US2978580A - Process and device for the addition of slow electrons to polyatomic or highmolecular compounds - Google Patents

Description

Aprll 4, 1961 M. VON ARDENNE 2, ,53

PROCESS AND DEVICE FOR THE ADDITION OF SLOW ELECTRONS T0 POLYATOMIC OR HIGH-MOLECULAR COMPOUNDS Filed Sept. 8, 1958 3 Sheets-Sheet 1 INVE TOR Mmvresp oNAADFNA/E April 4, 1961 Filed Sept. 8, 1958 M. VON ARDENNE 2,978,580 PROCESS AND DEVICE FOR THE ADDITION OF SLOW ELECTRONS TO POLYATOMIC OR HIGHMOLECULAR COMPOUNDS 5 Sheets-Sheet 2 5 Q A M INVENTOR.

M. VON ARDENNE 2,978,580

ECTRONS T0 POLYATOMIC OR HIGH-MOLECULAR COMPOUNDS 3 Sheets-Sheet 3 E mm EN u I N u lb u \w U m in S 8 PR n E NW ow A u u E Q maglvfl April 4, 1961 PROCESS AND DEvIcE FOR THE ADDITION OF SLOW EL Filed Sept. s, 1958 PROCESS AND DEVICE FOR THE ADDITION OF SLOW ELECTRONS T O POLYATOMIC OR HIGH- MOLECULAR COMPOUNDS Manfred von Ardenne, Dresden, Weisser Hirsch, Germany, assignor to VEB Vakutronik;l)resden, Dresden, Germany Filed Sept. 8, 1958, Ser. No. 759,811 I Claims priority, application Germany Apr. 25, 1958 14 Claims. (Cl. 250-413) The invention pertains to a process for the addition of slow electrons to polyatomic or high-molecular compounds, employing a hot-cathode low-voltage discharge and a device for carrying out the process preferably in precision mass spectrography or mass spectrometry of molecules.

Development of a molecule mass spectrograph, especially for high masses and high resolution, is of inestimable importance for the large field of organic chemistry and biochemistry. Research of this sort is particularly valuable in the fields of petroleum and fuel chemistry, vacuum pump oils, the chemistry of synthetics, pharmacy, special problems ofanalytical-chemistry, and for research and work on the chemical structure of macromolecules. Previous attempts to develop a molecule mass spectrograph for high masses failed particularly because the molecules to be measured were broken down into. nu-

merous fragments by the very act of ionization in the ion source. Up to now there has been no device and no process able to depict all molecule masses between'20 and 1000 or 10,000 in a single mass spectrum without splitting the molecules. Ionization by electron impact split the molecules to be analyzed. tron addition has made it possible to avoid splitting the molecules under very special conditions, but the intensity conditions made this approach impossible. These facts were pointed out a long time ago.

Theprincipal element of the molecule mass spectrograph is, as We know, a high-yield source of ions to'produce negative molecular ions by attaching thermal or nearly thermal electrons. The. electron'addition crosssection is of the order of electron energiesof 0.1 to a few electron volts for the molecules of'interest to us, say,

' between 10" and 10* cmfi. On the other hand, the

cross-section for the productionof positive gas andvapor ions by electron impact isof the order of 10* cm} for the optimum choice of electron energy. Since the crosssections available when working with negative ions pro ducedby electron addition are smaller than those available in mass spectrometry with positive ions by 2 to 6 orders of magnitude, it was obvious at the outset that Ionization by elecited States Patent '0 ice in the ionization chamber, the higher the emission cur rentdensity for a given addition cross-section and for a partial pressure of the vapor to be analyzed that is given for reasons of vacuum technique.

