Primary equilibrium, liquid chromatography

Figure 4.17 General phenonenaloglcal retention model for a solute that participates in a secondary chemical equilibrium in liquid chromatography. A - solute, X - equilibrant, AX analyte-equilibrant coeplex, Kjq - secondary chemical equilibrium constant, and and are the primary distribution constants for A and AX, respectively, between the mobile and stationary phases.

Certainly one of the major advantages of the aqueous-based mobile phases used in reversed-phase liquid chromatography is the ability to control secondary chemical equilibria. In liquid chromatography the primary equilibrium is the distribution of the solute between the stationary and mobile phases. Any other equilibrium involving the solute in the mobile or stationary phases is considered secondary. These secondary equilibrium processes change the chemical form of the solute, and can be used advantageously to change retention of solutes that... 

The basic mechanism of separation in MLC is fairly well understood and there is a reasonable theoretical foundation on which to build. MLC is a fascinating example of the use of a secondary chemical equilibrium in liquid chromatography. The primary equilibrium is the partitioning of the solute between bulk mobile phase and stationary phase, and the secondary equilibrium is the partitioning to micelles. This secondary equilibrium is affected by a great variety of parameters type and eoncentrations of surfactant and additives such as salts or organic modifiers (for instance, alcohols), and pH. The current knowledge on MLC interactions is exposed... 

The term column switching in liquid chromatography is used if two or more columns are connected to form a network. Column switching is a technique that changes the direction of flow of the mobile phase by valves, so the effluent from a primary column passes to a secondary column for a defined period of time as illustrated in Fig. 2. The objectives of column switching are to increase the chromatographic resolution and selectivity, to enrich trace amounts of sample, to protect sensitive detectors such as electrochemical detectors from contamination by coextractives, to prevent destabilization of the chromatographic equilibrium of the column by coextractives, and to achieve further objectives or a combination of... 

The retention of an eluite in liquid chromatography is based upon the distribution of the eluite between the stationary and mobile phase (primary equilibrium). By convention, any other equilibria that takes place in the mobile phase, or stationary phase, or both, are considered secondary . In the past, manipulation of the mobile-stationary phase equilibrium distribution of the solute by using secondary chemical equilibria (SCE) was widely utilized in order to overcome low column efficiencies [32]. In spite of the fact that we now have columns with higher inherent efficiencies, SCE is still a widely practiced technique. The wide range of different chemistries that can be used to alter the mobile-stationary phase equilibrium to achieve better resolution or... 

Several techniques are available for measuring values of interaction second virial coefficients. The primary methods are reduction of mixture virial coefficients determined from PpT data reduction of vapor-liquid equilibrium data the differential pressure technique of Knobler et al.(1959) the Bumett-isochoric method of Hall and Eubank (1973) and reduction of gas chromatography data as originally proposed by Desty et al.(1962). The latter procedure is by far the most rapid, although it is probably the least accurate. 

Chromatography, the process by which the components of a mixture can be separated, has become one of the primary analytical methods for the identification and quantification of compounds in the gaseous or liquid state. The basic principle is based on the concentration equilibrium of the components of interest, between two immiscible phases. One is called the stationary phase, because it is immobilized within a column or fixed upon a support, while the second, called the mobile phase, is forced through the first. The phases are chosen such that components of the sample have differing solubilities in each phase. The differential migration of compounds lead to their separation. Of all the instrumental analytical techniques this hydrodynamic procedure is the one with the broadest application. Chromatography occupies a dominant position that all laboratories involved in molecular analysis can confirm. 

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