Eluents Solvents for Extraction

The choice of eluent is controlled primarily by the desorptive effect required. The elutive effect is tested by applying spots of the substances 

Its potential interference in the subsequent quantitative stage must be considered when choosing the eluent. For example, if UV measurements are to follow, the solvent should absorb only weakly in this region or one has to evaporate the eluate to dryness and take up the residue in a more suitable solvent. 

Very fine particles of adsorbent which can scatter light, must be excluded by appropriate filtration or centrifuging at high speed [799] (cf. also p. 149). 

The interference of impurities extracted from the adsorbent is appreciably harder to avoid. The usual silica gels and aluminas contain inorganic impurities, in particular iron and chloride colour reactions influenced by iron show divergences when an eluent is used which dissolves iron [212]. Such divergences can be eliminated by using special preliminary purification procedures. The evaluation of bile acid chromatograms is an example illustrating this [797] the silica gel G was 

In practice, eluents of widely varying polarity have been used. Very weakly polar steroids (oestrenols) can be quantitatively eluted with hydrophobic solvents like methylene dichloride (cf Table 14) [217], whereas difficulties are reported in the elution of the more polar progesterone under the same conditions [691]. Chloroform has been used to elute dinitrophenylhydrazones from silica gel and opium alkaloids from alumina [470, 535]. Methanol and ethanol are often used for elution of substances of all types of compound class from silica gel [213, 215, 259, 434] or alumina [122, 437]. Butyl acetate has been chosen as the most suitable eluent for penicillin V [486]. Acetone has proved suitable for recovering ubiquinones [733]. Polar neutral, acid and basic aqueous eluents have also been employed, e.g., water for mucic acid derivatives [451] (cf Table 14), 1% Tween 80 solution for cobalamin [128], 0.2 N sulphuric acid for Vitamin Bg factors [702], 34% ammonium persulphate solution for nicotinic acid [702] and ammonium hydroxide for azo dyes [638]. Nitro-4-acetaminophenetoles have been extracted from alumina with the highly polar dimethylformamide [489]. 

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