US2782316A - Mass separation - Google Patents

Description

Feb. 19, 1957 c. F. ROBINSON 2,782,316

MASS SEPARATION Filed June 20, 1952 s Sheets-Sheet 1 39 la. 2 44 -4il //v1.r E 5 -r0 sxmusr l2 1 I I t I 7 g: i c 1- i 1} I i i 1 a l :f 3

1 i "K40 i 1 l I E l Y 38 gl 2 FIG. w

RF 3 see FIG. 2 OSCILLATOR i ac.

SUPPLY AWL/gyms SENSING gay 225 INVENTOR. CHARLES E ROBINSON AT TOIW Y Feb. 19, 1957 c. F. RQBINSON MASS SEPARATION 3 Sheets-Sheet 2 Filed June 20, 1952 m2 B 38 Q9 kzwzowmQ M m Q IN VEN TOR. CHARL ES E ROBINSON ATTORNEY R M kWbYIXW OR Feb. 19, 1957 c. F. ROBINSON 2,782,316

MASS SEPARATION Filed June 20, 1952 5 Shets-Sheet 3 8 RECORDER FIG. 7.

IN VEN TOR. CHARLES F: ROBINSO ATTORNEY United States Patent C l MASS SEPARATION Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application June 20, 1952, Serial No. 294,685

3 Claims. (Cl. 250-413) This invention relates to mass spectrometry and particularly to apparatus for analyzing fluid mixtures in accordance with the ionic resonant frequencies of the various components of the mixture.

The generalized principles of analytical mass separation are well known. Many different methods and forms of apparatus have been developed and disclosed. All of these diiierent procedures have in common the ionization of a mixture under investigation, selective temporal, directional or spatial separation of ions according to specific mass under the influence of suitably shaped electrical and magnetic fields, and selective sensing of one or more given specific masses.

A comparatively recent development in this "art which may be termed D. C. resonance mass spectrometry, makes use of characteristic ion oscillation in a D. C. field of parabolic configuration. In co-pending United States patent application Serial No. 252,044, filed October 19, 1951, by Clilford E. Berry and Charles F. Robinson, one method of D. C. resonance mass separation is disclosed in which ions of a given specific mass are sensed through the inductive loading of a tuned circuit sensible to ion oscillation in the parabolic D. C. field and tuned to the resonant frequency of the specific mass in resonance. This method is characterized by independence of any super-imposed R. F. stimulus and its adaptability to ion injection into the D. C. field from an external source.

Another form of D. C. resonance mass separation, as described in United States Patent No. 2,570,158, issued to P. O. Schissel on October 2, 1951, contemplates collection of ions of a specific mass. This is accomplished by disposing collector electrodes adjacent the opposite boundries of the D. C. field defined by the potential maxima of the field, and superimposing an R. F. driving field across a part or the whole of the D. C. field whereby ions in resonance with the R. F. field will acquire sufiicient energy to escape from the D. C. field and strike one or the other of the collector electrodes. it is this form of D. C. resonance mass separation to which the present invention relates and the following discussion and description are so directed.

The prior art has totally failed to take into account the efiects of electrons in this type of system. Since at least one electron is liberated in the formation of each ion, and inasmuch as such electrons are accelerated directly into the collector electrodes, spurious discharges inevitably result unless means are provided to counteract this effect. In some circumstances the spurious signals originating from electron discharge at the collector electrode may be of sufficient magnitude to completely mask positive ion discharge.

Another previously unresolved limitation inherent in this procedure is the indiscriminate phasing of the ions formed within the confines of the field. Ions formed at an inopportune phase angle require an inordinate number of cycles of oscillation to reach a collector electrode. Such ions contribute very little to the resolving power 2,782,315 Patented Feb. 19, 1957 and their space charge eiiects are out of all proportion to the amount of discharge current produced.

A third diiliculty is in the matter of ionization within the shaped field. To attain a suitable sensitivity an axial magnetic field has heretofore generally been employed to collimate the ions to limit migration other than on the ordinate axis of the parabolic field. Such a magnetic field makes it practically impossible to develop an ionizing electron beam normal to the field axis unles special techniques are employed.

The present invention is directed to improvements in methods and apparatus for D. C. resonance mass separation to minimize or remove the several limiting conditions outlined above.