How to concentrate slow electrons magnetically is. known already. This known measure did not sufli'ce, however, to achieve a very high density of slow electrons, because this was prevented by the electron space charge. The problem is solved in the invention by using a hotcathode low-voltage discharge to produce a quasi-neutralplasma with a low electron temperature and low potential gradient in an auxiliary gas, especially one possessing a low ionization voltage, after which the plasma is magnetically compressed by an inhomogeneous magnetic field directly in front of an ion-emission aperture, after which the molecular gas or molecular vapor to be analyzed is fed into the plasma that has been highly compressed magnetically directly in front of the ion-emission aperture, the negative ions and electrons being drawn oft" through this aperture and then passing through a known system of diaphragms and deflectors. Only when. the negative space charge is compensated by positive ions, that is, in a quasi-neutral plasma, it is possible to achieve a very high density. of slow electrons. The endeavor was made in this invention to secure these posi tive ions from a vapor exhibiting a particularly low ionization potential in order to prevent any. dissociation of the molecules to be measured or to keep this dissociation as small as possible in producing the quasi-neutral plasma and in producing positive ions for space charge compensation, or asthe result of collisions with the ions. Iti's' advisable to use cesium vapor, mercury vapor, or argonas the auxiliary gas. It has been found that operation is experimentally inconvenient when the cesium atom, which possesses the lowest ionization voltage, is used; That is why mercury vapor and argon have been adopted, it having been found that critical dissociation of themolecules to be measured in the low-voltage discharge mentioned above, failed to occur even when the in} homogeneous magnetic field is employed to compress the plasma magnetically directly in front of the ion-emission aperture. Another feature of the invention is the fact that discharge current densities of some 100 amp/cm. are achieved by reducing the wall and recomT-i bination'losses in the, magnetically pinched plasma rel gion. The molecules to be, analyze d'are vaporize'd,' beamed, orintroduced in-some other manner, for ex,- ample, mechanically, into the compressed 'el'ectron current of slow electrons directly in front of the ion-emission slit. It 'is advisable to introduce molecules of known c mass into the addition chamber as a calibrating substance, a

success could be achieved only if we were able to deextraordinarily electron additionin flieE-plane .ofthe-iomemission aper- 'ture of'the ion source, the easier it is to achieve the re-' quired resolving poWer':A:-=l-000'10 10000 by the{.use of ion ,objects The higher the density ,ofxslloWlectronstogether with themolecules to be analyzed. The' evapo' ration or'beaming of molecules should be done, however, only. during mass-spectrographic recording and the spectrum adjusting time. The molecules can be fed into the compressed plasma or the addition chamber preferably froma multichannel gasemitter, by means of a sharply focused beam of gas orvapor. If the substances. to be tion.. z ln the invention, it is advisable to have proteins} 7 finely dispersed in a solvent, which is then sprayedi'inz' 'droplets and condensed onic'ooling surfaces in avacuuin,

. analyzed are not vaporizable, the molecules 'mustl'be in fl troduced into the addition chamber mechanically, using a ultrasonic oscillators or other surfaces'ca'pable of vibraf-V the remaining moleculesbeing preferably forced into the". 7

j additionchamber mechanically. Another feature of the invention the fact that {thedischarge chamber of: the, L

' ion source and the .high-vacuumlradiation chamber,-say, v

of a mass spectrographorqa spectrometer, commuin es only throughthe. emission slit :of the ion source;

