Theoretical plates, in chromatography

In chromatography there are at least three equilibria analyte/mobile phase, analyte/stationary phase and mobile phase/stationary phase. The origin of the term theoretical plate in chromatography comes from the adaptation of an older plate theory for distillation described by Martin and Synge (Nobel Prize for Chemistry, 1952). This term which is universally used for historical reasons, has no physical significance. It may have been preferable to call it a Tswett ... 

Williamson and Craig showed that the fraction of solute contained in the various tubes in the two phases can be expressed by the terms of the binomial expansion [1/(D + 1) + DI D + 1)]", where the first term is the fraction unextracted and the second is the fraction transferred. A similar expansion had been used earlier by Martin and Synge to represent the amounts of sample in succeeding theoretical plates in chromatography. 

The concept of theoretical plates in chromatography is described elsewhere, and is not repeated here (18,19). Pieri et al. have described the application of the theoretical plate concept to column scale-up and demonstrated its utility in scale-up of pheromone separation over silica gel (15). We have found this same approach to be useful in separations over ion exchange resins used as chromatographic supports. In essence, this approach gives an estimate of column length for a given separation if the particle size is changed. This is based on empirical correlation of the number of theoretical plates, JV, ... 

The correct answer is to minimize those contributions to peak broadening These factors are interpreted in terms of contributions to the height equivalent to a theoretical plate (HETP). This line of reasoning leads to the need to define a HETP that, in turn, requires that we introduce the concept of a theoretical plate in chromatography. So let us get started ... 

Other definitions of HTU and NTU can be based on other forms of the overall mass transfer coefficients. The use of transfer units is a rough parallel with the use of stages in distillation or the term theoretical plates in chromatography. As such, it is a historical genuflection by the more recent absorption analyses in the direction of the older equilibrium stage separation analysis. 

Temperature Changes During the Passage of a Solute Through a Theoretical Plate in Gas Chromatography... 

HETP, height equivalent to a theoretical plate The length of a chromatography column divided by the number of theoretical plates in the column. 

There is much interest in high-efficiency- and high-speed separation media for liquid chromatography. The plate numbers available in practice have been in the range of 10,000-30,000 in HPLC for 20 years or so, but these are low compared to well over 100,000 theoretical plates in capillary gas chromatography or in capillary electrophoresis. This is caused by the limitation in the use of small-sized particles for HPLC, where a particle-packed column is commonly used under a pressure-drop of up to 40 MPa. An increase in column efficiency by using small particles, which is the approach taken in the past, is accompanied by an increase in the pressure-drop, as expected from Eqns. 5.2 and 5.3, below. Eqns. 5.1-3 describe the efficiency (plate height) and flow resistance of a column packed with particles [1-3], where N stands for the... 

In the form of equation (2-8) the definition of // is exactly identical to the plate height as it evolved from the distillation theory and was brought to chromatography by Martin and Synge [2]. If // is the theoretical plate height, we can determine the total number of the theoretical plates in the column as... 

Extraction chromatography (reversed phase partition chromatography) has been used in analytical and biochemistry to effect chemical separations. It is a method which combines the simplicity of ion exchange and the selectivity of solvent extraction. Ion exchange theory may be used to calculate the number of theoretical plates in the column and the enrichment coefficient. Extraction chromatography as a separation method has been recently reviewed by Cerrai (J) and Katykhin (7). 

Chromatography is a powerful separation method because it can be carried out easily under experimental conditions such that the two phases of the system are always near equilibrium. This is because the kinetics of the mass transfers between these phases is usually fast. The separation power of a column, under a given set of experimental conditions, is directly a function of the rate of the mass transfer kinetics and of the axial dispersion coefficient. The scientists involved in the development of stationary phases for chromatography have produced excellent packing materials that permit the achievement of a very large number of equilibrium stages (i.e., theoretical plates) in a column. Thus, as we show later in Chapters 10 and 11, the thermodynamics of phase equilibria is often the main... 

Almost all these recent studies have been concerned with the variation of sample distribution between bed and solvent as a function of separation conditions and sample molecular structure. The problem of the number of theoretical plates in the bed (as a function of separation conditions) has been approached from other directions. In the field of gas chromatography the experimental and theoretical unraveling of this problem proceeded quite rapidly after 1955. At the present time our knowledge in the latter area is beginning to have application to liquid chromatographic separations, since the underlying theory for both systems is basically similar. Recent experimental studies of bed plate numbers in... 

---