Layers reversed-phase chromatography

Bieganowska, M.L. Petmczynik, A. Thin-layer reversed phase chromatography of some alkaloids in ion-association systems. Part II. Chem. Anal. (Warsaw) 1994,39,445-454. 

Chapter 3 through Chapter 8 deal with the basic aspects of the practical uses of PLC. Chapter 3 describes sorbent materials and precoated layers for normal or straight phase (adsorption) chromatography (silica gel and aluminum oxide 60) and partition chromatography (silica gel, aluminum oxide 150, and cellulose), and precoated layers for reversed-phase chromatography (RP-18 or C-18). Properties of the bulk sorbents and precoated layers, a survey of commercial products, and examples of substance classes that can be separated are given. 

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

Reversed-phase chromatography is often used to separate both neutral and ionic organic compounds. In this section, some important aspects for the understanding of the behavior of ionic compounds in reversed-phase chromatography are discussed. The important concepts introduced here are the electrical double layer and the electrostatic surface potential. It will be shown that they are essential for the understanding of the elution profile of ionic compounds. These concepts are further explored in the next section where theoretical models for ion-pair chromatography are discussed. 

The model for ionic retention and ion-pair chromatography that are discussed in Sections 15.2 and 15.3 has been tested and applied to a number of different systems and works very well in most of the cases. From colloid and surface chemistry is known that the model has its limitations, and under certain chromatographic conditions, the presented model will not be valid. The limitations of the model when applied to reversed-phase chromatography of ions still need to be found. Some are self-evident, such as if the pairing-ion concentration is close or above the CMC or when the retention factor is very low so that the accumulation in the double layer is important in comparison to the adsorption, see Ref. [7] for a discussion concerning the accumulation in the double layer. 

M. L. Bieganowska, A. Petruczynik, and A. Doraczynska-Szopa, Thin-layer reversed-phase ion-pair chromatography of some sulphonamides, J. Pharm. Biomed. Anal., 77 241 (1993). 

Porous layer open tubular (PLOT) columns were used to separate basic proteins and peptides [15]. The use of these types of columns was prompted by their high permeability and by the relatively high loading capacity due to an increased surface area by the porous layer. The authors showed that under conditions of reversed-phase chromatography at acidic pH, the EOF mobility was over 8-fold higher than that in raw fused-silica capillary, which is an indication of the high surface charge present in the porous layer. As expected, the EOF... 

Because aqueous-organic mixtures are commonly used as eluents, it should be noted that RP-18 plates can be developed with solvents containing a maximum water content of approximately 60% (v/v), whereas on 50% modified silica layers, water percentages as high as 80% can be employed. Wettable RP-18W plates for normal- and reversed-phase chromatography can be eluted with purely organic and aqueous-organic solvents as well with purely aqueous eluents. 

This review describes currently useful methods for the separation and characterization of inorganic complexes. Some separations are required just to remove unreacted reagents or minor side products. Advances for these standard techniques (ion chromatography, normal and reversed-phase chromatography, thin-layer methods) are often limited by the availability of a specialized support material. However, newer, innovative methods for the separation of species have evolved, particularly separation methods needed in biomedical and industrial applications. 

Another modification of CE is micellar electrokinetic chromatography (MEKC), which is widely used for the separation of nonpolar compounds. The molecules in question partition into micelles (nonpolar layer) with mechanisms similar to those observed with reverse-phase chromatography. An anionic surfactant, sodium dodecyl sulfate, is commonly used as a micellar... 

The mechanism of separation in reversed-phase chromatography depends on the differential partitioning of the solutes into a bonded organic layer from the mobile phase (C8). 

Reversed-phase chromatography is now widely used for the fractionation of biological molecules. The technique is based on the use of a non-polar stationary phase and a polar mobile phase. The stationary phase is usually made of a hydrocarbonaceous layer, either n-octyl. Cfl, or n-octadecyl, Cib, ligands, chemically bonded to the surface of a silica matrix via siloxane bonds. Separation Is achieved by exploiting the difference in the hydrophobic properties of the molecules. 

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