US3469094A

US3469094A - Electrically adjustable beam defining slits and mass spectrometers using same

Info

Publication number: US3469094A
Application number: US635516A
Authority: US
Inventors: Raymond A Erickson
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-05-02
Filing date: 1967-05-02
Publication date: 1969-09-23
Anticipated expiration: 1986-09-23

Description

Sept. 23, 1969 k 5o 3,469,094

ELECTRICALLY ADJUSTABLE BEAM DEFINING SLITS AND MASS SPECTROMETERS USING SAME Filed May 2, 1967 2 Sheets-Sheet 1 INVENTOR RAYMOND A. ERICKSON ATTORNEY l 3, 1969 R. A. ERICKSON 3,469,094

ELECTRICALLY ADJUSTABLE BEAM DEFINING SLITS AND MASS SPECTROMETERS USING SAME Filed May 2. 1967 2 Sheets-Sheet 2 FIG.4

H65 FIG. 6

POWER -51 SUPPLY mvm mn RAYMOND AERIGKSON TTORNEY Patented Sept. 23, 1969 U.S. Cl. 250-41.9 7 Claims ABSTRACT OF THE DISCLOSURE Electrically adjustable beam defining slits are disclosed as employed for adjusting the thickness of an ion beam in a cycloidal mass spectrometer. One adjustable slit is employed at the exit of the ion source for determining the initial thickness of the ion beam. A second similar adjustable slit is employed at the entrance to the ion detector. Both slits are adjustable in concert by means of an electrical circuit over a range of 0.005". The slits comprise a fixed member, defining one side of the slit, and a movable member defining the other side of the slit. The movable member is supported on two parallel legs extending at right angles to the direction of movement of the movable slit defining member. The support legs deform to a generally S-shape to permit movement of the movable slit defining member. The deformation is such that the slit defining side edge of the movable member remains parallel to the fixed slit defining edge. A thermally expansive member, which is heated by an electrical current, operates via a lever arm against a spring bias force which tends to hold the movable slit defining member in its openmost position. Cooling of the thermally expansive member, as by loss of heating current, causes the thermal member to contract and to close the slit to its narrowest position. This is desirable since loss of heating current will cause the mass spectrometer to be adjusted to its condition of highest resolution.

In a preferred embodiment, the parallel support legs are tungsten leaf springs, whereby the support legs serve to provide the spring bias force.

DESCRIPTION OF THE PRIOR ART Heretofore, electrically adjustable ion beam defining slits have been used in mass spectrometers for adjusting their resolution. An example of such a prior art adjustable slit is described and claimed in copending U.S. application 539,917, filed Apr. 4, 1966, and assigned to the same assignee as the present invention. In this prior adjustable slit, the movable slit defining member comprised an elongated plate slidable within and confined by a way. The problem with this arrangement was that the movable slit defining member tended to bind on the sides of the way. Sometimes this binding action prevented further adjustment of the slits.

Also, in the prior adjustable slits, the mechanical linkage between the thermally expansive member and the spring biased movable slit defining member was such that if heating current were lost the expansive member opened the slit to its openmost position. This is generally undesired since it is preferred, in case of such a failure, to have the spectrometer operate in its condition of highest resolution, i.e., narrowest slit width. Also, the heating current for the thermally expansive member was relatively high, as of 3 amps, at its narrowest position for highest resolution. However, the magnetic field produced by the heating current flowing through the expansive element tended to perturb the uniform magnetic field in the mass analyzer region and such perturbing magnetic field was greatest when the spectrometer was to be operated at its condition of highest resolution.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision if an improved electrically adjustable ion beam defining slit and spectrometer means employing same.

One feature of the present invention is the provision, in an electrically adjustable beam defining slit, of a movable slit defining member supported via a pair of generally parallel elongated support legs movable in the direction of motion of the movable slit defining member, whereby the slit defining edge of the movable member remains parallel to the other slit defining edge over the range of adjustments of the slit widths.

