Plate number measurement

The characteristic chromatographic parameters that describe the chromatographic behavior of stationary phases and that therefore are suitable criteria for comparison purposes, are the plate number (measure of the bond broadening), the retention factor (measure of the strength of interactions of an analyte under defined conditions), and the separation factor (measure of the capability of the chromatographic system to separate two analytes under defined conditions). In addition, the asymmetry factor is frequently used. 

The resistance due to a circular junction is given by / = /2ak, where a is the radius of the junction and k is specific conductivity of the metal. For the case of two steel plates, the measured resistance is 5 x 10" Q for a load of 50 kg the yield pressure of steel is 60 kg/mm, and the specific resistance is 5x 10 Q/cm. Calculate the number of junctions, assuming that it is their combined resistance that is giving the measured value. 

The peak broadening measured for a plate number characterization is the sum of the variances a) for the column and the chromatographic equipment used ... 

The Waters company recommends a system check of the chromatographic equipment that is used for plate number determination and analyses (2) the columns in the GPC unit used are replaced by a zero dead volume union. Then the test sample is injected under the same conditions such as a plate number determination. The 5a peak width measured on a suitable recorded peak is evaluated this 5or width of a 20-/a1 injection should be lower than 150 /a1. 

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 result of this equation describes the quality of the separation on the basis of an ideal size exclusion mechanism with a given pore volume distribution. The quality of the packing is deliberately excluded from this consideration. This parameter should be measured separately and judged by the plate number. The ASTM standard method for HPSEC of polystyrene (4) contains the following equation for resolution (R,) ... 

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 ... 

Plate number The number of theoretical plates in a chromatographic column. This is a measure of the efficiency of the column. 

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 ... 

As the column becomes more efficient, N gets larger and H gets smaller. The plate number is dimensionless, but H has units of length, and is usually measured in mm or fim. There are other ways in which N can be measured, depending on whereabouts on the peak we choose to measure its width. Another way is ... 

Neither the capacity factor nor the separation factor take into account the effect of dispersion, which is measured by the plate number or the plate height of the column. These were defined in Section 2.3.2. 

When the column is ready to be used, the chromatogram of a suitable test mixture should be obtained. The plate number and retention times of the test solutes should be noted, and the peaks should have a satisfactory shape (minimal tailing). For measurement of the plate number, the recorder should be used at a high chart speed. Fig. 5.1b(i) and (ii) show test chromatograms for a C-18 column prepared by the above method, and Fig. 5.1c and 5.Id show the data that you should report with the chromatogram. The retention for an unretained peak is taken as the small baseline disturbance just before the first peak. 

As readily observed in most chromatograms, peaks tend to be Gaussian in shape and broaden with time, where W, becomes larger with longer This is caused by band-broadening effects inside the column, and is fundamental to all chromatographic processes.The term, plate number (N), is a quantitative measure of the efficiency of the column, and is related to the ratio of the retention time and the standard deviation of... 

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... 

Assuming an exclusion volume of 5 ml per column allows construction of Table I from Equation 1. Table I lists the bandwidth in microliters as a function of column plate number and the number of columns in series. The data assume that the plate number may be generated at total exclusion, as well as at total permeation actual measurements made using the smaller pore size column si ibstantiate this. 

Equation (2) shows that the resolution is a function of three different factors (1) the resolving power of the column as measured by the plate number that expresses the relative width of bands (2) the relative retention of the two compounds that measures how far apart the bands are from each other and (3) the magnitude of retention, as separation is a result of retention. The relative influence of these factors has been discussed by Snyder (72,13) in a form very easy to use in practice. 

Column efficiency could be measured in terms of plate number, N i.e.,... 

To compensate for, what appeared to be very misleading efficiencies values, the effective plate number was introduced. The effective plate number uses the corrected retention distance, as opposed to the total retention distance to calculate the efficiency. Otherwise the calculation is the same as that used in the normal calculation of theoretical plates. In this way the effective plate number becomes significantly smaller than the true number of theoretical plates for solutes eluted at low k values At high k values, the the two measures of efficiency tends to converge. In this way the effective plate number appears to more nearly correspond to the column resolving power. In fact, it is an indirect way of trying to define resolution in terms of the number of effective plates in the column. 

Measure the width of the 35C1 peak at half-height in Figure 26-31 and calculate the plate number. The capillary was 40.0 cm in length. Find the plate height. 

Efficiency of column. Usually measured by column theoretical plate number. Relates to peak sharpness or column performance. 

Standard Deviation Terms Measurements Plate Number n=... 

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