US2945126A - Mass spectrometer - Google Patents

Description

July l2, 1960 w. M. B RUBAKER ETAL MASS SPECTROMETER Filed June 23, 1958 All xbwww United States Patent O Mass srncTRoMErER Wilson M. Brubaker, Arcadia, and Charles F. Robinson, Pasadena, Calif., assignors to Bell Howell Company, Chicago, Ill., a corporation of Illinois Filed June 23, 1958, Ser. No. 743,902

8 Claims. (Cl. Z50-41.9)

This invention relates to circuits for regulating a mass spectrometer.

A mass spectrometer is an analytical instrument for sorting and measuring ions. Ordinarily, it includes an ion source having an ionization chamber in which molecules of the sample to be analyzed are bombarded With a stream of electrons to convert them into ions. The ions formed are expelled from the ionization chamber and propelled through an analyzer chamber under the inlluence of electric fields set up in the ion chamber. During passage through the analyzer chamber the heterogeneous beam of ions originating in the ionization chamber is subjected to a transverse magnetic teld or a combination of electrical `and magnetic iields to separate it into diverging ion beams, each of which is composed of ions of a given mass-to-charge ratio which differs from the mass-to-charge ratio of the ions forming the other beams. The diverging beams may be successively focused through an exit or resolving slit onto a collector electrode. Alternatively, a single one of the ion beams may be continuously focused on the resolving slit to the exclusion of the other beams. In either event, the selection of the diverging ion beam in many instances is achieved by adjustment of the potential applied to the accelerating electrodes. The current produced by the ion discharge at the collector electrode is indicative of the number of ions in a particular beam, and hence constitutes a measure of the partial pressure of the molecules, from which the ions were derived, in the sample being analyzed.

There is a speciiic type of mass spectrometer which to date finds relatively limited use and in which iirst order velocity and angular aberrations are reduced essentially to zero. This mass spectrometer instrument eifectively focuses with respect to both velocity and angular detlection, hence it has become known as a doublefocusing instrument. A double-focusing mass spectrometer includes an energy filter interposed between an ion source and a magnetic analyzer. By proper choice of geometrical parameters, this type of instrument is uniquely first order free of velocity and angular aberrations. A further description of such a double-focusing instrument can be found in the publication of Messrs. Mat-rauch and Herzog in the Zeitschrift fr Physik 89, p. 786, 1934.

In the conventional double-focusing mass spectrometer an apertured electrode separates the electric sector and the magnetic sector and only those ions focused by the electric section on this aperture gain access to the magnetic sector. tion of the propelling or accelerating potential to which they are subjected in the ion source and to the electrostatic iield in the electric sector. Careful control of these values is necessary to focus an ion beam of a given energy band and any deviation from such controlled value will result in an impairment of the instrument resolution. Heretofore it has been the practice to carefully regulate the voltage applied to the propelling and accelerating electrodes of the source and to take a rela- Ion beam focus at this point is a funcf4 ice tively small portion of this regulated voltage for application to the electric sector.

It has been proposed to stabilize the accelerating potential against deviations from a predetermined value by means of a voltage-dropping network wherein a portion of the accelerating voltage is compared with the voltage output of a standard cell. Any variation in proportionality between the portion and the standard voltage is balanced automatically by a servo motor or other means acting on the potential source supplying the accelerating electrodes. However, the accuracy of such `a system depends upon a resistance divider network, the long-term stability of which is questionable.

We have made a substantial improvement in this type of mass spectrometer which obviates the need for a rigorously regulated voltage source for the ion source and requires only the simpler regulation of the much smaller voltage applied to the electric sector. Since, in this type of instrument, beam focus is lirst order dependent on the voltage at the sector and only second order dependent on the accelerating voltage, and, since the sector voltage is only a fraction of the accelerating voltage, it is not only simpler to regulate but its careful regulation is of utmost importance. To accomplish this objective the mass spectrometer of the invention comprises an ion source, an electric sector, a magnetic sector and ion collector means. Repeller and accelerating electrodes are provided in the ion source and an apertured sensing electrode is interposed between the electric and magnetic sectors, the aperture therein being referred to as the energy slit. Circuit means is connected to the repeller and accelerating electrodes, the electric sector and the sensing electrode to develop ion repelling and accelerating fields in the ion source and an energy selecting lield in the electric sector so as to focus ions on the aperture of the sensing electrode and to sense ion discharge at this electrode. Means are further provided to accomplish a compensating adjustment of the ion source accelerating potential responsive to any ion discharge signal produced at the sensing electrode as a consequence of de-centering of the selected ion beam at the energy slit.

