Stable isotope incorporation triples the upper mass limit for determination of elemental composition by accurate mass measurement

By comparing electrospray ionization Fourier-transform ion cyclotron resona nce (FT-ICR) mass spectra and collision-induced dissociation (CID) FT-ICR m ass spectra of a phospholipid (851 Da) extracted from natural abundance and 99% C-13 bacterial growth media, we are able to reduce its number of possi ble elemental compositions (based on +/-10 ppm externally calibrated mass a ccuracy and biologically relevant compositional constraints) from 394 to 1. The basic idea is simply that the mass of a molecule containing N carbon a toms increases by N Da when C-12 is replaced by C-13. Once the number of ca rbons is known, the number of possible combinations of other atoms in the m olecule is greatly reduced. We demonstrate the method for a stored-waveform inverse Fourier transform-isolated phospholipid from an extract of membran e lipids from Rhodococcus rhodochrous hydrocarbon-degrading bacteria grown on either natural abundance or 99% C-13-enriched mixtures of n-hexadecane a nd n-octadecane. We project that this method raises the upper mass limit fo r unique determination of elemental composition from accurate mass measurem ent by a factor of at least 3, thereby extending "chemical formula" determi nation to identification and sequencing of larger synthetic and bio-polymer s: phospholipids, oligopeptides of more than three to four amino acids, DNA or RNA of more than two nucleotides, oligosaccharides of more than three s ugars, etc. The method can also be extended to determination of the number of other atoms for which heavy isotopes are available (e.g., N-15, S-34, O- 18, etc.). (J Am Soc Mass Spectrom 2000, 11, 835-840) (C) 2000 American Soc iety for Mass Spectrometry.