EFFECT OF ION-NEUTRAL COLLISION MECHANISM ON THE TRAPPED-ION EQUATION-OF-MOTION - A NEW MASS-SPECTRAL LINE-SHAPE FOR HIGH-MASS TRAPPED IONS

The decay amplitude envelope of an ICR time-domain signal determines i ts corresponding Fourier transform mass spectral line shape. The commo nly accepted FT-ICR frequency-domain unapodized Lorentzian spectral li ne shape originates from the Langevin ion-neutral collision model, in which an ion is treated as a point charge that induces an electric dip ole moment in a neutral collision partner. The Langevin model provides a good description of reactions of low-energy collisions of low-mass positive ions with neutrals. However, the Langevin model is inappropri ate for collisions of high-mass gas-phase biopolymer ions with low-mas s neutrals. Here, we examine ion trajectories for both Langevin and ha rd-sphere ion-neutral collision models. For the Langevin model, collis ion frequency is independent of ion speed, leading to a linear differe ntial equation of ion motion with a frictional damping term linearly p roportional to ion velocity. For the hard-sphere model, collision freq uency is proportional to ion speed and the frictional damping term is proportional to the square of ion velocity. We show that the resulting (non-linear) equation of ion motion leads to a non-exponential time-d omain ICR signal whose amplitude envelope has the form, 1/(1 + delta t ), in which delta is a constant. Dispersion-vs-absorption (DISPA) line shape analysis reveals that the 'hard-sphere' spectral line shape res embles that of overlaid narrow and broad Lorentzians. We discuss sever al important implications of the new 'hard-sphere' line shape for ICR spectral analysis, ICR signal processing, collision-based ion activati on, and ion axialization. Finally, in the hard-sphere limit, a non-lin ear frictional damping term will also apply to ions in a Paul trap. (C ) 1997 Elsevier Science B.V.