Elsevier

Chemosphere

Volume 41, Issue 5, September 2000, Pages 725-727
Chemosphere

Enantiomer fractions instead of enantiomer ratios

https://doi.org/10.1016/S0045-6535(99)00431-2Get rights and content

Abstract

The use of enantiomer ratios (ERs) to indicate the relative amounts of a pair of enantiomers in a sample has some disadvantages. Enantiomer fractions (EFs) are proposed as an alternative expression to eliminate the difficulties.

Introduction

Bioaccumulation and metabolism in biota are often different for enantiomers; therefore, a change of the relative amounts in which an enantiomeric pair is present can occur during disposition in the food chain. A significant deviation from the ratio in which the enantiomers are present in the technical or commercial mixture, in which they are usually present in equal amounts, suggests a specific metabolic transformation of one of the enantiomers. A constant ratio, on the other hand, points to biological persistence or a non-specific metabolic transformation. An aspect of additional interest is that enantiomers often have different toxic properties.

Usually, enantiomer ratios (ERs) are expressed as the peak area or peak height of the (+)-enantiomer divided by that of the (−)-enantiomer Mossner et al., 1992, Muller et al., 1992, Oehme et al., 1994, Glausch et al., 1996. When it is not known which conformation the enantiomers eluting from a chromatographic column have, ER is often expressed as the peak area or height of the first eluting enantiomer divided by that of the second one (Kallenborn et al., 1994)ER=Peakareaofenantiomer1Peakareaofenantiomer2.Other expressions used are the enantiomeric excess (e.e.) and the chromatographic purity (c.p.) Bicchi et al., 1994, Beesley and Scott, 1998e.e.=R−SR+S×100%,c.p.=RR+S×100%,where R and S are the well-known indications for the structural conformation of the enantiomers.

In the daily practice, ER is the parameter most frequently used. However, as will be outlined in Section 2, its use has some disadvantages and an alternative expression will therefore be proposed.

Section snippets

Discussion

Calculating the ratio of two enantiomers by means of ER gives an undefined result when the second enantiomer is not, or cannot be, detected. This was observed in a previous study (de Geus et al., 1998) and, therefore, the peak area of the second enantiomer was divided by that of the first one instead (ER′). Of course, this approach only shifts, and does not solve, the problem. The proper way to solve this problem is to divide the peak area of interest by the limit of detection expressed as peak

Examples

When calculating the mean ER value it is important to use the raw data instead of the calculated ER values, which is not always done correctly in the literature. On the other hand, the mean value can be calculated directly from the EF values and the raw data are not necessary, which obviously is an advantage. When, for example, a duplicate measurement is performed and the areas (in arbitrary units) of the enantiomers are 12.5 and 10.0 in the first run, and 10.0 and 12.5 in the second run, ERs

Summary

Using enantiomer fractions, EFs, rather than enantiomer ratios, ERs, has the advantage that plots of EF vs. the fraction of an enantiomer are linear, that there are no undefined values anymore, that correct mean and standard deviation values are obtained and that equal excesses of enantiomer 1 or enantiomer 2 will immediately be recognised because the deviation from the racemic value of 50% will be the same.

References (10)

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There are more references available in the full text version of this article.

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    In environmental and biodegradation analysis, for sake of comparison, the enantiomeric concentrations are presented in the form of EF. This notation makes straightforward the biodegradation plots [36,37,67]. Fig. 2 exemplifies the variation of EF in the biodegradation of ofloxaxin (racemate) and levofloxacin (S-enantiomer) in a period of time [67].

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