US2829259A

US2829259A - Mass spectrometer

Info

Publication number: US2829259A
Application number: US449807A
Authority: US
Inventors: Samuel N Foner; Richard L Hudson
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-08-13
Filing date: 1954-08-13
Publication date: 1958-04-01
Anticipated expiration: 1975-04-01

Description

April 1 .1958 s.v N. FON-ER ET AL, y 2,829,259,

MASS SPECTROMETER RECORDER F/G. I

SAMUEL N. FONEI'? RICHARD L. HUDSON INVENTRS BY jfmww ATTORNEYS April l, 1958 s. N. PONER ET AL l 2,829,259

MAss 'SPECTROMETER Filed Aug. 13, 1954' 2 Sheets-Sheet 2 T0 PHASE SENSITIVE DETECTOR SAMUEL N. FONER RICHARD L. HUDSN 1N VENTORS BY @www- ATTRNEYS United States Patent O e 2,829,259 MAss sPECrRoMETER Samuel N. Feuer, Silver Spring, and Richard L. Hudson,

Rockville, Md., assignors to the United States of America as represented by the Secretary of the Navy Application August 13, 1954, Serial No. 449,807

Claims. (Cl. Z50-41.9)

The present invention relates to improvements in mass spectrometers used for analyzing gases. In more detail, the invention relates to a mass spectrometer particularly useful for detecting short lived intermediates which are formed in chain processes such as occur in the case of combustion reactions.

The detection of reaction intermediates, such as the atoms and radicals formed in Chemical reactions, has long been a problem because of the short life ofsuch intermediates and their low concentration levelswhen compared with the background signals generated in spectrometers. the pumps used to maintain low pressure levels in the gas sampling system and the ion source are inefficient and do.

not remove all extraneous matter from the source. Thus, traces of oxygen, nitrogen, and other elements are normally present in the ion source, together with the radical or atom 'which it is desired to detect. When an oil diffusion pump is used to evacuate the source, the oil is also the origin of vcertain background signals. The pyrolysis products generated by a lament which is the source of electrons used to ionize the gas molecules, also provide background noise. with the hot filament are particularly annoying in the case of experiments withy atoms and radicals. A typical ex ample vis `carbon monoxide which is invariablyformed by the reaction which takes place at the filament when oxygen is in the system. Diftusely scattered and reacted molecules also seriously limit the useful sensitivity of prior art spectrometers.

The present invention hasl for its objectto provide a mass spectrometer'in which the ratio of signal intensity to background intensity is much greater thanthat of prior systems, thus making possible the detection of reaction intermediates.

A further object of the present invention is to provide a mass spectrometer with a much higher effective sensitivity due to the improved signal to noise ratio than has been attainable with prior spectrometers.

Still another object of the present invention is the provision of a mass spectrometer having an'improved gas sampling system by which the gas molecules can be quickly extracted from the reaction chamber and introduced into the ion source free of collision with the walls of the sampling system. .t I

A more specific object of the invention is the provision 0f a mass spectrometer including means for modulating` the molecular beam and employing'phase detection ofthe ion current to improve the signal to noiseratio.

Other objects and many of the attendant advantages of this invention will be appreciated readily as the same Fig; Zevis a view, partly diagrammatic and partly seci In part, these background signals arise becausel The products of reaction of the gas y electromechanical modulator.

2,829,259 Patented Apr. l, 1958 ICC tional, onanenlarged scale, showing the details of one aspect of the system shown in Fig. l; and, Fig. 3 is a schematic diagram showing some of the details of the gas sampling system and ion source that are not shown in Fig. l. i Briey, the present invention comprises a gas sampling system by which molecules are extracted quickly and directed along a collision-free path to the ion source. To achieve this, a gas sampling system comprising a plu-y rality of successive orificed chambers is provided, the orifices of the chambers being in alignment. Diffusion pumps are individually provided to evacuate the different successive chambers of the sampling system. The molecular beam provided by this sampling system is coaxial with an electron beam provided in the ion source, thereby increasing the probability of ionizing the gas contained in the molecular beam. In one chamber of the gas sampling system, an electromechanical device' is provided for periodically interrupting the molecular beam, thereby supplying a modulated molecular beam to the ion source. The ions formed by the interaction of the molecular beam and the bombarding electrons are passed through a conventional mass analyzer in which they are subjected to a magnetic field controlled in magnitude and direction for directing the ions to a suitable detector. The ion current detection system includes an electrometer amplifier and an electron multiplier which may be selectively connected to the mass analyzer to measure the ion current and a phase sensitive detector which is synchronized with a reference signal derived from the driving current of the Ion current signals which have random phase with respect to the modulator are` reship to the filament 41.

