Separation Methods

It is obvious that the kind of adsorbent used and solvent system composition determine the separation mechanism occurring in the chromatographic process. The adsorbent also determines the method of sample preparation. Thus, for adsorption and partition chromatography, alkaloids are mostly applied as bases in organic polar solvents; for ion exchange sorbents they are applied in the form of salts in aqueous solution.

Choosing the optimal chemical character of the stationary and mobile phase is especially important in the case of alkaloids because of the ionization ability of their molecules. Dissociation of bases in aqueous solution can be expressed by the following equation:

B # H2O ^BH+ # OH~ or, in the case of the conjugated acid BH#, by: BH + ^ B # H +

with a dissociation constant (acidic) Ka.

The dependence of the molar ratio of nondis-sociated molecules [B] to the total concentration of an alkaloid [B] + [BH + ] on the pH of the mobile phase is shown in the curves presented in Figure 2. The pKa values of chosen alkaloids are summarized in Table 1.

For TLC of alkaloids, numerous chromatographic systems have been reported. Some are presented in Table 2, together with their practical applications.

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