Baseline mass resolution of peptide isobars: A record for molecular mass resolution

Baseline resolution of two peptides, RVMRGMR and RSHRGHR, of neutral monoisotopic mass, similar to 904 Da, has been achieved by microelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry at a mass resolving power of similar to 3300000. 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.