, The-process o ifthe invention is carriedflout'with. the a of a deviceofthe 'inventiom What characterizes device is the fact that a magnetic pole-piece lens, preferably with the pole pieces of asymmetrical design, is installed to compress a quasi-neutral plasma generated in the discharge chamber, followed by a device for introducing the molecules, a discharge anode, and an accelerating electrode. The plasma-compressing pole pieces are designed in such a away as to keep the wall losses as small as possible by using a suitably large hole. The aforesaid magnetic field will also reduce the losses considerably. Another feature of the invention provides for electrically insulating the pole pieces on the cathode side, which pole pieces are employed to establish a potential having a value between the cathode potential and the anode potential of the discharge. The ion-emission slit, masked by a diaphragm in front of it, is likewise provided in the discharge anode, which is located behind the admitting device for the substance to be analyzed. It is advisable to design the emission slit member so that it can be slid through a distance that is larger than the diameter of the diaphragm. The molecular gas or molecular vapor to be analyzed is produced in a heated cup located directly in front of the ion-emission aperture. Several cups may also be provided in front of the ion-emission aperture to hold the calibrating substance and the substance under test. It is preferable to design the device in such a way that both the ion-emission slit member and the device for vaporizing the molecules can be rapidly replaced by means of an air lock. Another feature of the invention is the fact that it provides for a cathode system possessing several cathodes that can be separately heated or a single cathode and another air lock. When the process is employed for a precision mass spectrograph or for a mass spectrometer, the ion-emission slit is sharply imaged on the photographic plate that records the mass spectrum, preferably by the lens action of an electrostatic unit lens. The mass separation magnetic field is located directly behind the electrostatic unit lens that constitutes the ionemission slit.

In the basic geometry and mode of operation, the wall losses in the critical plasma region are greatly reduced by the aforesaid magnetic field, there being a low potential gradient in the region of the positive column from which the negative charge carriers are drawn off. Thus we are dealing with a form of discharge in which there is a high density of electrons in the vicinity of the anode, that is, in the vicinity of the emission aperture, with velocities corresponding to a Maxwell distribution of electron temperatures of some 104 degrees. In this distribution, there is an abundance of electrons with ener gies of 1 electron volt and less. The process of the invention and the device for performing this process have made it possible to draw off comparatively large currents of undissociated molecules from the emission aperture despite the smallness of the attachment cross-sections. Moreover, negative cleavage ions produced by the dissociative ionization of molecules by electron impact and a current component of 13.5 milliamperes of plasma electrons were drawn off from the emission slit of, e.g., -1000 cross-sectional area.

The device of the invention will be described in detail, using an embodiment shown schematically in the following'illustrations.

Fig. 1 shows the system or thedevice for the electronaddition ion source.

.Fig. 2 is an overall view of a precision mass spectrow p r Fig.3 is a diagram showing the path of the rays through a precision mass spectrograph.

A hot cathode tube is installed in the discharge chamber- 1 in the usual manner. The pressure inside the discharge chamber is about 10- mm. Hg. The discharge chamber 1 is surrounded by an energizing winding 3 and terminates in an asymmetrical iron pole-piece lens 4.

The discharge. chamber is also provided with an evacua-Y tion pump 5 and a valve 6. The auxiliary gas required to produce the quasi-neutral plasma is fed into the discharge chamber 1 through a line not shown or by means of the partial pressure of the mercury diffusion pump 5. The device 7 for vaporizing and introducing the calibrating substance or the substance under test is located directly behind the pole-piece lens 4, which also acts as a dynode. This device 7 can be rapidly replaced by means of an air lock 8. One or more electrically heated cups- 9, holding the substances to be vaporized, are in the imme-- diate vicinity of the pole-piece lens 4. Each of the cups- 9 can be moved separately. In the embodiment of the invention, an ion-emission slit member 10, which can slide in the longitudinal direction, fitted with a sliding device 11, an air lock 12, and a forevacuum valve 13, is-

