VAB

Figure 6 for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone. The column chosen was 25 cm long 4.6 mm i.d. packed with 5 |im silica particles bonded with the stationary phase Vancomycin (Chirobiotic V).

This stationary phase is a macrocyclic glycopeptide Vancomycin that has a molecular weight of 1449.22, and an elemental composition of 54.69% carbon, 5.22% hydrogen, 4.89% chlorine, 8.70% nitrogen and 26.50% oxygen. Vancomycin is a strongly polar stationary phase which contains 18 chiral centres surrounding three 'pockets' or 'cavities' which are bridged by five aromatic rings and thus can readily offer unique enantiomeric selectivity to a wide range of chiral substances. The column was operated in the normal phase mode using mixtures of n-hexane and ethanol as the mobile phase. Equation [7] is unambiguously validated by the curves relating the corrected retention volume to the reciprocal of the volume fraction of ethanol shown in Figure 6. It is also seen that an excellent linear relationship is obtained with an index of determination very close to unity.

Although silica gel was taken as an extreme example of a normal phase system, any stationary phase that is polar in nature (bonded silica or polymer based), when used with a predominantly dispersive solvent as the mobile phase, will constitute normal phase chromatography. Expressed in a slightly different way, although the interactive mechanism can be extremely complex and difficult to quantitatively predict, if the prevailing forces in the stationary phase are polar and the eluting forces in the mobile phase

If the corrected retention volume in the pure strongly eluting solute is very small compared with the retention volume of the solute in the other pure solvent, i.e. VA«VB which is very often the case in practical LC then eqn [6] simplifies to the simple reciprocal relationship:

Figure 6 Graph of corrected retention volume of the enantiomers of 4-benzyl-2-oxazolidinone against the reciprocal of the volume fraction of ethanol.

Equation [8] has been validated by the results from a number of workers in the field and an example of the expected correlation is given by the curves in

Figure 6 Graph of corrected retention volume of the enantiomers of 4-benzyl-2-oxazolidinone against the reciprocal of the volume fraction of ethanol.

are predominantly dispersive then it is a normal phase system. In practice, normal phase systems are used to separate mixtures of polar substances; they are largely ineffective in separating substances that are largely or exclusively dispersive. For reversed phase chromatography, the converse applies.

Seealso: N/Chromatography: Liquid: Column Technology; Theory of Liquid Chromatography.

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