US2768302A - Apparatus for mass spectral analysis - Google Patents

Description

Oct. 23, 1956 w. H. BENNETT 2,763,302

APPARATUS FLOR MASS SPECTRAL. ANALYSIS Filed Aug. 8, 1951 4 Sheets-Sheet 1 GLASS CYL llVDff? INYENTOR N WILLARD HBavmsTr Bfim M w I ATTORNEY;

Oct. 23, 1956 Filed Aug. 8. 1951 w. H. BENNETT 2,768,302

APPARATUS FOR "MAS-S SPECTRAL ANALYSIS I 4 Sheets-Sheet 2 IIIII/HI aua fJ/r NJW INVENTOR W/LLA/m H. DLN/YETT ATTORNEYS Oct. 23, 1956 Filed Aug. 8, 1951 4 Sheets-Sheet 4 INVENTOR W/LLAHD' H. ficlwvf'rr ATTORNEY} APPARATUS FOR MASS SPECTRAL ANALYSIS Willard H. Bennett, Fayetteville, Ark.

Application August 8, 1951, Serial No. 240,965

7 Claims. (CI. 25.0-41.9)

This invention relates to apparatus for mass spectral analysis.

One object of this invention is to avoid errors due to chemical reactions at the surface of the cathode, of gases being analyzed.

Another object of this invention is to provide a mass spectrometer suited for analysis with either positive or negative ions.

Yet another object of the invention is to provide a method and apparatus for mass spectral analysis which has only one mass line for each gas of the mixture, without the misleading indications of fragmentation ion indications.

Still another object of the invention is to provide a mass spectrometer with reduced background effects.

Another object of the invention resides in the provision of a method of mass spectrometry, applicable particularly to hydrocarbons, whereby the results may be ascertained by a direct reading.

In mass spectrometers of the prior art the gases to be analyzed contact the heated cathode. The high temperature of this cathode in some cases changes the composition of the gases being analyzed, whereby the results of the analysis show this new composition instead of or in addition to the desired one. That defect is avoided in the present invention by enclosing the cathode in a cup which is exhausted separately from the remainder of the tube, the cup having a small aperture through which electrons are fed into the other parts of the tube.

Background currents may be avoided in the collector system of this invention by employing a double grid in front of the collector electrode, and connected to the potential of the collector electrode, with an additional grid charged negatively interposed between the double grid and the collector element. This adidtional grid will repel all electrons from the collector.

In the analysis of gases such as hydrocarbons, errors often appear in the analysis because of fragmentation ions which appear due to the high potential of the electrons in the ion source. While it has been known that these appear, it has not been possible to eliminate them and hence the final spectrometer readings include unnecessary and misleading mass lines. In some instances it has been possible to identify and then ignore the unnecessary mass lines but this has not always been possible. According to the teachings of this disclosure these unnecessary mass lines are eliminated by selecting a potential for the ion source below the appearance potential of the fragment ions which are first to appear when the gas of the heaviest mass to be analyzed is placed in the spectrometer alone. Using that potential for the ion source one may then proceed essentially as outlined in detail in my prior copending application Radiofrequency Mass Spectrometer, S. N. 196,024, filed November 16, 1950, now abandoned, or as outlined in my published article in volume 21, pages 143-9, of the Journal of Applied Physics (February 1950).

'Ynited States Patent "ice Under these circumstances only one mass line will appear for each different gas in the mixture being analyzed.

This disclosure also illustrates a novel ion source utilizing parallel screens between which a beam of electrons is passed. The two screens are charged positively relative to the cathode but are at different positive potentials. Hence the positive and negative ions formed in the space between the grids (due to the electron bombardment of the gas under analysis) tend to move toward the two screens, the positive ions moving toward the screen of lower potential and the negative ions moving toward the screen of higher potential. The mass of the ions passing through these screens can be determined in known ways or by the apparatus described herein.

The apparatus claimed can be embodied in various forms and its adaptation in one form will now be described in detail.

In the drawings.

Figure 1 is a partly sectional and partly schematic drawing of one form of apparatus.

Figure 2 is a sectional view along line 22 of Figure 1.

Figure 3 is a right-hand end view of Figure 1.

Figure 4 is a schematic diagram of the apparatus when employed for positive ion analysis.

Figure 5 shows a series of curves used in explaining the invention.

Figure 6 is a partly sectional and partly schematic drawing of another form of apparatus.

Figure 7 is a plan view of the several grids used in the tube.

Figure 8 illustrates certain modifications of the system.

