Compact high-performance mass spectrometer

Details Compact high-performance mass spectrometer Compact high-performance mass spectrometer

1999-09-15

2003-02-25

Anderson, Bruce (Department: 2881)

Radiant energy

Ionic separation or analysis

Static field-type ion path-bending selecting means

C250S292000

Reexamination Certificate

active

06525314

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to mass spectrometers that use electrodynamic assemblies as mass filters, and in particular to tandem mass spectrometers that use multiple quadrupole mass filters.
BACKGROUND OF THE INVENTION
A traditional tandem mass spectrometer that uses multiple quadrupole mass filters comprises an ion source, a first mass analyzer, a collision cell, a second mass analyzer, and ion detector, typically laid out in a straight line. Since the quadrupole mass filters are generally 0.2 to 2 cm in diameter and 5 to 30 cm long, this straight line arrangement tends to produce an elongated mass spectrometer of one or more meters in length.
Mass spectrometry is an analytical technique for determining the composition of compounds present in a sample. In a single stage mass spectrometer using a quadrupole mass filter, compounds in a sample are ionized, accelerated and focused to form a stream or beam of ions that enters a first quadrupole mass filter. Appropriate adjustment of the alternating and constant voltages applied to the first or only quadrupole mass filter allows the user to select which ionic species are transmitted through the filter. Ions emerging from the filter are detected and converted to electrical impulses or current by known means such as an electron multiplier. Rapid scanning of these voltages further allows the user to produce a spectrum of the ionic species corresponding to the sample compounds.
For some purposes, it is useful to select a first ionic species in a first filter, fragment the ions of the first ionic species to produce ionized fragments, and then analyze the ionized fragments with a second quadrupole mass filter. The ionized fragments are typically detected by known means such as an electron multiplier. For example, analyzing these fragments can aid in elucidating the structure of an unknown molecule. One method of producing such fragments is through a technique of accelerating the selected ion to an energy between 2-100 eV and inducing collisions between sample ions and inert molecules in a collision cell.
Collision cells are well known in the art. A typical collision cell comprises a housing at an elevated pressure (10
−4
to 10
−2
Torr of Argon or Xenon) containing a set of parallel rods to which alternating electric potentials are applied. These potentials serve only to help contain and focus the sample and fragment ions. Sample ions enter at one end of the collision cell and fragment ions, and any remaining sample ions, emerge from the other.
Mass filters and collision cells typically employ electrodynamic assemblies to impose alternating electric fields on ions and fragments. These electrodynamic assemblies typically comprise an even number of electrodes arranged about a central axis. Ions travel about the axis where the ions and/or fragments are subjected to electric fields. Electrodynamic assemblies which employ four electrodes are known in the art as quadrupoles. Although cylindrical electrodes are common, the electrodes may assume a variety of shapes. Quadrupole assemblies are generally described by Paul et al. in U.S. Pat. No. 2,939,952.
In a quadrupole mass filter and in a collision cell, each electrode is typically 0.2 to 2.0 cm in diameter and 5 to 30 cm long. One common configuration of a quadrupole mass filter based mass spectrometer employs three quadrupole assemblies, each arranged in line about a common central axis to allow ions to travel a substantially straight path. The arrangement, simple in execution, requires a substantial amount of linear space. A linear space of one or more meters in length is frequently required.
An example of a prior art tandem mass spectrometer is the PE-Sciex model API 300, manufactured by PE-Sciex, Thornhill, Ontario, Canada.
FIGS. 15A and 15B
provide a schematic view and a cut away elevation view, respectively, of this instrument. The ion path elements of the API 300 mass spectrometer are laid out in a straight line. As illustrated in
FIG. 15A
, the ion path elements of the model API 300 include an ion source
101
, an ion filter
102
, a collision cell
103
, a fragment filter
104
and an ion detector
105
. The carriage assembly which supports most of the ion optics components is illustrated in FIG.
15
B. The carriage assembly is enclosed in a high vacuum chamber (not shown). A detailed description of a tandem mass spectrometer of this type is provided in U.S. Pat. No. 4,234,791, issued Nov. 18, 1980, to Enke et al.
It is desirable to make mass spectrometers that are more compact and that have higher performance. Many prior art mass spectrometers are elongated and occupy too much space on a typical laboratory bench. Some attempts to design a compact mass spectrometer have been made. Bear Instruments, Inc., of Santa Clara, Calif. used a curved first quadrupole filter (analyzer), a curved collision cell, and a curved second quadrupole filter (analyzer). A tandem mass spectrometer using this approach is disclosed in U.S. Pat. No. 5,559,327 to Steiner. The instrument was marketed by Bear Instruments, Inc. as “Bear Cub 800”. However, the introduction of curvature into a quadrupole filter was found to adversely affect stability of the quadrupole filter, thereby limiting the resolution of the mass spectrometer. Subsequently, Bear Instruments introduced the “Kodiak 1200 Quadrupole Mass Spectrometer”. This instrument uses straight filters in conjunction with a curved collision cell. The curved collision cell turns the ion beam through an angle of 180°. Although the curvature of a collision cell has less adverse effect on performance than curvature of a quadrupole filter the effect on the performance of the instrument and the costs of manufacture and alignment are not negligible. Accordingly, there is still a need for a compact high-performance mass spectrometer.
SUMMARY OF INVENTION
The present invention provides a compact high-performance mass spectrometer. To achieve a compact design with high performance, a preferred embodiment includes an ion deflector and a gas removal ring. The ion deflector allows a straight ion filter and a straight collision cell to be coupled in a folded configuration to make a compact design without the loss of performance associated with the use of curved quadrupole components. The gas removal ring, located proximate to an ion path aperture of the collision cell, allows an ion path aperture to be large for high sensitivity while minimizing performance degradation associated with the tendency of collision cell gas to escape via the collision cell ion path apertures to enter the high vacuum region and the detector.
As used herein, “compact” refers to occupying a small area of laboratory bench with emphasis on the mass spectrometer's, longest dimension. The longest dimension of a mass spectrometer is usually the sum of the lengths of the aligned components in the ion path trajectory. The longest components are typically the quadrupole components, i.e., the two ion filters and the collision cell. As used herein, “performance” refers to a combination of sensitivity and resolution.
The preferred embodiment of the mass spectrometer of the present invention includes an ion source, an ion filter, an ion deflector, a collision cell having a gas removal ring at each of its ends, a fragment deflector, a fragment filter and an ion detector. The collision cell includes a gas enclosure having an ion entry aperture and a fragment exit aperture. The ion source produces a stream of ions, each ion having a mass to charge ratio in accordance with its structure. The ion filter accepts ions from the ion source and selectively passes ions according to mass to charge ratio. Ions leaving the ion filter enter an ion deflector which deflects them through a first angle into the collision cell. In the collision cell, ions are fragmented to produce fragments. Fragments leaving the collision cell enter the fragment deflector which deflects them through a second angle into the fragment filter. The fragment filter selectively passes fragments according

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