Secondary-ion mass spectrometry (SIMS) of organic compounds. II. Mass spectra of peptides

Secondary-ion mass spectra of the dipeptides Gly-Gly, Gly-Leu, Phe-Gly, Meth-Phe, the tripeptides Gly-Gly-Gly, Leu-Leu-Leu, Gly-Leu-Ala, Gly-Pro-Ala, the tetrapeptides Phe-Gly-Phe-Gly, Pro-Phe-Gly-Lys, and the octapeptide Asp-Arg-Val-Tyr-Ile-His-Pro-Phe (angiotensin II) were recorded using bare silver targets and targets covered with mixtures of the peptides and glycerol. Spectra were compared with those obtained by chemical ionization (CI) with isobutane as a reactant gas, electron impact ionization (EI), and field desorption (FD). Secondary ionization (SI) was established to be less destructive than EI and CI and to yield more characteristic fragment ions than FD without loss of information on the molecular mass M; e.g. the abundance of the protonated angiotensin II molecule MH⁺ (1046 mass units) was measured to be 9.2% of that of the base peak. SI leads mainly to fragmentation of the peptide bonds. This permits application of SIMS to sequence analysis of minute quantities of oligopeptides.