US3588495A - Mass spectrometers having adjustable beam defining slits - Google Patents

Abstract

A MASS SPECTROMETER HAVING ANION SOURCE, AN ANALYZER, A COLLECTOR, AND SLITS ALONG A PATH OF ION TRAVEL FROM THE SOURCE TO THE COLLECTOR. THE SLITS ARE ADJUSTABLE TO HIGH AND LOW RESOLUTION POSITIONS. CIRCUITRY IS PROVIDED SUCH THAT THE ADJUSTMENT OF THE SLITS AND SCANNING OF THE ANALYZER OCCURS IN A COORDINATED FASHION AND THE ANALYZER IS SCANNED WHILE A SLIT IS MAINTAINED IN EACH OF ITS ADJUSTED POSITIONS.

Description

United States Patent [72] lnventors John Stephen llalllday [45] Patented June28, 1971 [73] Assignee Amociated Electrical Industries, Limited London, England [32] Priority June ll, I965 [3 3] Great Britain Continuation of application Ser. No.

557,272, June 13, 1966, now abandoned.

[54] MASS SPECTROMETERS HAVING ADJUSTABLE [Sl] lnt.Cl H01j39/34 [50] Field of Search A. 250/41 9 (3), 41.9 (158), (ISA). (1515), 41.9 (R), 105

Primary Examiner-James W. Lawrence Assistant Examiner-A. L. Birch Attorney-Watts, Hoffman, Fisher & Heinke ABSTRACT: A mass spectrometer having an ion source, an analyzer, a collector, and slits along a path of ion travel from the source to the collector. The slits are adjustable to high and low resolution positions. Circuitry is provided such that the adjustment of the slits and scanning of the analyzer occurs in a coordinated fashion and the analyzer is scanned while a slit is maintained in each of its adjusted positions.

MON/TOR COLLECTOR MA/N MAGNET/C BEAM DEFINING SLl'lS 10 Claims, 10 Drawing Figs.

[52] U.S.Cl 250/4l.9, 250/105 ELECTROSTATIC ANALYSER PROBE 7 I 7 "a; ION SOURCF DISPLAY ANALYSER PATENTEDJUN28I97i 3.588.495

SHEET 1 OF 5 FIG].

A IL I 0 mm 20-40 UNITS 0 UNITS 507 A LEcrR0s AT/c ANALYSER 508 MASS MAIN MAGNET/C ANALYSER PUMP L 45 AUX COLLECTOR INVENTORS DISPLAY 77 JOHN STEPHEN HALL/DAY THOMAS OLIVER MERREN JOHN ST AR HEATH 5y J M 7W, ATTVJ PATENTEU M28197] 3588, 195

sum 2 0F 5 FIG. 3.

INVENTORS JOHN STEPHEN HALL/DAY THOMAS OLIVER MERREN JOHN STEWART HEATH B) ATWS PATENTED JUN28 I971 SHEET 3 BF 5 FIG.

JOHN STEWART HEATH PATENTED JUN28 12m SHEET BF 5 \Embm mxqmm Q2663 zBz m N 3 Ed 3 Hi5? TEE q 5&8 6 Si n mEQmmw w mt INVENTORS JOHN STEPHEN HALL/DAY THOMAS OLIVER MERREN JOHN STEWART HEATH BY A M;

PATENTEU JUH28 |9T| 3; 588.495

' sum 5 OF 5 FIG. 7

MAGNET I 39 TRIGGER SAW TOOTH C/RCU/T C4 C4 GEN 38 FIGS FIELD T/ME C4 A El FIG. 9

C 4 CLOSED BOPEN C4 CLOSED BEAM .E L. LE 1 T FIGTIO a INVENTORS JOHN STEPHEN HALL/DAY THOMAS OLIVER MEHREN JOHN STEWART EATH B) W! 444%, ATW:

MASS SPEOMETEEQS HAVENG ADJUSTABLE BEAM DEWNWG SMTS This invention relates to mass Spectrometers. This application is a continuation of application Ser. No. 557,272 filed June 13, 1966 for Mass Spectrometers, now abandoned.