In one aspect the invention contemplates a method of mass separation comprising forming ions in a parabolic D. C. field by means of an ionizing electron beam, superimposing an alternating electrical field on the D. C. field to expel from the D. C. field those ions which are in resonance with the alternating field, collecting the expelled ions exteriorly of the D. C. field while separating electrons from the resonant ions between the point of ion formation in the ionizing electron beam and the point of collection of expelled ions.

In another aspect the invention contemplates a mass spectrometer comprising an analyzer chamber, a plurality of more or less planar electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic D. C. field across a space in the chamber, means for imposing a R. F. electrical field on the D. C. field, ion collection means including at least one collector electrode disposed adjacent a boundary of the D. C. field, means developing an ionizing electron beam directed transversely of the D. C. field and means adapted to limit electron travel in the direction of the ion collecting means. In a preferred embodiment, means are provided for preventing random exit of ions from the field. If an axial magnetic field is employed for this purpose, additional provision is made for permitting the ionizing electron beam to travel transversely of such a magnetic field.

The invention will be more clearly understood in all of its embodiments from the following detailed description thereof taken in relation to the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional elevation through a mass spectrometer in accordance with the invention;

Fig. 2 is a transverse sectional elevation taken on the line 22 of Fig. 1;

Fig. 3 is a graph of the direct current electrical potential established across the region of ion oscillation as accomplishing one means of separation of electrons and resonant ions;

Fig. 4 shows the momentary direct current field distribution periodically developed as a means for maintaining sharp resolution and at the same time separating electrons from resonant ions:

Fig. 5 is a longitudinal sectional elevation through an alternative form of mass spectrometer in accordance with the invention;

Fig. 6 is a diagram of electrode arrangement in yet another embodiment of the invention; and

Fig. 7 is a graph showing the potential distributions. established by the electrode arrangement in the embodiment of Fig. 6.

The mass spectrometer as shown in longitudinal sectional elevation in Fig. 1 and in transverse sectional elevation in Fig. 2 comprises an envelope it) provided with an exhaust line 11 for connection to an evacuating system (not shown) and a sample inlet line 12 for introducing a sample to be analyzed into the instrument. A

3 plurality of field-shaping electrodes 14, 15, 16, 17, 18, 19, 20, 21, 22 are arranged within the envelope in spaced parallel relation. These electrodes may be in the form of grids, as illustrated, annular rings of any generally planar form to permit ion oscillation in the.

region defined by the electrodes. A pair of collector electrodes 24, 25 are disposed in the envelope respectively adjacent the outer field-shaping electrodes 14 and 22, i. e. at opposite ends of the ordinate axis of the shaped field, the collector electrodes being adapted to collect and discharge ions expelled from the region of the fieldshaping electrodes in the manner hereinafter described. The two collector electrodes 24, 25 are interconnected in circuit with an amplification and sensing circuit 26, shown in block diagram, and through a dropping resistor 28 to ground. The amplifying and sensing circuit 26 and associated resistor 28 may take any suitable form, many such systems being well known in the art.

The field-shaping electrodes are interconnected in pairs, the pairing being between correspondingly positioned electrodes on opposite sides of the median plane, as defined by the central electrode 18. The several electrodes 14, 15, 16, 17 and the median electrode 18, together with their respective corresponding paired electrodes, 22. 21, 20, and 19, are connected across a voltage divider 30. The opposite sides of the voltage divider 30 are connected across a direct current supply 32 and inductively across a radio frequency oscillator 34.

For reasons hereinafter made apparent, the electrode system is biased by means of a battery 36 so that the collector electrodes 24, 25 are at a negative potential with respect to the field-forming electrode array.

One of the problems encountered in the operation of a mass spectrometer of this type is that of holding the oscillating ions within the shaped field as developed and defined by the electrode array. One means of over coming this problem is by the establishment of an axial magnetic field. This is accomplished in the embodiment of Fig. 1 by means of an electromagnetic coil 38 disposed coaxially around the envelope and energized from a voltage source 39, illustrated as a battery.

The development of such a coaxial magnetic field to collimate the oscillating ions gives rise to a further problem with respect to the development of an ionizing electron beam.

