Metrics for Expressing Enantiomer Composition of POPs

Two principal metrics are used for reporting enantiomer compositions of chiral POPs the enantiomer ratio (ER) and the enantiomer fraction (EF). Most earlier studies used ER, the ratio of the (+)-enantiomer concentration over that of the (—)-enantiomer. Chromatographic parameters for calculating concentrations, such as peak area or height, can also be used. If the elution order is unknown, then the ER is reported as the ratio of the first-eluted enantiomer (El) to the second-eluted one (E2) on a specific column and conditions  

The ER ranges from zero to infinity, with a racemate having an ER of 1. The EF [107,108], defined as the ratio of the (+)-enantiomer or El-enantiomer concentration to the 

The range in EF is from zero to unity, with a racemic value of 0.5. Enantiomer fractions are preferred to ERs, as the EF range is bounded, and a deviation from the racemic value in one direction is the same as that in the other. For example, if the (—)-enantiomer is twice the concentration as its antipode, the EF is 0.333, which is the same deviation (0.167) from a racemic EF of 0.5 as the opposite case of the (+)-enantiomer at twice the concentration as the (—)-enantiomer (EF = 0.667). The respective ERs would be 0.5 and 2. The corresponding deviations of 0.5 and 1, respectively, are not the same deviation from the racemic ER of 1. Thus, ERs can produce skewed data inappropriate for statistical summaries such as sample mean and standard error [109]. As a result, EEs are more amenable compared to ERs for graphical representations of data, mathematical expressions, mass balance determination, and environmental modelling [107, 109]. Individual ER and EF measurements can be converted [107, 108]  

Conversion of summarized values (e.g. mean ER cr to the equivalent EF tr) can lead to substantial discrepancies, and should be avoided [109]. In addition, the conventions used in describing ERs and EFs may differ between studies and analytical methods. For example, enantioselective separations on different stationary phases may result in reversal of elution order and lead to different values if elution orders are not known. In this discussion, EFs are used to the extent possible, and both EF and ER are defined using using Equations (4.1) and (4.3), respectively, for those analytes for which the elution order is known unless otherwise indicated. Otherwise, these metrics are defined using Equations (4.2) and (4.4) on the specified column. 

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