Analytical methods determination of enantiomeric purity

As mentioned earlier, optical rotations are particularly sensitive to temperature and concentration, and it has been estimated that errors in measured rotations from these combined effects are at least 4%. 

Although optical and enantiomeric purities are usually equated, this is not necessarily correct. Indeed it was with (1) that the inequivalence of enantiomeric purity and optical purity was first demonstrated unequivocally. If optical rotation does not vary linearly with concentration (and this may occur even in polar solvents) then an alternative method for measuring enantiomeric excess must be sought. 

It is often possible to correlate absolute configuration with the sense of optical rotation, and several empirical rules have been devised over the years based on this premise. Unfortunately this is not 

In summary, measurements of optical rotation may be used to determine enantiomeric purity, but only if the readings are taken carefully with a homogeneous sample under controlled conditions and with deference to the factors which affect measured rotations (temperature, concentration, wavelength, solvent). The determination of the enantiomeric purity on the basis of optical rotation measurements should be confirmed by an independent method before assigning absolute rotations. 

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