Sample preparation for routine high-performance liquid chromatographic determination of retinol palmitate in emulsified nutritional supplements by solid-phase extraction using monosodium l-glutamate as dissolving agent

Retinol palmitate (vitamin A, 73.3 μg/g) in an emulsified nutritional supplement was determined by solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC) with fluorescence detection (excitation 350 nm, emission 480 nm) using monosodium l-glutamate as a dissolving agent to obtain higher recovery of vitamin A from the emulsified sample solution. A Bond Elut C2 cartridge (500 mg) was chosen for SPE after comparison with 16 other types. A sample solution was applied to a conditioned Bond Elut C2 cartridge and then vitamin A was eluted with ethanol followed by HPLC. The proposed method was simple, rapid (sample preparation time by SPE: ca. 8 min, retention time: ca. 8 min), sensitive [detection limit: ca. 0.1 pg/injection (100 μl) at a signal-to-noise ratio of 3:1], highly selective and reproducible (relative standard deviation (RSD): ca. 2.9% (n=5), between-day RSD ca. 3.7 (5 days). The recovery of vitamin A was over 90% by the standard addition method.