Paper chromatography descending

The substance having same Revalue can be separated by two dimensional paper chromatography. Descending technique is better because compounds with low Ry-values which cannot be completely separated by ascending method can be separated by descending technique. 

Chromatography Descending paper chromatography (Whatman No. 1 paper) was used with the following solvent systems. 

Experiment I. The sample was first separated into fractions by a partition column, and the fractions were further fractionated by descending paper chromatography. A schematic representation of these procedures and the results is shown in Figure 2. 

The different methods of arranging the filter paper in the chromatographic chamber have been applied to sugar separations descending, ascending,39 40 and circular or horizontal development.41 These methods and other techniques will be discussed below. Detailed descriptions of the equipment needed for paper chromatography are readily available.8... 

With paper chromatography a descending technique may be more convenient (though an ascending method can of course also be used). 

Hilf et al.50 used the paper chromatography method by Algeri and Walker49 using descending chromatography. After development and detection of the barbiturate spot, the silver-barbiturate salt spot was cut out of the paper, shredded and extracted in order to obtain confirmation of identity by ultraviolet spectral ratios. 

There are several types of paper chromatography, and they are named according to the direction the mobile phase flows through the paper. These are ascending, descending, horizontal, and two-dimensional, and any of these with a reversed phase. 

Figure 21-2. Apparatus required for descending paper chromatography.

When using a mobile phase that formed two layers when it was prepared, it is considered good technique to use the nonmobile phase layer to establish equilibrium with the paper in descending chromatography. Why is this ... 

The apparatus used for continuous-flow paper electrophoresis can be used for separation with an elution rather than a continuous-flow method. The sample is spotted at the top of the paper and only the buffer solution is continuously added. If the electrophoresis is stopped before the sample components drip from the bottom of the paper, the technique combines the separating ability of descending paper chromatography (separation occurring vertically) with that of electrophoresis (separation occurring horizontally) to yield spots simultaneously developed in two dimensions. 

In the Ph. Eur., sodium pertechnetate [ Tc] and other Tc species are analyzed by descending paper chromatography (Council of Europe 1997 a). The solvent used consists of eight parts of methanol and two parts of water. The chromatogram is developed for 2 h, dried, and the distribution of radioactivity is measured with a suitable detector. Not less than 95% of the recovered radioactivity is located in the spot corresponding to pertechnetate ion, which has an Revalue of approximately 0.6. Reduced, colloidal forms remain at the start of the chromatogram. 

Paper Chromatography. The European Pharmacopeia describes descending paper chromatography using methanol/water (80 20 v/v) as solvent developing time is 2 h. The Tc-pertechnetate anion migrates with an Rf value of 0.6. More than 95% of the measured radioactivity corresponds to an Rf of 0.6 less than 5% are detected at the start. 

Descending. The descending technique, though more common in paper chromatography, can be used in TLC. The top of the plate, where the spots are located, has solvent from a trough fed onto it via a wick some solvent of the same composition is placed in the bottom of the tank but the plate is supported above the solvent level. In order to improve the resolution for particularly difficult separations a number of modifications to the above technique have been developed. 

The range of development techniques available in paper chromatography is similar to those employed in TLC, namely, ascending, descending, radial. 

Figure 21.10. Developing chambers for paper chromatography. A Ascending development B Descending development. Note that similar chambers are used in TLC. From D. Abbott and R. S. Andrews, An Introduction to C3iromatography, Boston Houghton Mifflin, 1965, by permission of the publisher.

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