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

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Abstract

By comparing electrospray ionization Fourier-transform ion cyclotron resonance (FT-ICR) mass spectra and collision-induced dissociation (CID) FT-ICR mass spectra of a phospholipid (851 Da) extracted from natural abundance and 99%13C bacterial growth media, we are able to reduce its number of possible elemental compositions (based on ±10 ppm externally calibrated mass accuracy and biologically relevant compositional constraints) from 394 to 1. The basic idea is simply that the mass of a molecule containingN carbon atoms increases byN Da when12C is replaced by13C. Once the number of carbons is known, the number of possible combinations ofother atoms in the molecule is greatly reduced. We demonstrate the method for a stored-waveform inverse Fourier transform-isolated phospholipid from an extract of membrane lipids fromRhodococcus rhodochrous hydrocarbon-degrading bacteria grown on either natural abundance or 99%13C-enriched mixtures ofn-hexadecane andn-octadecane. We project that this method raises the upper mass limit for unique determination of elemental composition from accurate mass measurement by a factor of at least 3, thereby extending “chemical formula” determination to identification and sequencing of larger synthetic and bio-polymers: phospholipids, oligopeptides of more than three to four amino acids, DNA or RNA of more than two nucleotides, oligosaccharides of more than three sugars, etc. The method can also be extended to determination of the number of other atoms for which heavy isotopes are available (e.g.,15N,34S,18O, etc.).

Member of the Dept. of Chemistry, Florida State University, Tallahassee, FL.