installed so as to be displaced 180 with respect to the device 7. The emission slit 10 may be an elongated very narrow slit having a width of, say, 2-20 microns, depending on the requirements and the nature of the substances under test and depending upon the resolving power required. A diaphragm 14 is located in front of the emission slit 10 (Fig. 1). The emission slit member is so designed as to be able to slide in the direction of the slit through a distance that is larger than the diameter of the front diaphragm 14 and the function of diaphragm 14 is to limit the effective height of slit 10. Directly behind the emission slit 10, there is another pair of iron pole pieces 15, which acts as an emission anode since pole piece 15 is at a positive potential relative to pole piece 4 and cathode 2. The addition chamber 16 is located between the emission anode 15 and the pole-piece lens 4. Behind the emission slit 10 and the emission anode 15, there is an accelerating electrode 17, followed by a pair of deflection plates 18. Insulation'ZO is provided between the electron addition ion source and an ordinary molecule spectrograph 19. The molecule spectrograph contains known devices, such as the auxiliary aperture diaphragm 21 and the main aperture diaphragm 22, cylindrical unit lenses 23, and a separator magnet 24. The platinum aperture diaphragm 22, which can be heated, is designed so that it can be slid into the system. Two diffusion pumps are also provided, connected to the molecule spectrograph' 19 through the pipes 25. An anticathode plate 26 with lead shielding is located behind the cylindrical unit lenses 23 just in front of the separator magnet 24. The necessary Schumann photographic plate is denoted by 27, while 28 denotes the secondary emission cathode made of the finest wire netting. The observation window for the ionic image converter fluorescent screen, with a millimeter scale, is denoted by 29, while 30 denotes the camera with the ionic image converter. The glass plate 31 has a fluorescent screen provided with an aluminum coating having a positive potential for conducting away the electrons impinging thereon. The plateholder drive 32, with an exposure counter, is attached at the level of the Schumann plate directly behind the separator magnet 24. The magnet chamber 33 is designed as a one-sided wedge as usual.

The hot-cathode low-voltage discharge is used to produce a plasma in the evacuated discharge chamber 1, this plasma being made quasi-neutral by means of an auxiliary gas that is fed into the discharge chamber either through a pipe line or through the mercury diffusion pump 5, it being remembered that the auxiliary gas chosen must be such as to ensure a low potential gradient'and a low electron temperature. This is-particularly the case with cesium vapor. The plasma is very highly compressed by the pole piece lens 4, especially by its asymmetrical pole pieces, and it is found that no critical dissociation of the molecules to be measured occurs even when cesium vapor is not employed, e.g., with the employment of mercury vapor or argon, as a result of the asymmetrical design of the pole chamber:16,.the generated slow "electrons of the highlycompressed quasi-neutral plasma are added ot the mol-' ecules under test, which have been introduced by a vaporizing device 7. The vaporizing device has a substance cup 9 that can be heated, its heater being, preferably slotted so that thehighest temperature exists where the crucible is suported, small currents sufiicing to produce a large heating effect. To get along with as little substance as possible and to diminish the rate at which the emission slit} is contaminated, the regular procedure in recording mass spectra is to record a spectrum without cup heating to begin with. This spectrum yields the mass lines of the background of the instrument, that is, practically nothing but lines in the region of masses below 100. The other spectra are then photographed by having the cup heating turned on during the exposure time itself. The exposure times ore of the order of 1 second. If we assume a vaporization time of 10 seconds, a surface of 0.03 cm. for the substance to be evaporated, a temperature of 400500 and a saturation pressure of 10" mm. Hg, we find that the amount of substance required for analysis is no more than 2.10- gram, for example, for an oil with a molecular weight of M=500. Instead of the single cup, two or more may be used, one containing the calibrating substance. The procedure is such that first the calibrating substance is brought in contact with the compressed plasma until evaporation occurs, as indicated by a rise of pressure within the discharge chamber 1. The position of the calibrated substance cupis graduated, after which the cup is returned to its initial position. The same procedure is,

repeated for the substance under test, both substances being brought to the vaporizing position. I

