Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Patent
1992-06-30
1994-02-01
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
250282, B01D 5944, H01J 4900
Patent
active
052834360
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method of generating a three-dimensional rotationally symmetric quadrupole electric field or an electric field of higher multipole moments inside an electrode structure forming the boundary of the field by application of a resultant electric potential .PHI..sub.q0 to the electrode structure.
Up to now, three-dimensional rotationally symmetric quadrupole fields were generated by an array of metallic electrodes with hyperbolic isopotential surfaces (U.S. Pat. No. 2,939,952 and U.S. Pat. No. 3,527,939). As an example in FIG. 1 the standard structure is shown, which consists of a ring electrode (1) of radius r and two end caps (2) of distance 2z.sub.0 .multidot.r.sub.0 and z.sub.0 are characteristic dimensions, which are related to the spacings of the hyperbolic surfaces from the center of the structure. The application of the three-dimensional rotationally symmetric quadrupole field to trap ions and charged particles and to study the properties of the trapped species and to generate mass spectra is well reported in the literature (Quadrupole Mass Spectrometry and Its Applications, P. H. Dawson, Ed., Elsevier, Amsterdam, 1976, and D. Price and J. F. J. Todd, Int. Mass Spectrom. Ion Processes, 60 (1984) 3).
For the generation of mass spectra chiefly four methods are described: in E. Fischer, Z. Phys., 156 (1969) 26, employing Fourier Transformation.
The generation of a three-dimensional electric quadrupole field by hyperbolically shaped metallic electrodes generates several severe problems: generated. field is easily influenced by charges accumulated on the surface of the electrodes. disturbed by other electric fields. position in the trap, resulting in a noise signal.
Finally, there is one further important disadvantage in generating a three-dimensional electric quadrupole field using hyperbolically curved electrodes: It is impossible to generate additional electric fields within the same interior region of the electrodes without any interference with the first electric field.
However, employing metallic electrodes with hyperbolic surfaces is not the only possibility of generating three-dimensional quadrupole fields, although up to now only electrode surfaces following the equipotential surfaces at the boundary of the electric field are commonly used because of prejudice.
Accordingly, it is an object of the invention to provide a method and the corresponding structures for generating a three-dimensional quadrupole electric field or an electric field of higher multiple moments which is much more exact, using no hyperbolically curved metallic electrodes and thus presenting the possibility of superimposing additional homogeneous electric fields without interference with the first electric field.
SUMMARY OF THE INVENTION
This object is achieved according to the invention by continuously varying the resultant electric potential .PHI..sub.q0 across the electrode structure.
Since the electric potential applied to the electrode structure is not constant, but varies continuously across the electrode structure, those surfaces of the electrode structure forming the boundary of the electric field must not be parallel to the equipotential surfaces of the electric field at its boundary. In other words, those parts of the electrode structure forming the boundary of the electric field do not necessarily have to be curved, but are only required to form contours corresponding to the boundary conditions of an implied resultant electric potential generating the quadrupole electric field or an electric field of higher multipole moments.
In one embodiment of the invention the resultant electric potential is continuously varied with position on the surface of the electrode structure adjacent to the electric field. In another embodiment the resultant electric potential is composed of a plurality of single electric potentials being applied each to separate electrodes forming the electrode structure. In both cases, an electric potential which continuously varies across the ele
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Anderson Bruce C.
Bruker-Franzen Analytik GmbH
Hackler Walter A.
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