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 influence 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 enhance the ejection of ions because the shift of the secular frequency beta(z)(omega/2) by the increasing oscillation amplitude pushes the working point in the a, q diagram into the instability area where strong defocussing forces exist. Negative even multipole fields, however, slow down the secular frequency if the amplitude is increased. The working point is drawn back to the stability border z=1 and is kept there, allowing the ion only to take up such an amount of energy as to maintain the oscillation frequency equal to omega/2 which is the frequency at the border. The ejection is thus delayed, the delay being dependent on 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 exhibits a basic quadrupole field with superposition of a weak negative octopole field, this trap shows a poor mass resolution for the mass selective instability scan, even with the ions damped by a gas. The resolution can be improved by the superposition of a positive octopole field e.g. by stretching the end cap distance, by modifying the hyperbolic angle or by shaping the electrodes to a non-hyperbolic form.