Mass Filter Apparatus Having An Electric Field The Equipotentials Of Which Are Three Dimensionally Hyperbolic

Abstract

A mass filter apparatus for use in mass spectrometers and the like having an electrode structure capable of generating an electric field, the equipotentials of which are three dimensionally hyperbolic and thus avoid the prior art difficulties associated with discontinuities at the entrance and exit ends of the filter apparatus.

Description

STATEMENT OF THE INVENTION

This invention relates generally to mass spectrometer apparatus and more particularly to a novel mass-filtering apparatus for use in a mass spectrometer.

PRIOR ART

A device designed to transmit charged particles selectively on the basis of their charge-to-mass ratio by means of electric fields alone and without requiring the use of any magnetic field was first described by W. Paul and H. Steinwedel, (Z. Naturforsch 82,448, (1953). The operating principle of their device was based on the fact that with certain relationships of DC and AC fields, charged particles of appropriate charge-to-mass ratio can move stably through a structure supporting these fields while other charged particles of different charge-to-mass ratio are unstable and are rejected from the stable path.

The most familiar prior art devices based on this operating principle are within the class of devices now known as quadrupole mass filters. These devices typically consist of a filter section using four rods of cylindrical or hyperbolic cross section, an ionizing section at one end of the filter section, and a collector section at the other end of the filter section. The four rods in the filter section are symmetrically disposed about a central axis in such a way that the fields are predominantly transverse to the central axis and are approximately described by the potential function .phi. where ##SPC1## where:

.+-. U is the DC potential of the rods;

.+-. V is the peak value of the AC potential of the rods, having angular frequency .omega.; and

r.sub.o is a scaling dimension.

The dynamics of a charged particle in the fields described by Equation (1) will conform to the solutions of the equations ##SPC2## where is the charge-to-mass ratio of the charged particle. The above equations (2) and (3) are forms of the Mathieu differential equation wherein stable solutions exist only for certain values of the various coefficients. For example, the solutions describing the motion in x and y will be stable when ##SPC3## The interval of mass values M.sub.1 .ltoreq.M.sub.S .ltoreq.M.sub.2, for which the motion will be stable, is greater as the inequality of the ratio in (5) increases, and the central value of the stable interval, M.sub.S, is dependent on the value of U or V.

One difficulty in the prior art lies in the fact that the structures used define fields which are correct only in the two dimensional domain. Although there exist fields in the center of these structures which are suited to the stable transmission of charged particles of selected charge-to-mass ratio, the fields at the ends of the structure are in fact disposed so as to reject many of the desired particles. This situation imposes severe constraints on the operation of conventional quadrupole mass filters, particularly when high resolution with high transmission is desired.

Several proposals have been made to mitigate this difficulty. In a notable example, Brubaker, (U.S. Pat. No. 3,129,327) has devised auxiliary electrodes to alter the fields at the entrance to the filter section. In another, Brubaker (U.S. Pat. No. 3,371,204) has utilized segmented quadrupole electrodes to improve the entrance conditions. However, none of the proposals attack the basic problem of providing fields which lead to stable motions at all points transversed by the desired charged particle.

OBJECTS OF THIS INVENTION

It is therefore a principal object of this invention to provide a mass filter means which is not subject to the unstable entrance conditions which are found in conventional quadrupole mass filters satisfying the geometric requirements only in a two-dimensional domain and which includes a structure which when energized produces electric fields which provide stable transit for certain selected charged particles at all points traversed between the source and collector.

Another object of the present invention is to provide a novel mass filter apparatus which is substantially free of electric field distortions at the ends of the field-supporting structure where charged particle injection and extraction take place.

Still another object of the present invention is to provide a mass spectrometer apparatus including a novel mass filter structure which provides stable transmission of certain charged particles over the entire distance between source and collector.

Still other objects and advantages of the present invention will become apparent after a reading of the following description of preferred embodiments which are illustrated in the drawing wherein:

IN THE DRAWING

FIG. 1 illustrates one embodiment of a mass filter in accordance with the present invention;

FIGS. 1A, 1B and 1C are cross sections of the embodiment shown in FIG. 1;

FIG. 2 is a stability diagram for the mass filter of the present invention;

FIG. 3 is an alternative embodiment of the present invention;

FIG. 4 is another alternative embodiment of the present invention;

FIGS. 4A, 4B and 4C are cross sections of the embodiment shown in FIG. 4; and

FIG. 5 illustrates an operable mass spectrometer utilizing the mass filter schematically illustrated in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to provide stability of transit through the apparatus, the particles should be launched generally along an axis of the structure and should begin their transit at an equipotential surface thereof. By choosing suitable field-creating electrode geometry allowing access from outside the structure to the particular equipotential surface selected for ion injection, the attachment of an ion source with a relatively large aperture will be permitted.

