Quadrupole RF ion traps for mass spectrometers

Details Quadrupole RF ion traps for mass spectrometers Quadrupole RF ion traps for mass spectrometers

1998-11-20

2001-10-02

Anderson, Bruce C. (Department: 2881)

Radiant energy

Ionic separation or analysis

Cyclically varying ion selecting field means

C250S282000

Reexamination Certificate

active

06297500

ABSTRACT:

The invention relates to quadrupole RF ion traps used in a mass spectrometer, either as storage elements or as mass separators for the measurement of the mass spectrum of stored ions. The invention particularly relates to ion traps, which should show a pure quadrupole field without superimpositions of higher multipoles or, on the other hand, a quadrupole field with superimposition of one or several higher multipole fields of a precisely defined intensity, but no others, particularly no higher multipole fields.
The truncation of ring and end cap electrodes to finite dimensions induces components of higher multipole fields within the ion trap, which may cause negative influences on the storage and scanning behavior. The invention consists of strongly suppressing the formation of higher multipole fields other than those required, by reduction of the gap width between the electrodes in the marginal area, compared to the gap width of electrodes modeled exactly according to the equipotential surfaces of the required field mixture of infinite expansion. A particularly strong suppression of higher multipole fields can be achieved by a wave-shaped constriction in the marginal area between the electrodes.
PRIOR ART
Theory and various applications of RF quadrupole ion traps as tandem mass spectrometers for MS/MS analyses, as reaction containers and measurement instruments for ion-molecule reactions, as tools for selective storage of ions with a uniform mass-to-charge ratio, or for the fragmentation of ions for analyses of their structure, are known from the standard book: “Practical Aspects of Ion Trap Mass Spectrometry”, Volumes I, II, and III edited by R. E. March and John F. J. Todd, CRC Press, Boca Raton, N.Y., London, Tokyo, 1995. The electrode form for the generation of an “ideal” quadrupole field was first described by Wolfgang Paul and Helmut Steinwedel in DE 944 900 and U.S. Pat No. 2,939,952. Accordingly, the ring and end cap electrodes within the ion trap must each have a rotationally symmetrical surface form with hyperbolic cross section, whereby the hyperbolas for the ring and end caps must belong to a hyperbola family with identical asymptotes, and the asymptotes have an angle of tang(&agr;)=2 to the axial direction.
A pure quadrupole field without superimposition of higher multipole fields is however only then generated by this arrangement if the electrodes extend infinitely, which cannot be realized for practical reasons. Any truncation of the electrode form to finite dimensions, necessary for a finite size of the instrument, but also for reasons of finite electrical capacity of the electrode structure, involves a distortion of the quadrupole field which corresponds mathematically to a superimposition with weak multipole fields of a higher magnitude.
The superimposition of the RF quadrupole field with higher multipole fields has severe, sometimes even dramatically severe effects on the stored ions, even if the multipole fields are relatively weak. The effect of the higher multipole fields only becomes apparent outside the center of the trap, i.e., if the ions are not calmly located in the center of the quadrupole field. The oscillations of the stored ions are normally decelerated by a damping gas so that they collect in the center of the ion trap. However, the amplitude of their secular oscillations are temporarily found to reach into the non-central areas of the ion trap. The latter is the case (a) when the ions are introduced from the outside into the ion trap or are generated outside the center inside the ion trap; (b) when the ions are excited by additional electrical fields in their secular oscillation (for example during collisionally induced fragmentation of the ions); and (c) when the ions are ejected from the ion trap mass-selectively for analysis.
An experimental analysis (Alheit et al., “Higher order non-linear resonances in a Paul trap”, Int. J. Mass Spectrom. and Ion Proc. 154, (1996), 155-169) demonstrates impressively how specific ions from an actually ideal, though spatially limited ion trap are ejected almost immediately if they are not collected in the center by a damping gas, due to numerous nonlinear resonances, generated by extremely weak higher multipole fields, occurring in regular patterns of the Mathieu stability diagram. Nonlinear resonances result when the overtones of the ion oscillations, which arise due to nonlinear (inharmonic) retroactive forces, encounter the frequencies of the so-called Mathieu side bands. In this way it is possible for the affected ions to acquire energy from the storage field and thus quickly increase their oscillation amplitude (see the above cited standard work, Chapter 3, regarding nonlinear ion traps).
The effect of higher multipole fields, relative to the suitability of the ion trap as a mass spectrometer, can be advantageous, but also extremely disadvantageous. The higher multipole fields have the strongest influence on the various types of mass-selective ion ejection. They can dramatically improve or diminish the mass resolving power of the scan (using a so-called scan method) at the same scan speed. They can even delay or accelerate the ejection of individual ion types with specific dielectric characteristics, as compared to other ions of equal mass-to-charge ratios. The mechanism for these so-called mass shifts (see Chapter 4 of the above cited standard work) is not yet understood. However, a false mass-to-charge-ratio is simulated in this way, and the mass spectrometer loses its intended function as a measuring instrument for the mass-to-charge ratio of the ions.
The generation of quadrupole fields with a required superimposition of specific multipole fields of even ordinal numbers, which are especially favorable for the method of “mass-selective in stability scans” according to EP 0 113 207, is known from EP 0 321 819 and is based upon a particular shape of electrode. Random superimposition with weak hexapole and octopole fields is possible without higher multipole fields, such as are required for the “nonlinear resonance ejection” scan method according to EP 0 383 961, is described in DE 40 17 264 and is also based upon a particular shape of the electrodes.
DISADVANTAGES OF PREVIOUS METHODS
The electrode surfaces for a pure quadrupole field according to DE 944 900 and those for superimposition with pure octopole and hexapole fields according to DE 40 17 264 are shaped respectively as finite sections of computed equipotential surfaces of the required fields, whereby the basis for the computation is that the equipotential surfaces extend infinitely. However, as already mentioned above, truncation of the electrodes to a practical size already involves an undesirable superimposition with higher multipole fields, which has in many cases a detrimental effect upon the scan method being used.
At the same time, multipole fields of measurable strength up to very high orders appear with alternating signs, i.e. the higher fields are partially added to, partially subtracted from the quadrupole field. In this way, the retroactive pseudoforces, responsible for the secular oscillations of the ions, no longer increase simply linearly with the distance from the center, but rather have a very complicated characteristic. As a consequence of this, a complicated and no longer manageable dependency of the secular oscillation frequency on the oscillation amplitude results, which finally determines the resolution of the ion-ejecting scan method.
Using simple mathematical simulation methods in computers, it is basically possible to optimize the octopole and hexapole fields for various scan methods. These simulations, however, roughly no longer agree with experimental results if higher multipole fields arise in weak, though influential, dimensions due to limitation of the electrodes. Exact simulation with fields using truncated electrodes is very difficult.
However, not only the mathematical simulations are impaired, but also many partially undesirable effects appear in the ion traps. These also affect—in addition to the above

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