US3075076A - Gas-analyzing method and apparatus - Google Patents

Description

\ Jan. 22, 1963 -K. G.GUNTHER GAS-ANALYZING METHOD AND APPARATUS 2 Sheets-Sheet 1 Filed Dec. 11, 1959 nvsumr/ou FIG. 2

a: 1 1 a MW w. m 4 WW! fl// W M/ W/ i W a 5 PW n4 17E vvu I FIG.3

United States Patent GAS=ANALYZING Ml'l'll-IQD AND APyARA'lUh Karl Georg Ghnther, Nurnberg, Germany, assignor to Siemens-Schuclrertwerire Ahtiengesellschait, Berlin- Siernensstadt, Germany, a corporation of Germany Filed Dee. ll, 19539, Ser. No. 859,03tl Claims priorit', application Germany Dec. 12, 1958 9 (6i. 250- il.9)

My invention relates to a method and apparatus for analyzing gaseous substance with the aid of a mass spec trometer or isotope separator of the type generally known from German Patent 944,900, June 28, 1956, or the corresponding U.S. patent application Serial No. 476,812, filed December 21, 1954, and also disclosed in the copending application of W. Paul et 211., Serial No. 782,838, filed December 24, 1958, U.S. Patent 2,950,389, Serial No. 476,812, issued as US. Patent 2,939,952, June 7, 1960.

According to these prior disclosures, a separation of ions having respectively diiferent specific electric charges is efiected by shooting the ions into a periodically variable electric field whose electric potential (p is a square function of the space coordinates x, y, z and has the general form p=f( (Win -7Z wherein f-(t) is any desired periodic function of time (t) and at, B, 'y are constants satisfying the equation owl-(i=7. Under these conditions, the ions in the periodic field travel either on stable or on instable paths depending upon their specific charge and thus are directed to separate electrodes. Specific charge means the ratio Electric charge Mass of particle as evidenced in the German latent 944,900. T..e reference disclosures describe difierent special cases as regards the configuration of the electric field, incniding a cylinder-symmetrical field in which the electric potential 1p has the form:

LTV2sm a y a The term r signifies the shortest distance of the electrode from .t-axis (field radius). Formula 2 corresponds to Formula (a) of German Patent 944,900.

As defined in said copending application Serial No. 782,838, U=the direct voltage, and V=the high frequency amplitude, applied as explained below.

Electrode arrangements for satisfying the foregoing requirements are described and their properties are dis cussed in the reference disclosures.

It is an object of my invention to provide a reliable method and apparatus for performing gas analyses on mass-spectroscopical principles in a simpler manner than by the mass spectrometers heretofore used for such purposes. More particularly, it is an object of my invention to devise a relatively simple mass spectrometer device for the measuring or supervision of degassing and drying processes in vacuum for technological purposes, such as the manufacture and operation of industrial vacuum devices.

According to my invention, such gas analyses in vacuum are performed with the aid of a mass spectrometer generally of the type described in the above-mentioned prior disclosures, but provided with a cold ion source and having a housing with a gas inlet duct for connection to the vacuum space under observation. The cold ion source may be of the high-frequency type. However, according to another, referred feature of the in Patented dart... 22, E963 ice vention, the cold source is an ion gun designed and operating in accordance with Pennings oscillatory electron principle. Regardless of the particular cold ion source employed, the invention further requires that the ion travel distance within the periodic electric field be sub stantially in accordance with the mean free path of the ions within the working pressure range of the mass filter. The dimensioning or operation of the cold ion source is to be such that the ions enter into the periodic electric old at such a speed that the number of possible oscillations or" the ions in the periodic electric field is at least approximately equal to 3.5 times the square root of the spectroscopic resolving power required. In order to avoid falsifying the measuring result, the cold ion source is to be designed and arranged in such a manner that no faster ions are shot into the periodic electric field. This can be secured, for example, by preventing the electrodes of relatively high potential from acting upon the outlet opening of the ion gun.

