Research Communications Enantiomer Fractions Are Preferred to Enantiomer Ratios for Describing Chiral Signatures in Environmental Analysis TOM HARNER,* ,† KARIN WIBERG, ‡ AND ROSS NORSTROM § Atm ospheric Environm ental Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada, Department of Chem istry, Environm ental Chem istry, Um ea ˚ University, SE-901 87 Um ea ˚, Sweden, and Environment Canada, Hull, Quebec K1A 0H3, Canada The enantiomer ratio (ER) is currently the standard descriptor of enantiomeric (chiral) signatures for environ- mental samples. In this paper, we argue for the adoption of the enantiomer fraction (EF) as the standard descriptor by showing drawbacks to the use of ER. The enantiomer fraction is superior because it provides a more meaningful representation of graphical data and is more easily employed in mathematical fate expressions. Several useful expressions are presented that allow EF to be used for tracking and apportioning chemical movement between environmental compartments and for investigating microbial degradation processes. Introduction Chiral analysis is becoming increasingly popular in the field ofenvironmentalscience for investigatingthe transport and fate of chemicals in various media (1). This technique has recently been applied to environmental samples to yield information on air -water exchange of R-HCHs in oceans (2) and lakes (3), the revolatilization of pesticides from soils (4), and the importance of microbial degradation in controlling environmentallifetimes ofpersistent chemicals (5). Analysis ofchiralcompoundsin variousbiologicalcompartmentsmay provide valuable insight to how chemicals are accumulated, degraded, and translocated within food chains (6). The property of chirality is attributed to a compound if it can exist as two non-superimposable mirror image forms s similar to our left and right hands. These two forms are designated as (+) and (-) enantiomers based on their interaction with plane-polarized light. Chiralcompounds of environmentalsignificance include R-hexachlorocyclohexane (R-HCH), cis- and trans- chlordane, o,p ′-DDT, heptachlor, and heptachlor- exo-epoxide (HEPX). Some PCBs and their metabolites (e.g., methylsulfone PCBs) exhibit axialchirality (atropisomerism) due to hindered rotation about the bi- phenyl σ-bond (6). Physical processes are not able to distinguish between the two enantiomeric forms of a compound.Consequently,chiralchemicalsare almostalways produced as a racemate in which 50% of the compound is the (+) form and 50% is the (-) form. In the environment, the racemic signature remains unchanged by physical removal mechanisms such as hydrolysis and photolysis reactions. However, the mechanisms of microbial degrada- tion and biologicalmetabolism maybe enantioselective and thus alter the enantiomer signature. Enantioselective per- meability through biological membranes has also been indicated.Totallyselective transfer of(+)- R-HCH across the blood -brain barrier occurs in seals and rats, whereas the ER in blubber is between 1 and 2 (7, 8). This altered signature or “fingerprint” can be exploited to track a compound’s movement and transformation. Chromatography, using a chiral stationary phase, is able to separate the (+) and (-) enantiomers in environmental samples. Until now, the most popular way for describing this altered signature was to use the concept of enantiomer ratio (ER) where [A + and A - correspond the peak areas of the (+) and (-) enantiomers; equal molar response factors are assumed]. The ER in the sample is often compared to the value in a standard that is typically racemic, i.e., ER ) 1.0. However, there are several limitations to using ER. When used graphically, the ER results in misleading representation of data. Because ofthe way it is defined, the ERcan range from 0 to infinity. The ER of R-HCH in seal brain is approximately infinity, and it is therefore not possible to represent it graphically (7). Therefore, a unit change in ER away from unity in the downward direction (i.e., <1) is not equivalent to the same unit change in the opposite direction. Compli- cations may also arise when the ER is employed in math- ematical expressions. We propose that a better representation of the chiral signature is the enantiomer fraction (EF) where where A 1 and A 2 are the first and last eluting enantiomers on chiral column x when the identity of the (+) and (-) forms is not known. Dividing the numerator and denominator by A 2 gives Nowdividingboth numeratorand denominatorbyERresults in the simple relationship EF ) 1/ (1 + 1/ ER). The EF can only range from 0 to 1.0 with EF ) 0.5 representing a racemic mixture. Each unit ofdeviation from the racemic value (0.5), both in the upward and downward direction, is equivalent. Because it is a proper fraction, the EF can also be applied more naturally in mathematical fate expressions. The EF of R-HCH in seal brain is ∼1(7), which is easy to represent graphically. In this paper, two examples are presented that highlight the advantages of EF versus ER. We also consider several usefulmathematicalfate expressions from the literature that employ ERs and rewrite these equations using EF format. Ultimately, we hope to make clear that the enantiomer fraction (EF) is the preferred descriptor of chiral signatures in environmental samples and should be adopted when presenting results in the literature. *Corresponding author e-mail: tom.harner@ec.gc.ca. † Atmospheric Environmental Service. ‡ Umea ˚ University. § Environment Canada. ER ) A + /A - EF ) A + / (A + + A - ) or EF x ) A 1 / (A 1 + A 2 ) EF ) (A 1 /A 2 )/ [(A 1 /A 2 ) + (A 2 /A 2 )] ) ER/ (ER +1) 218 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 1, 2000 10.1021/es9906958 CCC: $19.00  2000 American Chemical Society Published on Web 12/29/1999