Differentiation of Four Pairs of Furocoumarin Isomers by Electrospray Ionization Tandem Mass Spectrometry

A mass spectrometric method based on the combined use of electrospray ionization, collision-induced dissociation (CID) and ion-trap time-of-flight (IT-ToF) tandem mass spectrometry was used to identify four pairs of furocoumarin isomers. Our results show some characteristic mass spectrometric differences in isomeric furocoumarins. In the MS² spectra, the relative abundance ratios of m/z 131 [M + H – 2CO]⁺ to m/z 143 [M + H – CO₂]⁺ and m/z 115 [M + H – CO–CO₂]⁺ of psoralen are less than one, but both ratios are greater than one in the case of isopsoralen. The relative abundance of m/z 159 [M + H – CO₂]⁺ is much higher than that of m/z 175 [M + H – CO]⁺ in the case of bergaptol, but this trend is reversed in xanthotoxol. Moreover, three diagnostic ions of m/z 189 [M + H – CO]⁺, m/z 185 [M + H – CH₄O]⁺ and m/z 161 [M + H – 2CO]⁺ are observed in the MS² spectrum of xanthotoxin but not in that of bergapten. Further, two diagnostic ions of m/z 226 [M-H-CH₃-CO]⁻ and m/z 210 [M – H – CH₃ – CO₂]⁻ are observed in the MS² spectrum of isoimperatorin but not in that of imperatorin. The relative abundance of product ions associated with some diagnostic ions can be used to characterize furocoumarin isomers.