These and other aspects of our invention will be understood completely in the light of the following detailed description, as illustrated by the accompanying drawing in which:

Figure 1 is a schematic diagram of one embodiment of applicants invention;

Figure 2 illustrates the modulation of the ion beam 37 in Figure 1; and

Figure 3 is a curve illustrating an ion beam having a broad energy band which can be focused by the appara- -tus of Figure 1.

Referring now to the drawings and particularly to Fig. l, a double-focusing mass spectrometer has been shown schematically to include an ion source 10. The ion source includes a repeller electrode 12 and accelerating electrodes 14, 16. A gas sample to be analyzed is introduced into the region between the repeller electrode 12 and the first accelerating electrode 14, and s bombarded by electrons in an electron beam 17 to ionize the molecules in the gas sample. Ions thus formed are expelled through a slit `14A in the rst accelerating electrode by application of a potential to the repeller electrode which is positive relative to the electron beam potential. The expelled ions are propelled by the accelerating electrodes 14 and 16 into a beam 18 passing through slit apertures 14A and 16A.

The ion source electrodes are connected to a potential divider network 19 which is connected across the output of a D.C. amplier 20 and in series with a secondary winding 23 of a transformer 45. An alternating current source 22 is connected across a primary winding 23A of the transformer 45. The source 22 superimposes an A.C. ripple component on the D.C. output of the amplitier 20. The alternating current source 22 would be unnecessary in many instances since voltage sources such as the output amplifier often include residual unifiltered alternating current ripple. The divider network 19 and the connections to the ion source electrodes represent a conventional voltage supply circuit for a mass spectrometer wherein any change in the voltage applied across the resistor of the potential divider 19 results in a proportional change in the potentials applied to the accelerating electrodes.

An energy filter or electric sector 24 including electrodes 25, 26 is arranged to receive ions propelled from the ion source. The beam 18 issuing from the ion source and entering the electric sector enters an electrostatic field between the curved plates and the ions are deflected according to their energy. The electrodes 25, 26 are connected to a source 28 by the leads 30 and 32 connecting the plate 25 to the positive side of the supply source and the plate 26 to the negative side of the supply source. Preferably, source 28 is a separate source of adjustable but well-regulated voltage to maintain the electrostatic field in the electric sector constant and to provide a reference potential in a closed loop circuit regulating the acceleration potential.

A sensing electrode 34 is disposed at the focal line of the electric sector. An aperture or energy slit 36 in electrode 34 passes ions within a predetermined energy band in a selected beam 37 focused thereon by the electric sector. The ripple voltage superimposed on the source of acceleration potential oscillates or sweeps the beam 37. The magnitude of the superimposed A.C. ripple voltage is regulated so as to develop a beam sweep which is preferably less than the width of slit 36. Electric sector 24 separates the ions into beams according to their energy, the higher energy ions beam deflected less than the low energy ions. A beam of ions within a predetermined range of energy in which the ions alternate in energy magnitude with time, according to the superimposed A.C. component, will sweep across aperture 36 in the focus control electrode. The manner in which the A.C. ripple voltage sweeps the ion across the aperture 36 is illustrated in Fig. 2 in which the ions of higher energy assume a larger radius in the electric sector 24 than the ions of lower energy. For example, the ions being accelerated during the time that the ripple voltage is at a positive value will be deflected through a path illustrated by the top dashed line in Fig. 2, whereas, the ions being accelerated while the A.C. ripple voltage is at a negative value will be deflected through a smaller radius by the electric sector 24 and assume a path as indicated by the lower dashed line of Fig. 2. Assuming the amplitude of the A.C. ripple voltage is controlled, the sweep of the beam can be controlled to less than the width of the energy slit.

