Liquid-solid chromatography applications

Example of the application of liquid-solid chromatography to the analysis of amphetamines. (Chromatogram courtesy of Alltech Associates, Inc. Deerfield, IL). 

In adsorption chromatography the mobile phase is usually a liquid and the stationary phase is a finely-divided solid adsorbent (liquid-solid chromatography). Separation here depends on the selective adsorption of the components of a mixture on the surface of the solid. Separations based on gas-solid chromatographic processes are of limited application to organic mixtures. The use of ion-exchange resins as the solid phase constitutes a special example of liquid-solid chromatography in which electrostatic forces augment the relatively weak adsorption forces. 

Alternatively, Jaroniec and Martire have described liquid-solid chromatography in terms of classical thermodynamics (82). They show that a rigorous consideration of solute and solvent competitive adsorption in systems with a nonideal mobile phase and a surface-influenced nonideal stationary phase leads to a general equation for the distribution coefficient of a solute involving concurrent adsorption and partition effects. This equation is phrased in terms of interaction parameters and activity coefficients, which would need to be evaluated or estimated in actual applications. 

This technique, in its most popular application, is a modification of reversed phase liquid-solid chromatography. It is based entirely on concentration equilibrium and can be used to separate highly polar materials with a nonpolar surface. A counter ion to the ion desired to be separated is added to the mobile phase along with a buffer to maintain ionic strength and pH. A "paired ion" is formed that is neutral and can be separated from other similar compounds by a normal reversed phase column. A diagram of how this is done is shown in Figure 19-6. 

Sample retention volumes in GSC and liquid-solid chromatography (ESC) are obviously interrelated. Numerous experimental studies have demonstrated a general similarity of separation order in corresponding GSC and LSC systems [for a review see Ref. (5)]. Since liquid phase interactions have been shown to be unimportant in most LSC systems, insofar as sample K" values are concerned, the relationship between retention volumes in GSC and LSC should assume a relatively simple form. At first glance it might appear that Eq. (8-3) is directly applicable... 

A number of mathematical relationships have been presented in Chapters 6, 8, 10, 11, and 12 for the calculation of sample K" values in liquid-solid chromatography. Additional equations have been given for calculating several related chromatographic quantities. In this appendix we shall provide several examples of the application of these various relationships to some real and hypothetical problems. At the same time we shall review the sources of the various empirical parameters which enter into these calculations. Since silica and alumina are by far the commonest adsorbents in use today, our examples will be restricted to these two adsorbents. For additional examples of the calculation of sample KP values see Refs. (1-3) and preceding references. 

In the normal-phase mode ion pair adsorption or liquid-solid chromatography found a couple of applications but since the entry of bonded phases, reversed-phase LC has been a quite dominating separation technique and the main one employing the ion pair concept. Ion pair chromatography has been the most widely used name for this separation method, but terms such as dynamic ion exchange, ion interaction, and paired ion chromatography have also been used, where retention is due to electrostatic, hydrophobic, and polar interactions. 

Molecules of a liquid can stick to the surface of the solid and the stronger the intermolecular interaction is, the more sticky the liquid is on the solid. A polar compound would be expected to stick on a polar solid more tightly than a nonpolar compound would stick. If the polar solid is silica gel (Si02), a polar alcohol would stick very well but a nonpolar alkane would not. This property is important for separating molecules and also in many chemical reactions. These principles will be discussed in more detail for specific applications (liquid-solid chromatography, catalytic hydrogenation, certain metal catalyzed reactions) as they arise. 

The stationary phase must be compatible with the mobile phase. Lack of solubility of the stationary phase in the mobile phase and a large inter-facial area between the two phases are the two major requirements. Most adsorbents are applicable to any of the three types of mobile phase, giving rise, for example, to both gas-solid and liquid-solid chromatography. The specific sur-... 

Relationships such as Eqs. (45) and (46) have been utilized extensively in correlating solubility properties (such as gas/liquid and liquid/liquid partition coefficients), retention volumes in gas/solid chromatography, capacity factors in high-pressure liquid chromatography, etc.199 200 For instance, gas/liquid partition coefficients for each of 35 different liquid stationary phases were represented with R > 0.985.205 Other applications have been in biochemical and pharmacological areas,199 200 e.g., enzyme inhibition and pollutant effects. 

The more recent applications of open-column chromatography in fat-soluble vitamin assays utilize liquid-solid (adsorption) chromatography using gravity-flow glass columns dry-packed with magnesia, alumina, or silica gel. Such columns enable separations directly comparable with those obtained by thin-layer chromatography to be carried out rapidly on a preparative scale. 

Oliveira HM et al (2010) Exploiting automatic on-line renewable molecularly imprinted solid-phase extraction in lab-on-valve format as front end to liquid chromatography application to the determination of riboflavin in foodstuffs. Anal Bioanal Chem 397(1 ) 77-86... 

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