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

Baseline resolution of two peptides, RVMRGMR and RSHRGHR, of neutral monois otopic mass, similar to 904 Da, has been achieved by microelectrospray ioni zation Fourier transform ion cyclotron resonance mass spectrometry at a mas s resolving power of similar to3 300000. The elemental compositions of thes e molecules differ by N4O vs S2H8 (0.000 45 Da), which is less than one ele ctron's mass (0.000 55 Da)! This result establishes a new record for the sm allest resolved mass difference between any two molecules. This achievement is made possible by a combination of high magnetic held (9.4 T), large-dia meter (4-in.) Penning trap, and low ion density. The implications for prote omics based on accurate mass measurements are discussed briefly.