Do the Electrospray Mass Spectra Reflect the Ion Concentrations in Sample Solution ?

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The positive and negative ion formations from electrosprayed neutral, basic, and acidic aminoacid (M) solution were investigated as a function of concentrations of CH₃COOH and NH₃ added in solution. For neutral aminoacids, ammonia solution gave as strong (M+H)⁺ ion signal as acetic acid solution in the positive ion mode. In the negative ion mode, a strong (M-H)^- ion signal appeared from acetic acid solution. For basic aminoacids, ammonia solution of histidine and lysine gave strong (M+H)⁺ ion signals. For acidic aminoacids, fairly strong (M+H)⁺ ions for aspartic and glutamic acids were observed from ammonia solution. These experimental results clearly indicate that the ions produced from the electrosprayed charged liquid droplets do not reflect the equilibrium concentrations of ions in solution. The appearances of (M+H)⁺ ions from electrosprayed ammonia solution and (M-H)^- ions from electrosprayed acetic acid solution are due to the protonation and deprotonation of M by NH₄⁺ and by CH₃COO^- ions, respectively, in the electrosprayed liquid droplets.