Rapid extraction, discrimination and quantification of thermally unstable isomeric acteoside and isoacteoside in natural products by online extraction-quadrupole time-of-flight tandem mass spectrometry

Abstract

Rapid extraction, discrimination and quantification of isomers, especially for thermally unstable isomers, in complex natural products by direct MS are of great challenge due to the time-consuming sample pretreatment procedures, thermal instability, and identical mass behaviors. This work developed an online extraction-quadrupole time-of-flight tandem MS (OLE-QTOF-MS/MS) system for quantification of isomeric phenylethanoid glycosides (acteoside and isoacteoside; acteoside isomerizes to isoacteoside when heated) in Clerodendrum bungei within 1.5 min. Solid C. bungei was packed into a guard column, which was then positioned on the manual injection valve for OLE-QTOF-MS/MS analysis. The relative intensity ratio of diagnostic ion pairs (I₄₆₁/I₁₆₁) fragmenting at the heterogeneous linkage site changed linearly with the relative content of acteoside from 0% to 100% (R² = 0.9991), and then the content ratio of acteoside and isoacteoside in C. bungei was determined as 85.3 : 14.7 (m/m) with an RSD of 1.7%. Finally, the absolute quantification results (acteoside, 17.5 ± 0.8 mg g⁻¹ and isoacteoside, 3.0 ± 0.8 mg g⁻¹) were obtained through a standard addition method with RSD < 3.4% for intra- and inter-day variations. Results demonstrated that OLE-QTOF-MS/MS was a promising and practical method for simple, rapid and effective extraction and analysis of isomeric compounds in complex matrices.