Well defined chopping of the Paul trap rf potential up to a 1.5 MHz frequency combined with a time-of-flight method

A technique for extracting ions from a Paul tray is described wherein the t rapping potential is interrupted sharply when crossing the zero value. A ti me-of-flight profile, corresponding to the ion cloud kinetic energy distrib ution at the instant of trapping potential interruption, has been observed from the subsequent free motion of the ions. Unavoidable residual overshoot s of the storage voltage at this instant are taken into consideration in a three-dimensional (3D) simulation of 10(6) ion trajectories. Experimental p rofiles obtained with very good reproducibility can be matched with simulat ions (in the form of a histogram of ion number arriving at an electron-mult iplier detector in successive intervals of time). The fitting is achieved b y variation of the assumed ion kinetic energy and a coefficient representin g the anisotropy of the ion cloud. It was found that experimental profiles obtained by this ejection method has been modeled to a radio frequency of 1 .5 MHz corresponding to a confinement voltage of about 700 V-pp. The uncert ainty on the fitting of the profiles presented here leads to uncertainties in energy evaluation situated between 0.1 and 0.3 eV. (Int J Mass Spectrom 181 (1998) 173-180) (C) 1998 Elsevier Science B.V.