. The reduced plate height (h) and reduced velocity (v) were introduced by Giddings [6,7] in 1965 in an attempt to form a rational basis... 

Now, the column length (L) can be defined as the product of the minimum plate height and the number of theoretical plates required to complete the separation as specified by the Purnell equation. 

FIGURE 14.1 Determination of the theoretical plate number N by the half-height method. 

The peak measured for a plate number determination contains additional information about the packing quality of a column. The same peak may also be used to quantify information about the shape as well. The peak width on both sides of the perpendicular through the peak maximum is measured at a height of 10% of the maximum height (see Fig. 14.2). The quotient of the back by the front part of the peak is defined as the asymmetry factor (AF) ... 

The point of all this is simply that we must not use the apparent plate height or the apparent plate number as performance criteria in the unified chromatography techniques on the justification that they already work well for LC and that they work well for GC when a pressure correction is applied. A considerable expansion of theory and an effective means for evaluating equations (7.4) or (7.5) are required first. Likewise, as we consider multidimensional chromatography involving techniques existing between the extremes of LC and GC, we must not build judgments of the multidimensional system on unsound measures of the individual techniques involved. 

The column performance (efficiency) is measured either in terms of the plate height (H), the efficiency of the column per unit length, or the plate number (N), i.e. the nnmber of plates for the column. This number depends upon the column length (L), whereas the plate height does not. The mathematical relationships between the nnmber of plates, the retention time of the analyte and the width of the response is shown in the following equations ... 

The reduced plate height, h, is defined as the number of particles to a theoretical plate and is given by... 

The plate height, and thus the total number of theoretical or effective plates, depends on the average linear carrier gas velocity (van Deemter relationship) and, for a particular carrier gas, the efficiency will maximize at a particular flow rate. Only at the optimum carrier gas flow rate are n, N, and HETP Independent of the column length. The efficiency will also depend on the column diameter (see section 1.7.1) where typical values for n, N, and HETP for different column types can also be found. Values for n, N, and HETP are reasonably independent of temperature but may vary with the substance used for their determination, particularly if the test substance and statioKary phase are not compatible. 

In modern TLC the distribution of sample within a spot is essentially Gaussian and the number of theoretical plates (n, ) and the plate height (H,, ) observed can be conveniently expressed ]sy equation (7.7) and (7.8)... 

The more efficient the column, the smaller will be at a given value of Vr. To measure efficiency, we use quantities called the plate number (N) or the plate height (H) of the column, which are defined as follows ... 

Manufactured hplc columns have about 50 000 plates m 1 if packed with 5 fim particles and about 25 000 plates m-1 if packed with 10 particles, so that from a 12.5 cm column with a 5 fim packing we would expect a plate number of about 6500, corresponding to a plate height of 0.02 mm. Whether or not these high efficiencies are required depends on the sort of work you are doing a great deal of routine work in hplc is done at efficiencies far lower than this. 

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