Importance of bulk and surface excess concentration in fast atom bombardment mass spectrometry of small peptides

The surface tension of hydrophilic and hydrophobic peptides in glycerol solution at various concentrations is measured by the du Nouy ring method. From these values the surface excess concentration is calculated by means of the Gibbs adsorption equation. The behaviour of the surface excess peptide concentration on the fast atom bombardment mass spectrometric response (protonated molecular ion) as a function of the bulk peptide concentration in glycerol is discussed in terms of parameters commonly used in surface chemistry. The experimental results showed a straightforward correlation between the sputtering of protonated molecular ions of peptides and surface excess peptide concentration. Further, there is good agreement with a model involving desorption/ionization in the first layers of the liquid matrix surface. It is additionally demonstrated that the surface excess concentration of each peptide will be different in glycerol solutions of identical bulk concentration and, as a consequence, any differences in protonated molecular ion yield will reflect differences in the surface activity of the peptides, which explains suppression phenomena in peptide mixtures such as tryptic peptide digest. Based on this finding, it can be assumed that a difference observed in the mass spectrometric response does not represent a substantial difference in ionization efficiency.