US2844726A - Mass spectrometry - Google Patents

Description

July 22, 1958 MASS SPECTROMETRY Filed May 18, 1955 c. F. ROBINSON 2 Sheets-Sheet l SAMPLE INL ET IN VEN TOR.

CHARLIES n Rae/wow A TTORNEYS c. F. ROBINSON 2,844,726

MASS SPECTROMETRY July 22, 1958 2 Sheets-Sheet 2 Filed May 18, 1955 WMW A TTORNEYS United States Patent MASS SPECTROMETRY Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application May 18, 1955, Serial No. 509,142

9 Claims. (Cl. 250-419) This invention relates to mass spectrometers and particularly to improved means for adjusting the focus of an ion beam in a mass spectrometer.

A mass spectrometer is essentially an apparatus for producing ions and sorting them according to the ratio of their mass to their charge, i. e. according to their specific mass. A sample to be analyzed, such as a gas mixture, is ionized, usually by electron bombardment, and the resulting ions are propelled by an electrical potential into an analyzer. The ions enter the analyzer as an unsorted beam, and in the analyzer are sorted under the influence of an electromagnetic or electrical field, or both, into a series of divergent homogeneous beams of ions having the same specific mass. The sorted ions are collected and discharged, the quantity of each kind of ions being measured by the magnitude of the electrical current produced by the collection and discharge.

In the type of mass spectrometer to which this invention is directed primarily, ionization of the molecules is accomplished in an ionization chamber, or ion source, by bombardment with an electron beam. The resulting ions diffuse or are propelled into the analyzer chamber and are sorted under the influence of cross magnetic and electric fields, which cause the ion beams to travel in characteristic paths.

If a uniform electrical field is oriented normal to the magnetic field, across the space in which the magnetic field is imposed, the charged particles or ions introduced into such crossed electric and magnetic fields follow a cycloidal paththe path traced by a point at a given distance from the center of a circle rolling in a plane along a line in the plane, which in this instance is a plane perpendicular to the magnetic field. Such a path may also be considered as being rigorously circular'in a coordinate system moving with uniform velocity. The movement of the coordinate system or the center of the rolling circle is solely a function of the ratio of the electric and magnetic field strengths.

Thus, when ions of a particular mass are introduced into such crossed magnetic and electrical fields, the ions will complete one turn of their trajectory in a time which depends directly on the mass of the ion. From the periodic property of the cycloid a charged particle which is introduced into transverse magnetic and electrical fields and which crosses a plane normal to the electric field will again cross this same plane when the particle is going in the same direction of travel at a distance proportional to its mass-to-charge ratio. And if the electrical field strength is uniform, so that the coordinate systems or centers of the rolling circles corresponding to each ion move at the same velocity, ions of a particular mass will converge to a series of identical focal points after any whole number of turns in the magnetic field regardless of their velocity or direction of travel at the moment of introduction into the field.

In a typical instrument of this type the electrical field is established in an analyzer region by means of a plu- 2,844,726 Patented July 22, 1958 21rmEC X q where B=the magnetic field strength in Gauss; E: the electric field strength in statvolt/cm.; q=the ion charge in e. s. u.;

m=the ion mass in grams; and

c=the speed of light in cm./ sec.

Since in practice the focal distance X, is fixed, the ion mass m is determined by calculation from ascertained values of the electric field strength E and the magnetic field strength B.

If the electric field through which the particles move is not uniform, the behavior of the particles corresponds to rigorously circular motion in a coordinate system whose velocity is not uniform. In particular, if the electric field has a transverse component, the effect of this transverse component is to shift the focal point away from the focal plane in a direction normal to the focal plane. If the field gradients are small enough or the space divergence of the ion beam is small enough so that all of the particles in a given beam move through equivalent amounts ,of nonuniformity, the first order effect of nonuniformity in the electric field is not to destroy the quality of the focus, but to displace theposition at which this focus occurs. I have found that if a transverse electric field of controllable sign and magnitude can be imposed in a portion of the analyzer chamber the focal point can be displaced in a direction normal to the focal plane in a controllable way in order to make a first order correction for any accidental nonunifo'rmities in electric field which may exist due to manufacturing tolerances and the exigencies of a practical design. I have found also that a similar correction of any accidental nonuniformity in the electric field may be achieved by arbitrarily introducing a nonuniformity in the strength of the electric field through which the ions pass in their cycloidal trajectory.