A generic structure capable of providing stabilizing fields in a three-dimensional domain is one which provides fields which are described by the potential ##SPC4## One example of such a structure is shown in FIG. 1 of the drawing and consists of shaped electrodes 10, 12 and 14 the surfaces of which conform to the family of elliptical hyperboloids represented by the equations ##SPC5## For clarity, the intersections of these surfaces with the y-z plane, the x-y plane, and the x-z plane are shown in FIGS. 1A, 1B and 1C, respectively.

By the proper application of DC and AC potentials to the electrode surfaces 10, 12 and 14 described by equations (7) and (8), the fields described by equation (6) may be achieved within the structure at all points. It can be shown that a proper selection of values for U and V will provide the desired stability of transit along the z-axis for charged particles with charge-to-mass ration within a desired interval.

In this embodiment an entrance aperture 16 and an exit aperture 17 are provided at the intersections of the z-axis and the electrode 12. The dynamic equations which describe the motion of charged particles within this particular structure may be written as ##SPC6## No simple analytical expression will establish the values of U and V and their ratio to give the desired transmission properties. However, they may be described by noting that in the limiting case of k .fwdarw. .infin. , the limit of the ratio of U to V for stability in the x and y directions is the same as (5) above. Graphical representations of the areas of stable operation in the x-, y- and z-directions, respectively, may be found by appropriate scaling of the first stable region of the solutions of Mathieu's equation as shown in FIG. 2.

In accordance with the conventional treatments, the quantities a and q of the FIG. may be defined as follows: ##SPC7## On the basis of these definitions, equations (9)-- (11) have stable solutions in the regions indicated in the FIG. It will be noted that for any set of values a and q which fall within the area S, stability of transit will be provided for charged particles whose charge-to-mass ratio satisfies equations (12) and (13).

A mass analyzer utilizing this form of the invention includes a charged particle source 18 disposed at entrance 16, a set of electrodes 10, 12 and 14 constituting the mass analyzer, and a charged particle collector 19 disposed at exit 17 all housed in a suitable vacuum enclosure. The electrodes 10, 12 and 14 which constitute the mass analyzer are fabricated to conform to the surfaces shown in FIG. 1 and are described by the equations ##SPC8## Equation (14) describes electrode 12 which is a surface of a single sheet having a beltlike shape. Equation (15) describes a surface of two sheets forming the inverted caps 10 and 14 above and below the beltlike surface 12.

The charged particle source 18 is located on the z-axis at the aperture 16 in the beltlike electrode 12 at z = - r.sub.0 and is operated to introduce ions into the mass analyzer. The electrodes 10, 12 and 14 of the mass analyzer are excited by an appropriate generator of DC and AC potentials to establish an electric potential distribution within the structure according to equation (6) such that particles whose charge-to-mass ratio fall within the selected interval M.sub.1 .ltoreq. M.sub.S .ltoreq. M.sub.2 will execute stable trajectories through the mass analyzer structure.

The selected particles may then be collected on a collector electrode 19 placed outside the mass analyzer structure if the latter is also provided with an exit aperture 17 at z = + r.sub.0. In normal operation the collector electrode 19 is connected as usual to a sensitive electrometer. For greater sensitivity the simple collector electrode 19 may be replaced by a converter-electron multiplier detector which is well known in the art.

Because of the practical difficulties associated with the fabrication of the hyperboloidal surfaces 10, 12 and 14, it has been found desirable to consider alternative geometries which are capable of producing the required field configurations but are less difficult to fabricate. In FIG. 3 there is shown one such alternative in which the geometrical requirements of the analyzer are approximately satisfied. The parallel rods 20, 22, 24 and 26 are disposed in the familiar quadrupole configuration utilized in the prior art apparatus. However, the electrode rods 22 and 24, the axes of which lie in the x-z plane, are joined together by a hollow semicircular section 28 which approximates at the entry region the shape described by ##SPC9##

A charged particle source 30 is provided in the hollow semicircular section 28 to allow the injection of charged particles through an aperture 32 on its inside curved surface. In this example the structure may be considered as a geometrically similar approximation to the embodiment described above. The approximation represented by shaping the electrode section 28 will permit improvement over the conventional quadrupole structure with the most important feature of the approximation occurring at the entry region.