Under circumstances in which the beam is centered, the sweep will have no effect on the beam passing through the'aperture 36. However, if the acceleration potential varies sufficiently, excluding the alternating component, the selected ion beam will deviate from the centered position and strike one side or the other of the sensing slit during the sweep depending on the direction of acceleration potential variation, and a feedback signal will be produced on the sensing electrode which is coupled to an A.C. amplifier 38.

It should be noted that the potential applied to the electric sector can be adjusted to vary the position of the beam relative to the aperture 36. In the preferred arrangement, however, the voltage source 28 is well regulated and this source is used as a reference in the closed loop circuit or servo amplifier regulator system. In some instances, the potential applied to the electric sector is adjusted for the selection of ions in different energy bands to be focused on the aperture 36 of electrode 34.

The ion beam of the selected energy range passing through the sensing electrode enters a magnetic analyzer 40 in which a transverse magnetic field is formed n the conventional manner by a pair of magnetic pole pieces, one of which, pole piece 41, is shown in the drawing. In accordance with conventional practice, a second identical pole piece (not shown) is disposed parallel to and spaced from the pole piece 41 to form a magnetic field transversely to the direction of the ion beam passing therebetween. The magnetic sector effectuates mass separation and ions of predetermined mass forming a beam are focused on a collector electrode 42 through a mass resolving slit 43A of a mass resolving electrode 43. The collector electrode 42 is coupled to an amplifier and recorder 44 for amplifying and recording or otherwise sensing the discharge of ions on the collector electrode.

To hold the beam 37 or any other predetermined beam in focus on the slit 36 in the sensing electrode, a feedback circuit is provided which applies a correction signal to the accelerating potential as required. As previously noted, the output of a D.C. amplifier 20 is connected across a voltage divider 19 to which repeller electrode 12 and accelerating electrode 14 are connected such that the repelling and accelerating fields in the ion source are a direct function of the output of amplifier 20. The A.C. ripple signal is superimposed in a conventional manner from source 22 through a coupling transformer 45.

The feedback circuit for controlling and accelerating potential includes the sensing electrode 34 which develops a signal responsive to deviation of the ion beam from a focused position. This error signal is fed through amplifier 38 to a synchronous detector 46. The synchronous detector may be a conventional phase-sensitive demodulator wherein the polarity of the output signal is sensitive to the phase of the input and reference signals. (See, for example, U.S. Patent 2,827,611, issued to J. W. Beck March 18, 1958.) The detector is also coupled to the source 28 and by means of a conventional adding circuit produces a feedback signal which is a combined reference and correction voltage. This signal is rendered phase sensitive, i.e. responsive to the direction of beam defocus by comparison with a signal derived from A.C. source 22 and applied to the detector through line 47. The phase sensitive output of detector 46 is fed into a differential amplifier 48 through lead 49 where it is compared with a predetermined portion of the accelerating potential tapped off of divider 19 at tap 50. The output of the differential amplifier serves as the input to D.C. amplifier 20 which is the source of the accelerating potential. The amplifiers 20 and 48 are designed to be insensitive to the frequency of the voltage ripple produced across the potential divider network 19 by the A.C. source 22 so that the voltage regulating system does not compensate for the voltage ripple which modulates the energy imparted to the ion beam.

The D.C. source 28 may be conveniently referenced to ground by means of a resistor network 29, one point of which is connected to ground as shown in Fig. 1. The grounded tap on the divider network 19 and' the grounded tap on the divider network 29 provide a cornmon reference with which the voltages may be compared in the differential amplifier 48.

