Chiral packed capillary chromatography using an in-column immobilized selector : With special emphasis on vancomycin

Sammanfattning: Packed fused silica capillary columns have been used for enantioseparation of drugs and related compounds. The technique to prepare the chiral stationary phase (CSP) involved a reductive amination of aldehyde functionalized silica with a chiral selector. The immobilization was performed in the packed column, thereby minimizing the use of selector. An α-chymotrypsin CSP was used to investigate the influence of the pore size on the chromatographic performance. Both selectivity and efficiency were better on a 1000 Å pore size material.The retention mechanism of some arylpropionic acids on a vancomycin CSP was studied. A two-site retention model was sufficient to qualitatively describe the effects of different mobile phase compositions, including buffer, modifier and counter ion concentration as well as pH. A closer investigation of the immobilization reaction was also conducted for the vancomycin selector with the aim of elucidating if the linkage to the silica affected the enantioselective retention. To accomplish this one of the two amino groups was temporarily protected while the other was linked to the silica. However, no significant difference in enantioresolution between the two defined CSPs formed was obtained in comparison with the one resulting from the immobilization of native vancomycin.Different types of interactions seemed to be important in different modes of chromatography. Reversed phase LC primarily separated acidic compounds while polar organic phase LC (POPLC) was better suited to resolve basic compounds. Supercritical fluid chromatography (SFC) and normal phase LC could separate both acidic and basic as well as neutral compounds, but SFC had superior chromatographic performance resulting in both higher efficiency and selectivity.Acid and base interactions between analyte and selector seems to be the key process for chiral discrimination. It has previously been shown that the N-terminal of vancomycin was important for acidic compounds. With the use of an analogous selector, ristocetin A, it is here indicated that the C-terminal is equally important for resolution of bases.