Ejection of ions from ion traps by combined electrical dipole and quadrupole fields

摘要

The invention relates to an improved method and an apparatus for the mass-sequential ejection of ions from an RF quadrupole ion trap by electrical alternating fields which are generated in addition to the quadrupolar RF storage field and with different frequencies to it. In contrast to the already known ejection by a pure dipole field, the ions are here essentially ejected by a quadrupole field. The ions leave the ion trap through a perforated end cap and can be detected outside it with conventional means. A weak dipole field undertakes only excitation of the secular oscillation at the center, the amplitude increasing in linear manner in the stationary case. The more intense quadrupole field undertakes further widening of the oscillations with exponential growth in amplitudes. The dipole field is generated by an alternating voltage between the two end caps, while the quadrupole field is generated by an alternating voltage between the end caps on the one hand and the ring electrode on the other. The method is of particular use for the ions of very high masses ranging from approximately 5,000 u to 50,000 u. With the same mass resolution, it permits mass spectra to be recorded considerably quicker than the hitherto conventional use of pure dipole fields.