As mentioned, apparatus according to the invention are particularly well suited for supervision of degassing and drying processes in vacuum, in many fields of technology. For example, the apparatus is suitable for measuring and supervising the vacuum employed in the manufacture of electric capacitors and electric cables. Many other vacuum degassing processes, for example the degassing or electron-tube components and metal melts, require a continuous supervision which can be advantageously carried out with apparatus of the invention. Such apparatus are further applicable for measuring and continuously supervising the tightness of vacuum equipment, with great accuracy and with a relatively small expenditure.

While mass spectrometers have been previously ernployed for performing gas analyses in technological processes, the spectrometers heretofore available for such purposes require great expenditure in respect to space and material and can be used only for high-vacuum purposes at pressures below 10" mm. Hg. Mass filters of the type described in the above-mentioned prior disclosures are considerably simpler than those previously known; and the present invention further improves such mass filters toward attaining a considerable simplification in the supervision of technological vacuum processes and for use with vacuum pressures within a much wider range, in cluding higher pressure values than heretofore amenable to mass-spectrometric methods.

The invention will be further explained with reference to the embodiments of gas-analyzing devices according to the invention illustrated by way of example on the accompanying drawings, in which:

1 shows schematically a gas-analyzing device.

FIG. 2 is a longitudinal sectional view of a somewhat modified device.

KG. 3 is a cross section along the line 11-11 in FIG. 2.

FIGS. 4 and 5 are a longitudinal section and a cross section of an ion source applicable in devices according to FIGS. 2 and 3, the section being along the line V-V in FIG. 4.

FIGS. 6 and 7 show schematically two respective embodiments of complete vacuum apparatus equipped with a gas-analyzing device according to the invention.

The housing 1 of the device shown in FIG. 1 is essentially composed of three flanged-together portions la, lb and 10. The housing portion la has a duct 10! for connection to the vacuum space, and accommodates a cold ion source 2 of the Penning type. Such ion sources, operating on the oscillating-electron principle, are known as such, for example from Encyclopedia of Physics, edited by S. Fliigge, volume XXXlll, Optics of Corpuscles, 1956, pages 82 to 87. The-particular source 2 shown in FIG. 1 comprises a tubular anode 2a spaced and insulated from two coaxially aligned cathodes 2b, 20 which are electrically interconnected. The space between the two cathodes is subjected to the magnetic field of a permanent magnet NS. When apotential, for example of +1000 volts, is applied to the anode 2a relative to the cathodes 2b and 2c, the electrons from cathode 2b are accelerated toward the anode but the magnetic field forces the'electrons in the center to follow the direction of the field and to pass entirely through the tubular anode to the vicinity of the other cathode 20 which repels the electrons so that they return toward the first cathode 2b where they are again repelled, and so forth. If such an oscillating electron collides with a gas molecule, it may ionize that molecule by knoclcing out another electron Which'then contributes to sustaining the ionizing operation. The ions are drawn out of the anode space through an opening in cathode 2c by the action of an accelerating plate 2d and a collimating or focussing plate 2e. The ion beam then enters through the opening of -a diaphragm plate 3 into the middle housing portion 1b.

Mounted in housing portion 11') is a group of symmetrically distributed electrodes 4 for producing the cylinder-symmetrical periodic electric field. Preferably used are four electrode rods of circular cross section which extend longitudinally of the housing in parallel relation to the center axis and are uniformly distributed about that axis. The end portion 1c of the housing comprises a fieldexit diaphragm 5 and a collector electrode 6 which collects the impinging ions. Terminal connections for the electrodes 4 are shown schematically at 7.

