Combined HPLC−MS and HPLC−NMR On-Line Coupling for the Separation and Determination of Lutein and Zeaxanthin Stereoisomers in Spinach and in Retina

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Abstract
The determination and unambiguous identification of carotenoid stereoisomers from biological tissues, avoiding isomerization and oxidation due to the extraction process, is still a major challenge. Particularly, the analysis of lutein and zeaxanthin stereoisomers is of great importance, as these are the main constituents of the macula lutea, the central part of the human retina, and act as possible agents in the prevention and treatment of age-related macular degeneration (AMD). By combining a mild and quick extraction technique such as matrix solid-phase dispersion together with high-performance liquid chromatography (HPLC), the extremely light and oxygen sensitive lutein and zeaxanthin stereoisomers are extracted, enriched, and separated directly from the solid plant or tissue samples, excluding preparation of artifacts. HPLC separations are performed with C30 phases due to their enhanced shape selectivity compared to C18 phases and on-line coupled to mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy. By using HPLC−MS with atmospheric pressure chemical ionization, the lutein stereoisomers can be distinguished from the zeaxanthin stereoisomers within one chromatographic run in the upper picogram range, whereas HPLC−NMR coupling allows the unequivocal identification of each stereoisomer with a concentration in the upper nanogram range. This article provides an analytical method for the artifact-free determination of lutein and zeaxanthin stereoisomers directly from the solid biological tissue spinach as a source of carotenoids and retina as the sphere of activity for AMD. In addition, the structures of these stereoisomers were unambiguously elucidated by employing hyphenated analytical techniques.