The vast majority of drugs produced over the past few decades contain one or more chiral centers. Unfortunately, because the wrong enantiomer of a chiral drug can cause harmful side effects, very high enantiomeric purity of therapeutics is acknowledged as essential. Carl Pfeiffer first made the observation in 1956 that chirality can play a major role in the toxicity and specificity of therapeutic agents. The commonly referred to Pfeiffer's Rule states that "the greater the difference between the pharmacological activity of the D and L isomers the greater is the specificity of the active isomer for the response of the tissue under test." Thus, both producing enantiomerically pure formulations and testing for enantiomeric purity are critical. Unfortunately, both of these activities continue to pose significant challenges, even with current state-of-the-art analytical instrumentation.

The realization that enantiomeric purity plays a critical role in the specificity and toxicity of pharmacological agents has prompted the Food & Drug Administration to increase its regulatory oversight into the enantiomeric purity of approved pharmaceuticals. Recent FDA policy requires that any drug component comprising over 1% of the total composition must be tested with separate toxicology studies. A racemic drug candidate would thus require separate trials for each enantiomer. While enantiomeric purity is not mandated by the FDA, the huge cost of clinical trials virtually ensures that pharmaceutical companies will develop enantiomerically pure drug candidates, provided the enantiomers do not readily interconvert.

A variety of process solutions, such as resolution of racemates or asymmetric synthesis, are utilized by the pharmaceutical industry to provide these enantiomerically pure drugs. Owing to a lack of suitable catalysts or to insufficient selectivity, efficient enantioselective synthesis of these enantiomers still remains a major challenge. As a result, resolution methods continue to dominate in industrial production. Ongoing advances in the area of directed evolution and metabolic engineering show promise in changing this situation. Recent progress in the area of biocatalysis towards providing more stereoselective catalysts is encouraging, but the number of enzymes commercially available remains limited. In some special cases, where screens have been established for a substrates enantiomeric purity, directed evolution has shown the capacity for improving the enantioselectivity of enzymes on novel targets. Overall though, the general rule that "you get what you screen for" still holds, and prior to the introduction of MOPED, no generally applicable method for high throughput enantiomeric purity screening has been available to the researcher.