Effects of solvent composition on molecular ion response in electrospray mass spectrometry: Investigation of the ionization processes

Electrospray ionization of four structurally diverse natural products, actinomycin D, digitoxin, reserpine and quercetin was investigated. Specific attention was paid to the molecular ions formed under typical reversed‐phase high‐performance liquid chromatography (RP‐HPLC) conditions. The response characteristics of [M + H] + , [M + NH 4 ] + and [M + Na] + Ions, observed upon addition of formic acid, ammonium formate and sodium acetate were studied. It was found that the highest response for these molecular ions was obtained with proton, ammonium and sodium ion concentrations of approximately 10 −3 ∼ 10 −4 M. This critical concentration appears to be correlated with the equilibrium between the thermodynamically controlled molecular ions in solution and electrospray produced gas‐phase ions. Gas‐phase ion reactions in the ion transfer region were investigated for digitoxin, and the mechanism of gas‐phase ion formation is briefly discussed.