Another feature of the present invention is the same as the preceding feature wherein the support legs are fixed at their ends in such a manner that they are bent by displacement of the movable members and are made of a material which remains resilient up to a temperature of 300 C., whereby the support legs form leaf spring members for spring biasing the movable slit defining member.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of an electrically heated thermally expansive member mechanically coupled to the movable slit defining member for operating against a spring bias force to move the movable slit defining member, and wherein the mechanical coupling is such that cooling of the thermally expansive member causes the slit defining member to move in the direction to narrow the beam slit, whereby, in a mass spectrometer, highest mass resolution is obtained with a minimum of electrical heating current.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary side elevational view, partly broken away and partly schematic, of a cycloidal mass spectrometer employing features of the present invention,

FIG. 2 is a circuit diagram of the voltage supply for the mass analyzer electrode structure of the mass spectrometer of FIG. 1,

FIG. 3 is an enlarged plan view of the slit actuating mechanism of FIG. 1 taken along either one of the lines 33 in the direction of the arrows,

FIG. 4 is a sectional view of the structure of FIG. 3 taken along line 4-4 in the direction of the arrows,

FIG. 5 is an enlarged schematic view of the movable slit defining member and support structure with movements exaggerated to show its mode of operation, and

FIG. 6 is a circuit diagram of the electrically adjustable slits of FIGS. 3-5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes 1 are insulatively supported within a thin rectangular vacuum envelope 2, only partially shown, from a heavy demountable rectangular flange portion of the envelope not shown, which closes 0E one end of the vacuum envelope 2.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from a voltage source 3 (see FIG. 2) via leads 4 connected at nodes 5 of a voltage divider network 6. The different potential applied to the different rings 1 establishes a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole pieces 8 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably low pressure as of 10- torr. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the mass analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source ionizes the gas and projects it through a slit into the crossed magnetic field H and electric field E of the mass analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for a given intensity of E and H, will be focused at a detector slit 13 a certain focal distance from the source and at the same electric potential. An ion detector 14 is positioned behind the slit 13 to produce an output corresponding to number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

Referring now to FIGS. 3-5, there is shown the electrically adjustable ion beam defining slit mechanism 21 of the present invention. More particularly, the slit adjusting mechanism 21 is mounted transversely across one of the ring electrodes 1' and is designed to be thin in the direction of the electric field E such that it can be confined substantially entirely within the interior of the ring electrode 1', whereby the uniformly of the electric field E in the vicinity of the ion beam 20 is least perturbed. There are two such adjustable slits 13, one for the source 12 forming the beam exit slit 13 of the source 12 and one for the detector forming the beam entrance slit 13 for the detector 14. Both mechanisms 21 are substantially identical and connected in circuit to vary the slit widths alike and therefore will be described in detail as employed for the beam exit slit of the ion source 12.

The slit 13 is elongated in the direction of the magnetic field H. The slit 13 has its width defined by the space between the opposed marginal edges of two plate-shaped

slit defining members

22 and 23, respectively. The ends of the slit 13 are defined by the wall of a circular bore 24 in a base plate portion 25 of the plate 26 which is aflixed by a plurality of screws 27 within and across the interior of the first rectangular ring-shaped electrode 1'. The ion source 12 is aligned with the bore 24 and slit 13 such that the beam of ions 20 is projected through the slit 13, whereby the thickness of the ion beam 20 is determined by the width of the slit 13.

One of the beam defining plates 22 is fixed in position by being bolted via bolts 28 and nuts 29 to the base plate portion 25. The fixed plate 22 is positioned in a milled out portion 31 of the plate 26 and is beveled at its slit defining marginal edge. A second slit defining plate 23 is movable in a direction toward and away from the fixed slit defining plate 22 for adjusting the width of the slti 13 and, thus, the thickness of the ion beam passable therethrough.

