Effects of peptide hydrophobicity and charge state on molecular ion yields in plasma desorption mass spectrometry

Plasma desorption mass spectra were obtained for a series of peptides, grouped in four mass ranges having approximately 9, 20, 30, and 40 amino acid residues. Within each group, the individual peptides differed in hydrophobicity, charge state, and retention time, as measured on a reversed-phase HPLC column. Comparison of the molecular ion intensities in the positive ion mass spectra of peptides from each group showed a strong dependence upon hydrophobicity and no correlation with charge state. Plasma desorption mass spectra of mixtures of all the peptides within each mass range generally resulted in the desorption of a single residue and suppression of the ion signal from other components. In most cases, this could be correlated with hydrophobicity, as calculated from the Bull and Breese index; however, a better correlation existed when the results were compared with reversed-phase retention times. In general the spectra of mixtures were not influenced by charge state (except in the absence of hydrophobic peptides), as the same component in each peptide mixture produced the most abundant ions in both positive and negative ion spectra.