Liquid adsorption chromatography elution

These NELC techniques can be classified into five types. Each type requires gradient elution with two or more solvents. The first type is liquid adsorption chromatography (LAC) (Table I). In LAC, initial and final mobile phases should be good solvents for the sample copolymers. Silica gel is used as the stationary phase in most cases. The sample co-... 

Adsorption Mechanism. Adsorption mechanism mainly occurs with mineral porous gel particles well-known for their surface activity. This is the classical mechruiism of solid-liquid adsorption chromatography. The elution volume takes the form ... 

Each type of interaction chromatography requires a mobile phase that comprises two or more solvents. In liquid adsorption chromatography (LAC), the initial and final mobile phases are good solvents for the polymer. In most cases, silica gel is used as the stationary phase. The sample injected into the column adsorbs on the silica surface at the initial mobile phase. For very-low-molar-mass samples, the adsorption might be sufficiently weak and desorption in the same mobile-phase composition may take place. In this case, elution takes place under isoaatic conditions. 

In liquid-solid adsorption chromatography (LSC) the column packing also serves as the stationary phase. In Tswett s original work the stationary phase was finely divided CaCOa, but modern columns employ porous 3-10-)J,m particles of silica or alumina. Since the stationary phase is polar, the mobile phase is usually a nonpolar or moderately polar solvent. Typical mobile phases include hexane, isooctane, and methylene chloride. The usual order of elution, from shorter to longer retention times, is... 

Gas chromatography is a method of separation wherein gaseous or vaporised components are distributed between a moving gas phase and a fixed liquid phase or solid adsorbent. By a continuous succession of adsorption or elution steps, occurring at a specific rate for each component, separation can be achieved. The components can be detected by one of various methods as they emerge successively from the chromatographic column. From the detector signal, proportional to the instantaneous concentration of the dilute component in the gas stream, information about the number, nature and amounts of the components present is obtained. 

Experimental and theoretical studies of the separation of 1,7- and 1,10-phenanthrolines and some substituted derivatives in linear elution adsorption chromatography,44> 145 thin-layer chromatography,146-148 and liquid-liquid partition chromatography149 have been described. 

Reversed-phase chromatography is the term commonly applied to a system where a nonpolar liquid phase is coated on the solid support and elution carried out with an immiscible polar phase. Such systems are often necessary for separations which cannot be carried out by normal partition or adsorption chromatography. For TLC, the stationary phase is normally a liquid of high boiling point which does not readily evaporate from the adsorbent. Paraffin oil, silicone oil or n-tetradecane coated on silica gel or Kieselguhr are frequently used with water-based mobile phases such as acetone—water (3 2) or acetic acid-water (3 1). Reversed-phase chromatography is very useful for the TLC analysis of lipids and related compounds. 

Adsorption chromatography The process can be considered as a competition between the solute and solvent molecules for adsorption sites on the solid surface of adsorbent to effect separation. In normal phase or liquid-solid chromatography, relatively nonpolar organic eluents are used with the polar adsorbent to separate solutes in order of increasing polarity. In reverse-phase chromatography, solute retention is mainly due to hydrophobic interactions between the solutes and the hydrophobic surface of adsorbent. Polar mobile phase is used to elute solutes in order of decreasing polarity. 

The right combination of the two immiscible liquid phases, mobile and stationary, can lead to highly selective separations at ordinary temperatures for both volatile and nonvolatile solutes. An important characteristic is that th partition ratio ideally is independent of concentration the elution bands are therefore more symmetrical and less subject to tailing than those observed in adsorption chromatography. Consequently, better resolution is usually possible. The principal problem is that of stabilizing the stationary phase the stationary and mobile phases are not completely insoluble in each other, even if one is aqueous and the other a hydrocarbon. 

Gas chromatography has come into existence as an analytical method. The separation of analysed liquid or gaseous mixtures is the result of the difference in partition coefficients of their components between the gas (mobile) phase and liquid or solid surface (stationary phase). Depending on the kind of stationary phase the method is called partition or adsorption chromatography. The retention volume, defined as the volume of mobile phase causing the elution of the sample from the chromatographic column, is the value characterizing the interaction between the solute and stationary phase. 

---