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 upon 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) differing levels of hydrophobicity. The surface tension of 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 increased by decreasing the surface tension in two solutions, H₂O/MeOH and H₂O/MeCN. The use of H₂O/MeCN was more favorable for the signal intensity of analytes than that of H₂O/MeOH. A smaller vaporization enthalpy of MeCN compared to MeOH was proposed as one of the most plausible reasons. The order of the signal intensity of amino acids was Phe>Thr>Ala in relation to the order of hydrophobicity of amino acids. It can be practically concluded that the use of solutions with lower surface tensions and lower vaporization enthalpies results in higher signal intensities in ESI MS.