Referring to Figures 1 and 2, the tube employs a cathode 50 which has leads 50a. A metallic cylindrical shield 51 has lead 51a for connecting the shield 51 to a variable potential approximately the same as that of the cathode. Shield 51 has an aperture 51b which is about four millimeters in diameter. Cylindrical metal accelerating electrode 52 may be connected to a high positive potential through lead 52a. The electrode 52 has an aperture at its lower end of about two millimeters diameter. These parts are all mounted in an envelope 53 which has an inlet opening 54 through which gases to be analyzed may be admitted and it also has exhaust outlets 55 and 56 adapted to be connected to exhaust pumps for evacuating the tube. As is apparent, the exhaust outlet 55 primarily exhausts the interior of the cylindrical cup electrodes 51 and 52 for the following purpose. The gas to be analyzed may have its chemical composition somewhat changed if it strikes the very hot cathode. Therefore, in order to be certain that none of such gas with changed composition passes into! the main analyzing chamber, the tube is continuously being evacuated at exhaust outlet 55 whereby any gases whose composition were altered by cathode 5d are promptly removed from the envelope 53. The gases to be analyzed are continuously fed into inlet 54 and are continuously exhausted primarily through outlet opening 56.

The voltage on accelerating electrode 52 is held highly positive relative to cathode 50, say by fifty volts, by applying this much potential between the lead 52a and the center tap of the transformer winding connected to the filament leads 5%. Such connections are illustrated in Figure 4. As the electrons emerge through aperture 521) they pass between the two parallel grids 61 and 63. The average potential of these grids will be termed as i for simplicity. This potential in some cases is varied. Assume that it is desired to effect a value of P of 15 volts positive relative to the cathode 53, then one might apply a positive voltage of l4 volts to grid 61 and a positive voltage of 16 volts to grid 63. When one is interested in delivering negative ions to the analyzer he would make grid 63 with the higher positive voltage, say 16 volts in contrast to say 14 volts for grid 61. For positive ion analysis the grid 61 has the higher potential. A magnetic field of not more than about 100 gauss and generally about 20 gauss is applied perpendicular to the axis of the tube by means of a current in the two coils 21 and 22. As a consequence the electrons from cathode 53 form a narrow beam, known as a pencil of electrons, as they emerge from aperture 52b, at a speed of volts. They slow down to a speed of P volts as they enter the region between grids 61 and 63. in order that it may be determined how many electrons are being produced, an item which it is often desirable to know in calibrating and using the tube, as will hereinafter appear, a small metallic electrode 62 is connected in series with a meter I, a battery B, and the cathode 50. The magnetic field is temporarily removed and the voltage on electrode 52 is adjusted to give a maximum electron current, while the voltages on grids 61 and 63 are both set at 15 volts initially in order to focus the electron beam. When the beam is in proper focus the magnetic field is replaced and the voltages on grids 61 and 63 are rendered slightly different. When negative ions are desired for analysis, the grid 61 is less positive than grid 63, in other words grid 61 is negative with reference to grid 63 thus drawing all positive ions to the left and away from the analyzing grids. The negative ions are then drawn by grid 63 toward the analyzer to the right. When positive ions are to be analyzed the grid 61 is more positive and draws negative ions from the system whereas grid 63 is negative relative to grid 61 and draws positive ions into the analyzer. Adjacent grid 63 there are located grids 64 and 65 respectively connected to leads 64a and 65a, and these grids will be charged with potcntials intermediate those of grids 63 and 66. If one is analyzing with negative ions, as will hereafter appear, these grids will be charged positively; and if one is analyzing with positive ions these grids will be negative. In either case, however, grids 64 and 65 may be omitted but if these grids are omitted some of the sensitivity of the instrument will be lost.

The mass spectrometer shown in Figure 1 is of the three stage type and is basically described in my article entitled Radiofrequency mass spectrometer in the Journal of Applied Physics, vol. 21, No. 2, pages 143 to 149, February 1950, and is also described in my prior copending application entitled Radiofrequency Mass Spectrometer, Serial No. 196,024, filed November 16, 1950. in view of these prior disclosures the theory of the three stage spectrometer will not be related in detail, it being sufficient to say that it employs three groups of grids with three grids in each group. Grids 66, 66' and 66" are connected to lead 66a. Grids 67, 67 and 67" are connected to lead 67a and grids 68, 68 and 68 are connected to lead 68a. Leads 66a and 68a are connected to a source of negative polarity when positive ions are analyzed, and lead 67a is fed with radio frequency potential, as will be explained in greater detail later in connection with Figure 4. The three grids of each group are preferably close together as compared to the spacing between groups. The distance between grids 67 and 67 is preferably different than that between grids 67 and 67 and preferably according to the ratio of 7 to 5.

