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 b y ion cyclotron frequency drift during the detection period, due to ch ange in shape of the coherently orbiting ion packet during detection. The frequency drift may be quantitated by Fourier transformation of ea ch 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 digita l 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 c orrects for frequency drift of ions of other m/z values, as demonstrat ed for isotopic peaks from (M + K)+ from gramicidin S (m/z 1179). Narr owing of the FT-ICR mass spectral peaks results in a corresponding inc rease in peak height-to-noise ratio as well. In addition, we propose a theoretical model for frequency drift during detection of the ion cyc lotron resonance signal. Simultaneous relaxation of coherent cyclotron motion and compression of the axial distribution of an initially radi ally coherent ion packet account for ion cyclotron frequency drift dur ing detection. The potential energy generated by mutual ion-ion Coulom b repulsions varies with ion cyclotron orbital radius as ions undergo collisional damping. Excellent agreement with experimental frequency d rift vs time is achieved with the four parameters (only one of which i s adjustable) of the model: number of ions, initial cyclotron radius, initial ion z-distribution length, and collisional damping time consta nt.