Liquid chromatography electrospray ionization tandem mass spectrometric determination of quassin and neoquassin in fruits and vegetables

Quassia amara wood chips are used by organic farmers as a valid alternative to synthetic insecticides. The powder of Q. amara contains high levels of quassin, neoquassin and picrasinoside B. In this study we developed a liquid chromatography mass spectrometry method for the rapid and accurate quantification of the insecticide quassinoids on fruits and vegetables. Quassinoids were extracted from fruits and vegetables with acetonitrile and separated on a Zorbax Column Eclipse XDB C8 by isocratic elution with a mobile phase consisting of water and methanol with 0.1% of formic acid. Using a high-performance liquid chromatograph coupled with an electrospray ionization tandem mass spectrometer (HPLC/ESI-MS/MS), quassinoids were selectively and simultaneously detected monitoring the multiple reaction (MRM) transitions of proton adduct precursor ions: m/z 389.5→222.9 for quassin, 391.5→372.9 for neoquassin and 576.1→ 394.5 for picrasinoside B. For all quassinoids calibration was linear over a working range of 1 and 100 μg/kg with r > 0.991. Limit of determination (LOD) and limit of quantification (LOQ) for both quassinoids were 0.5 and 1 μg/kg respectively while for picrasinoside B they were 5 and 10 μg/kg. Quassinoid recoveries ranged from 85.3% to 105.3% with coefficients of variation between 2.5% and 12.8% for fruit and vegetables. The presence of interfering compounds in the fruit and vegetable extract was evaluated and found to be minimal. Due to the linear behavior it was concluded that the multiple reaction transitions of precursor ions can be used for analytical purposes, i.e. for identification and quantification of quassin, neoquassin, and picrasinoside B in fruit and vegetable extracts at trace levels.