The grids 69 as shown in Figure 1 are located between the last stage of the analyzer 68 and 67", 68a and the screen 7:) over the collector 71. This grid doublet is important in use with positive ions but may be omitted entirely when negative ions are analyzed or may be used as repelling electrodes by applying a suitable repelling potential Z thereto of negative polarity. This potential is selected as described in said article and in said prior application and is of a value to repel all background electrons and ions, leaving only those to be analyzed. Grid 76 is connected to cylindrical shield 70S and to lead 70a which for negative ion analysis may be connected to lead 69a. The collector 71 is connected to a source of positive potential by its lead 71a, and since it is made as positive as any other electrode in the tube, or more so, it will attract the negative ions that have passed grid 70 and repel all of the positive ions that have been accelerated by the other grids.

When positive ions are being analyzed the grid 69 is at the same high positive potential as the collector 71, thus providing a stopping potential Z. The function of the stopping potential Z is explained in said prior article and in said prior copending application. The grid 70 has a negative potential which is as negative as, or preferably more negative than, any other electrode in the tube to thus prevent negative ions from reaching the electrode 71.

Figure 3 illustrates in more detail, the leads from the tube. The several grids are knitted wire screens, as shown iin Figure 7, mounted in washer-shaped disc holders 720 which holders are supported by horizontal rods 72 (see Figures 1, 6 and 7). Instead of bringing the leads vertically through the side wall of the envelope they pass horizontally through the horizontal rods and out the right hand end of the envelope as shown in Figure 3.

Cylindrical wire screen 73 connects grids 66 and 66 and cylindrical wire screen 73a connect grids 68 and 68' inside the tube.

It should be noted that the uppermost two horizontal rods 72 are on either side of the direct path between cathode 50 and the space between grids 61 and 63, hence there is nothing to prevent electrons from the cathode from passing to electrode 62.

In Figure 4 a complete schematic diagram for operating the tube as a positive ion mass spectrometer is shown. The amplifier and measuring instrument 30 is connected between the collector 71 and the ground and measures the ions arriving at collector 71.

Variable frequency oscillator 81 provides the variable radio frequency potentials, and for many types of work a suitable frequency range for this may be from kilocycles to 10 megacycles. The output of the oscillator 81 is fed through condenser 83 and reactor 84. The drop across reactor 84 is fed to the middle grids 67, 67' and 67 and also to leads 66a and 6811 through resistors 66b and 68b. Vacuum tube voltmeter 82 measures the potential of the oscillator 81 whereby the latter may be adjusted and held constant.

It is possible to focus the electron beam by varying the contact arm 51c thus varying the potential on accelerating electrode 51.

Having described one suitable form of apparatus I will now describe how one may proceed to analyze a mixture of a series of hydrocarbon gases to determine the formulas of the gases present. One first selects the heaviest gas that might be present in the mixture and introduces a sample of it alone into the tube through inlet 54. Then the potential P is increased from zero by varying rheostat arm 51d (Figure 4) until only one mass line may be detected. The mass line is detected by vary ing the voltage, on lead 60 (which variation is indicated on meter V) through a wide range while observing the galvanometer 8% If during such a variation of voltage on lead 60 there is only one voltage value in said range at which an output indication is given at instrument 80, it can be said that there is only one mass line. Then the potential P is increased still further until during a sweep of voltages on lead 60 two mass lines appear. This second potential P is the appearance potential at which the first fragmentation ion of the gas appears. The potential P is then reduced gradually until this second mass line barely disappears. This potential, which is just below the appearance potential for the lowest fragmentation ion, is then used for analysis of the whole mixture of the series of gases. With this value of potential P, only one mass line will appear for each gas present in the mixture.

By way of illustration of this process and system, assume that of the several gases that may be involved the heaviest one is CxHy. With reference to Figure 5, we see that as the ionizing voltage (potential P) is increased from zero that We get an ion yield from the gas CmHy itself upon reaching potential Pl. As the potential is increased to P-2 we get fragmentation ions for the fragment CzH1 1 which appear due to bombardment of the parent gas. As the potential P is increased further to a value P3 we get ions due to the fragment CmHy-Z. The difference between potentials P1 and P-Z is the potential required to remove one hydrogen atom from a CzHy ion. The difierence between potentials P-1 and P-3 is the potential required to remove two hydrogen atoms from the CIHQ/ ion. The difference between potentials P-1 and P-4 is the potential required to remove one carbon and three hydrogen atoms from the CazHy ion.

