CHIRAL SEPARATIONS. CHIRAL DYNAMIC CHROMATOGRAPHY
8. DYNAMIC CHROMATOGRAPHY AS A TOOL TO QUANTIFY CATALYTIC SITES BONDED ON CHROMATOGRAPHIC SUPPORTSSITES BONDED ON CHROMATOGRAPHIC SUPPORTS
A severe problem in chromatographic analyses requiring rigorous quantification of chiral drugs is when the species to resolve may be involved into secondary equilibria during their separation. In fact, both the selector and matrix of the SP may be endowed on their surfaces with chemical groups responsible for possible catalytic effects. Such an event was quite recently specifically addressed in a study showing that even international pharmacopoeias can contain shortcomings arising from the aforementioned chemical lability (Cabri et al., 2008; D’Acquarica et al., 2010). Dihydroartemisinin is a powerful antimalarial drug that may easily interconvert between its two epimeric forms,aandb. It was proposed as an effective strategy to set optimized operating conditions (suitable choice of column length, MP flow rate, and temperature) aimed to slow down the secondary equilibrium. It was also stressed that commercial columns containing the same SP as the discriminating tool (all RP-C18 columns, in the specific case) can have a marked differential SPDPCeffect in promoting theb$aepimerization. In other words, the concentration of catalytic sites present on the SP surfaces not prepared according to standardized and uniform procedures may significantly differ from each other. However, the experi- mental determination of the2kvappDPC ½SSP term present inEq. (3.7) of Section 7may represent a valuable tool to gain quantitative information on the abundance of SSPsites. The results obtained could have direct implications in any analytical investigation aimed at quantitating this family of drugs.
In parallel studies performed on an anti-MAO drug (Cirilli et al., 2009b), within the limit of the adopted approximations (which include the assumption of amino groups as basic SSPsites bonded to the silica matrix, uncovered by the deposed chiral selector), the [SSP] concentration was estimated as 4.5103M. This, in turn, was assessed to correspond to uncovered amino groups included in a range from 3% to 5% of the whole parent amino groups originally bonded to the matrix. Therefore, such an approach discloses wide perspectives for studies aimed to give accurate characterization of the surface of chromatographic supports. Nevertheless, for a rigorous application of DC methods to these kinds of determinations, further dedicated studies have to be envisaged.
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FURTHER READING
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