Chromatographic separations, affinity liquid-solid

Solid phase extraction (SPE) involves the separation of components of samples in solution through their selective interaction with and retention by a solid, particulate sorbent. SPE depends on differences in the affinities of the various components of the sample for the sorbent. The mechanisms of the interactions are virtually identical to the sorption processes that form the basis of liquid chromatographic separations (p. 80). The choice of solvent, the pH and ionic strength of aqueous solutions, and the chemical nature of the sorbent surface, especially its polarity, are all of importance in controlling the selectivity and efficiency of an extraction. 

Chromatographic separation relies on the affinity of binding between different components of the API in liquid and the solid matrix column. The API is separated from the impurities by percolating the liquid through chromatographic columns filled with solid phase matrices. The matrices are made of different materials and separate the components on the basis of physicochemical properties such as charge, size and shape, hydrophobic and hydrophilic characteristics, complex formation with certain ions or metals, and interaction with dyes. 

Affinity Precipitation. All bioseparation processes include three stages preferential partitioning of target substance and impurities between two phases (liqnid-liqnid or liquid-solid), mechanical separation of the phases (eg, separation of the stationary and mobile phases in a chromatographic column), and recovery of the target snbstance from the enriched phase. Because smart polymers can undergo phase transitions, they could facilitate the second and the third stages of bioseparation processes. 

All chromatographic methods function on the same principle, which is the partitioning of components in a mixture between two phases (1) a stationary phase, which may be a solid, liquid, or gel, and (2) a mobile phase, which may be gas, liquid, solution, or a varying mixture of solvents. When a mixture is introduced into a chromatographic system, its components are alternately absorbed and desorbed, that is, partitioned between, the stationary and mobile phases. Partitioning is caused by different polarities of the stationary and mobile phases and the compounds being separated. Compounds in the mixture have different affinities for the phases and they will move at different rates in the chromatographic system and thus be separated. 

Adsorption chromatography relies on the different affinity of components of a mixture for a liquid moving phase and a solid stationary phase. The separation mechanism depends upon differences in polarity between the different feed components. The more polar a molecule, the more strongly it will be adsorbed by a polar stationary phase (Varki et al. 1999). Similarly, the more nonpolar a molecule, the more strongly it will be adsorbed by nonpolar stationary phase. It is often employed for relatively nonpolar, hydrophobic materials so that the solvent tends to be nonpolar while the stationary phase is polar. Proteins have a high affinity to polar chromatographic media and the recovery of the sample is usually difficult. Therefore, this method is not commonly used to purify and characterize proteins. 

Many of the organic constituents of FDR are explosive or explosive-related compounds and much of the work already done on the detection of explosive residues can be extended to include FDR. Explosives and their residues are usually analyzed using chromatographic techniques. Chromatography is the general name given to the methods by which two or more compounds in a mixture physically separate by distributing themselves between two phases (a) a stationary phase, which can be a solid or a liquid supported on a solid, and (b) a mobile phase, either a gas or a liquid which flows continuously around the stationary phase. The separation of individual components results primarily from differences in their affinity for the stationary phase. 

A third method of separation is called chromatography. Chromatography is the gen-erai name applied to a series of methods that employ a system with two phases (states) of matter a mobile phase and a stationary phase. The stationary phase is a solid, and the mobile phase is either a liquid or a gas. The separation process occurs because the components of the mixture have different affinities for the two phases and thus move through the system at different rates. A component with a high affinity for the mobile phase moves relatively quickly through the chromatographic system, whereas one with a high affinity for the solid phase moves more slowly. 

The GC technique is based on the separation of gaseous compounds according to their affinities for the stationary phase (polar columns, apolar, chiral, etc.). The analysis of liquid or solid compounds, therefore, requires heating to the gaseous state. It is the principal constraint in this technique, since we cannot use it except for the analysis of thermostable compounds. Retention time, a characteristic value of each constituent, can be used for the qualitative analysis. However, it depends on numerous operative conditions (equipment, analytical method, etc.) that make it difficult to use for the study of complex mixtures. It is generally preferable to use the retention indexes, which help to overcome this variability [KOV 58]. In feet, for a given compoimd, the retention index (RI) depends only on the nature of the stationary phase and not on chromatographic conditions. It is, therefore, possible to use the GC databases to attempt to identify the constituent [ADA 95]. Many of these databases are specifically dedicated to the analysis of secondary metabolites on different stationary phases. 

---