It is apparent that if the tube is operated at a potential greater than P2 that numerous mass lines will appear by reason of the several groups of fragmentation ions that are produced. These additional mass lines confuse the final result by showing mass lines for gases not actually present in the original mixture. However, if the potential P is held below the value P2, the only mass lines that will appear will be the proper ones for the gases introduced into the tube, without more.

It is understood that the process of analyzing hydrocarbon gases described in this application may not only be practiced with the apparatus shown herein but with other apparatus such as the tube of my said prior copending applications and that of said article.

An alternate form of tube is shown in Figures 6 and 7. In these figures all numbers below 51 represent dimensions in millimeters. In this case the cathode 100 has electrodes therefor 161 and 162 corresponding to the electrodes 51 and 52 of Figure 1. The inside cup 101 is continuously evacuated at outlet 105 while the gas to be analyzed is continuously admitted in inlet 104. The main exhaust outlet 106 is of course connected to the same, or a separate, exhaust pump as is outlet 195. The ion source employs three grids 107, 108 and 109 electrically connected together and these are to be held at a potential higher than that of the cathode. Drawing out grid 110 is connected to a high negative potential to draw ions from the ion source. Following grid 110 there are three groups of grids of three grids per group, these being numbered 111 to 119 inclusive. These grids function as described in said prior article and in said prior copending application.

Grids 120 and 121 are tied together and to the collector electrode 123 and are charged positively to act as a stopping potential Z. The grid 122 is charged negatively, in fact more negatively than any other grid of the tube, to thus repel all electrons from the collector.

While in my various forms of mass spectrometers as described herein and in my prior articles and other applications, it is desirable to have the distances between the middle grids of the groups integral multiples of the same unit of distance, this is not necessary if the radio frequency potentials are applied to the middle grids out of phase. For example, in Figure 8 the middle grids are fed with three phase radio frequency potential, and therefore the distances between grids are no longer integral multiples of a unit distance, but are so spaced that the ions of the desired gases being analyzed pass each grid at zero potential. Hence, any spacing of the middle grids can be used if the phase relations of the applied potentials are properly selected.

My copending application Serial No. 240,966 filed on even date herewith, entitled Method of and Apparatus for Mass Spectral Analysis with Negative Ions, the disclosure of which is incorporated herein by reference.

In certain circumstances it is desirable to first analyze a gas mixture by the positive ion method and then with negative ions. A multipole double throw switch may be provided to shift the hook-up of the tube from the circuit employed for positive ions to that employed for negative ions.

in some cases, in connection with my invention, the potential P may be above the potential at which certain fragmentation ions appear provided it is below the potential at which unwanted fragmentation ions appear. For example, in the analysis of certain oils, it is undesirable for any carbon fragmentation ions to appear but it does little harm for hydrogen fragmentation ions to appear. Hence, in such a case, in initially setting potential P, the potential would be raised until a mass line first appears which is more than 13 units below the heaviest mass line. At this point, the potential P is then slightly reduced until this extra mass line disappears.

As an example, assume a hydrocarbon gas having the following constituents: C10H22, CsHzo, Cal-11s, C'IHIS and Cal-I14. We would get a mass line at 142, 128, 114, 100, and 86. If the potential P was large enough to dislodge a few hydrogen ions from these gases, the only effect of that would be to broaden the mass lines or mass line groups, however they would still peak near the position for the parent unfragmented ions and would be readily distinguishable from the peaks corresponding to components with different numbers of carbon atoms in the molecule. Should the potential P be increased until carbon fragmentation ions appeared, a large number of mass lines might appear which would confuse the results. Hence, this disclosure contemplates operation at a potential below the point where carbon fragmentation ions appear.

i claim to have invention:

1. In a mass spectrometer, means for selectively ionizing a gas by electron bombardment including, electron emission means, electron accelerating means cooperating therewith, envelope means for maintaining gas to be analyzed in the path of the accelerated electrons, and means for selectively controlling the electron accelerating potential; and mass specrum analysis means responsive to the ions developed by the bombardment of the gas by the accelerated electrons.

