US3783279A

US3783279A - Hyperbolic field mass filter

Info

Publication number: US3783279A
Application number: US00120482A
Authority: US
Inventors: W Brubaker
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-03-03
Filing date: 1971-03-03
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01
Also published as: JPS5517465B1; FR2128554A1; FR2128554B1; DE2209951A1; GB1341357A

Abstract

A hyperbolic field mass filter (quadrupole or monopole) is described which includes at least two primary electrodes, an ion entrance aperture, an ion exit aperture and a.c. and d.c. bias sources coupled to the primary electrodes for producing a.c. and d.c. field components between the electrodes. At least one auxiliary electrode is disposed between the primary electrode and the entrance aperture together with means for biasing the auxiliary electrode with d.c. potentials to neutralize the d.c. field produced by the primary electrodes in the vicinity of the entrance aperture.

Description

United States Patent [191 Brubaker HYPERBOLIC FIELD MASS FILTER {76] inventor: Wilson M. Brubaker, 1954 Highland Oaks Dr., Arcadia, Calif. 91006 22 Filed: Mar. 3, 1971 21 Appl.No.: 120,482

52 U.S. c1 .Q ..250/292 [51] Int. Cl. 1 H0lj 39/34 [58] Field of Search 250/419 DS [56] References Cited UNlTED STATES PATENTS 1129,32? 4/1964 Brubaker 250/419 3,371,204 2/1968 Brubaker 250 419 Jan. 1,1974

Primary ExaminerWilliam F. Lindquist Attorney-Jackson & Jones 5 7 ABSTRACT A hyperbolic field mass filter (quadrupole or monopole) is described which includes at least two primary electrodes, an ion entrance aperture, an ion exit aperture and a.c. and d.c. bias sources coupled to the primary electrodes for producing a.c. and d.c. field components between the electrodes. At least one auxiliary electrode is disposed between the primary electrode and the entrance aperture together with means for biasing the auxiliary electrode with d.c. potentials to neutralize the d.c. field produced by the primary electrodes in the vicinity of the entrance aperture.

9 Claims, 4 Drawing Figures Arm/M Amy/r IIYPERBOLIC FIELD MASS FILTER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates 'to quadrupole and monopole mass filters and particularly to the disposition of auxiliary electrodes adjacent to the entrance aperture of the filter and the application of bias potential thereto to provide a delayed d.c. ramp mode of operation.

2. Description of the Prior Art Quadrupole and monopole mass filters sort out a beam of ions into a spectrum according to the mass (mass to charge ratio) of the incident ions. These mass analyzers are characterized by their utilization of combined a.c. and dc. fields, as contrasted to the use of magnets by most of the earlier mass analyzer types.

A quadrupole mass filter includes four primary electrodes in the form of parallelcylindrical rods arranged symmetrically about a central axis of the filter. Alternating and direct current voltages are applied in equal but opposite values to neighboring electrodes. The a.c. voltage component is in the radio frequency (r.f.) range.

The theory of operation of the quadrupole mass filter is set forth in the US. Pat. No. 3,129,327 which was granted to me in 1964. A summary of the theory is presented here. The force equations for the motion of 7 ions in the combined a.c. and dc. fields are readily transformed to the standard form of the Mathieu Equation. Solutions of the Mathieu Equation reveal regions of stability and those. of instability of the trajectories of the ions. The stability diagram as illustrated in FIG. 1 is divided into alternate bands of stable and unstable portions. The ordinate axis is the dimensionless vari-' able a, while the abscissa is the dimensionless variable q. These variables are defined in FIG. 1. For a given a.c. and dc. excitation level, the loci of the working points for all masses fall on' the scan line.

In the ion source the working point is at the origin, since the potentials of the quadrupole electrodes do not penetrate this region. As the'transmitted ion passes I from the ion source into the prior art filter through the fringe field of increasing strength, the associated working point moves from the origin to the vicinity of the apex along the scan line. In so doing it penetrates to a.c. field ratio near the ion entrance aperture which -is small as compared to the ratio in the uniform field region in the center of the filter. Under these conditions the working point moves from the origin to the apex of the triangle along a preferred path which lies entirely within the stable portion of the diagram. Thus the undesired y-directed impulse is avoided and greatly improved performance of the instrument is obtained.