The molecules may be introduced into the addition chamber 16 in another way as well. The molecular ionelectron beam is sent through the emission slit 10'and the emission electrode through the accelerating electrode 17 into the high-vacuum chamber or the magnet chamber 33 of the mass spectrograph 19. The pressure within the molecule mass spectrograph 19 is reduced to about 10' mm. Hg, this being done by two diffusion pumps through the pipe connections 25. The prescribed path is visible in Fig. l. The subsequent path of the molecular ion-electron beam is seen in Fig. 3. The pair of deflection plates 18 adjusts the beam of rays. The negative ions and electrons drawn from the ion-emission slit 1%) pass through an aperture diaphragm system 21, 22 and then pass through a cylindrical electrostatic unit lens 23, which forms an image of the ion-emission slit on the photographic plate or on the cathode of the ionic image converter. Directly behind the cylindrical electrostatic unit lens 23, there is the magnetic field required for mass separation, generated by the separator magnet 24. The geometry and the characteristics of magnet 24 should be so dimensioned that with, the provision of an ion accelerating potential of 50 kv. molecules with molecular weight up tomore than 10- can, if necessary, be deflected enough to ensure adequate'resolution. With a separator magnet that produces a magnetic field of H =l5,000 oer- ,steds and possesses a magnetic gap of S mm., the deflection in the plane of the photographic emulsion is about 18 mm. fora molecule of mass 100,000, while the width of the line can, in principle, be reduced to values of the order of 10 microns and belowby the employment of polarization-free heated platinum aperture diaphragms and a. suificiently fine ion-emission slit. The accompanying electrons that are also drawn off are sorted out directly after the molecular ion-electron beam enters the separator magnet field, so that'the accompanying electrons drawn off from the plasma merely appear as an additional load on the high-voltage source. To prevent the X-rays generated by these electrons from producing any additional blackening of the photographic emulsion, an anticathode plate 26 with an appropriate lead shield is provided in the immediate vicinity of the point where the beam enters the magnetic field. What is decisive for making the molecular lines based upon electron addition, which are also the residual gas pressure prevailing in the radiation I Interaction of the particles in the beam with chamber. the residual gas produces a troublesome background blackening of the mass spectrum. The background extends principally from the point of impact of the undeflected beamto the mass lineof maximum intensity in the spec- It is produced by neutral particles of high velocity, which stem from the massline of maximum intensity and are. rendered electrically neutral along the path of the beam as the result of the change of charge. The desired molecular spectra become clearly visible against the background once the two diffusion pumps provided are used to lower the working pressure within the radiating charn her to l-3 10" mm. Hg.

Operating with diminished ion beam aperture and hence longer exposure times are required for maximum resolving power. It is advisable to employ an electron-tube stabilizer to keep the ion accelerating potentialconstant for long periods of time. The ion-accelerating potential can be expected to remain sufficiently constant only if the stream of electrons and negative. ions drawn through the emisison opening remainsconstant. But this is the case only if the discharge in the electron-addition ion source glows uniformly, i.e. when no low-space or medium-frequency modulation of the drawn-off stream of negative carriers ispro-duced by relaxation oscillations, acoustic plasma oscillations, or strong ionic plasma oscillations.

An adequately uniform glow of the discharge is readily achieved when the electron-addition ion sourceis operated normally. A cathode-rayoscillograph may. also be employed to check the freedom-0f the low-voltage discharge from oscillations.

As described above, two deflection plates 18 are pro-'- vided directly behind theaccelerating electrode 17, parallel,

to the longitudinal direction of the emission slit, by means of which the center of the molecule-ion-electron beam can be accurately adjusted to the opening of the aperture diaphragm system 21, 22. This adjustment is made by adjusting to maximum intensity of the mass line visible in the ionic image converter. The molecular mass spectrograph is adjusted, the desired line length is set, and the intensity of the lines in the massspectrum is adjusted while observing the ion image'converter screen. As some of the lines are of very low intensity, two aperture diaphragms have been provided. One diaphragm 21 with a wide slit is provided for visual observation of the mass spectrum,

using the ionic image converter, while the other diaphragm 22, which is designed so as to slide'into the path of the beam and has a slitthat is about one-tenth as wide is proyidedfor photographing of the mass spectra with high resolution; 7 v Tests with the process of the invention and with the {device of the invention have yielded astonishing results for masses up to nearly 300. Nothing can be said asyet concerning the extent to which the mass lines thus reproduced in the molecular mass spectrogram represent dif- I ferent kinds of molecules that were originally present or s were produced by the thermal or discharge splitting off of hydrogen atoms. cleared up with ionic source discharges of very. difierent electron temperatures, using a greatly reduced thermal control'of the test vapor. Photography of molecular lines producedby cleavage would also constitute an important contribution, for their evaluation according to position and intensity might yield newinsightsinto the energy structure of polyatomic molecules and thus constitute a far-reaching extension of the results achieved hitherto with infrared spectrometry or spectroscopy.