2. In a mass spectrometer, means for selectively ionizing a gas by electron bombardment including, electron emission means, electron accelerating means cooperating therewith, envelope means for maintaining gas to be analyzed in the path of the accelerated electrons, and means for selectively controlling the electron accelerating potential over a range that includes a potential which is below the one at which fragmentation ions are produced; and mass spectrum analysis means responsive to the ions developed by the bombardment of the gas by the accelerated electrons.

3. In a mass spectrometer, means for selectively ionizing a gas by electron bombardment including, electron emission means, electron accelerating means cooperating therewith, envelope means for maintaining gas to be analyzed in the path of the accelerated electrons, and means for selectively controlling the electron accelerating potential over a range that includes a potential which is below the one at which any gas fragmentation ions are produced; and mass spectrum analysis means responsive to the ions developed by the bombardment of the gas by the accelerated electrons.

4. A mass spectrometer as defined in claim 2 in which said means for selectively ionizing a gas includes an ion source; said mass spectrum analysis means including all of the following: ion segregating and measuring means for indicating the flow of ions that exceed a predetermined energy, a plurality of ion energy increasing means located between the ion source and the ion segregating and measuring means for successively increasing the energy of ions of a predetermined mass during their 7 journey from the ion source to the ion segregating and measuring means, and means including a source of radio frequency energy for energizing the ion energy increasing means to increase the energy of ions of a predetermined mass more than of other masses.

5. In a mass spectrometer, an envelope having an inlet for receiving the gas to be analyzed, a cathode in said envelope, electrode means in the envelope and spaced from the cathode, a source of direct current potential connected between the cathode and said electrode means and including means for varying the potential applied between the cathode and the electrode means over a range wide enough to include potentials at which first fragmentation ions of hydrocarbon gases will not appear, and mass spectrum analysis means in the envelope responsive to the ions developed by the bombardment of the gas in the envelope by electrons from the cathode.

6. In a radio frequency mass spectrometer, an ion source, means for selecting and indicating the flow of ions that exceed a predetermined energy, first and second devices respectively energized by radio frequency potentials of the same frequency but of different phase for successively increasing the energy of the ions of a particular mass on their journey from the ion source to the first-named means more than they increase the energy of ions of other masses, and radio frequency generating means connected to said devices for supplying said radio frequency potentials thereto.

7. A radio frequency mass spectrometer comprising an evacuated envelope having an ion source including a cathode electrode, at least three groups of electrodes with each group having three electrodes, the spacing between groups being large as compared to the spacing between electrodes of the groups, means for applying direct current potentials to the outer two electrodes of each group, radio frequency potential generating means for applying radio frequency potentials of differing phases to the several middle electrodes, and means for deflecting away from the collector electrode all ions that have passed said electrodes which failed to attain a predetermined energy and for measuring the flow of the ions that passed said electrodes in excess of said predetermined energy, the spacings of the middle electrodes and the phase relations of the potentials applied thereto being so related that the energy of ions of predetermined mass increases as it passes each group of electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,373,151 Taylor Apr. 10, 1945 2,387,786 Washburn Oct. 30, 1945 2,400,557 Lawlor May 21, 1946 2,450,462 Washburn Oct. 5, 1948 2,535,032 Bennett Dec. 26, 1950 2,563,626 Stein Aug. 7, 195] OTHER REFERENCES Radio Frequency Mass Spectrometer, National Bureau of Standards Technical News Bulletin, vol. 32, September 1948, pages -108.

The Production of Heavy High Speed Ions Without the Use of High Voltages, by Sloan et al., published in Physical Review, vol. 38, dated December 1, 1931, pages 2021-2032.

Radio Frequency Mass Spectrometer, by Bennett published in Journal of Applied Physics, vol. 21, dated February 1950, pages 143-149.

Recent Advances in the Production of Heavy High Speed Ions Without the Use of High Voltages, by Sloan et al., published in Physical Review, vol. 46 No. 7 (second series), dated October 1, 1934, pages 539-542.

A Mass Spectrum Analysis of the Products of Ionization by Electron Impact in Nitrogen Acetylene, Nitric Oxide, Cyanogen and Carbon Monoxide, by Tate et al., published in Physical Review, vol. 48, dated September 15, 1935, pages 525-531.

Gas Analysis with the Mass Spectrometer, by John Hipple, published in Journal of Applied Physics, vol. 13, September 1942, pages 551-559.

Ionization and Dissociation by Electron Impact: The Methyl and Ethyl Radicals, by Hipple et al., published in Physical Review, vol. 63, Nos. 3 and 4, February 1 and 15, 1943, pages 121-123.

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