Differentiation of Furanocoumarin Isomers with Ratio of Relative Abundance of Characteristic Fragment Ions and Application in Angelicae dahuricae Radix

C-5-substituted and C-8-substituted furanocoumarin isomers, two important kinds of furanocoumarin, are widely documented as the main active constituents in Angelicae dahuricae radix. Due to the similar polarity and mass fragmentation pathways of such isomers, it is difficult to distinguish them using mass spectrometric methods. To address this issue, we developed a strategy employing combined full scan and product ion scan modes on an ultra high performance liquid chromatography-quadrupole time-of-flight tandem mass spectrometry (UHPLC-Q-TOF-MS) platform to differentiate four pairs of furanocoumarin isomer, viz. xanthotoxin and bergapten, imperatorin and isoimperatorin, psoralen and isopsoralen, and impinellin and isoimpinellin. A novel method using the ratio of relative abundance (RRA) of characteristic fragment ions was established to distinguish C-5-substituted and C-8-substituted furanocoumarin isomers, using the formula R = M/N, where M and N represent the ratios of relative abundance of characteristic fragment ions of a pair of furanocoumarin isomers. For R value greater than 1, compound M is substituted at C-5, whereas for R value less than 1, compound M is substituted at C-8. This method with good repeatability was applied to identify five pairs of isomeric furanocoumarins in Angelicae dahuricae radix. This is the first method to distinguish C-5-substituted and C-8-substituted furanocoumarin isomers, and can be used in complex matrix.