In my copending application, Serial No. 425,482, filed April 26, 1954, now U. S. Patent No. 2,794,126, there is shown apparatus for varying the magnetic field strength in order to compensate for nonuniformity of the electric field. The invention of that application was based upon the discovery that nonuniformities in the magnetic field are in certain instances similar in effect to electric fields. Thus, my invention herein described and claimed also provides means to correct accidental nonuniformities in the magnetic field which may exist due to manufacturing tolerances.

These and other aspects of my invention will be understood more clearly by reference to the accompanying drawings, in which:

Fig. 1 is a schematic sectional elevation of a cycloidal mass spectrometer showingsplit electrodes;

Fig. 2 is a schematic sectional elevation of a cycloidal mass spectrometer showing means of spoiling the uniformity of the electrical field;

Fig. 3 is a schematicsectional elevation showing means for generating a magnetic field; and

Fig. 4 is a graph showing the eficct of nonuniformity of the electric field upon the focus of a cycloidal mass spectrometer.

Referring first to Fig. l of the drawings, the basic concept of a crossed-fieldmass spectrometer is shown. An ion source is disposed to introduce ions into an influence-field through an aperture 11 in what may be considered a focusing plane. An electrical field is established across the space of ion travel by means of a plurality of electrodes 13, 14, 15, 16, 17, 18, 19, 20 with one boundary of the electrode 17 forming the aforementioned focal plane. A magnetic field is established transverse to the electrical field which, in the diagram, would be normal to the plane of the paper. A collector electrode 21 is spaced laterally from the ion source 10 with a second or resolving aperture 22 formed in the electrode 17 giving access to the ion collector. Ions of a given specific mass introduced into the region of the crossed fields from the ion source 10 will pursue a cycloidal trajectory as a consequence of their rigorous circular motion in a coordinate system moving laterally at a uniform velocity responsive to the influences of the magnetic and electric fields.

Such a trajectory will cause all of the ions of this specific mass, that is, all of the ions traveling in a cycloid of given pitch, to come to a focus in a point in space which in this instance is a point on the focal plane at which the resolving slit 22 is located, i. e. at a point exactly 360 of trajectory beyond the point of origin. It should be pointed out that the focal plane is normal to the imposed electrical field.

Fig. 4 shows the elfect in exaggerated manner of an accidental misalignment of the field-forming electrodes. As a consequence the focal plane 12a does not coincide with the plane 12 which in practice would be formed by the electrode 17 of Fig. 1. The ion beams shown in dotted lines in Fig. 4 and representing ions having the same mass are broadened before they reach the plane 12, which normally contains the resolving aperture designated as 22 in Fig. l. The result is to impair the resolution and in extreme cases the sensitivity of the instrument.

It is of course obvious that this error can be avoided by extreme precision in the design and construction of the instrument whereby absolute parallelism between the electrical field-forming electrodes is maintained to an extremely high degree of tolerance. Such practice of course greatly increases the cost of such an instrument and under many circumstances is not always possible of attainment. As previously described, I have found that displacement of the point of focus as a consequence of nonuniformity of the electrical field can be arbitrarily corrected by means of a so-called correcting field applied in a direction normal to both the orbit defining electrical and magnetic fields and as represented by the arrow E in Fig. 4. This correcting field may be induced electrically by artificially disturbing the orbit defining elec trical field through manipulation of the voltages applied to the field-forming electrodes, or to segments of the field-forming electrodes.

