Baseline Mass Resolution of Peptide Isobars: A Record for Molecular Mass Resolution

Abstract
Baseline resolution of two peptides, RVMRGMR and RSHRGHR, of neutral monoisotopic mass, ∼904 Da, has been achieved by microelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry at a mass resolving power of ∼3 300 000. The elemental compositions of these molecules differ by N4O vs S2H8 (0.000 45 Da), which is less than one electron's mass (0.000 55 Da)! This result establishes a new record for the smallest resolved mass difference between any two molecules. This achievement is made possible by a combination of high magnetic field (9.4 T), large-diameter (4-in.) Penning trap, and low ion density. The implications for proteomics based on accurate mass measurements are discussed briefly.

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