In the prior art the delayed d.c. ramp has been obtained by the use of an auxiliary set of electrodes which are energized with large a.c. potentials and nearly zero d.c. potentials. This circumstance requires the application of high frequency (r.f.) potentials to these electrodes which are at a negligible d.c. potential. This has been accomplished by using additional high voltage r.f. feedthroughs which pierce the'vacuum wall or by the use of resistor and capacitor networks located within the vacuum wall. Either of these expedients adds to the complication of the instrument and increases the a.c. power required to energize it.

SUMMARY OF'TI-IE INVENTION I have discovered a new and simple structure for providing the desired delay-dc. ramp mode of operation for quadrupole or monopole mass filters. In accordance with my invention, the mass filter includes entrance and exit apertures, at least two parallel primary electrodes and bias means for applying do. and ac. voltages to such electrodes to produce a hyperbolic electric field between the electrodes. At. least one auxiliary electrode is disposed adjacent to the entrance aperture and bias means are provided for energizing the auxiliary electrodes with do. potentials to neutralize the dc. field produced by the primary electrode in the vicinity of the entrance aperture.

The delayed d.c.ramp is obtained by applying d.c. potentials only to the auxiliary electrode or electrodes. The polarities of these potentials are opposite that of the corresponding primary electrodes. The field resulting from the'potentials applied to the auxiliary electrode cancels or neutralizes the dc. component of the field from the primary electrodes :in the vicinity of the entrance aperture. The a.c. field fromthe primary electrodes is of essentially normal strength in the vicinity of the entrance aperture. Thus the normalized d.c. field is BRIEF DESCRIPTION'OF THE DRAWINGS FIG. 1 is a stability diagram illustrating the operating characteristics of a quadrupole or monopolemass filter; 7

FIG. 2 is a schematic sectional elevation of a quadrupole mass filter and a circuit diagram for providing the bias voltages in accordance with my invention;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2; and

FIG. 4 is a schematic end view of a monopole mass filter.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 2 and 3, the quadrupole'mass filter includes a metallic housing. 10 which encloses four

primary electrodes

12, 14, 16 and 18.

Electrodes

12 and 16 are designated the X electrodes and electrodes l4 and 18 are designated the Y electrodes. The electrodes are suitably supported by high impedance insulators 19. The central axis of the filter is designated by as is illustrated in FIG. 2.

The primary electrodes are in the form of cylindrical conducting rods extending parallel to one another and disposed symmetrically about the central axis. The X electrodes lie with their centers in the X-Z plane, and the Y electrodes lie with their centers on the Y-Z plane. Ideally, the electrodes should provide a hyperbolic curvature. However, in practice, a circular curvature is a reasonably satisfactory approximation.

An entrance aperture plate 20, defining an ion entrance opening 22 centered on the Z axis is secured to one end of the housing by suitable metal screws. Ions of the substance to be analyzed are furnished by an ion source 24 which is secured to the plate 20 by a suitable means. The source 24 is connected to a suitable power supply (not shown) by a cable 26.

An exit aperture plate 28 is located at the end of the housing remote from the entrance plate 20. The plate 28 defines an ion exit aperture 30 centered on the Z axis. A rear wall 32 closes the housing and carries an ion collector 34 supported on a feedthrough insulator 36.

Ions of the selected mass-to-charge ratio enter the filter through the entrance aperture 22, traverse the filter and impinge on the collector 34. The ion current may be measured by a conventional measuring circuit 38. The conductive housing 10 and

plates

20, 28 and 32 are all at ground potential. The interior of the housing is maintained at a low pressure by means of a suitable vacuum pump (not shown).

Four

auxiliary electrodes

42, 44, 46 and 48 are positioned symmetrically about the filter-axis Z adjacent the entrance aperture and between a plane defined by the ends of the primary electrodes (adjacent the entrance aperture) and the ion source 24, as shown in FIG. 2. The

auxiliary electrodes

42, 44, 46 and 48 are mounted on the entrance aperture plate by means of suitable insulators 55. As is shown, the ion entrance plate 20 is provided with four cavities or recessed portions adjacent the ion entrance aperture 22. The auxiliary electrodes 4248 are positioned within individual cavities so that the inner surface of the auxiliary electrodes (facing the interior of the mass filter) are substantially flush with the inner surface 49 of the entrance plate 20. i