A mass spectrometer enables an analysis of a substance to be made by projecting ions along a beam and deflecting the beam laterally, such deflection may be carried out either elec trostatically or magnetically. The beam at its source is passed through a narrow aperture to confine its width and similarly at the collector end it is passed through a narrow aperture so that only a part of the cross section of the beam is collected at an instant. By varying the lateral deflection progressively, the beam is scanned and hence elements of different mass will be collected successively; this enables an analysis of the elements in the substance under investigation to be made.

It follows that if the apertures are small then the cross-sectional area of the beam impinging on the collector is small, so that as the deflection is varied to scan the beam a series of narrow peaks will be received, and if this reception is displayed against a time base the peaks will be quite narrow. If on the other hand the apertures are large, the resolution will be low and instead of narrow peaks appearing on the display the peaks will be wide and also of a greater amplitude.

It follows that low resolution will give greater Etivity to enable elements to be detected which would not be capable of detection with a high resolution, and on the other hand it is difficult accurately to determine the relative time instants at which the peaks occur so as to identify the elements; such identification usually being made by comparison with the time instances of peaks received from a known substance used as a reference.

On a low resolving power spectrum, metastable ions will be observed as broad peaks, possibly several masses wide. These metastable peaks are an aid to structural interpretation and are the result of the decomposition of an ion after acceleration in the source but generally before the magnetic analyzer. Whilst it will be difficult to measure the peak positions with sufficient accuracy with a low resolution, the peaks will be much more clearly defined with a high resolution. Moreover, some elements may give several peaks representing the same nominal mass and a high resolving power is needed to discriminate between them.

According to the present invention, a mass spectrometer is provided with means for adjusting the efi'ective size of the apertures between limiting positions or states and means for rapidly changing over between the positions or states so as to obtain a rapid change from high resolution to low resolution or vice versa.

According to one arrangement, the resolution is changed for successive scans so that successive scans are made with high resolution and low resolution alternately. In other arrangements, the apparatus may be arranged normally to operate at a high resolution or a low resolution as the case may be, and means may be provided for changing over at will to the other type of resolution when this is required. The resolution may be controlled by slit apertures, the width of which is varied; alternatively electron lenses may be used and the effective aperture width controlled electrically in a known manner.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings in which:

FIG. I shows graphically the types of display obtained with high resolution and low resolution;

FIG. 2 shows the general arrangement of a known form of a mass spectrometer;

FIG. 3 is a detail view to a larger scale than FIG. 2 showing apparatus for varying the width of a slit mechanically;

FIG. 4 is an extension of FIG. a,

FIG. 5 is a horizontal sectional view taken on the line IV-IV of FIG. 3;

FIG. 6 shows electrical control apparatus for varying the slit widths;

MG. 7 is a detail view showing the variable aperture at the source end of the beam path;

FIG. 8 shows in block form a circuit for synchronizing the aperture control with the deflection scan;

FIG. 9 shows graphically how the aperture control can be related to the deflection scan; and

FIG. 10 shows diagrammatically the arrangement of an electrostatic variable aperture.

In FIG. 1, graph A shows an image obtainable with a high resolution and graph B similarly shows a low resolution image. In graph A, the peaks are quite narrow and therefore their positions along a time base may be measured accurately for identification purposes. Graph 8 shows the same spectrum as shown in graph A but with a low resolution. It will be noted that the peaks are higher and wider in graph B and that there are some peaks visible which do not appear in graph A. However, it will also be noted that owing to the increased width of the individual peaks in graph B it is not possible accurately to determine the positioning of the peaks along a time base and it is most desirable that both types of resolution should be readily available.

The mass spectrometer shown in FIG. 2 includes an ion source chamber 1 into which a specimen carrying probe 3 can be inserted so that ions can be liberated from the specimen. An electrode 5 to which an accelerating voltage of say 8 kilovolts is applied serves to repel these ions from the chamber 1 as a beam which passes first through an electrostatic analyzer 7 including opposed conductive plates 8 between which a potential difference is maintained, then through the slit of a monitor collector 9 into a magnetic analyzer 11. In magnetic analyzer ll an electromagnet coil 12 establishes a strong magnetic field which extendsin a direction transverse to the path of the ions, and since the ions are charged particles, their paths will be curved in the magnetic field. The deflected ions, or certain of them if different groups of ions are deflected through different degrees, pass through an adjustable slit 113 in a member 14 and are picked up by a collector electrode 15 associated with an electron multiplier 17, the output ofwhich is fed to a display 45.

