Phase correction for collision model analysis and enhanced resolving powerof Fourier transform ion cyclotron resonance mass spectra

Phase correction of FT-ICR data yields an absorption spectrum that offers a gain by up to a factor of 2 in mass resolving power (at half-maximum peak height), compared to conventional magnitude-mode display. That improvement is equivalent to doubling the applied magnetic field strength, without loss in signal-to-noise (S/N) ratio, provided that the time-domain data are pad ded with an equal number of zeroes before FFT, Our simple, visual, user-int eractive algorithm quickly corrects for zero-or der and first-order variati on of phase with frequency. We find that the theoretical mass resolving pow er enhancement for pressure-limited absorption-mode over magnitude-mode lin e shape depends on the collision mechanism: factor of 1.40 for hard sphere vs 3(1/2) for Langevin (ion: induced dipole), Thus, the experimental enhanc ement in mass resolving power (factor of 1.43 +/- 0.09) for isotopically re solved peaks in the FT-ICR mass spectra of electrosprayed bovine carbonic a nhydrase (similar to 29 kDa) directly supports the hard-sphere collision mo del. Optimal implementation of phasing requires the following: (a) a delay between excitation and detection of less than half of one sampling interval to avoid baseline "roll" and Gibb's oscillations; (b) accurate analog-to-d igital conversion; (c) a sufficiently long acquisition period to yield seve ral data points per absorption-mode peak width at half-maximum peak height; and (d) avoidance of FT-ICR apodization functions (e.g., Hamming and Hanni ng) that suppress the initial time-domain data. Pulsed single-frequency exc itation (duration much less than the reciprocal of the Nyquist bandwidth) c an eliminate higher than first-order variation of phase with frequency. Pha sed FT-ICR spectra should prove especially desirable for analysis of comple x mixtures, for resolving isotopic distributions in electrosprayed multiply charged macromolecules and for characterizing ion collisions (and thus ion size and shape).