Plate height principles

If we have reasoned correctly, we now have what amounts to a principle of corresponding states for chromatography. Given a reduced velocity v, the reduced plate height should be approximately the same for virtually all well-packed columns. In other words, a single h versus v curve should come close to representing all such chromatographic systems. 

The concepts of reduced velocity v and reduced plate height h are powerful ideas that allow us to compare columns to each other under a broad range of mobile-phase conditions and over a range of particle sizes. We use the principle of corresponding states to form dimensionless parameters from the HETP and the linear velocity. The HETP has the dimension of length. To make it dimensionless, we simply divide it by the particle diameten... 

Since electromatography uses a stationary phase just like in conventional LC, the principles of band broadening in CEC and LC are similar. The plate height can be expressed using a modified Van Deemter equation ... 

From an engineer s point of view it is highly desirable to describe the chromatographic system by reduced parameters. This helps to understand some very basic principles of chromatography and to compare very different systems. Reduced parameters are dimensionless. Instead of the plate height H and mobile phase linear velocity u, one defines the reduced plate height h and the reduced velocity v by the following two equations ... 

The UPLC is based on the same principle as HPLC, namely adsorption chromatography using an adsorbent of very fine size, to increase the surface area and thus adsorption. The stationary phase in UPLC consists of particles less than 2.0 pm, while HPLC columns are typically filled with particles of 3-5 pm. The underlying principles of this evolution are also governed by the van Deemter equation, which is an empirical formula that describes the relationship between linear velocity (flow rate) and plate height (HETP or column efficiency). Naturally, the column length has to be less and much higher pressure is needed to maintain percolation... 

The primary driver factor responsible for the growth and development of this technique has been the evolution of packing materials used to effect the separation. The underlying principles of this evolution are governed by the van Deanter eqna-tion, which describes the relationship between linear velocity and plate height. 

Band broadening in CEC can again be described by the van Deemter equation, and in principle all three terms contribute. However, since both the A and C terms decrease when smaller particles are used (see Equations [3.33] and [3.37]), use of such particles results in smaller plate heights. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

Figure 2 shows the principle of electrochemical Kosters laser interferometry for the determination of changes of surface energy by the resulting deformation of an elastic plate. The height AZc of the center of the plate with respect to a plane at a given radius yields Ag from the appropriate form of Hookes law... 

The bubble-cap column (Fig. 1-26) is a simple illustration of this principle. Each of the stages A, B, and C consists of a disk with a small cylinder sealed to a hole in its center and covered with a domeshaped cap. Let us consider the process of distilling through such a column after the distillation has been in progress for some time and each of the plates is filled with liquid up to the height of the cylinder. 

The following so-called dynamic capillary method was developed by Van Hunsel Joos (1987b) and complements the area of application with respect to other methods. This method allows measurements from 50 ms up about 1 s, similar to the inclined plate and growing drop techniques described above, and can be used at liquid/liquid and liquid/gas interfaces without modification. The principle of the experiment is schematically given in Fig. 5.23. Two fluids are contained in a tube of diameter R. The interface (or surface in case of studies at the water/air interface) is located in such a way that its interfacial tension can be measured by the capillary rise of the lower liquid in a narrow capillary c, which connects the both fluids. The height of the capillary rise h is determined via a cathetometer Cat. 

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