Mass spectrometers as described above are well known in the art, and the output from the electron multiplier 17 is used after amplification to provide a record of the ions passing through the adjustable slit in member 13 on say a cathode ray tube display or on a record chart. The angular deflection of an ion in passing through the magnetic analyzer 11 will depend upon the accelerating voltage, since that determines the speed of the ions, the intensity of the magnetic field in the analyzer 11, and the mass of the ion.

One method of scanning a large range of a mass spectrum is to maintain the voltages used in the electrostatic analyzer 7 and on the accelerating electrode 5 constant, and to scan by slowly decreasing the current used in the electromagnet coil 12 of the magnetic analyzer 11. This progressively changes the deflections of all the ions passing through the magnetic analyzer, so that the output from the collector 15, which may for instance be part of an electron multiplier, indicates the number of ions passing through the slit member 13. If the output from collector i5 is presented on a cathode ray tube in the display 435 as the vertical deflection with a horizontal scanning speed corresponding to the decay of the magnetic field in the magnetic analyzer H, the trace shows peaks if ions having such a mass number that they are deflected to pass through the slitted member 13 are present.

Magnetic scanning, as described above, is used where sensitivity and quantitative accuracy are required and the scanning rate can be relatively low. Where a somewhat faster scanning rate is required, one can resort to voltage" scanning, in which the magnetic field in the magnetic analyzer is maintained constant but the voltage applied to the accelerating electrode 5 and the electrostatic analyzer 7 are varied together to produce the desired variation in the deflection in the beam in the magnetic analyzer. However, this tends to give a poorer resolution between adjacent peaks and the sensitivity of the mass spectrometer varies from end to end of the part of the spectrum which is scanned.

in tiapparatus shown in MG. 2, there will be the adjustable slit s3 at the collector end and a second adjustable slit W at the source end. which is shown in greater detail in H0. 7. The longitudinal axes of these slits will be perpendicular to the plane of the paper. The slits will be adjusted in a manner which will now be described.

in PKG. d a slit 2ll is defined by the gap between two blochs 20. it is assumed that the beam will pass in a direction normal to the paper vertically downwards through the slit 2i between these two blocks, the width of the slit 211 being indicated by the gap between the arrows in. The blocks have slanting sides "which slide over jewel supports 22. A rod 23 presses down against the upper faces of the blocks 20 so that when the rod 23 pushes the blocks 20 downwards against the action of springs 2d the width of the slit 211 will increase, and similarly when the rod 23 is raised the blocks 20 will be pressed upwards by the springs 2 t and will tend to move together and reduce the width of the slit 21. Thus, as the rod 23 is reciprocated, it causes reciprocation of the blocks 20. As the blocks 29 reciprocate, the supports 22 cam and guide the blocks 20 toward and away from one another according to the direction of reciprocation.

The control of the vertical positioning of the rod 23 is as follows: an electric motor 23 (FIG. 3) operating through a slipping clutch 2h drives gear wheels 2'7 and 2h. The gear wheel 28 is mounted on a shaft 29 and carries a sleeve 30, and the shaft 29 makes a screw-threaded engagement 311 with a fixed bearing member 32. Thus when the shaft 29 and sleeve 30 are rotated, the end of the shaft 29 will move upwards or downwards by a small amount and the lower end of the shaft 29 pressed through a ball 33 against the upper end of the rod 23 (also shown in lFllG. t) will cause the rod 23 to move upwards or downwards. in this manner the rotation of the shaft 29 from the motor 23 controls the width of the slit 211.

in order to limit the travel of the adjustment in each direction, the sleeve 3th carries a pin 3d (H6. 3); a cylindrical block 33 surrounds the sleeve 30 and this is cut away through a hemisphere to form houlders 36 against which the pin 34 willabut, and so the travel of the pin 3d and hence the rotation of the shaft 29 will be limited. The blOCk 33 may be rotated and clamped in various positions to adjust the angular positioning of the shoulders 36 whilst a fine adjustment is provided by member 37 which is rotated to adjust the position of the nose 3h against which the pin 3d abuts.

