Sequence analysis of underivatized peptides by negative ion chemical ionization and collision induced dissociation

The sequence of amino acids in a series of underivatized di- and tripeptides has been determined via collision induced dissociation of [M—H]− ions produced by chemical ionization using [OH]−. The collision induced dissociation spectra of individual [M—H]− ions are determined via mass analysed ion kinetic energy spectrometry. Isomeric peptides can be readily distinguished by this method and characteristic sidechain reactions permit the distinction between leucine and isoleucine. The capability for selection of ions of specific m/z value enables mixtures of non-isomeric peptides to be handled by the above technique. Dipeptides containing certain aromatic amino acids (e. g. tryptophan and histidine) eliminate water thermally but this does not necessarily preclude the identification of their constituent amino acids.