US2794126A - Mass spectrometry - Google Patents

Description

May 28, 1957 c. F. ROBINSON MAss sPEcTRoMETRY Filed April 26, 1954 2 Shee'ts-Sheei l wwl/ EOFAWFM Ilan Om, Mh ul QR mm. QV km. .y/// n m6. @mv a n ../.Nb/ mv m RY mm.. /bv

@N NIV INVENTOR. CHARLES F. ROBINSON BY Q a. 4 t

AT TORNE V May 28, 1957 c. F. ROBINSON MASS SPECTROMETRY 2 Sheets-Sheet Filed April 26, 1954 F/G. PLATE` LEVEL CORRECT/NG F/ELD 22) /9 F/ G, .SPL/175s T/PPED of? RESOL v/NG sL/T M/SPLACED INVENTOR. CHARLES F. ROBINSON LTlORNEV United States Patent 2,794,126 MASS sPEcrRoMErni Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamies Corporation, Pasadena, Calif., a corporation of California Application April 26, 1954, Serial No. 425,482

4 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometry and particularly to means for developing focus-correcting fields in such instruments.

Mass spectrometry is the art of analysis by ionization and selective detection of the relative abundance of ions of differing mass to charge ratio. In general, a mass' spectrometer involves an ion source in which a sample to, be analyzed is ionized by means of an electron beam, an analyzer chamberinto which ions are propelled from the ion source and in which they are subject-to the influenceof electrical or magnetic fields, or both. Under the infiuencelof such fields, and depending upon their application and orientation, the ions are caused to pursue, characteristic paths of travel in the analyzer chamber .as` a function of their mass to charge ratio. Means are normally provided for selectively collecting ions as a function of spatial, temporal ordirectional separation between ion masses inthe analyzer chamber. s Y p l One form of instrument of this type has been referred, to as the crossed field mass spectrometer. If a charged particle is introduced into a magnetic field it will move in a circular path to return to its point of origin. This is true regardless of the `mass of the particle with particles of increasing mass traveling in circles of increasing radius, but in each instance returning to the point of origin. If a, uniform electric field is imposed across the space defined by the magnetic field and normal to the magnetic field, the ions pursue a path which may be considered as rigorously circular in a coordinate system moving with uniform velocity. The movement of the coordinate sysduced into such a field system they will complete one turn n.

of their circular motion in a time which depends directly on the 'mass of the particle, and if the electric field strength is uniform so that the coordinate systems corresponding to each particle move at the same velocity the particles willl converge to a series of rigorous point foci after any integral number of turns in the magnetic field, regardless of their velocity or direction of travel at the moment of introduction into the field. Since the time required for ions to complete one turn of their circular motion depends directly on the mass, and since under the condition of uniform field strength specified the rate of motion 2,794,126 Patented May as, 1957 'ice i be used, making energy spreads critical in instruments of the coordinate system is invariant to the mass of the particle involved, the focal point of the heavy particles will be displaced farther from the point of origin than the focal point of the lighter particles. This is the basic concept ofthe crossed field mass spectrometer.

An instrument of this type has two principal characteristie advantages. It is not subject to aberration in any form and is therefore insensitive to the energy spread of ions introduced into the field. For this reason space l A originate and terminate at the base line or zero level of the record rather than approaching the recordl zero level asymptotically as is thecase in an instrument whose focal point is sensitive to the energy of the particle. `This has the'a'dvantage of allowing more accurate measurement of closely adjacent masses and is a further important consequence of the insensitivity of this type of instrument to thermal energies.

If for any reason the electric field through which theA particlesv are propelled is not uniform, the behaviorof thel particles corresponds to rigorously circular motion, in acoordinate system Whose velocity is not uniform. In. Particular, if the electric field has a component in a directionnormal toits desired orientation, the effect oflthis component is to shift the focal point of the moving ions..` Thus the first-order effect of non-uniformity in the electric field `is not to destroy the quality ofthe focus but to displace the position at which this focus occurs compared to an instrument at which a collector electrode is located at the .properpoint `of focus based upon considerations of uniformK electric and magueticfield. Such focusdisplacement consequent upon any non-uniformity `ortr'ans-7 verse, 'component of the' electrical field is to impair the resolutionof the instrument since ythe focal point Ino longer Y coincides with `the resolving slit.` Y

inasmuch as the displacement of the focal point responi ysivevto non-uniformity of the electrical field is a first-V uniformity in the electrical field may be overcome so as; v

in effectA to return the point of ion focus to the collector resolving slit, which discovery makes it possible to construct an instrument without therequirement of'impractical design, fabrication and assembly tolerances.