During operation of the device, a sinusoidal volt-age of high frequency and a superimposed direct voltage are impressed upon the electrodes 4 to produce an electric periodic field in accordance with the above-stated condition (2). As a result, there is a stable range in which the oscillation amplitude of ions of a given electric charge does not exceed a given maximum value. Hence, only such ions can pass from the ion source 2 between the electrodes 4 to the collector electrode 6. The other ions, having diiterent electric charges and performing instable oscillations after entering the periodic electric field, assome oscillation amplitudes of such large magnitude as to impinge upon the field electrodes 4. Due to the abovementioned dimensioning rule for the ion travel distance between source and collector, the mass filter is directly applicable up to pressures of approximately mm. Hg. This requires giving the field-electrode rods 4 a length of 10 to cm. The provision of a cold ion source is an essential prerequisite for suitability of the device up to pressures of approximately 10* mm. Hg. Ion sources with incandescent cathodes, as generally used in mass spectrometers, would rapidly burn through at these high pressures.

However, a device according to the invention is also applicable for still higher pressures, it a throttle passage is interposed between the container for the gaseous sub stance to be tested, and the duct portion of the housing through which the vacuum space in the housing communicates with that of the gas'container. In addition, an auxiliary vacuum pump must be connected with the vacuum space in the spectrometer housing as will be further described below with reference to FIG. 7.

The use of relatively short field electrode rods requires adapting the entering speed of the ions accordingly. This is because a sufficient mass separation can be obtained only if the ions in the periodic electric field can perform a sufiioient number of oscillations. For that reason, the number of these oscillations must be at least equal to 3.5 times the square root of the-required mass-spectroscopic resolving power. As in the method according to the application Serial No. 476,812 (US. Patent 2,939,812), the resolving power is defined by the ratio m/Am, wherein m isthe mass of the ions to be collected and Am is the mass difference relative to the other ions which can still be separated from those to be collected. For example, if a resolution of 50 is required the above-mentioned condition requires that the shooting-in speed of the ions must be so rated that the ions can perform at least 25 oscillations while passing through the electric periodic field between the electrodes 4. It may be mentioned that gas collisions in the electric field have relatively slight effects because of the focussing of the field.

In a device according to the invention, the total travelling path of the ions is kept as small as feasible. This is particularly important with respect to the distances between .the ion source and the field-electrode rods, and the distance between the field rods and the collector electrode, because no iocussing forces are active at these locations. As a result, the collision losses are reduced considerably.

The device described above with reference to FIG. 1, is shown more realistically in FIGS. 2 and 3, the same reference characters being applied to corresponding elements respectively. The four electrodes 4 are spaced from each other a distance about equal to the electrode diameter. The electrodes are held in fixed position by insulating spacer discs 4a of ceramic material and are preferably adjustable. The direct and alternating voltages required for producing the electric field between the electrodes 4 are supplied thereto by terminals 7 which are located in housings 7a vacuum-tightly connected with the housing portion 1b by means of screw caps 7b. The collector electrode 6 is shown grounded through an external resistor 11. The voltage drop of resistor 11 due to the discharge current from electrode 6 to groundis measured by an instrument 12 such as a voltmeter or recorder. The diaphragms 3 and 5 shield the ion source and the collector electrode from the high-frequency field between the electrodes 4. As mentioned above, those ions that are excited by the high-frequency field to oscillate along their trajectory with unlimited amplitudes, cannot reach the collector electrode 6 but impinge upon the field electrodes 4.

The housing portion 1a of the device shown in FIGS. 2, 3 may be provided with an ion source as shown in FIG. 1. A somewhat modified source, also of the Penning type, is illustrated in FIGS. 4 and 5 where the same reference characters as in FIG. 1 are used for respective corresponding elements. The tubular anode 2a of the ion source has a slot 2 through which the ions are withdrawn radially of the cylindrical anodespace.

