Chromatographic plate number

The linear velocity of the mobile phase is an important determinant of both speed of analysis and the chromatographic plate number, which affects peak resolution. Both are critical features of the values of chromatographic analysis. 

Molar amount of product Molar mass Number of electrons Number of moles of product Projected lifetime of plant Chromatographic plate number Mass flow Electrical charge Volumetric flow rate Charge density Heat flow... 

Increasing the electrode length may, however, result in a much poorer response, as the signal to noise ratio (S N) is reduced. Recalling that the flow-through detector is on line to a HPLC column, the concentration time profile may be considered as a Gaussian distribution. The chromatographic plate number may be used to characterize the column ... 

Column Efficiency. Under ideal conditions the profile of a solute band resembles that given by a Gaussian distribution curve (Fig. 11.1). The efficiency of a chromatographic system is expressed by the effective plate number defined from the chromatogram of a single band. 

As a secondary consideration, the chromatographer may also need to know the minimum value of the separation ratio (a) for a solute pair that can be resolved by a particular column. The minimum value of (a) has also been suggested [8] as an alternative parameter that can be used to compare the performance of different columns. There is, however, a disadvantage to this type of criteria, due to the fact that the value of (a) becomes less as the resolving power of the column becomes greater. Nevertheless, a knowledge of the minimum value of (cxa/b) can be important in practice, and it is of interest to determine how the minimum value of (aA/B) is related to the effective plate number. 

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. 

Naturally, several other possibilities can be used to increase the number of dimensions. Between the first and second developments, or sample, the characteristics of the chromatographic plate or the properties of the sample can also be modified. Although interfacing of on-line OPLC with one- or two-dimensional TLC is not particularly difficult, it is not yet widely practiced. It must be concluded that full exploitation of the versatility of MD-PC is at an early state of development as a consequence several significant changes in practice might be expected in the next few years (10). 

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

The object of a chromatographic separation is to achieve satisfactory resolution of solutes in the minimum time. Resolution is influenced by the capacity factor of the solutes and the selectivity and plate number of the column. 

HETP = height equivalent to a theoretical plate. It is derived from the plate theory of distillation which is a confusing concept having no basis in fact in the context of modem chromatographic separations. Nevertheless the terms plate number and plate height are still very widely used. 

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... 

It is well known that UV detectors used in liquid chromatographs are concentration-sensitive devices. Injection of the same mass of a particular compound onto two columns with identical plate number and length but different inner diameters, will result in a higher response from the column with the smaller inner diameter. The gain in the signal is inversely proportional to the square of the ratio of the inner diameters of the two columns. The situation is different for a mass spectrometer, which is a mass-flow sensitive detector. Under constant flow conditions,... 

The peak broadening for the entire chromatographic system,. columns plus the instrument, may thus be estimated from the bandwidth contribution of each component of the system. The effective plate number of the system may then be calculated from Equation 1. 

We have seen in Eq. (2) that, once the chromatographic system is chosen, the resolution of the components of the mixture is possible only if the plate number of the column exceeds the value given by Eq. (12)... 

It is also of interest to the chromatographer to know the minimum (a) value of a pair of solutes that can be separated on a particular column. In fact, this has been suggested, (11), as a basis for comparing the resolving power of different columns. The disadvantage of this type of criteria is that the value of (a) becomes smaller the higher the resolving capacity of the column. Nevertheless, the minimum value of (a) is important in practice and it is of interest to see if it can be related to the effective plate number of the column. 

A chromatographic property of interest is the separation between spots (solutes). The most simple definition is resolution R= z ( - - z 2 H2 (J2 (5 ) with a, the bandwidth of the developed spot. Assuming ai=cj2 and using zJo= NRf [2] N is the plate number) the formula becomes ... 

However, the T-distribution permits an extension of the plate theory, which is also usable in case of asymmetric peaks. The chromatogram (1 component) is considered to be the result of a pure time delay and a T-distribution response. The procedure implies the fitting of a function f(t) given in Eq. (15) to the chromatographic peak. The asymmetry of the peak determines the new plate number n, decreasing with increasing asymmetry. 

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