As shown in the drawing, an electron gun 40 comprising an emitter 41 and a shield electrode 42 is disposed in the envelope 10 to direct an electron beam 43 transversely of the shaped field and at the median plane thereof toward an electron target 44. Refinements of differential pumping. electron gun construction. and the like. are well known in the art and form no part of the present invention. Moreover, there is no requirement that the electron beam be located at the median plane of the field although such orientation is presently preferred. An electron beam in the presence of an axial magnetic field of suflicient magnitude to accomplish the collimation referred to above will be unable to traverse the region between the electron gun and target electrode at any practical operating voltage, the magnietic field having the efiect of turning the electron beam about itself and driving it back to the emitter.

I have now devised a means of overcoming the effects of the axial magnetic field as related to the electron beam, which means also has the further advantageous effect of permitting modulation of the beam so as to accomplish desired phasing of ion formation. This means is illustrated in Figs. 1 and 2 with the electrical connections involved being more clearly evidenced in the transverse sectional elevation of Fig. 2. A pair of plate electrodes 46, 47 are disposed adjacent the median plane of the shaped field and are arranged parallel to each other on opposite sides of the beam. The electron emitter 41 is connected across a voltage source 50, the shield electrode 42 being biased positively with respect to the emitter by a voltage source 52 to develop a propelling potential for directing the emitted electrons toward the target electrode 44. The plate electrode 47 is held at a high negative potential with respect to the potential of the emitter 41 and the plate electrode 46 is connected across a source 54 of square wave potential.

At any instant at which the potential on the plates 45, 47 are the same, the potential therebetween is negative with respect to the source of electrons, and no electrons can traverse the gap defined by the plates. In the moment in which electrode 46 is driven positive with respect to the electrode 47, responsive to the square wave potential applied thereto, then the space potential intermediate the electrodes 46, 47 can be made positive with respect to the electron source, and electrons can traverse the gap to the target 44. This arrangement accomplishes two purposes. First, it provides a very strong electrostatic field transverse to the electron beam which can be used to buck out or obscure the effect of the axial magnetic collimating field. By way of example of the parameters involved: With an axial magnetic field of 1000 gauss, an electrostatic field of approximately 6000 volts per centimeter will provide a transverse force sufiicient to just cancel the transverse magnetic force on an electron beam of volts energy and thus to permit such an electron beam to travel in a straight line to target 44. Secondly, the resultant modulation of the electron beam correspondingly modulates the ion beam, and by proper frequency control of the square wave modulating potential the ion beam is selfchopped or phased to avoid the space charge problem induced by out-of-phase particles.

In considering the operation of the described instrument, reference is made to a particle of mass m (in grams) carrying a charge q (E. S. U.) placed in a region where the electrical potential (statvolt) along the X axis (ordinate axis of the parabolic field) is symmetrical with respect to the origin or median plane (X=0). For the purposes of mass resolution in accordance with the present invention it is desired to develop a potential distribution in which the period of oscillation of the particle will be amplitude independent. It can be shown mathematically that a parabolic potential distribution represented by the function:

V==AX (l) where V=electrical potential at a point X, A=a numerical constant, and X =distance from potential minimum,

is the most general distribution which will yield strictly simple harmonic motion of the particle. If an A. C. potential of frequency w is superimposed on the parabolic D. C. potential with a completely arbitrary distribution along the X axis, it can be shown that the component of the A. C. potential, which is in symmetry with the median plane, is effective only in accelerating even harmonic particles of the resonant mass and the unsymmetric components are effective for accelerating fundamental and odd harmonic particles. Hence, it is possible to com pletely exclude one class or another by shaping the R. F. potential either symmetrically or anti-symmetrically with respect to the median plane of the X axis. A representative expression can be developed mathematically descriptive of the A. C. field distribution required to eliminate harmonics, one such field being defined by the function V=A2X Sin (Zwt-I-(p) where A2=a numerical constant,

X =space coordinate of the field,

w=the natural resonant frequency of the particle in the D. C. field V=AX go=a constant phase angle.

The particularA. C. field distribution defined by'Equation 2 avoids all harmonic interference and may be developed in the system as illustrated in Figs. 1 and 2.

As shown in Fig. 1 the R. F. oscillator 34, by means of which the A. C. field is developed across the electrode array, is connected to the electrodes in the same pattern as the D. C. supply 32 with a consequent similar symmetrical distribution of the A. C. field along the axis. By symmetrical distribution is meant that the potential at two points equally spaced on opposite sides of the median plane is approximately the same and not necessarily that the potential gradient between the median plane and the extremities of theaxis are uniform. In fact, in a parabolic distribution, as contemplated, uniformity does not exist but the potential field is symmetrical within the above definition.