lFlCi. t5 shows an electrical control circuit for the motors 25 being operated; the slit controlling apparatus shown in MG. 4! willjbe provided both at the source end of the beam and at the collector end and these two apparatus can be controlled synchronously by the circuit shown in FIG. s. in the arrangement shown in F116. 6 control is effected from the contacts Cd which will be actuated synchronously with the scan. it is assumed that the source motor SM and the collector motor CM will each have two windings which when energized will cause the a motor concerned to rotate respectively in opposite directions. in the position shown in MG. ti, in which the selector switch is set for alternate high and low resolution during successive scans, closure of contacts, Cd at the scan will energize RL; this will change over the contacts RLT and lRL2 so that the source motor winding l and collector motor winding ll will now be energized. Rotation of these motors will change the slit opening until the pin 3d (FIG. 3) abum against one of thcshouldcrs 336, the motor will continue rotating for a while, thisggtationheing permitted by the slipping clutch 2s (FIG. 3) until the motors have completed a specified number of turns when the limit switches LMt and Lit i3 will open and cut offthc supply. At the same time the brake windings will be dcenergizcd so that the brahes will be applied. if now at the start of the next scan the contacts C4 open, RL will be deenergizcd so that contacts RM and R142 will revert to the positions shown in FIG. ti. When this happens the motor windings 2 will be energized so that both the motors will start up in reverse and the shaft 29 (FIG. 3) will be rotated in the reverse direction until the pin 3d abuts against the opposite stop face 3b of the bloch 33 (F116. 3); this sequence will be repeated recurrently and it will be seen that apertures will be alternatively opened and closed. The selector switch shown in FIG. h is shown as having three positions; in the alternate position shown the sequence that is above described will occur. When moving the switch to a high resolution, the slits will remain continuously in the high resolution state and similarly may be controlled to remain continuously in the low resolution state by moving the selector switch to the appropriate position. in the arrangement described above provision has been made for changing between the aperture slit openings. The invention is not limited to this but additional intermediate openings may be obtained and these may be obtained by selection or they may be obtained automatically in sequence; thus instead of moving between an extreme high resolution state and an extreme low resolution state the change may occur in steps. Alternatively, the high resolution state may be preselected to a required value and similarly the low resolution state may be preselected. in the alternate position of the selector switch the change over may occur alternatively between the two preselected states.

in employing magnetic scanning the spectrum may be observed either during the decay sweeps of the magnetic flux or during the recovery sweeps, i.e., the flux build up sweeps. it follows therefore that the decay sweeps may occur at high resolution and recovery sweeps at low resolution as shown in graph B of H6. 9, or the states may be reversed.-

Alternatively successive decay sweeps or successive recovery sweeps as the case may be occur alternatively at high and low resolution.

Preferably scans are observed on decay sweeps.

lFllG. lltl shows an electrostatic variable aperture which may be used in place of the mechanically varied aperture shown in MG. d. The aperture shown in MG. Mi comprises an entrance slit plate Ml, a limiting screen or platedld together with an electrostatic lens between the plate db and the screen M.

The electrostatic lens is formed by the earthed slit apertured plates dill and d3 and the field forming slit apertured plate d2.

The aperture in plate d0 limits the maximum aperture whilst the electrostatic lens has a focal plane in front of the screen td, thus the ion paths are dispersed and not all can ass through the aperture in plate dd. By varying the potential applied to the plate 42 the proportion of ion paths which can pass through screen M can be varied and thus the effective width of the aperture can be varied.

For further description of the electrostatic type of aperture reference may be made to FIG. 2 of British Pat. No. 759917.

The output from the mass spectrometer may be passed to display apparatus indicated at M in FIG. 2 or alternatively to recording apparatus in which it may be recorded for instance on magnetic tape and thus may be in analogue or digital form.

Many modifications and variations of the invention will be apparent to those skilled in the art in view of the foregoing detailed disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than is specifically shown and described.

