ELIMINATION OF FREQUENCY DRIFT FROM FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTRA BY DIGITAL QUADRATURE HETERODYNING - ULTRAHIGH MASS RESOLVING POWER FOR LASER-DESORBED MOLECULES

At sufficiently low pressure, FT-ICR mass resolving power is no longer pressure-limited. Rather, the observed spectral peaks are broadened by ion cyclotron frequency drift during the detection period, due to change in shape of the coherently orbiting ion packet during detection. The frequency drift may be quantitated by Fourier transformation of each of a series of consecutive segments of the time-domain ICR signal, followed by fitting the frequency vs time behavior to a polynomial in time. Correction for that frequency drift is then achieved by a digital quadrature procedure, followed by multiplication by a weight factor which removes the frequency drift. We demonstrate a 750-fold reduction in FT-ICR mass spectral peak width for pseudomolecular (M + K)+ ions of laser-desorbed leucine enkephalin (m/DELTAm = 1 300 000)! Moreover, correction based on the frequency drift of ions of a given m/z also corrects for frequency drift of ions of other m/z values, as demonstrated for isotopic peaks from (M + K)+ from gramicidin S (m/z 1179). Narrowing of the FT-ICR mass spectral peaks results in a corresponding increase in peak height-to-noise ratio as well. In addition, we propose a theoretical model for frequency drift during detection of the ion cyclotron resonance signal. Simultaneous relaxation of coherent cyclotron motion and compression of the axial distribution of an initially radially coherent ion packet account for ion cyclotron frequency drift during detection. The potential energy generated by mutual ion-ion Coulomb repulsions varies with ion cyclotron orbital radius as ions undergo collisional damping. Excellent agreement with experimental frequency drift vs time is achieved with the four parameters (only one of which is adjustable) of the model: number of ions, initial cyclotron radius, initial ion z-distribution length, and collisional damping time constant.