THE NONLINEAR ION-TRAP .4. MASS-SELECTIVE INSTABILITY SCAN WITH MULTIPOLE SUPERPOSITION

Non-linear ion traps are generated by the superposition of weak higher multipole fields on the basic quadrupolar field. The name stems from the non-linear increase of the field strength in such ion traps. The i nfluence of the higher multipole fields on the ejection of ions by the instability at the border beta(z) = 1 of the stability region during r.f. voltage scans is investigated. Positive even multipole fields enh ance the ejection of ions because the shift of the secular frequency b eta(z)(omega/2) by the increasing oscillation amplitude pushes the wor king point in the a, q diagram into the instability area where strong defocussing forces exist. Negative even multipole fields, however, slo w down the secular frequency if the amplitude is increased. The workin g point is drawn back to the stability border z=1 and is kept there, a llowing the ion only to take up such an amount of energy as to maintai n the oscillation frequency equal to omega/2 which is the frequency at the border. The ejection is thus delayed, the delay being dependent o n the oscillation amplitude with which the ion arrived at the border. Odd higher multipole fields cause a similar delay, independent of the sign of the multipole field. Because the truncated Paul ion trap exhib its a basic quadrupole field with superposition of a weak negative oct opole field, this trap shows a poor mass resolution for the mass selec tive instability scan, even with the ions damped by a gas. The resolut ion can be improved by the superposition of a positive octopole field e.g. by stretching the end cap distance, by modifying the hyperbolic a ngle or by shaping the electrodes to a non-hyperbolic form.