Analysis of calcium adduct molecules of paeoniflorin and albiflorin using electrospray ionization quadrupole time-of-flight mass spectrometry

Paeoniflorin and albiflorin were investigated by electrospray quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS) in positive-ion mode, when ultrapure water was selected as solvent. In full-scan mode, the ions [M+H2O+Ca]2+, [M+2H2O+Ca]2+, [2M+Ca]2+, [3M+Ca]2+, and [4M+Ca]2+ as the primary ions were detected. Low-energy collision-induced experiments for the precursor [M+H2O+Ca]2+ and [2M+Ca]2+ ions provided many interesting product ions. Most product ions are formed by combinations of a paeoniflorin (or albiflorin) molecule, benzoic acid anion, aglycone moiety, glucosyl group, H2O, or Ca2+ ion. Strong noncovalent bond formed by Ca2+ and O atom of neutral molecules and proper coordination number might contribute to producing these complex ions. A labeling experiment involving H/D exchange of paeoniflorin indicates the H2O molecule did not come from solvent. Although the fragmentation patterns of the isomers of paeoniflorin and albiflorin are similar, they can be completely distinguished by the MS/MS spectra of either [M+H2O+Ca]2+ or [2M+Ca]2+.

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► Calcium adduct molecules as the most abundant primary ions were detected in positive-ion mode.
► Many interesting complex product ions were detected in MS/MS mode.
► Complex product ions might contribute to understanding the unique character of calcium adduct molecules in the gas phase.
► Sufficient information obtained from calcium adduct molecules is especially valuable for rapid identification of the isomers.