Ion Pair Chromatography

The use of ion pair chromatography (IP-TLC) of alkaloids may be carried out on normal and reversed phases. This technique has been applied to analyse basic drugs, including alkaloids, on silica gel using normal-phase systems. The best results are obtained by applying chloride and bromide as counterions of at least 0.1 mol L"1 concentration in the spreading slurry or in the solvent.

Reversed-phase IP-TLC is far more widely used. The counterion reagents which may be present in the mobile phase and serve for impregnation in the nonpolar stationary phase may be di-(2-ethylhexyl) or-thophosphoric acid (HDEHP), camphoric and cam-phorosulfonic acids, sodium dodecylsulfate and simple hydrophilic anionic reagents such as perchloric acid, oxalic acid and trichloroacetic acid. The acidic environment of the mobile phase ensures ioniz-ation of the acidic counterions and enables the creation of an ion pair with the alkaloids protonized under these conditions. The behaviour of some isoquinoline bases using RP-18 plates and alkylsul-fonates as counterions has also been investigated.

Although retention and separation selectivity in IP-TLC depend on many factors, optimization of such chromatographic systems is basically concerned with pH changes, concentration and the chain length of the counterion or the concentration of organic modifier in the mobile phase.

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