Separating efficiency of a column

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]... 

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. 

The equation is used to calculate Tmin and coating efficiency, CE. The latter is used to indicate the separating efficiency of a column compared to the theoretical efficiency ... 

The number of separating stages found with a test mixture depends on its equilibrium curve and the conditions of distillation maintained, and it is obviously also dependent on the dimensions of the column. If the manufacturer of a column merely quotes a number of theoretical stages, without further information, the statement is of little vahie. Only if the data in Table 27 are available can the separating efficiency of a column be fully assessed and the reported figure be reproduced. 

To find the separating efficiency of a column under operating conditions, the procedure described in section 4.10.4 for determining stage equivalents should be followed. 

A somewhat more realistic measure of the separation efficiency of a column is the effective theoretical plate number N pp ... 

The plate number A/ is a compound-specific measure (it, therefore, applies to each individual peak) for the separation efficiency of a column under clearly defined mobile phase and temperature conditions. It will change over the column lifetime and can also be influenced by the HPLC instrument. The plate number increases proportional to the column length L, provided all other conditions remain constant (except for the colunm pressure). It also increases at constant column length when the stationary phase particle size, particle architecture, or bonding chemistry is optimized in way that accounts for less band dispersion. The respective columns exhibit increased separation efficiency per unit column length. Once the plate number of a column or method is increased, one can separate more analytes or separate analytes with better resolution under otherwise constant conditions. The formula to calculate the plate number from the peak parameters retention time and peak width at base Wi, or peak width at half height Wj, is as follows ... 

In order to maintain good column performance the separation efficiency of SEC columns should be checked regularly. Because some column manufacturers do not test columns individually prior to shipping, a new column should always be tested before first use. All PSS SEC columns are tested individually before they are shipped. PSS delivers all columns in the solvent the user wants to run it in and tests the column using these conditions. This guarantees maximum certainty for the user to receive exactly what they pay for. Additionally, the risk of reconditioning columns from one to another solvent is taken over by PSS as the manufacturer. 

Normally a calibration curve—molar mass against the total retention volume—exists for every GPC column or column combination. As a measure of the separation efficiency of a given column (set) the difference in the retention of two molar masses can be determined from this calibration curve. The same eluent and the same type of calibration standards have to be used for the comparison of different columns or sets. However, this volume difference is not in itself sufficient. In a first approximation the cross section area does not contribute to the separation. Dividing the retention difference by the cross section area normalizes the retention volume for different diameters of columns. The ISO standard method (3) contains such an equation... 

Injection systems of a capillary gas chromatography should fulfil two essential requirements (i) the injected amount should not overload the column (ii) the width of the injected sample plug should be small compared with band broadening due to the chromatographic separation. Good injection techniques are those which achieve optimum separation efficiency of the column, allow accurate... 

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... 

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. 

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 ... 

As with any form of chromatography, the separating efficiency of capillary columns in gas chromatography is strongly dependent on the column stationary phase, carrier gas flow rate, and temperature. Because of the high separation efficiency of capillary columns, only a limited number of stationary phases can be substituted for the numerous phases used in most packed column applications. The choice of a stationary phase is commonly dictated by experience. A phase that has been successfully used by others is usually a good choice. Fre-... 

In GC and LC the adsorbent is fixed into a cylinder that is usually made of glass, polymer or stainless steel (column). In this column the adsorbent is present as a porous or non-porous randomly arranged packing or as a monolithic block. Because of the high separation efficiency of packed columns made of small particles this type of chromatography is called high-performance liquid chromatography (HPLC). 

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. 

It follows that the lowest dilution or the highest peaks are obtained if a short and, most importantly, thin column with as high a theoretical plate number as possible, is used. The origin of the separation efficiency of the column must come from its fine,... 

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