One embodiment of my invention is shown in 'Fig. 1. Ions formed Within the source 10 are impelled into the analyzer chamber body 46 through the slit 11 by repeller 29 where they follow cycloidal paths in the well known manner. Potentiometers 50 and 51, together with resistors 52 and S3 drive a set of field trimmer plates through the use of which-a transverse field gradient can be imposed between the segments 16a, 17a and 18a and the electrodes 16, 17 and 18. Potentiometers 50 and 51 and resistors 52 and 53 are of such value that when the potentiometers are in their center positions, electrodes 16, 17 and 18 are at the same potentials as 16a, 17a and 18a respectively. Potentiometers 50 and 51 are ganged together in such a way that an increase in the resistance of one corresponds to the decrease in the resistance of the other so that rotation of their common shaft makes the group of electrodes 16, 17 and 18 positive or negative with respect to the segments 16a, 17a and 18a respectively, thus imposing a transverse field of either sign. The effect of the transverse field is to move the focal point of the beam toward or away from the plane of electrode 17 which contains the resolving aperture 22 so that if an accidental nonuniformity causes the focal point to be displaced in one direction or another, as shown in Fig. 4, manipulation of potentiometers and 51 can be employed to return the focal point to the plane of the resolving aperture.

Referring to Fig. 4, the focal plane 12a of the dotted trajectories can be brought into coincidence with the focal plane 12 of the solid trajectories by imposing compensating nonuniformity in the electric field strength over a portion of the area through which the ions pass in their cycloidal trajectory.

To accomplish such nonuniformity, as shown in Fig. 2, the several electrodes 30, 31, 32, 33, 34, 35, 36, 37 are connected to sliders 38, 39, 40, 41, 42, 43 on the voltage divider 46. For simplicity, additional elements of the circuitry, as shown in Fig. 1, are not reproduced in Fig. 2. This form of the invention relates to my discovery that accidental nonuniformity in the electric field can be compensated by arbitrarily imposing a nonuniformity in the strength of the electric field through which the ions pass in their cycloidal trajectory.

The trajectories shown in Fig. 2 represent trajectories in which the electrical and magnetic field strengths are uniform throughout. And, as heretofore pointed out, the velocity of the coordinate system corresponding to these trajectories is a function of the ratio of E/B, or the ratio of the electrical field strength to the magnetic field strength. If the velocity of such a coordinate system is changed after the beam passes through the focal plane, the result will be a disturbance of the trajectory when compared to a trajectory associated with a coordinate system travelling at uniform velocity.

Such a change in velocity will result if the electric field strength is not uniform throughout the path of the ions. For example, if the electric field strength below the focal plane is higher than that above the focal plane, the velocity of the coordinate system will be higher than normal when the ions of the beam of interest are below the focal plane and will cause the focal point to be displaced. Thus, the introduction of a means to control the strength of the electrical field above and below the focal plane provides a means to displace the focal point in order to obtain good focus in the plane of the resolving slit.

Apart from the specific apparatus of the invention, a crossed field mass spectrometer (shown schematically in Fig. 1) comprises an evacuable envelope 26 provided with conduit means 27 for connection to an evacuating system (not shown) and a sample inlet 28. Through a conduit (not shown) the sample inlet is connected to the source 10. The electrodes are suitably supported in the envelope and spaced and insulated from each other by well known means. The ion source 10 is supported adjacent the inlet aperture and includes a chamber 54, an electron gun 55, an electron target 56 and a repeller electrode 29 arranged in a more or less conventional fashion so that molecules in the chamber 54 are ionized by an electron beam 58 and under the influence of the repeller electrode 29 and the ions are propelled out of the source into the chamber 46 defined by the electrodes.

Electron gun 55 and the target 56 are interconnected through an emission regulator circuit 57 so that the ionizing electron beam 58 is maintained at a substantially uniform intensity. Many emission regulator circuits are known in the art as conventional adjuncts to a great number of commercial mass spectrometers.

The appropriate potentials are impressed on the several electrodes by means of a voltage divider network 60. Magnet poles 63 and 64, shown in Fig. 3, are supported externally of the envelope 26 to develop a magnetic field across the region defined by the field-forming electrodes and transversely of the electrical field formed by the electrodes. A D. C. power supply 58 is connected across a capacitor 59, a voltage divider 69 is connected in parallel across the capacitor 59, and the several electrodes are connected to the divider network 60 as illustrated. A mass spectrum can be scanned by charging capacitor 59 and allowing the charge to decay across the voltage divider 60 by opening switch 65.