In this example, an exit geometry resembling a conventional quadrupole mass filter may be employed in spite of the field distortions since the stability requirements at the exit are considerably less critical than at the entrance region and may be otherwise improved by introducing appropriate auxiliary fields, by means of the ion collector which is used, for the extraction of the ions. In fact, complete symmetry about the x-y plane in this design may be less desirable since the approximation used at the entry is adequate to improve the entry conditions into stable transit orbits, but is not adequate to provide good focusing of particles into a small exit. Thus, for this geometry, the subsequent sections of the mass analyzer and the detection means may be operated exactly as are quadrupole mass analyzers of conventional design.

A more desirable embodiment is based on the geometrical considerations of FIGS. 4, 4A, 4B and 4C. In this embodiment the desired hyperboloidal potential distribution can be created by an electrode structure which is comprised of a number of electrical potential supporting elements in the form of elliptical rings R which are relatively disposed so as to lie on the boundaries of two elliptical cones described by the equation ##SPC10##

A set of such elements are shown perspectively in FIG. 4 and the intersections of these elements with the x-y, y-z and x-z planes are shown respectively in FIGS. 4A, 4B and 4C.

The elliptical rings R of FIG. 4 are respectively energized so as to produce within the enclosed area equipotentials of the form ##SPC11## where n is an index number corresponding to a particular ring R. Moreover, the potentials applied to the respective rings R are such that the gradient of potential along the rays of the elliptical cones described by equation (20) is uniform. If the elements of the electrode structure, i.e., the rings R, are so disposed that their boundaries mark equal intervals on the conoidal surface, the desired hyperboloidal potential distribution may be established within the structure by an excitation which supplies the elements R with potentials which are uniformly distributed between ##SPC12## The positioning of the elliptical electrodes R however, is in no means restricted to an equally spaced distribution over the imaginary conical surface and may just as well be spaced in accordance with any suitable distribution scheme. The illustrated equal spacing is merely for convenience and to allow the use of a less complex voltage supply means for providing the field creating potentials to the respective electrodes.

In FIG. 5 there is shown a mass filter structure 31 which is capable of producing substantially the same equipotential distribution as that produced by the plurality of elliptical rings R depicted in FIGS. 4, 4A, 4B and 4C. In this embodiment a set of elliptical cylinders C of different lengths and radii are concentrically mounted so that one end of each interior cylinder C terminates on the surface of an imaginary elliptical conoid. A second set of similar cylinders C are likewise disposed opposite the first set so that the opposing terminal edges of the cylinders C substantially reproduce the ringed structure shown in FIG. 4. The exterior elliptical cylinder C.sub.7 provides the potential support surface equivalent to the ring R.sub.7 of FIG. 4 and includes an entrance aperture 32 and an exit aperture 34 through which the charged particles are passed.

A source 36 of charged particles which, for instance may be a source of ions, is disposed proximate the entrance aperture 32 and is electrically connected to the cylinder C.sub.7 so as not to cause a perturbing field to be created therebetween. Opposite the source 36 and adjacent exit aperture 34 there is disposed a collector electrode 38 which collects the particles passing through the aperture 34. The source 36, mass filter 31 and collector 38 are all enclosed in an envelope 39 which is evacuated by a suitable vacuum pumping means. The collector 38 is operatively connected to the input of an amplifier 40 which amplifies the collected ion current before it is fed to a suitable detector means 42 the output of which is recorded by a recorder 44.

In order to create the desired potential distribution among the cylindrical electrodes C a plurality of conductors 46 are utilized to couple the various cylinders C to a voltage divider 48 which is energized by an AC- DC power source 50. A plurality of capacitors 52 are provided for coupling the AC energy to the electrodes C in a manner to produce the desired distribution of AC potentials in the same proportionate distribution as is the case with the differing DC potentials applied thereto.

In operation, mass filters in accordance with the present invention provide for motion which is, for particles of the desired charge-to-mass ratio, bounded in the x and y directions. The motion is unbounded in the z-direction primarily for the reason that the injection energy of the particles is greater than the energy of motion in the z-direction which may be bounded between the injection and detection apertures. There is, in fact, no strict requirement that the operating point be chosen such that the motion in the z-direction be bounded in any limits; the operating point being determined by the ratio of DC to AC potentials applied to the electrodes and the sense of the DC potentials.

