Ion trap mass spectrometer of high mass-constancy

Abstract

The invention relates to high performance ion traps used as mass spectrometers which in spite of a variable thermal load require a high constancy of the mass scale calibrated in. Ion traps consist at least of one ring electrode, two end cap electrodes, and suitable fixing elements which determine the distance between the electrodes. When exposed to a thermal load, the parts of the ion trap are subject to thermal expansion, which leads to a change in field intensities even if the applied RF voltage is constant, and thus to an apparant shift of masses. The invention consists of selecting the thermal expansion of the ion trap parts in such a way that when a constant RF voltage is applied, the field intensity within the trap remains constant by first approximation, in spite of the altering geometric form and expansion with changing operating temperature. In this way, displacement of the mass scale is avoided. To compensate an unavoidable thermal expansion .DELTA.r.sub.0 of the ring electrode with an inscribed radius r.sub.0 by a ratio .DELTA.r.sub.0 /r.sub.0, the distance z.sub.0 of the end cap poles from the center of the trap must become smaller by the proportional ratio .DELTA.z.sub.0 /z.sub.0 =-.DELTA.r.sub.0 /r.sub.0. This compensation can be achieved by a suitable design with suitably selected expansion coefficients for the ion trap electrode material and the material of the fixing elements.