Fractionation methods countercurrent

Some of the substances that have been separated by this method are given in papers referred to by Morris and Morris (1964) amino acids, peptides (particularly those having molecular weights ranging from 500 to 5000), polypeptide antibiotics, proteins (including enzymes), carbohydrates (although for most compounds in this chemical class other fractionation methods are much more frequently applied), purines, pyrimidines, nucleic acid derivatives, tRNA s that are specific for various amino acids, organic acids, steroids, lipids, antibiotics that are not peptides, porphyrins, pterins, vitamin B,2 and other vitamins, lipoic acid, and alkaloids. The countercurrent-distribution procedure of Holley et al. (1965) is widely used, sometimes with modifications. Korte et al. (1965) have separated three isomers of tetrahydrocannabinol. 

Fractionation methods related to countercurrent distribution have been reviewed and automatic equipment and techniques described (Dutton and Scholfield, 1963 Dutton et al., 1961 Dutton and Cannon, 1956 Scholfield et al., 1961a,b) Scholfield and Dutton, 1958 Scholfield and Hicks, 1957 Therriault, 1963). 

The hydrolysis is performed as a continuous countercurrent reaction in tall reaction towers (height 15-20 m, diameter 0.7 m). The reaction time amounts to 60-90 min. Reaction products are as well obtained an aqueous glycerin solution (about 15%) as on a mixture of raw fatty acids [50]. The free fatty acids are carefully distilled with the aid of a thin film evaporator (2-10 mbar, 260°C maximum) [51]. Crystallization and transwetting are additional methods for fractionation of fatty acid mixtures. 

The basis for the second purpose is the use of a two-phase distribution in conjunction with a countercurrent process of some sort, so thar separation may be made in spite of closely related partition ratios. This method may be called Extraction for Fractionation Purposes... 

Partition Methods. Walter (1952) devised a countercurrent distribution procedure for the fractionation of casein in a two-phase system containing water-ethanol-phenol at pH 8.2. The concentration of /3-casein increased in the phenol phase. Ellfolk (1957) employed a two-phase system consisting of collidine, ethanol, and distilled water at 20 °C. The a-caseins were concentrated in the water-rich phase, while the /3-caseins were concentrated in the collidine-rich phase. 

Perpendicular flow occurs in chromatography, countercurrent distribution, field-flow fractionation, and related methods. Below we explain the basic mechanism by which flow assumes its vital role in these separation techniques. 

Countercurrent chromatography (CCC) is a chromatographic method which separates solutes more or less retained in the column by a stationary phase (liquid in this case) and are eluted at the outlet of column by a mobile phase. Two treatments of column effluent have been used until now in CCC. Either the column outlet is directly connected to a detector commonly used in high-performance liquid chromatography (HPLC) (on-line detection) or fractions of the mobile phase are collected and analyzed by spec-trophotometric, electrophoretic, or chromatographic methods (off-line detection). 

The analysis of the mobile-phase fractions collected at the outlet of column is the oldest method used in CCC (droplet countercurrent chromatography and rotation locular countercurrent chromatography) to elucidate the quality of separation and to characterize solutes. With modern CCC, such as CPC, CCC type J, and crossaxis, numerous apphcations have been described for preconcentration and preparative chromatography. 

In attempts to synthesize the oxazolone-thiazolidine of formula 2, by condensing the dimethylcysteine with the appropriate oxazolone, it was noted that the reaction products always showed a very low antibiotic activity though they had properties similar to penicillin. However, at first all attempts to concentrate the activity by various fractionation procedures, including chromatography, failed and it was doubted in many circles whether or not the activity could really be due to penicillin. This uncertainty was resolved by du Vigneaud and co-workers (S) who employed the method of countercurrent distribution (see Chapter I), and were able to isolate a crystalline sodium salt from the mixture. This salt had full antibiotic activity and was identical in every respect with naturally-occurring benzylpenicillin. The synthetic acid was present in the reaction mixture at a concentration of only about 0.1%. 

One of the important methods of critical fiuid fractionation involves the countercurrent separation of phospholipids from a vegetable oil. A system to achieve this end is presented in Figure 7. Here, high pressure CO2 is fed into a pressure vessel packed with segmented gauze mesh packing (the refining... 

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