The movable slit defining plate 23 is beveled at its slit defining marginal edge and is supported from a support structure 32 via a pair of generally parallel support legs 4 33 and 34. The

support legs

33 and 34 are clamped at their ends to the movable slit defining member 23 and to the support structure 32 via two pairs of wedges .35 which clamp the

legs

33 and 34 to the side edges 36 of slots 37 cut in the movable slit defining member 23 and support structure 32. The wedges 35 are forced apart by screws 38.

The support structure 32 which is generally plate-shaped is secured to the base plate portion 25 via a pair of screws 39. The support structure 32 is initially adjusted for .1 position which will place the slit defining marginal edge of the movable member 23 at a distance from the fixed plate 22 which is greater than the maximum expected adjustment of the slit width.

The

support legs

33 and 34, in a preferred embodiment are made of a resilient high temperature material such as tungsten and are made to deform alike into a generally S-shape with movement of the slit defining member 23. as depicted in FIG. 5. The deformation of the

support legs

33 and 34 provides a spring bias force on the movable slit defining member. In a preferred embodiment, this spring bias force tends to open the slit 13 over the operable range of slit widths from 0.0000" to 0.005". The movable slit defining member 23 slides on the base plate portion 25.

A thermally expansive member 41, as of for example a directly heated 0.015" diameter stainless steel wire, serves to actuate the movable slit defining member 23 against the spring bias force via a pivoted lever arm 42. More specifically, the thermally expansive wire 41 is fixed at one end to an insulated binding post 43 and at the other end to the free end of the lever arm 42 which is pivoted at its other end about a pin 44 threaded into the base plate portion 25.

A jack screw 45 is threaded through the lever arm 42 and bears against the end of the movable slit defining member 23, thereby transmitting the spring bias force on the movable slit defining member 23 to the thermally expansive wire 41 via the lever arm 42. The jack screw 45 is initially adjusted such that, when the wire 41 is operated at zero or negligible heating current and, thus, at ambient temperature of the source 12, the slit width will be at some narrow width, as of 0.0002", for maximum resolution of the mass spectrometer. A second screw 46 is threaded through the lever arm 42 and bears against a shoulder 47 of the base plate 26 to serve as a mechanical stop to prevent the wire 41 from pulling the slit defining member 23 into the fixed member 22 when the wire 41 is cooled below ambient temperature, as during shut-down times.

The width of the beam defining slit 13 is adjusted by adjusting the heating current raised through the wire 41 from the binding post 43 to the lever arm 42 and thence via

screws

45, 46 and pin 44 to the grounded base plate 26. As the heating current is increased the wire 41 expands in length to permit the spring bias force produced by the

deformed support legs

33 and 34 to pull the movable slit defining member 23 away from the fixed slit defining member and, thus, open the width of the slit 13.

One advantage of this arrangement of parts is that the heating current is at its minimum for the narrowest position of the beam defining slit 13 which corresponds to maximum mass resolution of the mass spectrometer. Thus, the magnetic field produced by the heating current has its least perturbing effect upon the uniformity of the magnetic field H in the analyzer region when operating at highest resolution.

Another advantage of the adjustable beam defining slit of FIGS. 3-5 is that the slit defining marginal edge of the movable slit defining member 23 retains its parallelism with the fixed slit defining edge during movement of the movable member 23 (see FIG. 5). This results from the parallelism of the two

support legs

33 and 34 and from the fact that the

legs

33 and 34 are connected to the support structure 32 and movable member 23 at points falling at the corners of a parallelogram. This is essentially the principle used in a parallel-rule and points out that the

deformable legs

33 and 34 may be replaced by pinned connecting links. However, in such an alternative embodiment, the spring bias force must be separately provided since, in the embodiment of FIGS. 3-5, the

deformable legs

33 and 34 also provide the needed spring bias force which is transmitted to the movable slit defining member 23 and wire 41. For the case of the pivoted connecting links, the separate spring bias force may be provided by a separate spring operable on the lever arm 42 or movable member 23 in a manner similar to that described in the aforecited copending patent application 539,917.

