Influence of Solvent Composition and Surface Tension on the Signal Intensity of Amino Acids in Electrospray Ionization Mass Spectrometry

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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.