DETERMINATION OF ASPARTIC-ACID, PHENYLALANINE, AND ASPARTYLPHENYLALANINE IN ASPARTAME-CONTAINING SAMPLES USING A PRECOLUMN DERIVATIZATION HPLC METHOD

The sweetener aspartame (APM) and its hydrolysis products, aspartylphenylalanine (AP), aspartic acid (Asp), and phenylalanine (Phe), react with naphthalene-2,3-dicarboxaldehyde (NDA) in the presence of cyanide ion (CN) in a mildly alkaline medium to give highly fluorescent 1-cyano-2 substituted benz[f] isoindole (CBI) derivatives. These CBI derivatives are readily resolved by reverse-phase high-performance liquid chromatography, giving detection limits in the subpicomole range. Since APM can readily hydrolyze to AP or cyclize to give diketopiperazine (DKP) under alkaline conditions, accurate determination of the levels of AP in analytical samples is possible only after APM is separated from Asp, AP, and Phe by solid-phase extraction, followed by alkaline hydrolysis to give AP, which reacts quantitatively with NDA to form the fluorescent CBI product. Assay procedures are described for the determination of Asp, Phe, AP, and APM in diet beverage samples. © 1990, American Chemical Society. All rights reserved.