phases was first introduced by Armstrong [39]. One method of preparation is to covalently bond Vancomycin to the surface of silica gel particles. Vancomycin contains 18 chiral centers surrounding three 'pockets' or 'cavities' which are bridged by five aromatic rings. Strong polar groups are proximate to the ring structures to offer strong polar interactions with the solutes. This type of stationary phase is stable in mobile phases containing 0–100% organic solvent. The proposed structure of Vancomycin is shown in figure 49. A, B and C are inclusion cavities. Molecular weight 1449. Chiral centers 18. pK's 2.9, 7.2, 8.6, 9.6, 10.4, 11.7. Isoelectric point 7.2 Courtesy of ASTEC Inc.   Figure 49 The Proposed Structure of Vancomycin

Vancomycin is a very stable chiral stationary phase, has a relatively high sample capacity, and when covalently bonded to the silica gel, has multiple linkages to the silica gel surface. A, B and C are inclusion cavities. Molecular weight 1449. Chiral centers 18. pK's 2.9, 7.2, 8.6, 9.6, 10.4, 11.7. Isoelectric point 7.2   Courtesy of ASTEC Inc.   Figure 52 The Proposed Structure of Vancomycin   It can either be used with mobile phases with a high water content, as a reversed phase, or with a high solvent content, as a largely polar stationary phase. For example, when used as a reversed phase the strongly polar THF–water mixtures would be very effective mobile phases. Conversely, when used as a polar stationary phase, n-hexane–ethanol mixtures would be more appropriate. Vancomycin has a number of ionizing groups and thus can be used over a range of

stationary phase. The mobile phase (termed by the manufacturers as elution by the polar organic mode) is strongly polar so the major retentive mechanism is likely to be predominantly dispersive in nature modified by the selective interactions afforded by the chiral centers of the Vancomycin (see The Mechanism of Chromatographic Retention ).   Courtesy of ASTEC Inc.   Figure 38. The Chromatographic Properties of the Enantiomers of Nicardipine when Retained on the Chirobiotic Vancomycin

spatial arrangement of chemical groups or atoms around a single atom, referred to as the stereogenic center or the chiral center(s). Such differences impart very subtle physical chemical variations between the individual isomers and, thus, must be carefully exploited if a chromatographic separation of the individual enantiomers is to be achieved. A number of antibiotics have been shown to be very effective stationary phases for the separation of chiral mixtures. An example of one of these is Vancomycin the structure of which is depicted in figure 37. Vancomycin has three fused rings (shown as rings A, B and C in figure 37) and 18 chiral centers which endows it with the characteristic strong chiral selectivity exhibited by many antibiotics.   Figure 37. The Structure of Vancomycin

nbsp;As an example of the application of the opimizing equations the retention data reported by Beesley and Scott (15) for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone will be employed. The column that was used was 25 cm long, 4.6 mm I.D., packed with 5 mm silica particles bonded with the stationary phase Vancomycin (Chirobiotic V, Advanced Separations Technology Inc., Whippany New Jersey). Vancomycin is a macrocyclic glycopeptide thaving a molecular weight of 1449.22, and containing 18 chiral centers surrounding three 'cavities' bridged by five aromatic rings. The column was operated in the normal phase mode using mixtures of n-hexane and ethanol as the mobile phase. As the interactive character of the solvent mixture was highly dispersive, the mechanism of retention, and the chiral

a very stable chiral stationary phase, has a relatively high sample capacity, and can be covalently bonded by multiple linkages to the silica gel surface. It can be used with mobile phases with a high water content, as a reversed phase, or with a high solvent content, as a largely polar stationary phase. For example, when used as a reversed phase THF–water mixtures are very effective mobile phases. Conversely, when used as a polar stationary phase, n-hexane–ethanol mixtures are often employed. Vancomycin has a number of ionizing groups and thus can be used over a range of different pH values (pH 4.0 to 7.0) and exhibit a wide range of retention characteristics and chiral selectivities. Ammonium nitrate, triethyl-ammonium acetate and sodium citrate buffers have all been used satisfactorily with this stationary phase. The effect of the chosen buffer has little or no effect on chiral selectivity only on pH control. An example of the use of Vancomycinas a stationary phase to separate