Voltage sources 60 and 62 provide a.c. and d.c. potential, respectively, for the primary and auxiliary electrodes as is illustrated in the circuit diagram of FIG. 1. Diametrically opposed

electrodes

12 and 16 are connected via a common lead 66 to the bias sources 60 and 62 through a transformer 64. The

primary electrodes

14 and 18 are connected to the bias sources by means of a lead 68. The a.c. and d.c. bias sources are shown as being provided with a ganged control arm 70 so that the magnitude of the d.c. potential and the amplitude of the a.c. bias may be varied together to cause ions of a selected mass to be transmitted through the filter to the collector. It should be noted that the ganged control of the a.c. and d.c. bias sources is used for illustration only. The d.c. potential would normally be obtained from the a.c. source 60 by rectification, and would be proportional to the a.c. potential. Such means are described and illustrated in my US. Pat. No. 3,129,327. As is illustrated in FIG. 3, the X electrodes (12 and 16) are provided with a positive d.c. potential and the Y electrodes (14 and 18) are provided with a negative d.c. potential.

The auxiliary electrodes 42-48 are connected to the d.c. source 62 by means of suitable leads and potentiometers 72 and 74. A pair of r.f. decoupling capacitors 76 and 78 are connected between opposite terminals of the d.c. source 62 and ground to provide an effective ground for any a.c. signals from the a.c. source 60. Diametrically opposed

auxiliary electrodes

42 and 46 are electrically connected as a pair to the movable tap of the potentiometer 74 via lead 75 for receiving a negative d.c. bias.

Auxiliary electrodes

44 and 48 are connected as a pair to the movable tap of potentiometer 72 via lead 77 to receive a positive d.c. bias. The corresponding primary and auxiliary electrodes (12 and 42, etc.) are thus provided with d.c. bias voltages of opposite polarity. The taps on potentiometers 72 and 74 are adjusted to produce a d.c. field in the vicinity of the entrance aperture 22 which neutralizes or cancels out the d.c. field produced by the primary electrodes 12-18 so that the d.c. field is substantially zero in the vicinity of the entrance aperture 22.

The a.c. fields in the vicinity of the entrance aperture are relatively unaffected by the presence of the auxiliary electrodes. Hence, ions entering the filter respond to the a.c. fields promptly, and to the d.c. fields, only after they have penetrated more deeply into the filter.

As is illustrated in FIG. 1, for low values of the a.c. field (near the entrance aperture) even a very small d.c. field will cause the trajectories to be unstable in the Y-Z plane. To increase the stability in the Y-Z plane, I have provided a reverse polarity d.c. bias on the auxil' iary electrodes to reduce the d.c. field near the entrance aperture to substantially zero. In this manner I have achieved a highly efficient quadrupole mass analyzer without the need for applying a.c. voltages to the auxiliary electrodes to insure that the ions are subjected to a large a.c. component immediately upon entering the analyzer. This structure eliminates the need for high frequency leads or insulators for the auxiliary electrodes. Also the auxiliary electrodes may be conveniently mounted within the cavities of the entrance aperture plate 20 with the bias leads 75 and 77 being incorporated into the power supply cable for the ion source.

By mounting auxiliary electrodes 40-48 so that their inner surfaces are flush with the inner surface of the entrance plate 20, the auxiliary electrodes do not shield the region of the entrance aperture from the a.c. fields produced by the primary electrodes. This manner of mounting the auxiliary electrodes provides a structure which is simple and inexpensive to manufacture. The auxiliary electrodes are carried by the entrance plate 20 and the ion source and the connecting leads for the electrodes may be integrated into the leads which energize the ion source.

A monopole mass filter is illustrated in FIG. 4 and includes primary electrodes 14' and 80. Electrically the monopole mass filter is exactly one-fourth of a quadrupole filter. It was observed by Ulf Von Zahn and reported in Review of Scientific Instruments, Vol. 34, pages 1-4, 1963 that the planes of symmetry in a quad rupole mass filter appear midway between the primary electrodes and that these planes of symmetry are equipotential surfaces. The replacement of three of the primary electrodes with a conducting surface in the form of a 90 angle plate 80 as shown in FIG. 4 provides the same type of hyperbolic field as that provided in the quadrupole mass filter illustrated in FIG. 2. The primary electrode 14 is connected to suitable a.c. and d.c. potential sources. Such sources are illustrated in FIG. 4. The electrode 80 is connected to ground. This geometry adds an additional constraint to those of the normal quadrupole mass filter in thations can cross the instrument axis (the apex of the electrode 80) only at the extremities of the filter. Under these conditions the loci of the working points of the transmitted ions lie within a narrow band slightly displaced from the Y- stability limit. The axial velocity of the ions becomes an important factor in determining the transmission of the ions through the filter, and the ratio of d.c. to a.c. potentials is of lesser importance. The use of an auxiliary electrode 44 which is energized with a d.c. voltage of opposite polarity to the d.c. voltage applied to the electrode 14' and of sufficient magnitude to neutralize the d.c. field adjacent the entrance aperture permits the monopole filter .to also operate in a delayed d.c. ramp mode. The use of an auxiliary electrode and an appropriate bias source to neutralize the d.c. field adjacent the entrance aperture provides the same advantages for a monopole as have been described above for quadrupole filters.

