Simultaneous determination of tryptophan, kynurenine, kynurenic and xanthurenic acids in honey by liquid chromatography with diode array, fluorescence and tandem mass spectrometry detection

A liquid chromatography method using diode array-fluorescence detection and atmospheric pressure chemical ionization mass spectrometry (LC-DAD-FLD and LC–APCI-MS/MS) was developed to quantify the levels of tryptophan (TRP), kynurenine (KYN), kynurenic (KYNA) and xanthurenic (XA) acids in honey. This procedure involved isolating the compounds of interest via solid-phase extraction (SPE) with mixed-mode polymeric cartridges. Chromatographic separation of the analytes was performed in isocratic mode on a Synergi 4μ Hydro-RP 80Å (150 × 4.60 mm i.d.) analytical column at 30 °C. The mobile phase of 20 mM ammonium formate (pH 4) and methanol was passed at a flow rate of 0.5 mL/min. In replicate sets of spiked honey samples, the average analyte recoveries ranged from 60 to 98% for TRP, 55 to 120% for KYN, 65 to 106.5 for KYNA and 56 to 114% for XA. Detection limits ranged from 4 to 36 μg/kg for LC-DAD-FLD to 0.2 and 1.0 μg/kg for LC–APCI-MS/MS. A strong matrix effect was found when MS/MS was employed, necessitating calibration using the standard addition method on matrix-matched standards for each honey type. The method was used to quantify each of the compounds of interest in 17 honey samples of distinct botanical origins.