Minimizing Peak Coalescence: High-Resolution Separation of Isotope Peaks in Partially Deamidated Peptides by Matrix-Assisted Laser Desorption/Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

摘要

Resolution of greater than 100 000 is routinely achieved by MALDI-FT-ICR, based on measured peak widths. However, the ability to separate peaks that require this resolution is difficult to obtain in practice due to peak coalescence, a result of coupling of the cyclotron motion of ions with similar frequencies. This phenomenon is accentuated for high space charge, high trapping plate voltages, and high mass. Very !ow trapping plate voltages with properly chosen transient measurement times are shown here to yield ultrahigh-resolution separation of closely spaced peaks in peptide mixtures. Measurements of the isotope peaks for partially deamidated preparations of substance P or partially reduced/partially deamidated la-Gly[Arg]↑(8)-vasopressin show as many as six isotope peaks at one nominal mass. In one example, the ↑(13)C isotope peak was separated from the 15N isotope, a that required a resolution in excess of 180 000. Measurements were made with an external source MALDI-FT-ICR mass spectrometer with a 4.7 T magnet. These data suggest the need for high-resolution measurements for the determination of exact masses for peptide mix-tares.