My improved mass filter provides the desired delayed d.c. ramp mode of operation by means of an inexpensive and simple structure. The filter operates at high ef ficiency as a result of a reduction in the a.c. excitation power required compared to that of the prior art systems of achieving the delayed d.c. ramp mode of operation. i

What is claimed is:

1. In a hyperbolic field mass spectrometer including an ion source, an ion detector, means defining ion entrance and exit apertures aligned with a central axis of the spectrometer and positioned adjacent the ion source and detector, respectively, at least two primary electrodes extending parallel to said axis, the ends of said primary electrodes defining a plane substantially normal to said axis adjacent the entrance aperture, at least one of the primary electrodes being in the form of a rod and means for applying d.c. and a.c. voltages between the primary electrodes to produce a hyperbolic electric field therebetween, the improvement which comprises:

a. at least one auxiliary electrode, each auxiliary electrode having a major portion thereof positioned between said plane defined by the ends of the primary electrodes and the ion source so that the a.c. field in the vicinity of the ion entrance aperture produced by the a.c. voltages on the primary electrodes is substantially unaffected by the presence of the auxiliary electrodes, and

b. means including a d.c. voltage source for applying a potential to each auxiliary electrode to neutralize the d.c. field produced by the primary electrode in the vicinity of the entrance aperture.

2. The combination as defined in claim 1 including two pairs of diametrically opposed parallel primary rod electrodes symmetrically spaced about said axis and at least two 'pair of auxiliary electrodes.

3. The combination as defined in claim 1 wherein the spectrometer includes an entrance plate forming the entrance aperture, the ion source being carried by the entrance plate, the entrance plate defining a cavity adjacent the entrance aperture and means for mounting the auxiliary electrode within said cavity so that the surfaces of the auxiliary electrode and the aperture plate adjacent the primary electrode are substantially coincident, the mounting means including means for electrically insulating the auxiliary electrode from the entrance plate.

4. The combination as defined in claim 3 including two pairs of diametrically opposed. parallel primary rod electrodes symmetrically spaced about said axis and two pairs of diametrically opposed auxiliary electrodes, an auxiliary electrode being associated with each primary electrode and means for energizing the auxiliary electrodes to neutralize the d.c. field component produced by the primary electrodes in the vicinity of the aperture.

5. In a multi-pole mass spectrometer the combination which comprises:

a. a housing having an ion entrance plate at one end which defines an ion entrance aperture and an ion exit plate at the other end which defines an ion exit aperture,

b. two pairs of diametrically opposed parallel primary rod electrodes spaced symmetrically about acentral axis which extends from the entrance to the exit apertures, the ends of the primary electrodes defining a plane substantially normal to said axis adjacent the entrance aperture,

c. bias means for applying d.c. and r.f. voltages to the primary electrodes to produce a multi-pole electric field between the primary electrodes,

d. an ion source carried by the entrance aperture plate for injecting ions into the spectrometer,

e. an ion detector positioned adjacent the ion exit aperture for detecting ions,

f. at least two pair of auxiliary electrodes secured adjacent the ion entrance aperture plate and electrically insulated therefrom, each of the auxiliary electrodes having a major portion thereof positioned between said plane defined by the ends of the primary electrodes and the ion source so that the a.c. field in the vicinity of the ion entrance aperture produced by the r.f. voltages on the primary electrodes is substantially unaffected by the presence of the auxiliary electrodes, and

f. means for applying a d.c. potential to the auxiliary electrodes whichis of opposite polarity to the d.c. voltage applied to the corresponding adjacent primary electrodes to neutralize the d.c. field component produced by the primary electrodes in the vicinity of the entrance aperture.

6. The combination as defined in claim 5 wherein the auxiliary electrodes are positioned between the plane defined by the ends of the primary electrodes and the ion source.