vThe invention contemplates in a mass spectrometer including an analyzer chamber, the combination comprising means producing a magnetic field across the anal lyzerchamber, and means operable to distort the magnetic field to produce in the analyzer chamber an influence equivalent tolan electrical field whose direction is normalto that of the applied magnetic field. Y I have found that non-uniformity artificially induced in the magnetic field may be made equivalent to anelectrical field and may thus be used to influence ions in the analyzer chamber in aplane normal to the .magnetic field in the manner of a normal component of electrical energy. The invention is, for convenience, described in relation to a crossed field massA spectrometer to correct a non-uniformity in the moving coordinate system resulting from an vaccidental disturbance of the electrical field. However, 'the invention is directed to the accomplishment of this objective in' any type of mass spectrometer. v

The invention will be more clearly understood by refl erence to the following detailed description thereof taken in conjunction with the accompanying drawings, in which:

Fig. l is alongitudinal sectional elevational through a` crossed field mass spectrometer of the type here under consideration;

Fig. 2 is a transverse sectional elevation taken on the line 2-2 of Fig. 1; Y

Fig. 3 is ahorizontal section taken on the line 3-3 of v Fig. 2;

Fig. 4 is a diagram illustrating the conduct of ions in a crossed field of uniform orientation; l

Fig. 5v is a-diagram of the conduct of Vions in a crossed field in which the electrical field is non-uniform as a consequence of lack of parallelism between the field-forming electrodes; and

Fig. 6 is a schematic diagram of the electrical circuitry of the spectrometer.

Referring first to Fig. 4 of the drawing, the basic concept of 'a crossed iield mass spectrometer is shown. An ion source 10 is disposed to introduce .ions into an influence-field through an aperture 11 in what may be considered ya focusing plane 12. An electrical eld is established across the space of ion travel by means of a plurality of electrodes 13, 14, `15, 16, 17, 1S, 19 with one boundary of the electrode 16 forming the `aforementioned focal plane 12. A magnetic eld is established transverse to the electrical field which, in the diagram, would -be normal to the plane of the paper. Acollector electrode 20 is spaced laterally from the ion source 10 and on the opposite side of the focal plane 12 with a second or resolving aperture 21 formed in or adjacent the boundary of the focal plane Idefi-ned by the electrode 16 giving access to the 'ion collector. Ions of a given specific mass introduced into the region of the crossed iields 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 intiuences of the magnetic and electric lfields.

Such a trajectory will cause all of the ions of this specitic mass, that is, .all of the ions traveling ina cycl'oid of given pitch, to come to a focus in a point in sp-ace which in this instance is a point on the `focal plane at which the resolving yslit 21 is located, i. e. at a point exactly 360 of trajectory beyond the point :of origin. It is noted that the focal plane is normal to the imposed electrical field.

IFig. shows the effect yin exaggerated manner of an accidental misalignment of the field-forming electrodes. In the drawing of Fig. 5, electrodes 13, 14, 15, 17, 18 and 1-9 are inclined with respect to the electrode 16, and as a consequence the `focal plane 112 illustrated in this instance by Ia dotted line does not coincide with the electrode 16 in which, or at the bound-ary of which, the inlet and resolving apertures are formed. Since the ions introduced from source through the inlet :aperture 11 must come to `focus after 360 of revolution and at the focal plane, they do not vfocus at 'the resolving aperture 21 and as a consequence the beam which strikes the resolving aperture is broadened, impairing 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 any circumstances is not always possible of attainment. As previously described, I have found that ydisplacement of the point of focus as a consequence of non-uniformity of the electrical eld can 'be arbitrarily corrected by means of a `so-called correcting eld applied in a ldirection normal to yboth the orbit defining electrical and magnetic fields and as represented by the anow 22 in Fig 5. This correcting field may be induced electrically by artilicially disturbing the orbit defining the electri-cal neld through manipulation of the voltages applied to the field-forming electrodes, or to segments of the held-.forming electrodes. Such a method 'of correction is described rin my copending application Serial No. 509,142, tiled May 18, 1955. The present invention is directed to the discovery that non-.uniformities in the magnetic ield :are in certain instances equivalent in their effect to electrical iields. That this is true and that this characteristic of the magnetic tield can be `made use of in accordance with the invention is established mathematically as follows: EIn a conventional mass spectrometer magnet in which the spacing d between the pole faces may be Variable, the magnetic lield strength B is given by the following expression:

B=k/d (1) where k is a constant -of proportionality. lIf it is assumed that the pole faces are plane but tipped with respect to `one another, then the gap spacing d is given by the expression:

d=do(1+ocX) ('2) where do is the gap spacing at a reference plane; a yis the .angle `of tip in radians divided by the gap spacing at the reference plane; and

X is a dimension perpendicular to the axis of tilt.

iIn general, the radius of curvature r of a charged particle of mass m, energy V, and charge q, moving in a magnetic ield of `strength B is given by:

where Vo is the particle energy as it crosses an arbitrary reference plane. From this the radius of curvature can be derived as:

E T To( 1 Comparing Equations 4 and 6, it is apparent that there is `a formal i-dentity between the behavior of charged particles in these two situations and thus the trajectories of the two charged particles, one moving with uniform energy in this `type of non-uniform magnetic tield and the other moving in a uniform magnetic Ifield plus a small transverse electric field, yare completely superposable as long as the gradient a in the magnetic ield in the rst case and the electric iield strength E in the second case are related by:

Based on this discovery I have found that the provision of a slight tip of the magnet pole faces about an axis perpendicular to `the plane deiined by the entrance yand resolving slits in a crossed iield mass spectrometer .affects the ion motion in exactly the same way Ias the introduction of a small electric field parallel to this plane. Therefore any accidental electric field component developed parallel to this plane as `a consequence of mis- :alignment `or other `substantially unavoidable manufacturing tolerances can be -compensated by tipping the magnet pole Efaces about an axis perpendicular to it. This means of alignment of `a crossed field mass spectrometer has .advantages over electrical methods of correction in that the physical structure of the field plates is greatly simpliied since there is no segmentation required and the number of electric leads to be lcarried through the vacuum envelope is materially reduced. This method has the further advantage that the c-ompensation is more nearly rigorous than is achievable by imposition of correcting electrical iields. However, either the electrical method or the magnetic method herein described has the advantage over an uncompensated instrument ofper-i mitting greater geometric tolerances in production.

Equation 7 shows also that the apparent electric field E introduced by tipping one magnet pole face is a linear function of the ion energy at any selected fixed point of the trajectory so .that this method is suitable for compensating through an extended range of masses whether scanning is done electrically or magnetically. A crossed field mass spectrometer in accordance with a preferred embodimentof the invention is illustrated in longitudinal section in Fig. 1, transverse section in Fig. 2, `and in Fig. 3 in horizontal section taken on the line 3-3 of Fig. 2. `Fig. 6 is a schematic circuit diagram of the instrument.

The crossed field mass spectrometer comprises an evacuable envelope 26 provided with conduit means 27 for connection to an evacuating system (not shown) and a sample inlet means 28. A plurality of electrodes 30, 31, 32, 33, 34, 35, 36 are supported in the envelope from a framework 37 by a series of pins 38, 39, 40, 41, 42, 43. The several electrodes are spaced and insulated from each other by insulating spheres 44, 45, etc. The electrode structure defines a chamber 46 having an inlet slit 47 and a resolving slit 48 spaced from each other on a common or so-called focal plane. An ion source 49 is supported adjacent the inlet slit 47 and, as shown schematically in Fig. 6, includes a chamber 50, an electron gun 51, an electron target 52 and a repeller electrode 53 arranged in a more or less conventional fashion so that molecules in the chamber 50 are ionized by an electron beam 54 and under the influence of the potential between the repeller electrode 53 and the walls of the source, the ions ionized are propelled through an outlet aperture 55. The outlet aperture 55 is apparent in Fig. 1.

Electron gun 51 and the target 52 are interconnected through an emission regulator circuit 56 so that the ionizing electron beam 54 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. y

The appropriate potentials are impressed on the several electrodes 30, 31, 32, etc. by means of a voltage divider network as shown schematically in Fig. 6. A D. C. power supply 58 is connected across a capacitor 59. A voltage divider 6i) is connected in parallel across the capacitor 59, and the several electrodes 30, 31, 32, 33, 34, and 35 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 6i). The several field-forming electrodes will remain at the same relative potentials bu-t the eld strengths will diminish as the capacitor 59 discharges and different ion trajectories will be brought to focus at the resolving aperture 48.

