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:
“Genius has infused itself into nature. It indicates itself by a small excess of good, a small balance in brute facts always favorable to the side of reason.”
—Ralph Waldo Emerson (1803–1882)