In the preferred operation, the voltage of the D.C. source 28 is used as a reference and is an independent variable. With no beam at the energy slit 36 of the sensing electrode, and hence no error or off-center feedback signal, the input signal to the differential amplifier is the difference between the potential of the source 28 and a predetermined portion of the acceleration potential whereby the output of the D.C. amplifier 20 and the acceleration potential is at the proper value to cause the beam to fall on the slit 36.

If the selected ion beam 37 is not centered on the energy slit 36, an error feedback signal resulting from the discharge of a portion of the uncentered beam at the sensing electrode is coupled to the A.C. amplifier 38, amplified and applied to the synchronous detector. The detector produces a D.C. output, the polarity of which is dependent upon the direction of unsymmetrical displacement of the sweep of the ion beam from its centered position in energy slit 36. For example, the ion beam oscillating in the slit 36 and becoming uncentered outwardly and upwardly, as illustrated by the drawing, indicates too large an acceleration potential. In the illustrated arrangement, the feedback signal resulting from such de-centering of the beam sweep decreases the acceleration potential. Since the phase of the feedback signal from the sensing electrode determines the positive or negative output of the synchronous detector, the feedback signal resulting from the ion beam being deflected outwardly and' upwardly in the focusing aperture is of a phase producing a negative output from the synchronous detector which, when added to the reference voltage derived from the voltage supply 28, decreases the sum of the combined reference and feedback signal voltage coupled to the differential amplifier 48. The decrease in the combined feedback signal voltage coupled to the differential ampliiier decreases the diiference between the input signals, i.e. portion of acceleration potential from tap 50 and combined feedback signal and the resulting regulating signal output voltage of the differential amplifier which is coupled to the D.C. amplifier 20. This, in turn, decreases the output of the amplifier 20 and the potential across the divider 19. Since the potential across the divider 19 determines the potential of the accelerating electrodes, a decrease in potential across the divider decreases the accelerating potential which, in turn, decreases the acceleration of ions and the velocity tending to return the ion beam to the center of the sensing aperture 36.

An insufficient acceleration potential will cause the selected beam of ions to be uncentered inwardly in the sensing aperture. Converse to the above situation, this produces a positive error signal which adds to the reference voltage and enlarges the accelerating potential in a compensating manner.

The feedback signal derived from the sensing electrode 36 and coupled to the synchronous detector produces a positive or negative signal dependent upon the position of the beam. The signal voltage developed by an uncentered sweep of the beam in the sensing electrode and the portion of the accelerating potential coupled from the potential divider 19 at tap 50 are feedback signals, either of which can be used independently of the other to control the position of the sweep of the ion beam. In the present application, the feedback signals are added to assure immediate response to variations in accelerating potential due to changes in the resistor element of the potential divider 19 or other factors changing the accelerating potential.

'Ihe sensing electrode detects directly the sweep position of the ion beam. The acceleration potential can be controlled directly and independently by the positive or negative output of the synchronous detector and Without combining it with the deflection or reference potential. Also, the acceleration potential can be controlled independently by the combined feedback signal including the deflection voltage and signal output of the synchronous detector. In addition, the acceleration potential can be independently controlled by comparing the deflection potential and a portion of the acceleration potential taken at the divider tap 50 and the differential voltage applied to the input of the D.C. amplifier 20 where it is amplified and applied across the potential divider 19.

The foregoing has assumed the preselection of a single range of energies in the ion beam. The range of energies may be selected by variation or adjustment of the ratio of the accelerating potential and voltage of the electric sector as by a variation of the voltage at source 28 which is preferably a separate well-regulated source of D.C. voltage. The closed loop regulating system will regulate the accelerating potential to the new reference voltage. The above discussion of the operation of the closed-loop voltage regulating system assumes a narrow range of energies in the ion beam such that the entire beam will pass through the slit 36 in the sensing electrode 34. It is obvious, however, that the system will operate to center the maximum portion of any ion beam (having a wide energy band) on the slit 36. For example, assume that the energy band takes the form shown in Fig. 3 wherein the abscissa represents the energy of the individual ions and the ordinate represents the num ber of ions. The self-regulating system of Fig. 1 will operate to center the ion beam of Fig. 3 so that the maximum number of ions in the beams will be centered approximately within the center of the slit 36. In this condition an equal number of ions will be falling on the sensing electrode 34 during each half cycle of the reference source 22, and thus the feedback signal resulting from the sensing electrode 34 will be zero.