As previously mentioned, one of the principal problems encountered in R. F. mass separation of the type herein contemplated is the migration of electrons toward the collector electrodes. One electron is developed upon the formation of each ion and under normal circumstances such electrons are accelerated with considerable energy in the direction of the collector electrodes. The present invention contemplates several alternative means and methods for preventing actual impingement of such vagrant electrons on the collector electrode.

One such means is shown in Fig. 1 and is illustrated diagrammatically in Fig. 3. Referring to Fig. 3, the parabolic D. C. field is represented by curve A plotted with respect to the conventional X and Y axes, the Y axis representing increase in potential away from the origin. The collector is illustrated with respect to its potential relationship to the shaped field of curve A and, as illustrated, is maintained at a negative potential with respect to any part of the shaped field. This is accomplished in the apparatus of Fig. 1 by means of the bias battery 36 which imposes a positive bias on all of the electrodes of the field-shaping array. As a consequence, electrons formed anywhere in the field will have insufficient energy to overcome the negative bias of the collector electrodes and will be repelled therefrom. This method of eliminating vagrant electrons is entirely satisfactorily when operating the instrument at comparatively low resolution. The use of a small negative bias on mass spectrometer collector electrodes to insure ion discharge thereon is old in the art (cf. Schissel U. S. 2,570,158). However, the bias contemplated herein is of a difierent order of magnitude, i. e. negative With respect to every part of the system and for a different purpose, i. e. to prevent discharge of electrons whatever their energy may be.

At high resolution under which conditions resonant particles oscillate through a great many cycles within the shaped field, a phase error is introduced as a consequence of particle entry into a so-called region of detuning indicated by shaded portions on the diagram of Fig. 3. Each time a resonant particle enters this region of detuning a slight phase shift is induced and at high resolution the number of entries are such as to accumulate a large enough phase error that such a resonant particle will no longer gain energy from the A. C. field and will never reach the collector at all.

To overcome the efiects of such phase shift the instrument may be operated to maintain the potential relationships as illustrated in Fig. 4 wherein the resonant ion collector is again shown as being at a negative potential with relation to the rest of the system. In Fig. 4 the shaped field is represented by the parabolic curve B such that nonresonant ions are limited in their oscillation to an amplitude a. By pulsing the field-shaping electrodes lying in the region marked 0, resonant ions oscillating in this region can be periodically dumped" to the collector electrode at the same time non-resonant ions can be dumped to a non-resonant ion collector by the pulsation of the fieldshaping electrodes in the region marked a as illustrated.

F a D. C. supply 76 and oscillator 78.

6 problems. In other words, the conditions obtained in Fig. 3 may be developed only periodicallyin the situation depicted in Fig. 4 so that the phase shift problem is not encountered. Many means for accomplishing this pulsation will be obvious to anyone skilled in the art.

Another means of avoiding electron interference is illustrated in the apparatus of Fig. 5. The instrument there shown includes an envelope 70 provided with an exhaust conduit 71 for connection to an evacuating system (not shown) and a sample inlet conduit 72. A fieldforming electrode array 74 including electrodes 74A, 74B, 74C, 74D, 74E, 74F, 74G, 741-1 and 741 is disposed within the envelope and may be identical to the electrode arrangement of the apparatus of Fig. 1 in connection to An electron gun 79 is provided for developing an electron beam for direction against a target 80. The electron shielding arrangement illustrated in Figs. 1 and 2 and described in detail with relation thereto is represented in Fig. 5 by a single plate electrode 82.

In this embodiment an electromagnetic coil $4 is disposed around a central section of the envelope 76 more or less axially co-extensive with the electrode array '74 to develop a coaxial magnetic field in the region of the shaped field to hold ions along the longitudinal axis of the shaped field. Collector electrodes 86, 87 are spaced from opposite ends of the field-forming electrode array, the spacing being such as to remove these electrodes from the influence of the coaxial magnetic field. Separate magnet means 99, 90A and 91, 91A are disposed respectively so as to develop a transverse magnetic field adjacent each I of the collector electrodes 86, 87, through which trans- This constitutes one means of minimizing space charge verse field the ions and also electrons ejected from the shaped electrical field must pass. Each of the collector electrodes 86, 87 is oriented in a plane inclined from the plane of the drawing, as for example at an angle of approximately 20, whereby, as a consequence of the transverse magnetic fields, ions will be deflected in the direction of the electrodes and electrons will be deflected in an opposite direction and thereby prevented from striking the electrodes.