As mentioned, when the device is in use, the connecting duct 11!, shown in FIG. 6 at a somewhat different location, forms a communication with the vacuum vessel 20 under observation. According to FIG. 6, the ion source 2 is connected to a unit 21 which has terminals 22 for connection to a utility power line and which supplies the necessary direct voltage, for example of 1000 volts, to the ion source. The unit 21 is shown to be provided with an instrument 23 for indicating or recording the total ion flow of the source or the proportional total gas pressure at the source. The field electrodes 4 receive alternating voltage from a high-frequency source 24 whose frequency is adjustable as schematically represented by a variable tuning capacitor 25. Superimposed upon the high-frequency voltage is a direct voltage of adjustable magnitude. This is schematically represented bydirect-voltage source 26 and a voltage-adjusting rheostat 27, although it will be understood that the direct-voltage component of the electrode voltage may be produced by rectifying an adjustable share of the alternating voltage as is explained in the above-mentioned copending application Serial No. 782,838. The collector 6 is connected to an amplifier 28 which operates 'a recording microampere meter 29' for indicating the separatedion flow, or the partial gas pressure proportional to that flow.

. When using the device, it is to be adjusted to the mass of the particular molecule to be ascertained. For example, when supervising a drying process, the device is adjusted to the mass 18 of the water (steam) molecule. The

intensity of that particular mass is observed on recorder 29 as a function of time. The stabilization to a given mass value is effected by the corresponding choice of the high frequency or high-frequency amplitude and the voltage of the superimposed direct field. The ions of this stabilized mass value impinge upon the collector and are measured with the aid of the microampere meter. For supervision of degassing operations, the high-frequency voltage can be varied within certain periods of time in order to make the device responsive to the entire mass spectrum. When employing the device as a detector for leaks in vacuum vessels, the field frequency is analogously adjusted to the mass of the test gas being used.

As mentioned above, a device according to the invention can be used for vacuum pressures above mm. Hg by interposing a throttle passage between the mass filter and the vacuum vessel, and maintaining the proper pressure in the mass filter by means of an auxiliary pump. FIG. 7 shows such a throttle passage at 26a in conduit 1d. Connected to the housing 1 of the mass filter is a diffusion pump 31 and a prepump 32, both cooperating to keep the vacuum pressure in housing It sufficiently low and proportional to the pressure in the vacuum vessel 24). In all other respects the apparatus shown in FIG. 7 is equipped and operated as described with reference to FIGS. 1 to 6.

I claim:

1. Mass-spectroscopical gas analyzing apparatus, for

separating ions of respectively difierent specific electric charges, by causing ions to assume oscillations having amplitudes correlative with their specific charges; comprising a structure providing an evacuable chamber and having a duct for communicating said chamber with a vacuum space under observation; a cold-type ion source operating on the oscillating-ion principle; said source comprising a magnetic field device, a hollow anode in said magnetic field, and opposed cathodes between which the anode is located; a collector electrode spaced from said ion source in said chamber; a group of field electrodes extending lengthwise in said chamber betwecnsaid source and said collector electrode in symmetrical and radially spaced relation to the common axis thereof, the field electrodes being adapted for producing a cylinder; symmetrical electric field; and electric-field excitation means connected to said field electrodes for supplying a component direct voltage and a component high-frequency voltage of adjustable frequency, to provide a resultant cylinder-symmetrical electric periodic guiding field between said electrodes, whereby, in accordance with frequency and direct voltage adjustment, ions of a given mass can pass to the collector electrode; shielding means between the field electrodes and the cold ion source, and between the collector and the field electrodes; the effective length of the field electrodes being such that the ion travel distance within said guiding field is of the same order of magnitude as the mean free path of the ions in the apparatus, in the working pressure range thereof; said cold ion source providing an ion exit speed at which the minimum oscillatory frequency of the ions in the periodic field is about equal to 3.5 times the square root of the resolving power of the apparatus, the resolving power being defined as the ratio m/Am, wherein m is the mass of the ions to be collected and Am is the mass difference relative to the other ions which can still be separated from those to be collected.

