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/m Deltam(50%) > 350 000 for 200 < m/z < 1000), it would be necessary to spread out a conventional mass spectrum over similar to 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 degrees, 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., similar to1 Da between odd and even masses, corresponding to either even vs odd number of nitrogens or C-12(c) vs C-12(c-1), C-13(1) elemental compositions of the same molecule; similar to2-Da separations, corresponding to a double bond or ring; similar to 14 Da separations, corresponding to one CH2 group) but also the "fine structure" (i.e., different mass defects for different elemental compositions) across each I-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 families-class NnOoSs), type (Z in CcH2c+ZNnOoSs).