7. The combination as defined in claim 5 wherein the auxiliary electrodes comprise two pairs of diametrically opposed electrodes, each of the auxiliary electrodes being associated with one of the primary electrodes.

8. The combination as defined in claim 7 including a capacitor connected between each pair of auxiliary electrodes and ground to provide a low impedance path for the r.f. voltages.

9. The combination as defined in claim 7 wherein the entrance plate has an inner surface adjacent the primary electrodes, the entrance plate defining at least one cavity adjacent the entrance aperture for receiving the auxiliary electrodes and means for mounting the auxiliary electrodes on the entrance plate so that the inner surface of the entrance plate is substantially coincident with the surface of the auxiliary electrodes adjacent the primary electrodesto prevent the auxiliary electrode from shielding-the entrance aperture from the a.c. fields produced by the primary electrodes, the mounting means including means for electrically insulating the auxiliary electrodes from the entrance plate. a

Claims

1. In a hyperbolic field mass spectrometer including an ion source, an ion detector, means defining ion entrance and exit apertures aligned with a central axis of the spectrometer and positioned adjacent the ion source and detector, respectively, at least two primary electrodes extending parallel to said axis, the ends of said primary electrodes defining a plane substantially normal to said axis adjacent the entrance aperture, at least one of the primary electrodes being in the form of a rod and means for applying d.c. and a.c. voltages between the primary electrodes to produce a hyperbolic electric field therebetween, the improvement which comprises: a. at least one auxiliary electrode, each auxiliary electrode having a major portion thereof positioned between said plane defined by the ends of the primary electrodes and the ion source so that the a.c. field in the vicinity of the ion entrance aperture produced by the a.c. voltages on the primary electrodes is substantially unaffected by the presence of the auxiliary electrodes, and b. means including a d.c. voltage source for applying a potential to each auxiliary electrode to neutralize the d.c. field produced by the primary electrode in the vicinity of the entrance aperture.

2. The combination as defined in claim 1 including two pairs of diametrically opposed parallel primary rod electrodes symmetrically spaced about said axis and at least two pair of auxiliary electrodes.

4. The combination as defined in claim 3 including two pairs of diametrically opposed parallel primary rod electrodes symmetrically spaced about said axis and two pairs of diametrically opposed auxiliary electrodes, an auxiliary electrode being associated with each primary electrode and means for energizing the auxiliary electrodes to neutralize the d.c. field component produced by the primary electrodes in the vicinity of the aperture.

5. In a multi-pole mass spectrometer the combination which comprises: a. a housing having an ion entrance plate at one end which defines an ion entrance aperture and an ion exit plate at the other end which defines an ion exit aperture, b. two pairs of diametrically opposed parallel primary rod electrodes spaced symmetrically about a central axis which extends from the entrance to the exit apertures, the ends of the primary electrodes defining a plane substantially normal to said axis adjacent the entrance aperture, c. bias means for applying d.c. and r.f. voltages to the primary electrodes to produce a multi-pole electric field between the primary electrodes, d. an ion source carried by the entrance aperture plate for injecting ions into the spectrometer, e. an ion detector positioned adjacent the ion exit aperture for detecting ions, f. at least two pair of auxiliary electrodes secured adjacent the ion entrance aperture plate and electrically insulated therefrom, each of the auxiliary electrodes having a major portion thereof positioned between said plane defined by the ends of the primary electrodes and the ion source so that the a.c. field in the vicinity of the ion entrance aperture produced by the r.f. voltages on the primary electrodes is substantially unaffected by the presence of the auxiliary electrodes, and F. means for applying a d.c. potential to the auxiliary electrodes which is of opposite polarity to the d.c. voltage applied to the corresponding adjacent primary electrodes to neutralize the d.c. field component produced by the primary electrodes in the vicinity of the entrance aperture.

9. The combination as defined in claim 7 wherein the entrance plate has an inner surface adjacent the primary electrodes, the entrance plate defining at least one cavity adjacent the entrance aperture for receiving the auxiliary electrodes and means for mounting the auxiliary electrodes on the entrance plate so that the inner surface of the entrance plate is substantially coincident with the surface of the auxiliary electrodes adjacent the primary electrodes to prevent the auxiliary electrode from shielding the entrance aperture from the a.c. fields produced by the primary electrodes, the mounting means including means for electrically insulating the auxiliary electrodes from the entrance plate.