- Simultaneous analysis of retinol, all-trans- and 13-cis-retinoic acid and 13-cis-4-oxoretinoic acid in plasma by liquid chromatography using on-column concentration after single-phase fluid extraction. Teerlink, T., Copper, M.P., Klaassen, I., Braakhuis, B.J. J. Chromatogr. B Biomed. Sci. Appl. (1997)