After having read the above disclosure, many more alterations and modifications of the invention will be apparent to those of skill in the art and it is to be understood that this description of preferred embodiments is for purposes of illustration and is in no manner intended to be limiting in any way. Accordingly, I intend that the appended claims be interpreted as covering all modifications which fall within the true spirit and scope of my invention

Claims

I claim:

1. A mass filter providing stable transit over its entire operating length for charged particles having a certain mass-to-charge ratio comprising:

potential supporting electrode means for creating within a given volume of space an electric field the instantaneous equipotentials .phi. of which are described by an equation of the form ##SPC13## where:

U is a predetermined DC potential; and

V is the peak value of a predetermined AC potential having angular frequency .omega..

where:

k is a constant;

x, y and z are the orthogonal coordinates of points on a given equipotential .phi., and r.sub.o is a scaling dimension; and

entrance aperture means disposed at one extremity of said electrode means for allowing the introduction of charged particles into said given volume of space;

exit aperture means disposed of another extremity of said electrode means remote from said entrance aperture means for allowing the particles for which said field offers a stable transit through said volume of space to exit from said volume of space; and

means for simultaneously supplying said predetermined AC and DC potentials to said electrode means for creating said electric field.

2. A mass filter as defined in claim 1 wherein said electrode means includes a first electrode the surface of which is in the shape of an elliptical hyperboloid defined by the equation ##SPC14## and second and third electrodes having surfaces in the shapes and dispositions of the elliptical hyperboloid of two sheets defined by the equation ##SPC15##

3. A mass filter as defined in claim 2 wherein said entrance aperture passes through said first electrode at ##SPC16## and said exit aperture passes through said first electrode at ##SPC17##

4. A mass filter as defined in claim 3 wherein said potentials applied to said electrode means are of the form ##SPC18##

5. A mass filter as defined in claim 1 wherein said electrode means are comprised of a plurality of elliptical rings disposed relative to each other in a manner so as to lie on the surfaces of a pair of imaginary right elliptical cones positioned base to base, the axes of said cones being coincident with the y axis, and said rings lying in planes parallel to the x-z plane.

6. A mass filter as defined in claim 5 wherein said rings are functionally simulated by the truncated boundaries of a plurality of concentrically disposed elliptical cylinders, said truncated boundaries being contiguous with the surface of said imaginary cones.

7. A mass filter as defined in claim 5 wherein said elliptical rings are uniformly spaced about the y-axis and the potential applied to said rings is of the form ##SPC19## where:

.phi..sub.n is the potential applied to the n.sup.th ring from the y-axis;

U.sub.n is the DC potential appointed for the n.sup.th rings; and

V.sub.n is the peak value of the AC potential having angular frequency .omega. appointed for the n.sup.th rings.

8. A mass spectrometer for analyzing charged particles comprising:

a source of charged particles;

a collector means for collecting certain ones of said charged particles; and

a mass filter separating said source and said collector means and having an entrance aperture adjacent said source and an exit aperture adjacent said collector means,

said mass filter further including potential supporting electrode means for creating within a given volume of space defined by said electrode means an electric field the instantaneous equipotentials of which are defined by the equation ##SPC20## where:

U is a predetermined DC potential;

V is the peak value of a predetermined AC potential having an angular frequency .omega.;

k is a constant;

x, y and z are the orthogonal coordinates of points on a given equipotential .phi.; and

r.sub.o is a scaling dimension.

9. A mass spectrometer as defined in claim 8 wherein electrode means are comprised of a plurality of elliptical rings disposed relative to each other in a manner so as to lie on the surfaces of a pair of imaginary elliptical cones positioned base to base, the axis of said cones being coincident with the y-axis, and said rings lying in planes parallel to the x-z plane.

10. A mass spectrometer as defined in claim 9 wherein said rings are functionally simulated by the truncated boundaries of a plurality of concentrically disposed elliptical cylinders, said boundaries being contiguous with the surface of said imaginary cones.

11. A mass spectrometer as defined in claim 9 wherein said elliptical rings are uniformly spaced about the y-axis and the potential applied to said rings is of the form ##SPC21## where

.phi..sub.n is the potential applied to the n.sup.th ring from the y-axis;

U.sub.n is the DC potential appointed for the n.sup.th rings; and

V.sub.n is the peak value of the AC potential having angular frequency .omega. appointed for the n.sup.th rings.