We claim:

i. in a mass spectrometer having an ion source, a collector, a means for deflecting a beam laterally, and at least one means for varying the width of at least one aperture in an ion beam path, said means for varying the width of at least one aperture comprising:

a. an electric motor including a drive shaft;

b. a first gear mounted on said drive shaft;

c. a fixed bearing member;

d. a shaft having first and second ends, said shaft threadably engaged with said fixed bearing member allowing longitudinal movement of said shaft relative thereto;

c. a second gear mounted on said first end of said shaft and in engagement with said first gear;

f. a sleeve surrounding and secured to said shaft near said first end thereof;

g. a rotatable. circular bloclt surrounding said sleeve and having a block recess around a portion of the periphery of said circular block, said recess bounded on its ends by shoulders;

h. said rotatable sleeve including a pin for engagement with said shoulders;

i. a ball bearing engaging a face of said second end of said shaft;

j. a member having a wedge-shaped end, the other end of said member engaging said ball bearing; and

k. a pair of blocks defining an aperture therebetween. said blocks together forming a wedge-shaped recess corresponding to said wedge-shaped end of said member for engagement therewith, said member adapted to be moved in line with said aperture for causing said blocks to move relative to one another and vary the width of said aperture.

2. A mass spectrometer comprising:

a. an ion source;

b. a collector responsive to the impingement of ions;

c. conduit means establishing a path for the travel of focused ions from the source to the collector;

d. an analyzer along said path for deflecting ions emitted by the source according to the mass/charge ratios of the ions;

e. a slit structure positioned along said path for adjustably delineating the resolution of ions traveling along the path;

f. slit condition control means operably connected to the slit structure to vary the resolving power of the slit structure automatically between scans of a continuing study involving a series of scans conducted while a beam of ions generated from a given sample is produced; and

g. presettable means operably connected to the slit condition control means for establishing at least two preadjusted and preset slit conditions for controlling said condition control means such that said slit structure is repetitively changed from one to another of said at least two conditions, whereby said analyzer deflects said ions across said slit structure during one scan while said slit structure is in one of said at least two conditions and deflects said ions across said slit structure during another scan while said slit structure is in another of said at least two conditions.

3. The mass spectrometer of claim 2 wherein the slit structure electrostatically controls ion resolution.

4. The mass spectrometer of claim 2 wherein the slit structure comprises a plurality of relatively movable elements delineating a slit and the elements are relatively movable to adjust the transverse dimension of the slits and thereby the resolution of the ions.

5. The mass spectrometer of claim 4 wherein the position control means includes a motor with a rotatable output shaft slit actuating structure, a slip clutch coupling the shaft to the actuating structure, and wherein the control means includes a motor energizing means operating the motor for a predetermined period of time each time a slit resolution adjustment is made such that the shaft continues to rotate after the slit adjustment has been terminated by the presettable means.

6. The mass spectrometer of claim 2 wherein there are two slit structures and said structures are synchronously adjusted into position by said condition control means and said presettable means.

7. The method of analyzing a sample with a mass spectrometer including an ion source, an analyzer, a collector, and a slit structure along a path of ion travel from the source to the collector comprising the steps of:

a. presetting a slit control mechanism to establish a plurality of predetennined slit conditions of differing degrees of resolution;

b. ionizing a portion of said sample in the chamber to form a quantity of ions;

c. establishing a beam of ions from the source to the collector;

d. controlling the resolution of the beam by automatically actuatin'gl the slit control mechanism between scans to change e slit structure from one predetermined condition to another to adjust the resolution of the instrument to a selected one of said differing degrees of resolution; and

e. scanning aid beam across said collector during one scan while said slit structure is in one of said predetermined conditions and then scanning said beam across said collector during another scan while said slit structure is in another of said predetermined conditions while continuing a study of ions from said sample whereby both relatively high and relatively low resolution analysis of said sample is made in a single study involving a plurality of scans.

8. The method of claim 7 wherein a second such slit structure is adjusted to one of a plurality of predetermined conditions to further vary the resolution of the spectrometer.

9. The method of claim 8 wherein the two slit structures are adjusted simultaneously and in synchronism.

10. The method of claim 7 wherein the analyzer is scanned as the slit structure is maintained in each such predetermined condition.

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