Location and Removal of Separated Compounds

After the preparative chromatographic plate has been developed and the mobile phase evaporated, the separated bands must be located and the desired compounds removed from the plate. If the compounds of interest are coloured, their position on the layer can be located under white light. If they are fluorescent, or become so after post-chromatographic derivatiz-ation, their position on the layer can be determined under UV light. Conversely, a PPC plate containing a fluorescent material will indicate the separated compounds as dark zones on a bright background when examined under UV light. Pre-coated plates containing 254 nm or 365 nm fluorescent indicators should be used if possible because they provide a mode of detection which is generally nondestructive.

If the compounds themselves are not visible or fluorescent, detection can be performed by use of iodine vapour or by use of destructive reagents (e.g. vanillin-sulfuric acid). If such a reagent is used for detection of the separated compounds, a vertical channel must be scraped in the layer about half a centimetre from the edge of the streak. After covering the major portion of the layer with a suitable glass plate, the part of the layer which is not covered is sprayed, and thus serves as a guide area. If heating is necessary for detection, the sprayed portion of the plate must be detached from the rest, by use of a glass cutter, because heating the developed preparative plates may lead to decomposition of the compounds of interest.

After location of the desired compound, the subsequent steps are: (1) mechanical removal of the adsorbent zone, (2) extraction of the compound from the stationary phase with a suitable solvent, (3) separation from the residual adsorbent, and (4) concentration of the solvent. The areas of the layer containing the compounds of interest are then scraped off cleanly down to the glass with a suitable scraper. One of the best methods is to put the adsorbent with the compound to be extracted in an empty receptacle containing a sintered glass filter to retain the adsorbent and to extract the compound with a solvent and the aid of vacuum.

The substance should be highly soluble in the solvent or solvent mixtures used for extraction; the solvent should also be as polar as possible. Chloroform is widely used for nonpolar substances, and ethanol or acetone for polar compounds. If water is the chosen solvent, it should be removed by lyophiliz-ation. Because silica is significantly soluble in methanol, this solvent should be avoided. The mobile phase used for the separation is highly recommended also for extraction. As a rule of thumb the volume of solvent (Vsolvent) required when the chromatographic mobile phase is chosen for extraction is as in equation [1]:

It should be noted that the longer the substance is in contact with the adsorbent, the more likely decomposition is to occur. Once the solution of the compound to be isolated is obtained (free from adsorbent) the extract must be evaporated to dryness. The evaporation temperature should be as low as possible, to avoid decomposition.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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