Ion Exchange Chromatography

Ion exchange techniques are applied for both crude fractionation and separation and determination of alkaloids.

The typical ion exchange sorbents used for TLC of alkaloids have been as follows: anion exchangers AG 1-X4 and Cellex D, and cation exchangers with cellulose (alginic acid and sodium carboxymethylcel-lulose), paraffin (Rexyn 102) and polystyrene (Dowex 50-X4) matrices.

While choosing the best eluent for ion exchange chromatography, pH values should be carefully considered. They are closely correlated with the number of charges in the alkaloid molecules and at the same time decide the retention values. The trends for most alkaloids fit the type of curves shown in Figure 3.

One of the popular adsorbents which may function as an ion exchanger is aluminium oxide (AI2O3) with an aqueous mobile phase. Depending on the kind of aluminium oxide used, a cation- or anion-exchanging mechanism may occur. Thus, in aqueous alcoholic solution basic alumina functions as a cation exchanger (I), but acidic alumina acts as an anion exchanger (II). With neutral alumina, both types of reactions may take place depending on the conditions used:

(I) Al-O-Na # (BH) + Cl_ pAl-OH + B + Na+ # Cl"

(II) Al-Cl # BH + OH" p Al-OH # (BH)+ Cl"

Figure 3 RM versus pH curves for some alkaloids on alginic acid thin layers (after Lepri L, Desideri PG and Lepori M (1976) Chromatographic Behaviour of Alkaloids of Thin Layer of Cation Exchangers. Journal of Chromatography 123, 175. Amsterdam: Elsevier).
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