Separation technique, zone refining

It may be added here that a combination of more than one separation or purification method may have to be used at times. Solvent extraction, for example, is very often only the first step in a multiple-method separation. Similarly, zone refining is the final step in getting an extremely pure sample of a substance which has already been obtained in more than 99per cent purity through the use of other purification techniques. 

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

ZONE REFINING. One of a class of techniques known as fractional solidification, in which a separation is brought about by crystallization of a melt without solvent being added. See also Crystallization. A massive solid is formed slowly and a sizable temperature gradient is imposed at the solid—liquid interface. 

Even if sufficient sample size, in volume, may not be available, enrichment techniques that concentrate trace metals in microliter samples are sometimes quite useful because modern instrumental detection systems such as AAS, ICP-AES, ICP-MS, etc. do not need a large sample size. Moreover, if trace metals that have been separated from their major substances can be concentrated in an extremely small area of the polytetrafluoroethylene (PTFE) tube in HSCCC, this would be an ideal flow-injection analysis system for determination of inorganics. From this point of view, the recently developed pH-zone refining technique has great potential for enrichment, especially for instrumental inorganic trace analysis. 

The CCC technique is well suited for the separation of peptides and related compounds because it eliminates sample loss and denaturation caused by the solid support used in conventional liquid chromatography. The method separates multigram quantities of peptide samples using pH zone-refining CCC. 

These two countercurrent chromatography (CCC) techniques are mutually related pH-peak-focusing CCC is for analytical-scale separations and pH-zone-refining CCC is for preparative-scale separations. 

Also, a zone refining partition coefficient technique has been developed to separate the alkaloids present in a crude extract of Crinum moorei (32). 

Pfann (1) first described the essential features of zone refining and pointed out its potential as a separation technique. 

Recently two useful CCC techniques have been developed pFl-zone-refining CCC, which separates a large quantity of charged samples, and chiral CCC, which uses a ligand called chiral selector to separate enantiomers according to their affinity to the chiral selector. 

The main difference between the two techniques is that in zone refining only a part of the charge is melted at any time. This change alone tremendously increases the efficiency of crystallization as a separation and purification method. An impurity travels with or opposite to the movement of the zone depending, respectively, on whether it lowers or raises the melting point of the substance to be purified. In this way the impurities are concentrated in one or... 

Zone refining is a powerful tool for separation and concentration. Following preconcentration, conventional analytical techniques are used to identify and quantify the impurity. This has enabled certain observed abnormalities in properties to be correlated with the presence of impurities in ultratrace amounts that had earlier remained undetected. In another version, the distribution coefficient, k, of a metallic solute is determined by adding its radioisotope under the conditions of the zoning experiment. If the concentration of the nonradioactive form of the solute in the starting material is measurable by an available analytical technique but subsequently falls below the detection limit after refining, its final concentration can still be calculated from the value of k and the zone refining curves. 

Because of its rapid and high separation efficiency, the multilayer coil CPC has been extensively used for separation and purification of variety of compounds using suitable organic/aqueous solvent systems. The application also covers special CCC techniques such as peak-focusing CCC and pH-zone-refining CCC (see pH-Peak-Focusing and pH-Zone-Refining CCC, p. 1808) chiral and affinity CCC (see Chiral CCC, p. 413) foam CCC (see Foam CCC, p. 905), liquid-liquid dual CCC and CCC/MS (see CCC/MS, p. 323). 

Ito, Y. Shinomiya, K. Fales, H.M. Weisz, A. pH-Zone refining CCC. A new technique for preparative separation. In Modem Countercurrent Chromatography Conway, W.D. Petroski, R.J., Eds. ACS Symposium Series No. 368 American Chemical Society Washington, D.C., 1995 156-183. 

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