Phase correction for collision model analysis and enhanced resolving power of 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 padded with an equal number of zeroes before FFT. Our simple, visual, user-interactive algorithm quickly corrects for zero-order and first-order variation of phase with frequency. We find that the theoretical mass resolving power enhancement for pressure-limited absorption-mode over magnitude-mode line 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 enhancement in mass resolving power (factor of 1.43 +/- 0.09) for isotopically resolved peaks in the FT-ICR mass spectra of electrosprayed bovine carbonic anhydrase (approximately 29 kDa) directly supports the hard-sphere collision model. 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-digital conversion; (c) a sufficiently long acquisition period to yield several data points per absorption-mode peak width at half-maximum peak height; and (d) avoidance of FT-ICR apodization functions (e.g., Hamming and Hanning) that suppress the initial time-domain data. Pulsed single-frequency excitation (duration much less than the reciprocal of the Nyquist bandwidth) can eliminate higher than first-order variation of phase with frequency. Phased FT-ICR spectra should prove especially desirable for analysis of complex mixtures, for resolving isotopic distributions in electrosprayed multiply charged macromolecules and for characterizing ion collisions (and thus ion size and shape).