Ultrahigh‐resolution matrix‐assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectra of peptides

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Abstract
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 non‐apeptide 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/Δm50%≈ 1 500 000 for bradykinin (m/z≈ 1060) after frequency drift correction and 100 000 for insulin B‐chain, in which Δm50%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.

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