Electrode 33 is provided with a cavity 62 into which the resolving aperture 48 opens and in which a collector electrode 63 is mounted. lons focusing on the resolving alperture 48 will collect on and discharge at the collector electrode 63. Suitable electrical leads are brought through a wall of the envelope 26 in conventional manner for connection to the various portions of the ion source of the field-forming electrodes and the collector electrode 63. An electrical conduit 64 is shown as accomplishing this purpose, the individual electrical leads brought into conduit not being distributed in the drawing for purposes of clarity. Magnet poles 66, 67 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. As viewed in Fig. 2, the electric field developed by the field-forming electrodes 3l), 31, 32, etc. is in the direction of the arrow X, and the magnetic field is in the direction Y. The error field with which this invention is concerned is in a direction normal to both the axes X and Y which, in Fig. 2, means in a direction perpendicular to the plane of the drawing. As explained above, such an error field aga-agar@ isl produced as aiconsequence of any mis-alignment of theeld-forming electrodes, some mis-alignment being almost,

forming electrodes 30, 31, 32, etc. Pole piece 66 can be` mounted in a similar fashion.

In the operation of the illustrated device, sample molecules introduced to the ion source arey ionized by. the electnon beam and under the infiuence of a propelling po.

tential established across the ion source are expelled from the source into the analyzer chamber. Responsive to the transversely oriented magnetic and electrical fields impressed across the analyzer chamber, the ions pursue cycloidal trajectories in the chamber. 'Ihe pitch of the ion trajectories is a function both of mass and field strength, the latter being controlled to focus ions of a given predetermined mass at the resolving slit. A sharp focus is obtained by adjusting the pole faces of the magnet means. As explained above, any deviation of these pole faces from parallelism distorts the magnetic field in a manner to simulate a relatively weak electrical field normal to the transverse electrical and magnetic fields. This so-called distortion field is employed and regulated to achieve sharp ion focus at the resolving slit. The in focus ions are collected at the collector electrode and the resultant discharge current is sensed in any conventional fashion. To scan a mass spectrum, either of the transverse magnetic and electrical fields may be varied to successively focus ions of different mass. A sharp focus is retained by virtue of the prior adjustment of the magnet pole pieces.

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 field component is applicable to any modified form of crossed field instrument, or more generally, to any situation in which it is desired to simulate a weak electric field transverse to a magnetic field.

I claim:

l. In a mass spectrometer including an analyzer charnber, the combination comprising means for impressing an electrical field in one direction across the analyzer chamber, magnet means for developing a magnetic field across the analyzer chamber transverse to the electrical field, and means inclining the magnet means from a plane normal to the axis of the magnetic field whereby the magnet means produces in the analyzer chamber an influence equivalent to a relatively weak electrical field disposed transversely of both the magnetic and the imposed electrical field.

2. In a mass spectrometer, the combination comprising an envelope, an ion source, means producing an electrical field across a space in the envelope, means producing a magnetic field across the space normal to the electric field, means for propelling ions from the source into the space, means disposed in the spiace to collect those ions which traverse a given restricted region of the space, and means operable to distort the magnetic field to produce in the space an influence equivalent to that of an additional electrical field whose direction is normal to that of both the applied fields.

3. In -a mass spectrometer, the combination comprising an envelope, an ion source, means producing an electrical field across a space in the envelope, magnet pole piece disposed on opposite sides of the envelope with the pole faces nominally parallel to each other and in planes parallel to the electrical field to produce a magnetic field across the space in a direction normal to the electrical field, means for propelling ions from the source into the space, means disposed in the space to collect those ions which traverse a given restricted region of the space, and means operable to vary the position of at least one of the pole pieces to incline its respective face from its norminal position so as to produce in the space an inuence equivalent to that of an additional electric field whose direction is normal to that of both the applied fields.

4. In a mass spectrometer including an analyzer chamber, the combination comprising means for developing 10 5 electrical field.

References Cited in the file of this patent UNITED STATES PATENTS 2,193,602 Penny Mar. 12, 1940 2,221,467 Bleakney Nov.v 12, 1940 2,471,935 Coggeshall et al May 31, 1949 2,615,128 Ruderfer Oct. 21, 1952

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