Internal reference, absolute configuration determination

Determinations of absolute configuration by internal comparison are becoming rather frequent (see Table 5) but they may be unreliable since occasionally authors seem to only assume the absolute configuration of the chiral reference component in the structure. 

Absolute configurations can be determined by X-ray crystallography, either by the method of anomalous X-ray scattering or by that of internal comparison with a reference of known absolute configuration. 

Besides the chiral resolution of pollutants, GC determination of enantiomeric ratios (ERs) can be used for the quantification of the enantiomers in complex matrices, with a known quantity of the pure enantiomer added as an internal standard. With a method of enantiomer labelling, a known quantity of an enantiopure standard is added to the mixture (or an aliquot of it) and the amount of the enantiomer originally present is calculated from the change in the ER after addition of the standard. This method provides precise knowledge of the ERs of the sample and the standards [87]. In addition, the absolute configurations of the pollutants may be determined directly, and free of chiroptical evidence, by GC via co-injection of reference pollutants with known stereochemistry [87]. 

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