Kendrick Mass Defect Spectrum: A Compact Visual Analysis for Ultrahigh-Resolution Broadband Mass Spectra

At currently achievable Fourier transform ion cyclotron resonance broadband mass spectrometry resolving power (m/Δm50% > 350 000 for 200 < m/z < 1000), it would be necessary to spread out a conventional mass spectrum over ∼200 m in order to provide visual resolution of the most closely resolved peaks. Fortunately, there are natural gaps in a typical mass spectrum, spaced 1 Da apart, because virtually no commonly encountered elemental compositions yield masses at those values. Thus, it is possible to break a broadband mass spectrum into 1-Da segments, rotate each segment by 90°, scale each segment according to its mass defect (i.e., difference between exact and nominal mass), and then compress the spacing between the segments to yield a compact display. For hydrocarbon systems, conversion from IUPAC mass to "Kendrick" mass (i.e., multiplying each mass by 14.00000/14.01565) further simplifies the display by rectilinearizing the peak patterns. The resulting display preserves not only the "coarse" spacings (e.g., ∼1 Da between odd and even masses, corresponding to either even vs odd number of nitrogens or 12Cc vs 12Cc-113C1 elemental compositions of the same molecule; ∼2-Da separations, corresponding to a double bond or ring; ∼14 Da separations, corresponding to one CH2 group) but also the "fine structure" (i.e., different mass defects for different elemental compositions) across each 1-Da segment. The method is illustrated for experimental electrospray ionization FTICR ultrahigh-resolution mass spectra of a petroleum crude oil. Several thousand elemental compositions may be resolved visually in a single one-page two-dimensional display, and various compound familiesclass (NnOoSs), type (Z in CcH2c+ZNnOoSs), and alkylation seriesmay be identified visually as well.