In the adjustable slit mechanism of FIGS. 3-5, essentially all the metal parts are made of 304 non-magnetic stainless steel except for the

spring support legs

33 and 34 which are made of tungsten ribbon 0.006 thick, 0.060" wide, and 0.5" long.

Referring now to FIG. 6 there is shown the electrical circuit for adjusting the source and detector beam defining slits 13. The two thermally

expansive wires

41 and 41 are connected in parallel to a power supply 51 via a current meter 52 and variable resistor 53. The current is connected to the wires 41 and 41' via

binding posts

43 and 43. The current through the

wires

41 and 41 is varied by adjusting resistor 53 to vary the widths of the two slits 13 in concert.

Although the adjustable beam defining slits 13 have been described as employed for controlling the ion beam thickness in a cycloidal mass spectrometer they may be employed to advantage in other types of mass spectrometers. However, they are particularly well suited for use in cycloidal mass spectrometers because optimum performance of the spectrometer is critically dependent upon a well defined beam of ions of uniform and well controlled thickness.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a mass spectrometer apparatus, means for producing a beam of ions, means for analyzing the ions according to their charge-to-mass ratio, means forming a first beam edge defining member having a marginal edge portion disposed in the beam path, means forming a second beam edge defining member having a marginal edge portion disposed in the beam path and facing the marginal edge portion of said first beam defining member to define a narrow adjustable beam defining slit therebetween, said second beam defining member being movable toward or away from said first beam defining member for adjusting the width of the beam defining slit therebetween, the improvement comprising, means forming a fixed support structure, means forming a pair of generally parallel elongated support legs interconnecting said movable second beam edge defining member and said fixed support structure, said pair of support legs being affixed to said support structure and to said movable second beam defining member at four points which fall at the corners of an imaginary parallelogram, means for moving said second movable beam defining member in a direction transverse to the direction of elongation of said support legs and in the plane of the parallelogram to cause said support legs to move in unison in a direction transverse to their direction of elongation and in the plane of the parallelogram such that said support legs retain their generally parallel condition with movement thereof and such that the movable beam defining marginal edge of said movable second member remains parallel to the first beam defining marginal edge with movement of said second beam defining member.

2. The apparatus of claim 1 wherein each of said support legs is non-pivotably connected at one of its points of connection to said support structure and said second movable beam defining member, and said support legs being deformed by movement of said second beam defining member relative to said support structure, and said support legs being made of a resilient material such that the deformation thereof produces a spring bias force on said second beam defining member.

3. The apparatus of claim 2 wherein each of said support legs are non-pivotably connected at two points of connection to said support structure and said second beam defining member such that said support legs are each similarly deformed into a generally S-shape by relative movement between said support structure and said second beam defining member.

4. The apparatus of claim 3 wherein said support legs are made of tungsten.

5. The apparatus of claim 1 including, means for spring biasing said second beam defining member in a direction away from said first beam defining member, and wherein said means for moving said movable beam defining member includes, means forming a thermally expansive member, means forming a mechanical coupling between said thermally expansive member and said spring biased second beam defining member, said mechanical coupling means serving to couple the spring bias force to said thermally expansive member to put said thermally expansive member in tension, whereby contraction of said thermally expansive member with cooling thereof moves said second beam defining member toward said first beam defining member to narrow the beam defining slit therebetween.

6. The apparatus of claim 5 wherein said thermally expansive member is an electrically resistive wire, and means for passing an adjustable electrical current through said resistive wire for varying its length and, thus, the width of the beam defining slit.

7. The apparatus of claim 1 wherein the mass spectrometer is a cycloidal mass spectrometer.

References Cited UNITED STATES PATENTS 2,852,684 9/1958 Payne. 2,977,470 3/1961 Robinson.

RALPH G. NILSON, Primary Examiner S. C. SHEAR, Assistant Examiner