The process of the invention and the corresponding 'dei; observation hithertoun vice have opened up to human known regions of nature. I claim:

1. Method for adding Jslow electrons to -polyatomic It 'may be that these problems canbe 4 s or high-molecular compounds for the precision mass spectroscopy of molecules, comprising the steps of producing a low voltage electron discharge froma hot cathode into an auxiliary gas'to form a quasi-neutral plasma of low electron temperatures and low potential gradient, applying an inhomogeneous magnetic field to the plasma which is shaped so as to magnetically compress the plasma in an electron addition chamber directly in front of an ion emission opening, adding the molecules to be analyzed to the magnetically compressed plasma directly in front of the ion emission opening, drawing off the negative ions and electrons of the plasma through the ion emission opening into a spectroscopic mass analyzing system.

2. Method according to claim 1, wherein the auxiliary gas is one of the elements of the group consisting of cesium vapor, mercury vapor or argon.

3. Method according to claim 1, wherein the strength and the shape of the inhomogeneous magnetic field is such as to reduce the wall and recombination losses to an extent enabling the discharge current density to reach 100 amp./cm.

4. Method according to claim 1, including the step of introducing molecules of known masses into said chamber in front of the ion emission opening together with the molecules to be analyzed.

5. Method according to claim 4, wherein the molecules to be analyzed are introduced into the plasma only during the time that the apparatus is being adjusted and a measurement is being made.

6. Method according to claim 1, wherein the molecules are introduced as a sharply focused gas or vapor beam.

7. Method according to claim 1, wherein unvaporizable molecules to be analyzed are introduced by a vibrating surface near the ion emission opening.

8. In a mass spectroscope having a mass analyzing system, means ahead of said analyzing system for producing a quasi-neutral plasma, said means comprising a thermionic cathode, a chamber containing an auxiliary :3 5;! gas having a low ionization voltage, and means for producing a discharge of slow electrons from said cathode into said auxiliary gas, means for introducing molecules to be analyzed into said chamber, said chamber having an ion emission opening, means for compressing said plasma in a region in said chamber immediately adja cent said opening, and means for drawing ions through said opening into the analyzing system.

9. Apparatus according to claim 8, wherein said plasma compressing means includes a magnetic lens having an asymmetrical pole piece adjacent said opening.

10. Apparatus according to claim 9 wherein the means for drawing the ions through the opening includes a magnetic pole piece on the emission side of said opening and an accelerating electrode immediately beyond said last mentioned magnetic pole piece.

11. Apparatus according to claim 8 including a slotted diaphragm shutter mounted over said opening.

12. Apparatus according to claim 8 wherein said auxiliary gas is a member selected from the group consisting of cesium vapor, mercury vapor and argon.

13. Apparatus according to claim 8, including means for introducing into said chamber molecules of an additional known substance, whereby said known substance provides a calibration.

14. Apparatus according to claim 8, wherein the means for introducing the molecules to be analyzed includes a cup, means for sliding the cup toward the compressed plasma, and means for heating the cup for vaporizing the substance to be analyzed.

References Cited in the file of this patent UNITED STATES PATENTS 2,806,161 Foster Sept. 10, 1957 2,816,243 Herb et al. Dec. 10, 1957 2,831,996 Martina Apr. 22, 1958 2,841,726 Knechtli July 1, 1958 2,883,568 Beam et al. Apr. 21, 1959

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