Efficiency of a column

An alternative to determining packed height is through the use of an empirical term, height equivalent to a theoretical plate (HETP). This term can be measured in a fashion similar to that used for the overall plate efficiency of a column (eq. 44) ... 

The efficiency of a column (n), in number of theoretical plates, has been shown to be given by the following equation,... 

Height equivalent to a theoretical plate (HETP) A measure of the efficiency of a column usually expressed in millimeters. HETP — LIN, where L is the length of a column and N is the number of theoretical plates. The reciprocal of HETP is also used to describe efficiency and is often expressed by the terms plates per meter. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

The peak width relates to retention time of the peak and the efficiency of a chromatographic column. The efficiency of a column is directly related to the total number of theoretical plates (N) offered by it. 

Performance Inside a column the concentration of a band of solute decreases as it moves through the system. The column performance or the efficiency of a column entirely depends on the amount of spreading that takes place. The measurement is represented in Figure 30.6, below ... 

The efficiency of a column can be assessed in a similar manner to that described for HPLC and values for the resolution index of two solutes, the number of theoretical plates and the height equivalent to a theoretical plate may also be calculated. Although it is easier to measure gas pressure, it is the actual gas flow, which is affected by the particle size and compression of the packing, that should be used in column assessment investigations. 

If the correctly sized flow cell and connecting tubing are not used, the high efficiency of a column or high theoretical plate number columns cannot be effectively used. The detector cell volume contributes hold-up volume. The larger is the cell volume, the greater the peak broadening. The cell volume... 

Theoretical Plate In a distillation column, it is a plate onto which perfect liquid-vapor contact occurs so that the two streams leaving are in equilibrium. It is used to measure and rate the efficiency of a column at separating compounds. The ratio of the number of theoretical plates to the actual number of plates required to perform a separation is used to rate the efficiency of a distillation column. Actual separation trays in refinery distillation units are usually less effective than theoretical plates. 

Equation (8) describes the relationship between the efficiency of a column in theoretical plates and the efficiency given in effective plates. It is also seen that the calculation of the number of effective plates in a column does not provide an arbitrary measure of the column performance, but is directly related to the number of theoretical plates In the column as defined by the plate theory. It should be noted that as (k ) becomes large, (n) and (N) converge to the same value. 

COLUMN EFFICIENCY. Two methods are available for expressing the efficiency of a column in terms of HETP one is to measure the peak (Figure 1.18) width at the baseline (Equation 2.55) the other is to measure the peak width at half-height (Equation 2.57). 

SEPARATION FACTOR. The reader will recall that the separation factor, a, in Section 2.1.4, is the same as the relative volatility term used in distillation theory. In 1959, Purnell (32,33) introduced another separation factor (S) term to describe the efficiency of a column. It can be used very conveniently to describe efficiency of open tubular columns ... 

Efficiency of the chromatographic system can be determined from the number of theoretical plates per meter. Although this term primarily describes the property and resolution efficiency of a column, other extra column variables, such as the... 

The options we enumerate for improving efficiency of a column are by no means exhaustive or unique. We simply state them here to alert the reader of possible improvement options. We note that "cookbook" techniques to improve efficiencies are foredoomed to obsolescence since technology advances and they do not capture "out-of-the-box" solutions, which can shift paradigms. 

The efficiency of a column used for HPLC describes the ability of the column to produce sharp, narrow peaks. Typically, the efficiency is represented at the plate number, N. The plate number can be estimated by... 

The efficiency of a column is a number that describes peak broadening as a function of retention, and it is described in terms of the number of theoretical plates, N. Two major theories have been developed to describe column efficiency, both of which are used in modern chromatography. The plate theory, proposed by Martin and Synge,31 provides a simple and convenient way to measure column performance and efficiency, whereas the rate theory developed by van Deemter et al.32 provides a means to measure the contributions to band broadening and thereby optimize the efficiency. 

The efficiency of a column, N, is a number that describes peak broadening as a function of retention and is dependent on the entire chromatographic system. The most common method for calculating N is the tangent method. 

The separation efficiency of a column for liquid chromatography and the relation with the mobile-phase velocity (u) can be described by the Van Deemter equation, which in lumped terms reads [2]... 

For a given type of stationary phase, the efficiency of a column is mainly determined by the column length and the packing procedure. The quality of a packing technique can easily be derived from well-defined parameters (i) the... 

In theory a theoretical plate is equivalent to the length of a column necessary to allow one equilibration of the solute to occur between the stationary and the mobile phases. To increase the efficiency of a column, the number of theoretical plates is increased. In practice, this can be accomphshed by increasing the lengtli of the column. It should be remembered, however, that, as shown in equation (3), resolution is proportional to the square root of column efficiency. Therefore doubling the column length would only result in a 40% increase in resolution. 

In Section 6B-2, we pointed out that the breadth of a Gaussian curve is described by the standard deviation a and the variance chromatographic bands are usually Gaussian and because the efficiency of a column is reflected in the breadth of chromatographic peaks, the variance per unit length of column is used by chromatographers as a measure of column efficiency. That is, the column efficiency H is defined as... 

The influence of the different mass transfer parameters on the overall efficiency of a column is shown in Fig. 2.11, where the efficiency represented by the plate height is plotted versus the mobile phase velocity. 

The number of theoretical plates N is a measure of the peak broadening of a solute during the separation process (for definitions see Chapter 2). The efficiency of a column can be given for any solute of a test mixture but is strongly dependent on the retention coefficient of the solute. 

Two sample components may only be separated from each other if their k values differ. The effective plate number, NefT, or the effective plate height, Heis used to describe the separation efficiency of a column. 

Plate number The plate number, or the number of theoretical plates is widely used to described the efficiency of a column, it can be expressed as ... 

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