Ca. Hughey et al., Kendrick mass defect spectrum: A compact visual analysis for ultrahigh-resolution broadband mass spectra, ANALYT CHEM, 73(19), 2001, pp. 4676-4681
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 clo
sely resolved peaks. Fortunately, there are natural gaps in a typical mass
spectrum, spaced 1 Da apart, because virtually no commonly encountered elem
ental compositions yield masses at those values. Thus, it is possible to br
eak 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 f
rom IUPAC mass to "Kendrick" mass (i.e., multiplying each mass by 14.00000/
14.01565) further simplifies the display by rectilinearizing the peak patte
rns. 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 composi
tions 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 dif
ferent elemental compositions) across each I-Da segment. The method is illu
strated for experimental electrospray ionization FTICR ultrahigh-resolution
mass spectra of a petroleum crude oil. Several thousand elemental composit
ions may be resolved visually in a single one-page two-dimensional display,
and various compound families-class NnOoSs), type (Z in CcH2c+ZNnOoSs).