Chromatographic determination of thiols after pre-column derivatization with o-phthalaldehyde and isoleucine

The reaction of primary amines with excess o-phthalaidehyde (OPA) and thiol yields unique isoindole derivatives that are readily separated by reversed-phase liquid chromatography. In a previous work, a spectrophotometric procedure was proposed for the assay of N-acetylcysteine by derivatization with OPA and isoleucine at pH 9.5, with satisfactory results. The chromatographic determination of this and other low molecular-weight thiols, after isoindole formation with isoleucine, using mobile phases of acetonitrile-water at pH 3 and spectrophotometric detection, is now examined. From the assayed thiols (thioglycolic acid, 3-mercaptopropionic acid, tiopronin, N-acetylcysteine, N-acetylpenicillamine, glutathione, captopril, cysteamine, cysteine, and penicillamine), the latter three did not seem to form isoindole derivatives measurable at 335 nm. The factors affecting the formation and stability of the derivatives were examined. Glutathione, tiopronin, and captopril produced double peaks, whereas N-acetylcysteine, N-acetylpenicillamine, 3-mercaptopropionic acid, and thioglycolic acid gave rise to single peaks at reasonably low times, using a mobile phase containing 40% (v/v) acetonitrile at pH 3. The procedure was applied to the chromatographic determination of N-acetylcysteine in pharmaceutical formulations with good results.