jected by this detecting system as noise. the phase sensitive detector is then supplied to a suitable recorder, thus completing the massspectrometer of the present invention.

yIn more detail, referring to the drawings, it will be seen that the gas sampling system 10 of the present invention includes a chamber 12 in kwhicha chemical reac tion takes place. The gases formed by the reaction are led in a beam from chamber 12 through a suitable sam-- pling orifice 14 intoa chamber 16. A mechanical modulator or chopper 18 is positioned to interrupt the molecular beam led from chamber 12, at a frequency which is` preferably the resonant frequency of the chopper itself;

The chopper is actuated by a suitable chopper driver 20 to which oscillatory currents are supplied by an amplitude l.

stabilized oscillator 22. As will be described later, a

pickup may be positioned in chamber 16 for detecting theI vibrations of the chopper 18 and providing a feedback signal utilized by the oscillator.

pleting the gas sampling system. As can be seen in Fig. 3, the`

chambers

16 and 28 are provided with connections 34 for attachingl diffusion pumps, not shown, which evacuate the chambers.

rl`he modulated `molecular beam is thus conducted through the slit 30 into chamber 3-6 which houses an ion source generally indicated by reference numeral 38.

ly to Fig. 3, a filament 41 serves as a source of electrons,l and an accelerating grid 42 is provided adjacent the source. For controlling the electrons, an auxiliary grid 43 is arranged at each end of the chamber 40 andl an electron collector 44 is provided in spaced relation? Of course, suitable potentials '-'f n are applied to the grids and the collector, and in addi@ The output of tion,aimagneticitield from asource not'shown is provided for collimating the electrons from the filament 41. The

molecular beam is coaxial with the electron beam formed in -the `ion-source thereby increasing the probability "of ionizing the incoming gas molecules. -T he `ions formed inthe chambrl40 are expelledby a transverse electric field throu`gh`aslit'50 formed inthe wall ofsaid chamber and into `the tube 52 which is a part of the mass analyzer genallyindicated by `reference `character 54.

Asisconventionah an electromagnet 55 is arranged adjacent ito the tube 52 forproviding a magnetic eld to control "the path ofthe ions emitted Vfrom `the ion source v38. The ions Vpassing through the tube 52are focusedona 'slit 56 by the magnetic field of the electromagnet 55 *8nd, by varying the strength of the field, ions of different mas'sf'can' be caused `to enter the chamber 581whichhou`sesmeanstocollect the ions or pass them `on 'to `a suitable detector. A gate `69 is pivotally mounted Vin the chamber158 'and may bemoved from a position `shown in dotted lines to full line position by an Yexternal operating mechanism (not shown). When in the ldotted line position, "the gateA conducts the ion current tothe inputterminals-ofan electrometer amplierh64 4of conventional t`y'pe. `On the `other hand, when the gate60 is in the full linepositiom the ions will pass between electrodes 65 and AA66,"th`rough"opening 68, and into yan electron multiplier `detector 70, which is also of conventional structurep `Whichever is selected, the electrometer amplifier 64 or the electron multiplier detector 70,'can `be connected by `a two position switch 72 "to a phase sensitive detector74. Either of these devices 64 or detects and `arripliies the ion current flowing through`the-slot 56 of the mass analyzer 54. The electrometer `amplifier is particularly useful when it is desired to supply an :amplifier signal representing ion current to a recorder bran oscilloscope for immediate visual examination. The electron multiplier detector is particularly suited in other applications, as for example, where it is desired to use an ion counter or similar device.

