ULTRAHIGH-RESOLUTION MATRIX-ASSISTED LASER-DESORPTION IONIZATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTRA OF PEPTIDES

Quasimolecular peptide [M + H](+) ions are generated by matrix-assisted (2,5-dihydroxybenzoic acid/D-fructose) laser desorption/ionization (frequency-tripled Nd:YAG at 355 nm) from leucine enkephalin, bradykinin, the nonapeptide Arg-Leu-Cys-Ile-Phe-Ser-Cys-Phe-Arg, angiotensin I, bovine insulin chain B and some of their deuterated derivatives. The ions are cooled and axialized by azimuthal quadrupolar irradiation in the presence of argon collision gas in the 'source' compartment of a dual cubic Penning trap. Following ion transfer to the low-pressure 'analyzer' trap, ultrahigh FT-ICR mass resolving power is obtained for protonated oligopeptide quasimolecular [M + H](+) ions: e.g. m/Delta m(50%) approximate to 1500000 for bradykinin (m/z approximate to 1060) after frequency drift correction and 100 000 for insulin B-chain, in which Delta m(50%) is the magnitude-mode peak full width at half-maximum peak height. These results constitute the highest mass resolving power yet demonstrated for internally-generated MALDI ions at 3 T, and compare favorably with results obtained at much higher-magnetic field with externally-generated ions. High mass resolution is important for resolving adducts and chemical modifications of a peptide or protein and (as demonstrated here) for facile determination of the degree of deuteration from an H/D exchange experiment. Limitations to FT-ICR mass resolving power by Coulombic ion-ion interactions at high ion density in the trap are demonstrated experimentally and discussed.