Determination of synephrine enantiomers in food and conjugated synephrine in urine by high-performance liquid chromatography with electrochemical detection.

Determination of synephrine enantiomers was made with HPLC with electrochemical detection using a chiral ligand-exchange column (Sumichiral OA-6000). The calibration curve for each enantiomer showed good linearity (r = 0.999) at 1.0-500 pmol injected with a detection limit of 1.0 pmol (signal-to-noise ratio (S/N) = 3). Relative standard deviation (RSD) was 1.8% at 50 pmol d-synephrine and 1.6% at 50 pmol l-synephrine. The contents of synephrine enantiomer in food such as Citrus unshiu fruit, orange juice, and marmalade were determined. The present method was also used to determine conjugated synephrine enantiomers in urine following the ingestion of C. unshiu pulp. l-Synephrine in fruit was converted to the conjugated form of synephrine, and l-synephrine underwent chiral inversion to d-synephrine in vivo.