A survey of this Journal shows that only a few experiments involving radioactive organic compounds have been published in the last 30 years (1-9). References 10 and 11 cite numerous experiments utilizing radioactive inorganic elements. The experiment described here analyzes a commercial uniformly carbon-14 labeled L-valine sample (L-[U-14 C]Val) for its enantiomeric purity. This type of information is crucial to researchers who require enantiomerically pure radioactive amino acids for investigating stereochemical aspects of biosynthetic pathways (12,13).The experiment utilizes a reverse-isotope dilution technique that was performed on the microscale level in three threehour periods by pairs of students in an advanced chemistry laboratory at SUNY Oswego. The experiment involves the following steps:1. Diluting 0.5-1 µCi of L-[U-14 C]Val with nonradioactive racemic DL-valine, 1 followed by conversion to the highly fluorescent 5-dimethylamino-1-naphthalenesulfonyl (dansyl, DNS) derivatives using DNS-chloride (14). 2. Resolving the enantiomeric DNS-DL-Val by reversedphase thin-layer chromatography (TLC) via diastereomeric inclusion complexes with β-cyclodextrin (β-CD) as a mobile-phase additive (15, 16). 3. Scraping the pure DNS-D-and -L-Val from the plate and subjecting each sample to liquid scintillation counting (LSC) to determine the relative amounts of radioactivity present in each.