2. The apparatus defined in claim 1, the electropotential p having the following form:

3. Mass-spectroscopical gas analyzing apparatus, for separating ions of respectively difierent specific electric charges, by causing ions to assume oscillations having amplitudes correlative with their specific charges; comprising a structure providing an evacuable chamber and having a duct for communicating said chamber with a vacuum space under observation; a cold-type ion source and a collector electrode axially spaced from each other in said chamber; two pairs of cylindrical field electrodes extending lengthwise in said chamber between said source and said collector electrode in symmetrical and radially spaced relation to the common axis thereof; electric-field excitation means connected across said respective pairs of field electrodes and having a component direct voltage U and a component high-frequency voltage of amplitude V, to provide a resultant periodic cylinder-symmetrical electric field between said electrodes in which the electric potential (p has the following form:

shielding means between the field electrodes and the cold ion source, and between the collector and the field electrodes; the effective length of the field electrodes being such that the ion travel distance between said source and said collector electrode is of the same order of magnitude as the mean free path of the ions in the working pressure range of the apparatus; said cold ion source providing an ion exit speed at which the minimum oscillatory frequency of the ions in the periodic field is about equal to 3.5 times the square root of the resolving power of the apparatus, the resolving power being defined as the ratio m/Am, wherein m is the mass of the ions to be collected and Am is the mass difference relative to the other ions which can still be separated from those to be collected.

4. Mass-spectroscopical gas hnalyzing apparatus, for separating ions of respectively difierent specific electric charges by causing ions to assume oscillations having amplitudes correlative with their specific charges; comprising a structure providing an evacuable chamber and having a duct for communicating said chamber with a vacuum space under observation; a cold-type ion source and a collector electrode axially spaced from each other in said chamber, the cold-type ion source operating on the oscillating-ion principle whereby gas molecules are ionized, comprising 5 magnetic-field device, a hollow anode in said magnetic field, and opposed cathodes between which the anode is located; two pairs of cylindrical field electrodes extending lengthwise in said chamber between said source and said collector electrode in symmetrical and radially spaced relation to the common axis thereof; electric-field excitation means connected across said respective pairs of field electrodes and having a component direct voltage U and a component high-frequency voltage of amplitude V, to provide a resultant periodic cylindersymmetrical electric field between said electrodes in which the electric potential c has the following form:

shielding means between the field electrodes and the cold ion source, and between the collector and the field electrodes; the effective length of the field electrodes being such that the ion travel distance between said source and said collector electrode is of the same order of magnitude as the mean free path of the ions in the working pressure range of the apparatus; said cold ion source providing an ion exit speed at which the minimum oscillatory frequency of the ions in the periodic field is about equal to 3.5 times the square root of the resolving power of the apparatus; the resolving power being defined as the ratio m/Am, wherein m is the mass of the ions to be collected and Am is the mass difierence relative to the other ions which can be still be separated from those to be collected.

5. The apparatus defined in claim 4, and accelerating electrode means and a diaphragm in said chamber for passing ions from said source toward said field electrodes and said collector electrode for producing a cylindersymrnetrical electric field, and electric-field excitation means connected to said field electrode means for supplying a component direct voltage and a component highfrequency voltage of. adjustable frequency, to provide a resultant cylinder-symmetrical electric periodic guiding field between said electrodes, the electropotential (,0 thereof being of the following form:

U+V sin mp -2 2 V r whereby, in accordance with frequency adjustment, ions of a given mass can pass to the collector electrode; the ion travel distance within said guiding field being of the same order of magnitude as the mean freepathof the ions in the apparatus, in the working pressure range thereof; said cold ion source providing-anion exit speed at which the minimum oscillatory frequency of the ions in the periodicfield is about equal to 3.5 timesthe square root of the resolving power of the apparatus, the resolving power being defined as the ratio -m/ Am, wherein m is "the mass of the ions to be collected and Am is the mass difference relative to the other ions which can still be separated from those'to'be collected.

