Polarity of Analgesics


September 6, 2013 – 11:04

Cin.uni-tuebingen | Networks / Methods in Neuroscience / Research

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Revised labscript :

Determination of analgesics by thin layer chromatography (TLC)

1. Introduction:

Thin layer chromatography is a special application of adsorption chromatography, in which a thin layer of adsorbent coated onto a flat surface is utilized, instead of a column of adsorbent, as used in column chromatography. The most commonly used adsorbent in TLC is silica gel and the flat surface is a plain rectangular or square glass plate.

The separation of the components of a mixture depends on adsorption-desorption equilibria between compounds adsorbed on the solid stationary phase and in the moving liquid phase. The extent of adsorption of a single component depends upon the polarity of the molecule, the activity of the adsorbent, and the polarity of the mobile liquid phase.

The separation of the components in a mixture is dependent on the relative values of the adsorption-desorption equilibrium constants for each of the components in the mixture. In general, the more polar a functional group in the compound, the more strongly it will be adsorbed on the surface of the solid phase. The activity of the adsorbent (adsorptive power) depends on the type of material and on the mode of its preparation. The choice of the proper adsorbent will depend on the types of compounds to be chromatographed (1, 2).

Elution, or development of the chromatogram, is accomplished by capillary movement of the solvent up the thin layer of adsorbent. The sample is applied in a small drop a short distance from one end of the plate, and the solvent evaporated off. The plate is then placed vertically into a closed jar with its lower edge dipping into a pool of eluting solvent. Separation is stopped by removal of the plate when the solvent front approaches the top edge.

If the components are colored, they can be located visibly, but more often they are invisible and must be located by other means. Illumination with ultra-violet light will excite many compounds to fluoresce. Another possibility is to impregnate the plate in advance with a fluorescent dye; the presence of an ultra-violet-absorbing compound on the adsorbent will result in a dark spot on exposure to ultra-violet light, as the compound quenches the fluorescence of the dye. If these approaches still do not make the TLC components visible, a color- or fluorescence-producing reagent can be sprayed onto the dried plate, to render the spots visible. Once the spots are located, their Rf values can be calculated from the equation:

Source: delloyd.50megs.com

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