Electrode 17 is provided with a support s2 on which a collector electrode 21 is mounted. Ions focused on the resolving aperture 22 will collect on and discharge at the collector electrode 21. The amount of current is sensed by means (not shown) which are well known in the art.

One form of crossed-field mass spectrometer has been illustrated and described in detail. However, it is understood that the invention is not directed to the specific form of instruments as presented since the discovery of the existence and correction in an instrument of this type of, an error producing electric or magnetic field component is applicable to any modified form of crossedfield instrument or, more generally, to any type of doublefocussing mass spectrometer in which it is desired to simulate a weak electric field transverse to a magnetic field.

I claim:

1. In a crossed-field mass spectrometer the combination of an analyzer chamber, a plurality of electric field establishing electrodes disposed about said analyzer chamber for establishing a potential gradient, means generating a magnetic field having a direction parallel to said electric field establishing electrodes, an ion source for propelling a beam of ions into said analyzer chamber, an apertured plate suspended within said analyzer chamber between said electric field establishing electrodes, and means generating a compensating electric field for causing said ion beam to be focussed in the plane of said apertured plate.

2. Apparatus in accordance with claim 1 in which said means establishing a compensating electric field comprises a set of segmented electrodes suspended within said analyzer chamber for generating a potential gradient in a direction transverse of the electric field generated by said plurality of field establishing electrodes.

3. Apparatus in accordance with claim 1 in which said means for generating a compensating electric field comprises means for energizing said plurality of electric field establishing electrodes to provide an electric field having a first predetermined potential gradient on one side of said apertured plate and providing an electric field having a second predetermined potential gradient on the other side of said apertured plate.

4. In a crossed-field mass spectrometer, the combination of a plurality of parallel electrodes for establishing an electric field having a given direction, means generating a magnetic field transverse of the given direction of said electric field, said electric and magnetic fields being adapted to cause an ion beam to follow a cycloidal tra jectory, and means for generating a compensating electric field for causing an ion beam not following said cycloidal trajectory to be focussed in a predetermined focal plane.

5. Apparatus in accordance with claim 4, in which said means for generating a compensating electric field comprises at least one segmented electrode, and means energizing said segmented electrode to generate an electric field having components in a direction transverse of the given direction of said first named electric field.

6. Apparatus in accordance with claim 4, in which said compensating electric field generating means comprises said plurality of parallel electric field generating electrodes to provide an electric field having a first predetermined magnitude on one side of said focal plane and an electric field having a second predetermined mag nitude on the other side of said focal plane.

7. In a crossed-field mass spectrometer, the combination of an apertured plate having an ion beam inlet aperture and at least one ion beam resolving aperture, means injecting an ion beam through the inlet aperture of said apertured plate, a plurality of electrodes mounted parallel to and on each side of said apertured plate to define an analyzer chamber, means energizing said plurality of electrodes to generate an electric field having a potential gradient perpendicular to said apertured plate, means generating a magnetic field transverse of said electric field potential gradient for operating in conjunction with said electric field for causing an ion beam passing through the inlet aperture of said apertured plate to follow a cycloidal path in said analyzer chamber, and electric means compensating for irregularities in said electric and magnetic fields for causing said ion beam to be focussed in the plane of said apertured plate at said resolving aperture.

8. Apparatus in accordance with claim 7, in which said electric compensating means comprise at least one segmented electrode mounted within said analyzer chamber for establishing an electric gradient transverse of said first named electric field potential gradient.

9. Apparatus in accordance with claim 7, in which said electric compensating means comprises apparatus for variably energizing said plurality of electrodes located on one side of said apertured plate to generate an electrostatic field having a first predetermined potential gradient and for energizing said plurality of electrodes located on the other side of said apertured plates to generate an electric field having a second predetermined potential gradient.

1 References Cited in the file of this patent UNITED STATES PATENTS 2,221,467 Bleakney Nov. 12, 1940

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