7. The method of separating gas ions ofrespectively difierent, specific electric charges in vacuum, by causing gas ions of respectively different specific electric charges to assumeoscillations having amplitudes correlative with said charges, which comprises passing the ions through an electric field having a field potential U+V sin wt 11 -5? T'T said field being a .cylindrically symmetrical electric field having a high-frequency component of adjustable frequency and a direct field component imposed by a direct current voltage, whereby, in accordance with frequency and direct current voltage adjustment, ions of a given specific electric charge are directed on a path, for collection thereof, maintaining said ion path at a length corresponding to the mean free path of the ions in the desired working pressure range, and accelerating the ions from the source-prior to their entering the electric field to a speed at which the minimum number of possible oscillations of the ions in the field is about equal to 3.5 times the square root of the required mass resolving power, the

resolving power being defined as the ratio m/nm, wherein m is the mass of the ions to be collected and Am is the mass difference relative to the other ions which can still "be separated from those to be collected.

8. The method of analyzing gas in vacuum according to claim 7, which comprises preventing faster ions from passing from the ion source into the electric field.

9. The method defined in claim 7, the said gas ions being derived by passing a gas being tested through a cold ionizing step in which the gas molecule is collided with oscillating electrons.

References Cited in the fileof this patent UNITED STATES PATENTS 2,569,032 Washburn Sept. 25, 1951 2,769,910 Elings Nov. 6, 1956 2,839,706 Anderson et al June 17, 1958 2,939,952 Paul et al. Iune'7, 1960 OTHER REFERENCES C. F. 'Giese: Strong Focusing Ion Source for Mass Spectrometers, pages 260 and 261 of vol. 31, No. 4, April 1959 issue of The Review of Scientific Instruments.

Claims (1)

1. MASS-SPECTROSCOPICAL GAS ANALYZING APPARATUS, FOR SEPARATING IONS OF RESPECTIVELY DIFFERENT SPECIFIC ELECTRIC CHARGES, BY CAUSING IONS TO ASSUME OSCILLATIONS HAVING AMPLITUDES CORRELATIVE WITH THEIR SPECIFIC CHARGES; COMPRISING A STRUCTURE PROVIDING AN EVACUABLE CHAMBER AND HAVING A DUCT FOR COMMUNICATING SAID CHAMBER WITH A VACUUM SPACE UNDER OBSERVATION; A COLD-TYPE ION SOURCE OPERATING ON THE OSCILLATING-ION PRINCIPLE; SAID SOURCE COMPRISING A MAGNETIC FIELD DEVICE, A HOLLOW ANODE IN SAID MAGNETIC FIELD, AND OPPOSED CATHODES BETWEEN WHICH THE ANODE IS LOCATED; A COLLECTOR ELECTRODE SPACED FROM SAID ION SOURCE IN SAID CHAMBER; A GROUP OF FIELD ELECTRODES EXTENDING LENGTHWISE IN SAID CHAMBER BETWEEN SAID SOURCE AND SAID COLLECTOR ELECTRODE IN SYMMETRICAL AND RADIALLY SPACED RELATION TO THE COMMON AXIS THEREOF, THE FIELD ELECTRODES BEING ADAPTED FOR PRODUCING A CYLINDERSYMMETRICAL ELECTRIC FIELD; AND ELECTRIC-FIELD EXCITATION MEANS CONNECTED TO SAID FIELD ELECTRODES FOR SUPPLYING A COMPONENT DIRECT VOLTAGE AND A COMPONENT HIGH-FREQUENCY VOLTAGE OF ADJUSTABLE FREQUENCY, TO PROVIDE A RESULTANT CYLINDER-SYMMETRICAL ELECTRIC PERIODIC GUIDING FIELD BETWEEN SAID ELECTRODES, WHEREBY, IN ACCORDANCE WITH FREQUENCY AND DIRECT VOLTAGE ADJUSTMENT, IONS OF A GIVEN MASS

Images