TABLE OF CONTENTS Although normal silica gel 60 is a very versatile sorbent for the separation of many aromatic and aliphatic saturated and unsaturated organic compounds, there are occasions when silica gel has insufficient resolving power to achieve adequate separation of unsaturated organic species, particularly where cis/trans isomerisa-tion may be involved. Argentation or silver nitrate TLC overcomes this problem in a unique way and enables excellent separations of many types of lipids, fatty acids, olefins, steroids and triterpenes. [85] [93]
The separation is based on the known interaction of the silver ion, Ag + with ethylenic ?-bonds present in the solute molecules. The strength of the interaction will depend on a number of factors:
The number of olefmic bonds present.
Steric hindrance surrounding the olefmic bonds.
The position of the olefmic bonds in the structure.
The stronger the interaction and hence complexation, the more the compound is retained on the layer. The technique is very powerful as it will even allow cis and trans isomers of some organic compounds to be resolved. The difference in retention observed for such geometric isomers is normally due to the degree of steric hindrance around the double bond. Good examples of this effect are found in the separation of unsaturated fatty acids and lipids. In some cases the silver nitrate can be added to the solvent mixture used for development (where aqueous-based solvents are used). This will be described later in Chapter 5. However, in by far...
