The enantiomeric excess of a substance is a measure of how pure it is. In this case, the impurity is the undesired enantiomer (the "opposite-handed" mirror image of a chiral compound).
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Famous quotes containing the word excess:
“Solomon’s ... excess became an insult upon the privileges of mankind; for by the same plan of luxury, which made it necessary to have forty thousand stalls of horses,—he had unfortunately miscalculated his other wants, and so had seven hundred wives....
Wise—deluded man!”
—Laurence Sterne (1713–1768)