An alternative electrode array is illustrated in Fig. 6 and is characterized by its function of inherently tending to retain ions along the longitudinal axis of the array even in the absence of an axial magnetic field. In the electrode system shown schematically in Fig. 6, two groups of annular planar disk-type electrodes are alternately arranged in the envelope, with the correspondingly positioned electrodes of each group on opposite sides of the median plane being paired as in the previous embodiments. Thus, electrodes 1% are paired and connected across a voltage divider Hi2 which is in turn connected across a source 103 of D. C. voltage and across an R. F. oscillator 104. Similarly, electrodes 106 interleaved between each of the electrodes 160 are likewise interconnected in symmetrical pairs and across a voltage divider 107. The voltage divider 197 is in turn connected across a D. C. voltage source 108 and across the R. F. oscillator 104. In this manner each of the two sets of electrodes is provided with approximately parabolic potential distribution within itself, but the electrodes 106 are biased by the battery 110 with respect to the electrode group Referring to Fig. 7, which is a plot of the potentials developed within the electrode array of the apparatus of Fig. 6, curve A is representative of the potential distribution developed by the grids 186, curve B is repre sentative of the potential distribution developed with the grids 1G0, and curve C represents the resultant po tential to which the ions are sensitive.

An ionizing electron beam 101 is directed transversely across the field at or adjacent the median plane thereof, and in this case no special means need by provided for overcoming the effects of an axial magnetic field since such a field is not required. Collector electrodes 112,

'7 113 are positioned and interconnected in the manner of the previous embodiments with the electrode grid system biased positively with respect thereto.

In effect, the two sets of electrodes provide electric fields at the periphery of the parabolic field of alternate sign with a consequent increase in the field strengths adjaeent the circumferential boundaries of the electrode array so as to retain ions therein. Ion retention is due to the fact that when an ion moves more or less parallel to the axis but near the edges of the electrodes, the net action of the field at that point is to deflect the ion toward the central axis whether or not the ion undergoes a net acceleration or deceleration.

The manner of dividing the R. F. field among grids 100, 106, shown in Fig. 6, is such that the R. F. field is approximately parabolic even adjacent the field boundaries. This method is most effective in avoiding accelera tion of non-resonant particles. Other methods of dividing the R. F. field may be employed without departing from the invention.

In any of the embodiments of the invention the sample spectrum may bescanned by varying the frequency of the A. C. field to bring ions of ditfering mass into resonance therewith or by varying the amplitude of the D. C. field as desired.

Other forms of apparatus or electrical circuitry may be derived for etfcctuating ion collimation and electron control within the scope of the invention which teaches methods and apparatus for improving the effectiveness of D. C. resonance mass spectrometry.

I claim:

1. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic D. C. field across a space in the chamber, means for superimposing an R. F. field on the D. C. field, ion collection means including at least one collector electrode disposed exteriorly of and adjacent a boundary of the D. C. field, means developing an ionizing electron beam directed transversely of the D. C. field to ionize gas molecules therein, and means forming a field to prevent electrons from impinging on the collector means.

2. Apparatus according to claim 1 wherein the means adapted to limit electron travel in the direction of the collecting means comprises means developing a magnetic field across the chamber transversely of the longitudinal axis of the parabolic D. C. field and in the region between the boundary of the D. C. field and the respective adjacent collecting means whereby electrons and ions expelled from the field in the direction of the collecting means are spatially separated so that electrons do not impinge on the collecting means.

3. Apparatus according to claim 1 wherein the means adapted to limit electron travel in the direction of the collecting means comprises means biasing said plurality of electrodes positively with respect to the collecting means so that the collecting means is at a negative potential with respect to every part of the shaped field and electrons regardless of their point of origin in the shaped field will have insutficient energy to overcome the collector bias.

References Cited in the file of this patent UNITED STATES PATENTS 2,541,656 Long Feb. 13, 1951 2,570,158 Schissel Oct. 2, 1951 2,627,034 Washburn et al. Jan. 27, 1953

Images