The significance of monoisotopic and carbon-13 isobars for the identification of a 19-component dodecapeptide library by positive ion electrospray Fourier transform ion cyclotron resonance mass spectrometry.

Harnessing the ultra high resolution capabilities of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and positive ion electrospray, we have demonstrated the significance and utility of cumulative mass defect high resolution mass separation stable isotope distribution, exact mass measurement and elemental formula as a means of simultaneously identifying 19 components of the dodecapeptide library Ac-ANKISYQS[X]STE-NH2. With an instrument resolution of 275 000 (average), isobaric multiplets attributed to monoisotopic and carbon-13 components of peptides: Ac ∼ SLS ∼ NH2; Ac ∼ SNS ∼ NH2; Ac ∼ SOS ∼ NH2; Ac ∼ SDS ∼ NH2; within the mass window of 1380–1385 Da, and Ac ∼ SQS ∼ NH2; Ac ∼ SKS ∼ NH2; Ac ∼ SES ∼ NH2; Ac ∼ SMS ∼ NH2, within the mass window 1395–1400 Da, were mass resolved, accurately mass measured and identified from the computed molecular formulas. This experimental procedure enabled the separation of monoisotopic and carbon-13 isobars yielding enhanced selectivity and specificity and serves to illustrate the significance of monoisotopic and carbon-13 isobars in final product analysis. Chromatographic separation (HPLC) was of limited utility except for monitoring the overall extent of reaction and apparent product distribution. Positive ion electrospray-FTICR-MS and fast atom bombardment (FAB) MS were used to assess final product quality and apparent component distribution. Copyright © 2000 John Wiley & Sons, Ltd.