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Chromatographic Mechanisms

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Chromatographic Mechanisms ( chromatographic-mechanisms )

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Chromatographic Mechanisms Chromatographic techniques are based on four different sorption mechanisms, surface adsorption, partition, ion exchange and size exclusion. Surface Adsorption Chromatography The separation mechanism depends upon differences in polarity between the different feed components. The more polar a molecule, the more strongly it will be adsorbed by a polar stationary phase. Similarly, the more non-polar a molecule, the more strongly it will be adsorbed by non-polar stationary phase. During a surface adsorption chromatography process, there is competition for stationary phase adsorption sites, between the materials to be separated and the mobile phase. Feed molecules of low polarity spend proportionally more time in the mobile phase than those molecules that are highly polar, which are retained longer. Therefore the components of a mixture are eluted in order of increasing polarity. The choice of stationary phase is governed by the polarity of the feed components. If the feed components are adsorbed too strongly, they may be difficult to remove. Weakly polar mixtures should be separated on highly active absorbents, or little or no separation will occur. The choice of mobile phase is equally important. The polarity of the mobile phase should be chosen to compliment the choice of stationary phase. In general, good separation is achieved by using fairly polar stationary phases and low polarity mobile phases such as hexane. The 2 most common adsorbents used in chromatography are porous alumina and porous silica gel. Of lesser importance are carbon, magnesium oxide, and various carbonates. Alumina is a polar adsorbent and is preferred for the separation of components that are weakly or moderately polar, with the more polar components retained more selectively by the adsorbent, and therefore eluted from the column last. In addition, alumina is a basic adsorbent, thus preferentially retaining acidic compounds. Silica gel is less polar than alumina and is an acidic adsorbent, thus preferentially retaining basic compounds. Carbon is a non-polar (apolar) stationary phase with the highest attraction for larger non-polar molecules.

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