TABLE OF CONTENTS There are at least 25 inert materials that are available as sorbents in TLC, some of which have been more widely used than others. A number of the more important ones will be reviewed in this chapter. Clearly for optimum separations, it is important that the correct material is chosen. Some sorbents have a specific range of application (e.g. silica gel impregnated with caffeine for polyaromatic hydrocarbons, or silica gel impregnated with a chiral selector for the separation of enantiomers of amino-acids and derivatives). By contrast silica gel or aluminium oxide are used for a wide range of applications. Silica gels and aluminas can also be split into a number of distinct, separate sorbents depending on pore size, particle size, and pH. Before choosing the sorbent, consideration must be given to the compounds to be separated. Characteristics, such as the polarity, solubility, ionisability, molecular weight, shape and size of the analytes are all important in deciding on a separation mechanism, and hence largely define both the type of sorbent and the solvents used both for the preparation of the sample and in development.
In 1973 Scott [1] examined over 1100 papers to determine which sorbents were the most regularly used in TLC. Silica gel was by far the most popular ( ?64%), followed by cellulose ( ?9%), and alumina ( ?3%). Since then silica gel has remained the most widely used, but noticeable changes have occurred with the appearance of...
