RESUMO
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.
RESUMO
The influence of solvent composition and surface tension on the signal intensity of deprotonated molecules [M-H]- in electrospray ionization mass spectrometry (ESI MS) was evaluated using alanine (Ala), threonine (Thr) and phenylalanine (Phe), which have differing levels of hydrophobicity. The surface tension of the ESI solution was varied by changing the ratio of the organic solvents methanol (MeOH) and acetonitrile (MeCN) in water (H2O). In ESI MS, the signal intensity of all the amino acids was increased with decreasing surface tension for the two solutions, H2O/MeOH and H2O/MeCN. The use of H2O/MeCN was more favorable for achieving a strong signal for the analytes compared to H2O/MeOH. The smaller vaporization enthalpy of MeCN compared to MeOH was proposed as one of the most plausible explanation for this. The order of the signal intensity of amino acids was Phe>Thr>Ala, the same order as their hydrophobicity. It can be practically concluded that the use of solutions with lower surface tensions and lower vaporization enthalpies would result in higher signal intensities in ESI MS.
