Chromatographic absorbents

Both diastereomers are relatively reactive, complicating their isolation and purification. The half-lives of anti-and syn-BPDE in water at pH 7 are 2 hr and 30 min., respectively. Although they tend to decompose on chromatographic absorbants, these diol epoxides can be purified by rapid chromatography on low activity alumina columns or by HPLC in the presence of triethylamine as a stabilizer. 

Hill JM. Silica Gel as an Insoluble Carrier for the Preparation of Selective Chromatographic Absorbents. The Preparation of 8-Hydroxyquinoline Substituted Silica Gel for the Chelation Chromatography of Some Trace Metals. Journal of Chromatography 1973 76 455-458. 

A number of other methods have been reported for the acceleration of Diels-Alder reactions, including the use of a microwave oven,877 water as a solvent (a hydrophobic effect),878 5 M LiC104 in Et20 as solvent,879 absorption of the reactants on chromatographic absorbents,880 and the use of an ultracentrifuge881 (one of several ways to achieve reaction at high pressures).882... 

The exchange of D20 with a variety of hydrido-metal complexes is discussed. While the exchange of HRe(CO)s with D20 is rapid, that of a variety of hydrido-metal cluster complexes is not observed. The exchange ofD20 with H2Re2(CO)s or H2Os3(CO)l0 on the other hand is found to be catalyzed by Florisil chromatographic absorbent. Mass and Raman spectrometric means of characterization are discussed. 

Hydrogen-Deuterium Exchange of H20s C0) q and H Ee CO) on Florisil Chromatographic Absorbent... 

In this technique the chromatographic absorbent is designed to make use of specific biochemical inter-... 

Jones and Pytkowicz, 1973 Hatheson, Coleman and Bell Chromatographic Absorbent Silica Gel wash with 0.10 N NaOH and distilled water Artificial seawater ND ND... 

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

Numerous high pressure Hquid chromatographic techniques have been reported for specific sample forms vegetable oHs (55,56), animal feeds (57,58), seta (59,60), plasma (61,62), foods (63,64), and tissues (63). Some of the methods requite a saponification step to remove fats, to release tocopherols from ceHs, and/or to free tocopherols from their esters. AH requite an extraction step to remove the tocopherols from the sample matrix. The methods include both normal and reverse-phase hplc with either uv absorbance or fluorescence detection. AppHcation of supercritical fluid (qv) chromatography has been reported for analysis of tocopherols in marine oHs (65). 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Since only relatively few substances are capable of emitting fluorescent radiation, they can be particularly selectively detected. This means that the selectivity of the chromatographic separation, which is always aimed at, is meaningfully extended by the selectivity of detection. Accompanying substances that absorb radiation but do not emit light do not interfere when the analysis is made by the selective determination of fluorescence ... 

An example of the results obtained in the form of a chromatoelectropherogram can be seen in Figure 9.6. The contour type data display showed the three variables that were studied, namely chromatographic elution time, electrophoretic migration time, and relative absorbance intensity. Peptides were cleanly resolved by using this two-dimensional method. Neither method alone could have separated the analytes under the same conditions. The most notable feature of this early system was that (presumably) all of the sample components from the first dimension were analyzed by the second dimension, which made this a truly comprehensive multidimensional technique. 

Ion chromatography has been successfully applied to the quantitative analysis of ions in many diverse types of industrial and environmental samples. The technique has also been valuable for microelemental analysis, e.g. for the determination of sulphur, chlorine, bromine, phosphorus and iodine as heteroatoms in solid samples. Combustion in a Schoniger oxygen flask (Section 3.31 )is a widely used method of degrading such samples, the products of combustion being absorbed in solution as anionic or cationic forms, and the solution then directly injected into the ion chromatograph. 

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