Thin Layer Liquid Chromatography

A variety of adsorbents can be used to separate out various molecular weight polyols. If the Rp values are found to be close together with a particular adsorbent a change to another type usually improves the separation. A list of Rp values of the more common low molecular weight polyols is given in Table 11.9. 

The alcohols can be detected by means of a strong oxidizing agent, e.g.  

Dicarboxylic acids can be separated on an adsorbent consisting of a 10 3 mixture of Kieselguhr G and polyethylene glycol (mol. wt 4000) using as eluant a 90 7 3 mixture of diisopropylether, formic acid and water. Detection can be effected by the use of a 0-04% solution of bromocresol purple in 50% methanol at pH 10 (sodium hydroxide solution). The R, values obtained are given in Table 11.10. 

In another analytical procedure, the diamine from the diisocyanate is obtained from the filtrate of the acidified alkaline hydrolysis product and can be determined quantitatively by colorimetric methods. If preliminary analysis has indicated 1,5-naphthalene diisocyanate, the filtrate is oxidized with periodic acid and gives a violet-pink colour. For other diisocyanates, colour formation also suitable for colorimetric analysis can be obtained by 

A full analysis of a known polyurethane composition containing 1,5-naphthalene diisocyanate and polyethylene adipate gave results in which the total adipic acid-glycol polyester content was of the order of 77% and the 1,5-naphthalene diisocyanate 15 5%. The added total is thus 92 5%, leaving 7 5% unaccounted for, but bearing in mind the complexity of the analysis, this must be considered a relatively satisfactory result. 

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Chemically, crude oil can be divided into fractions of different polarities using column or thin-layer liquid chromatography. 

Usually, different types of solvent mixtures are used in normal-phase liquid chromatography, especially in thin-layer liquid chromatography. For example, sedative medications were separated hy thin-layer liquid chromatography using different types of eluents (developing solvents). 

Thin-layer liquid chromatography has been employed for the determination of alkylated cresols and amine antioxidants [102, 103] in polybutadiene, phenolic antioxidants in PE [104-106] and PP [106], dilauryl and distearyl thiodipropionate antioxidants in polyolefins, ethylene-vinyl acetate copolymer, acrylonitrile-styrene terpolymer and PS, UV absorbers and organotin stabilisers in polyolefins [102], and accelerators such as guanidines, thiazoles, thiurans, sulfenamides, diethiocarbamides, and morpholine disulfides in unvulcanised rubber compounds. 

SERS has also been applied as a sensitive, molecule-specific detection method in chromatography, e.g. thin layer, liquid, and gas chromatography. SERS-active colloids were deposited on the thin layer plates or mixed continuously with the liquid mobile phases. After adsorption of the analytes, characteristic spectra of the fractions were obtained and enabled unambiguous identification of very small amounts of substance. 

Normal-phase liquid chromatography is thus a steric-selective separation method. The molecular properties of steric isomers are not easily obtained and the molecular properties of optical isomers estimated by computational chemical calculation are the same. Therefore, the development of prediction methods for retention times in normal-phase liquid chromatography is difficult compared with reversed-phase liquid chromatography, where the hydrophobicity of the molecule is the predominant determinant of retention differences. When the molecular structure is known, the separation conditions in normal-phase LC can be estimated from Table 1.1, and from the solvent selectivity. A small-scale thin-layer liquid chromatographic separation is often a good tool to find a suitable eluent. When a silica gel column is used, the formation of a monolayer of water on the surface of the silica gel is an important technique. A water-saturated very non-polar solvent should be used as the base solvent, such as water-saturated w-hexane or isooctane. 

Following extraction/cleanup, amphenicols can be separated by thin-layer, liquid, or gas chromatography and measured by spectrophotometric, electron capture, or mass spectrometric detectors (Table 29.2). 

The use of modern physical methods (NMR, UV, and IR spectroscopy, mass spectrometry, and gas-liquid (GLC) and thin-layer (TLC) chromatography is becoming increasingly noticeable. By 19522 few systematic studies of the preparation of derivatives of benzo [6]thiophene had been undertaken, no attempt had been made to alkylate benzo[6]thiophene by means of the Friedel-Crafts reaction, and Friedel-Crafts acylation had been little studied. Halogenation of benzo[6]thiophene had only been superficially investigated and... 

Emission and atomic absorption spectroscopy for detecting organic impurities and determining metal ions. Chromatography, including paper, thin layer, liquid (high, medium and normal pressure), flash and vapour phase. 

Normal-phase column liquid chromatography using unmodified silica gels has been used for the purification of both natural and synthetic compounds. It is a powerful tool for stereoisomer separations. The instability of unmodified silica gel means that its use for quantitative analysis in column liquid chromatography is avoided because of poor reproducibility. Normal-phase separation has, however, been performed using thin-layer liquid... 

The stationary phase can be inside a tube as a packing or a thin layer (column chromatography) or on a flat surface. The interactions of the components of the mixture depend on the nature of the stationary phase. Adsorption on surfaces, solvation in a liquid layer or solid layer (membrane), and reversible chemical reaction between ionic parts of molecules are examples of interaction mechanisms. 

The various experimental procedures that have been developed for its determination in different biological materials could be conveniently divided into bloassay or biological methods, chemical methods, radioactive tracer determinations, enzymatic techniques, paper, column, thin layer, gas chromatography, high performance liquid chromatography (HPLC) and automated methods (ref. Table 2.4 for references to these techniques and their grouping). The methods for determination of ascorhigen have also been listed separately in Table 2.4. A. 

Golkiewicz, W. and Polak, B., Relationship between capacity factors and composition of a mobile phase in thin-layer liquid exchange chromatography of enantiomers of some amino acids, J. Planar Chromatogr., 7, 453, 1994. 

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