Unexpected hydrated ions in the detection of sodium adduct coumarins using electrospray tandem mass spectrometry

A series of coumarins were analyzed using electrospray ionization quadrupole time-of-flight tandem mass spectrometry in the positive-ion mode. Unexpected hydrated ions ([M + H₂O + Na]⁺) was observed upon collision-induced dissociation of the sodiated ions ([M + Na]⁺) of eight coumarins. Several factors which affected relative abundance of [M + H₂O + Na]⁺ ions such as collision energy, concentration and solvent were investigated. None of them have effect on the relative abundance of [M + H₂O + Na]⁺. However, the peak of hydrated ions was not detected in the further collision-induced dissociation of protonated ions of coumarins. Apigenin and Quercetin sharing similar benzopyrone structural unit with coumarins are selected for tandem mass spectrometry analysis. There were no hydrated ions in their tandem mass spectrometry spectra of the precursor [M + Na]⁺ ions. Thus, both coumarins and sodium were necessary for the formation of [M + H₂O + Na]⁺. Together with the result that hydrated ions are not formed by hydrolysis reactions, a six-membered ring structure which involves with the formation of [M + H₂O + Na]⁺ was presented. And D-labeling experiment indicates that the H₂O molecule did not come from solvent.