Influence of hydrophilic additives on the signal intensity in electrospray ionization of flavonoid glycosides

Rationale The influence of hydrophilic additives glycine, glucose, and glycerol on electrospray ionization (ESI) signal intensity of flavonoid glycosides and a nonreducing disaccharide is examined. The addition of excess glycine to the ESI solution would affect signal intensity more than glucose and glycerol due to its strong hydration capability. Methods The ESI signal response upon the addition of excess additives prepared was estimated in both selected ion monitoring and scan mode. All the mass spectrometry data were acquired in negative ion mode, because negative ion mode is recommended for saccharide compounds. Results The addition of glycine to the ESI solution of flavonoid glycosides and trehalose enhanced signal intensity, whereas the addition of glucose and glycerol had little effect. The signal intensity of rutin was higher than that of naringin and hesperidin, in accordance with their solubility in ESI solution. Trehalose molecules specifically interacted with glycine molecules to form a 1:1 trehalose–glycine complex, whereas the flavonoid glycosides did not produce such complex ions. Conclusions The ESI signal enhancement of the saccharides with the additive glycine can be explained by its strong hydration capability, with the deprotonated carboxylic oxygens of zwitterionic glycine molecules strongly interacting with water hydrogen atoms resulting in strong hydration enthalpy. Consequently, glycine molecules set the analytes free from solvation with water molecules in the ESI droplets.