Semi-preparative applications

One of the major advantages of supercritical media is the possibility to regulate the solubility of different compounds in the medium. This can be easily accomplished with high accuracy by a change in pressure. This property makes supercritical fluids extremely suitable for selective extractions. A method for the selective extraction of lipid classes has been developed at the Arrhenius Laboratories and its performance was demonstrated by the extraction of digalactosyldiacylglycerol (DGDG) from oat bran (Andersson, Demirbiiker and Blomberg, 1992). 

In SFE, in situ derivatization of the analytes is sometimes applied in order to facilitate the extraction of target analytes (Hawthorne et al., 1992). Such a procedure leads to improved extraction selectivity. The application of enzymatic reactions is a powerful means to increase further the selectivity in 

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An additional problem exists in which impurities in the displacer itself complicate separation.54 Also, the displacer itself must be removed from the column, which lengthens regeneration time and can adversely affect throughput. Ironically, while the difficulties involved in identifying displacers and in column regeneration have retarded use of displacement as a preparative method, there has been renewed interest in using displacement chromatography in analytical and semi-preparative applications for enrichment of trace compounds.55 56... 

Huher U. 2000. Semi-preparative purification of synthesis products with the Agilent high-throughput analysis system. Application Note. 

Paper and thin layer chromatography (TLC) are essentially analytical techniques that can also be used semi-preparatively. The paper or thin layer material serves as the solid phase and the mobile phase is the solvent, or mnning buffer, which is transported along the stationary phase by capillary forces. We discuss here only thin layer chromatography (Grinberg 1990 Touchstone 1992) which has superseded paper chromatography in almost all applications. 

The amide phase systems are also applicable to preparative scale separations. A semi-preparative bonded column (10 mm i.d. X 25 cm) was prepared, yielding a loading capacity of ca. 1 mg per 1 g packing material. Enantiomeric and diastereomeric pairs of benzyloxycarbonyl and tert-butyloxycarbonyl protected di- and tri-peptides were resolved successfully using this chiral amide-bonded column system. 

Detectors which measure a physical property of the eluent such as the refractive index, rate of radioactive decay or the speed of sound waves can be used with chromatographic systems. However, in most instances the detection limits are relatively high, which reduces the application range to preparative and semi-preparative separations. 

However, many of these applications belong to a (semi)-preparative approach, e.g., the isolation of specific proteins from raw extracts. If the term affinity chromatography is used in the literature, in nearly all cases it refers to variant A. Variant B is quite rare and more or less only of mechanistic interest at the moment. 

First, considerably greater emphasis has been placed on semimicro techniques and their application to preparations, separations, analysis and physical determinations such as those of molecular weight. We have therefore greatly expanded the section on Manipulation on a semi-micro scale which was in the Third Edition, and we have described many more preparations on this scale, some independent and others as alternatives to the larger-scale preparations which immediately precede them. Some 40 separate preparations on the semi-micro scale are described in detail, in addition to specific directions for the preparation of many classes of crystalline derivatives required for identification purposes. The equipment required for these small-scale reactions has been selected on a realistic basis, and care has been taken not to include the very curious pieces of apparatus sometimes suggested as necessary for working on the semi-micro scale. 

A notable change in methods of isolating alkaloids from plant materials has been described by Applezweig, depending on the use of a suitable ion-exchange material and capable of application on a semi-micro scale or for industrial use. It has been applied to the preparation of the total alkaloids of cinchona bark (totaquina) and according to Sussman, Mindler and Wood, is also used industrially for the recovery of hyoscine. 

Pyridine base eliminations of a-bromo ketones cannot be recommended for general use because of the side reactions already discussed. The semi-carbazone-pyruvic acid method should be employed if strict absence of isomerization is required in the dehydrobromination of 2- or 4-bromo-3-ke-tones. This procedure is not applicable for the preparation of -3-ketones,... 

The construction of calibration curves is recommended in nephelometric and turbidimetric determinations, since the relationship between the optical properties of the suspension and the concentration of the disperse phase is, at best, semi-empirical. If the cloudiness or turbidity is to be reproducible, the utmost care must be taken in its preparation. The precipitate must be very fine, so as not to settle rapidly. The intensity of the scattered light depends upon the number and the size of the particles in suspension, and provided that the average size of particles is fairly reproducible, analytical applications are possible. 

Among the spectroscopic methods applicable to polysaccharides, u.v. spectrophotometry is of little value for characterizing heparin, whose main, electronic chromophore (the C02 group) displays a band at 220 nm, that is, in a region where all glycosaminoglycans absorb (also through their N-acetyl chromophores), and where minor proportions of unsaturated or aromatic contaminants cause serious interference.77 With pure heparin preparations, the carboxylate chromophore is most useful for chiroptical measurements, and a semi-quantitative evaluation of the extent of N-acetylation of 2-amino-2-deoxy-D-glucose residues is also possible.78... 

In the case of paints and printing inks, the initial preparations will be in the semi-solid state because solvents are needed both in the process of dispersing the pigment in the paint or ink medium and for application purposes. These solvents dry out after the paint or ink is applied. When making coloured plastic articles, both heating and solvents may be used to aid dispersion in the plastic medium as part of the moulding process. However, from the viewpoint of the optical properties in all of these pigment uses, what is most important is that each of these media has a refractive index close to 1.5. 

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