It should be noted that the instrument described above is initially designed to focus a beam of ions with any predetermined energy band on the slit 36. Thus, when the instrument is placed in initial operation, at least a portion of the desired ion beam will pass through the slit 36 to permit the closed-loop regulating system of Fig. l to center the beam in this slit. It is obvious, however, that where an ion beam having a Wide energy band such as shown in Fig. 3 is employed, there will be an error signal detected by the sensing electrode 34 to permit the system to center the beam with respect to the slit 36 even though the beam is initially out of focus to a large extent.

Although the invention has been described with particular reference to a regulating system employing a synchronous detector, differential amplifier and D.C. amplifier, it is realized that the mechanical equivalents of the foregoing electrical or electronic equipment may be employed. For example, the output of the A.C. amplifier 38 may be applied without rectification to one phase of a two-phase servo motor which controls the accelerating voltage through an appropriate potentiometer.

The double-focusing type of mass spectrometer renders the analysis independent of initial velocities of the ions. The mass resolution is independent of the A.C. ripple voltage superimposed on the ion accelerating voltage for sweeping the ion beam. However, the voltage supplied the electric sector 24 should be exceedingly well regulated and filtered since ripples or poor regulation will be reflected in the final mass resolution on the collector 42. Since the voltage applied to the electric sector in many instances is approximately 5% of the accelerating potential, it is relatively easy to stabilize and regulate it directly rather than stabilize or regulate the higher voltage of the accelerating potential.

Furthermore, application of the invention is not limited to the particular apparatus shown. It is entirely practical to produce the necessary sweep of the ion beam by means other than impression of an A.C. voltage on the accelerating electrodes. For example, auxiliary electrodes arranged transversely with respect to the path of the ion travel will induce the same type of sweep when energized from a source including an A.C. voltage component. Also, transversely arranged electrodes may serve to maintain or restore centering of the beam by impressing a D.C. potential thereon and varying this potential by a superimposed A.C. ripple in a manner illustrated in conjunction with the accelerating electrodes.

The preferred embodiment of the invention, therefore, involves sweeping the ion beam in any desired manner and adjusting the beam to be kept centered on the focus control electrode aperture or decentered slightly, the latter mode of operation being merely a variant of the first.

We claim:

1. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy band according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, means for developing a voltage control signal by adding a signal from said source and a signal derived from the sensing electrode representative of any de-centering of said given ion beam thereon, accelerating voltage supply means for applying voltages to the accelerating electrodes, and means regulating the accelerating voltage supply means as a function of said voltage control signal.

2. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy band according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, means for developing a voltage control signal by adding a signal from said source and a signal derived from the sensing electrode representative of any de-centering of said given ion beam thereon, accelerating voltage supply means for applying voltages to the accelerating electrodes, means regulating the accelerating voltage supply means as a function 'of said voltage control signal, and means for superimposing an A.C. ripple on the accelerating voltage.

3. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy band according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, means for developing a voltage control signal by adding a signal from said source and a signal derived from the sensing electrode representative of any de-centering of said given ion beam thereon, accelerating voltage supply means for applying voltages to the accelerating electrodes, a differential amplifier for comparing and amplifying the difference between the voltage control signal and a predetermined porti-on of the accelerating Voltage, and means coupling the differential amplifier to the accelerating voltage supply means for regulating such means.

4. Apparatus according to claim 3 wherein said accelerating voltage supply means is a D.C. amplifier adapted to receive as its input signal the output of the differential amplifier.

5. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy baud according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, an amplifier connected to amplify ion discharge signals produced at the sensing electrode responsive to uncentering of an ion beam focused on the aperture thereof, means for developing a voltage control signal by adding a signal from said source and said amplified discharge signal, accelerating voltage supply means for applying voltages to the accelerating electrodes, and means regulating the accelerating voltage supply means as a function of said voltage control signal.

6. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy band according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, an amplifier connected to amplify ion discharge signals developed at the sensing electrode responsive to uncentering of an ion beam thereon, means for developing a voltage control signal by adding a signal from said source and said amplified discharge signal, a differential amplifier adapted to amplify the difference between said voltage control signal and a predetermined portion of the accelerating voltage, and accelerating voltage supply means including a D.C. amplifier for applying voltages to the accelerating electrodes, the output of the differential amplifier being connected as the input of said D.C. amplifier.

7. In a mass spectrometer the combination comprising in serial arrangement an ion source including ion accelerating electrodes, an electric sector adapted to pass only ions within a predetermined energy band, a mass analyzer adapted to separate ions in said given energy band according to mass, an apertured sensing electrode interposed between the electric sector and the mass analyzer, a regulated voltage source for supplying potentials to the electric sector, an amplifier connected to amplify ion discharge signals developed `at the sensing electrode responsive to uncenteringl of an ion beam thereon, means for developing a voltage control signal by adding a signal from said source and said amplified discharge signal, a differential amplifier adapted to amplify the difference between said voltage control signal and a predetermined portion of the accelerating voltage, accelerating voltage supply means including a D C. amplifier for applying voltages to the accelerating electrodes, the output of the differential amplifier being connected as the input of said D.C. amplifier, and means superimposing an A.C. ripple on the accelerating voltage.

8. In a mass spectrometer the combination which comprises: an ion source including ion accelerating electrodes; an electric sector disposed adjacent the ion source and adapted to pass only ions within a predetermined energy band; means adapted to separate ions in said given energy band according to mass; an apertured sensing electrode interposed between the electric sector and the last named means; a regulated voltage source for supplying potentials to the electric sector; accelerating voltage supply means for applying voltages to the accelerating electrodes; means for superimposing an A.C. ripple voltage on the accelerating voltage to modulate the ion beam in accordance with the A.C. ripple voltage; synchronous detection means coupled to the A.C. ripple voltage means, the regulated voltage source and the sensing electrode for producing a voltage control signal proportional to the sum of a signal from the regulated voltage source and a signal representative of the difference in the ion current collected by the sensing electrode during each half cycle of the A.C. ripple voltage; and means for applying the voltage control signal to the accelerating voltage supply means to regulate the accelerating voltage to center the ion beam with respect to the aperture in the sensing electrode.

Perkins et al. Oct. 7, 1952 Hipple Sept. 25, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Patent No2,945,126

I Wilson M Brubaker et al.

Y 9 n Y Y Y Y Y l0 July 12l 1960 It is hereby certified that error appears in the printed specification of the' above numbered patent requiring correction and that the said Letters .Patent should read as corrected below.

Column 3, lines 7 and 8, for "unifiltered" read El for "ion" read ions Signed and sealed this 20th day of December (SEAL) Attest:

KARL AXLINE Attesting Officer ROBERT C. WATSN Commissioner of Patents UNITED STATES PATENT oEEICE CERTIFICATE OF CORRECTON Patent No. 2, 945,l26 July l2, 1960 Wilson Mo Brubaker et al.

It is hereby certified that error appears in 'the printed specification of 'the' above numbered patent requiring correction and that the said Letters `Patent should read as corrected below.

Column '3, lines 7 and 8, for "unfiltered" reed unfiltered line 45., for "ion" read ions --T' signed and sealed this 20th day of December 1960.

(SEAL) Attest:

KARL H.. AXLINE ROBERT C. WATSGN Attesting Oicer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Patent No. 2,945,126 July 12 l960 Wilson IVI.. Brubaker et ele It is hereby certified that error appears in the printed specification o' thev above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, lines 7 and 8, for "uniiltered" read t unfiltered line 45, for "ion" read ions Signed and sealed this 20th day of December 196()e (SEAL) Attest:

KARL H.. AXLINE ROBERT C. WAT-SGN Attesting Officer Commissioner of Patents

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