MINIMIZING PEAK COALESCENCE - HIGH-RESOLUTION SEPARATION OF ISOTOPE PEAKS IN PARTIALLY DEAMIDATED PEPTIDES BY MATRIX-ASSISTED LASER DESORPTION IONISATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY/

Resolution of greater than 100 000 is routinely achieved by MALDI-FT-I CR, based on measured peak widths. However, the ability to separate pe aks that require this resolution is difficult to obtain in practice du e to peak coalescence, a result of coupling of the cyclotron motion of ions with similar frequencies. This phenomenon is accentuated for hig h space charge, high trapping plate voltages, and high mass. Very low trapping plate voltages with properly chosen transient measurement tim es are shown here to yield ultrahigh-resolution separation of closely spaced peaks in peptide mixtures. Measurements of the isotope peaks fo r partially deamidated preparations of substance P or partially reduce d/partially deamidated Ala-Gly-[Arg](8)-vasopressin show as many as si x isotope peaks at one nominal mass. In one example, the C-13 isotope peak was separated from the N-15 isotope, a separation that required a resolution in excess of 180 000, Measurements were made with an exter nal source MALDI-FT-ICR mass spectrometer with a 4.7 T magnet. These d ata suggest the need for high-resolution measurements for the determin ation of exact masses for peptide mixtures.