Supports for Stationary Phases

Supports used for the stationary phases include aluminum foil, terephthalate films, thin glass plates and braided PTFE or glass fiber. Without the use of these inert substrates, it would be quite impossible to handle the finely divided sorbents. 

Terephthalate films are becoming less frequently used. Their translucency is regarded as an advantage by some users, but their considerably poorer thermal stability is a disadvantage. 

Braided PTFE and glass fiber products are no longer obtainable commercially in Europe. 

The thickness of the sorbent layer for analytical purposes lies between 50 and 100 (un (extra thin films for automated multiple development, AMD, are described in Section 11.1), 200 [un for HPTLC plates and 250 pm for the normal TLC plates. The layer thickness on aluminum foil is 200 pm, while layers that can be used for preparative work can be up to 2 mm thick. 

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Used as a solid support for stationary phases such as liquid paraffin used in analysis of fixed oils... 

Solid support for stationary phases robust inert material used as support for a liquid mobile phase, has a large surface area, common materials are silica gel for HPLC, processed brick and celite particles for GC. 

It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

It should be recalled that all substances that are used as stationary phases, or as supports for bonded phases, that have pores commensurate with the size of the molecules being separated, will exhibit exclusion properties. Thus, even if the solutes are retained largely as a result of the interactions of the solute molecules and those of the two phases if, due to their size, some molecules can interact with more stationary phase than others, then the retention will also be controlled to some extent by exclusion. The term exclusion chromatography is, therefore, usually confined to those separations where retention is controlled... 

Mellors, J.S., Jorgenson, J.W. (2004). Use of 1.5-pm porous ethyl-bridged hybrid particles as a stationary-phase support for reversed-phase ultrahigh-pressure liquid chromatography. Anal. Chem. 76, 5441-5450. 

The capacity ratio of a solute, (k ), was introduced to develop a chromatographic measurement, simple to calculate, independent of flow-rate and one that could be used in solute identification. Although helpful, the capacity ratio is so dependent on the accurate measurement of extra column volume and on very limited solute exclusion by the support and stationary phase, that it is less than ideal for solute identification. An alternative measurement, the separation ratio (a) was suggested where, for two solutes (A) and (B),... 

Other Immobilization Techniques Along with noncovalent and covalent immobilization methods, other techniques have been developed for the preperation of affinity supports. Such methods include entrapment, molecular imprinting, and the use of the ligands as both the support and stationary phase. Although these methods are not as common as the approaches already examined, they have important advantages in some applications [8]. 

The flow channel in FFF is typically 25-100 cm long and 1-3 cm in breadth. The thin dimension (thickness) of its ribbon-like structure is generally 50-500 /um, or 0.05-0.5 mm. The channel is open and unobstructed—it contains no packing material. There is no need for packing to support a stationary phase (as in chromatography) because there is no stationary phase retention is induced by the external field. 

The development of novel supports and stationary phases, in particular for protein separation, and the understanding of the separation mechanisms are still in their infancy further improvements must be made. 

The polaragraphic analysis of NMe was studied in buffered solns. The half wave potential was found to vary with pH hence the necessity of using a buffered scln (Ref 22). Gas CjirGiTiatGg retention times for NMe at 50-90° and 110— 50° are listed in Ref 38, various supports and stationary phases were used. Small amts were quanty detd by coupling with p-diazobenzene-sulfonic acid and detg the colored prod spec-trophotometrically (Ref 37)... 

Liquid-liquid partition chromatography systems have been used with or without stationary-phase support. Support-free LLPC has been classified as countercurrent chromatography (CCC). High-speed CCC techniques, based on planetary motion and coaxial orientation of the coiled column, have been developed that achieve both high partition efficiency and excellent retention of the stationary phase, thus circumventing the need for stationary-phase supports [6],... 

Column extraction based on the theory of LLE is a widely used technique in biochemistry, toxicology, pharmaceutical analysis, and other fields. Extrelut by Merck (Darmstadt, Germany) and Extube by Varian (Harbor City, California, LfSA) are commercially available prepacked columns used in these applications (Fig. 2A). Extraction is performed by fixing an aqueous solution or suspension to a supporting material (stationary phase) and allowing the other immiscible solvents (mobile phase) to pass over it. The two phases are in contact, thus permitting a continuous and multistep extraction to occur. This technique can replace the conventional LLE in a separating funnel, and thus it becomes more efficient and practical as no emulsions can be formed, less solvent volumes are used, and preparation time is reduced. Body fluids (e.g., urine) are the best example for the application of incolumn LLE. 

Clearly, further improvements in the reliability and accuracy of the IGC method depend on the development of more suitable columns to support the stationary phase. Several authors have speculated that the use of capillary columns, or open tube columns would eliminate some of the concerns cited above, and would be advantageous for IGC applications (33-351. The principal attraction of a capillary column is the possibility of achieving more uniform dispersal of the polymeric phase. Ideally, the polymer would cover the wall as a uniform annular film. Such a geometrical configuration would simplify modelling of the transport processes within the column, and improve the inherent reliability and accuracy of IGC measurements. 

Cellulose powder. Supplied ready for use and usually requires no further treatment, not even the addition of the stationary phase, since this is acquired from the aqueous solvent. The use of cellulose in columns is an alternative to the use of cellulose in the form of thin layers coated on glass plates (Chapter 3). Cellulose columns are essential if a preparative separation is required, and they have also been found more convenient for quantitative estimations. A difference between the techniques which may affect the solvent flow, is that in a column the support and stationary phase are in contact with mobile phase before the separation starts, whereas in thin layer the mobile phase has a definite boundary which moves ahead of the solutes. 

N. Tanaka, T. Fukutome, K. Hosoya, K. Kimata and T. Anaki, Polymer-supported pseudo-stationary phase for electrokinetic chromatography. Electrokinetic chromatography in a full range of methanol-water mixtures with alkylated starburst dendrimers, J. Chromatogr. A, 716, 57-67, 1995. 

Unfortunately, both and will vary between different columns and, due to the partial exclusion that can occur with porous supports and stationary phases, may vary between different solutes. For this reason, the separation ratio (a) was introduced as an identification parameter. For two solutes, (A and B), the separation ratio is defined as... 

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