In-phase ("dispersion"), 90° out-of-phase ("absorption"), and "absolute-value" (square root of the sum of the squares of the "absorption" and "dispersion") ion cyclotron resonance (i.c.r.) spectra of CH3+ and CH4+ have been obtained by broad-band frequency-swept r.f. irradiation of an ionized methane sample, followed by broad-band detection, digitization of the transient i.c.r. response, Fourier transformation using a digital computer (1), and application of a linear phase correction across the F.t.-i.c.r. spectrum. The "absorption" spectrum is the same as would be obtained by conventional marginal-oscillator field-sweep i.c.r. detection and provides a means for identifying spurious "foldover" peaks; the "dispersion" response is not observable with a marginal oscillator detector. The principal advantages of the new "absolute-value" display are (1) the signal-to-noise ratio is significantly better than for the conventional "absorption" mode, at the expense of a slight broadening of the i.c.r. spectral line shape; and (2) "absolute-value" display will very likely be the method of choice in frequency-sweep F.t.-i.c.r. spectroscopy (2), in order to circumvent the complicated non-linear phase corrections which may otherwise be required to obtain an "absorption" mode display.
Interlaboratory comparison of humic substances compositional space as measured by Fourier transform ion cyclotron resonance mass spectrometry (IUPAC technical report)