NORMAL-PHASE LC-MS DETERMINATION OF RETINOIC ACID DEGRADATION PRODUCTS

The degradation products formed when 13-cis retinoic acid (13-cis RA) and all-trans RA were exposed to fluorescent light and air were invest igated. These retinoids are known to undergo Z-E isomerization (due to the existence of four unsaturated double bonds) and oxidation when ex posed to light and air. Analysis by LC was carried out on a 25 cm x 4. 6 mm Zorbax Rx-SIL (5 mu m) with a mobile phase (1.4 ml min-1) of hept ane-THF-acetic acid (96.5:3.5:0.015) and an in-line UV (365 nm) detect or. The LC eluate was coupled through a Vestee universal interface to a Finnigan 4023 mass spectrometer. EI-mass spectra were obtained at 77 eV from m/z 200 to 350 with multiplier voltage of 1200 V. Solid sampl es of 13-cis RA and all-trans RA exposed to light and air and also sol utions of these retinoids in the mobile phase exposed to the same cond itions were used for the analysis. Tentative identities of the degrada tion products from the mass spectra suggest the isomerization of the r etinoids (Z-E isomerism) and the formation of the 5,6-epoxides of thes e isomers. Identities of the 5,6-epoxides were confirmed with chromato graphic and mass spectral data from synthetic samples of the epoxides. Isomerization occurred more readily in solution than in the solid for m and the 13-cis RA isomer oxidized more readily than the all-trans is omer.