The 'swtch`72 will conduct the signal representing the detected ion current to a phase sensitive detector 74. A reference `signal'for the phase sensitive detector is supplied from`the `oscillator 22 'which provides the driving current `to the chopper driver`20. The phase sensitive detector 74 detects uonlyithat partof thesignal representin'g `iou curret Which is in phase with the vibrations of the 'chopper modulator `8; The output of thedete'ctor 74 isfedto'a recorder 78.

The :details ofthe chopper and the associated equip ment for modulating the molecular beam are shown in Fig. 2. The chamber 1'6 of the gas sampling system is shown as being generally rectangular in cross `section and having an opening 79 in its top wall. A parot apertured lugs 80 and 82 lp'rovided on the opposite side walls of the :chamber 16 slidably receive `

posts

84 and 86,uv respectively. `At their upper ends the

posts

84 and 86 supporta plate 88 which carries the chopper 18 and a driving mechanism generally indicated by reference numeral 90.` The plate V88 is provided with slots for receiving the upper ends of the

posts

84 and 86, and screws I 92 and 94 threaded in apertures providedin a `rim 96 on the plate 88 permit lateral adjustment of the chopper 18 `so thatthe latter interrupts the molecular beam in such amanner as to provide a symmetrical waveform when the ioncurrent'subsequently formed is detected.

A bellows surrounds the opening 79 provided in the topwall of chamber `16, the lower end of the bellows being attached to said top wall and the upper end there of being attached to the, underside of the supporting plate The chopper 18 is inthe nature of a vibrating reed type `of apparatusand includes an elongated, partially` hollow section 104 which terminates at the lower end in a broad `portion -106 positioned to periodically in- `terrupt the molecular beam passing through orilice 26.

The` hollow section 104 receives, in its upper end, the lower end 113 of a rod-shaped element 114 which is made of suitable magnetic material such as soft iron, said lower end 113 being machined to t snugly inside the hollow section 104. The periphery of the rod-shaped section 114 of the chopper 18 is notched as shown at 111 and 112 so that the chopper 18 vibrates about this point as a node. At its upper end, the chopper 18 has an enlarged section 108 which is received snugly in a sleeve member integral with the suporting plate 8S so that the chamber 16 is sealed.

The chopper driving mechanism 90 actuates the chopper 18, preferably at Aits fundamental frequency. The driver includes a coil which partially ills the space provided internal to a permanent magnet yoke 122, suitably polarized as indicated. The yoke 122 is provided with

apertures

123 and 123 which snugly receive the enlarged section 108 of the chopper 18 and the sleeve 110 formed on plate 88, respectively. Potting compound indicated by

reference numerals

124 and 126 iills the space between the coil and the permanent magnet 122, and the space between said permanent magnet and an external shell 128, respectively. An oscillator is coupled by a transformer 132 to the coil 120, to supply operating current thereto.

To provide the required phase relationship between the vibrations of the chopper and the oscillatory currents used to drive the chopper, it may be desirable to employ a feedback arrangement including a capacity type pickup 134 `positioned adjacent the chopper and a preamplifier 135 inserted between the pickup and the oscillator 130. The capacity pickup 134 includes a plate 138 carried at the lower end of a rod 1.40 of conducting material, the rod being supported in an insulating seal 142 in the plate 88. An electrical lead 144 having a coupling condenser 145 inserted therein interconnects the plate 138 and the` grid 146 of a triode vacuum tube 148, which isconnected as a cathode follower. A source of positive voltage is connected directly to the plate of tube 148 and through a current limiting resistor 152 to the plate 138 of the capacity pickup 134. A blocking capacitor 154 is connected between the source of positive voltage and ground, and an electrical lead 156 connected to the plate 88 completes the interconnection of the capacity pickup and the preamplifier 135.

For providing a suitable bias, the grid 146 is connected to the junction of

resistors

158 and 160 which comprise a voltage divider connected between the source of` vpositivevoltage and ground. Bias for the cathode 162 is provided by resistor 164 and an. output connection 166 is Imade to the. cathode 162. With the choice of the proper circuit components, the output of the preamplier 135 is fed to the oscillator 138 in the proper phase for maintaining oscillations. The oscillations thus generated are coupled by the transformer 132 to the chopper driver 90, causing the chopper to vibrate. A second transformer 170 is used to provide a reference signal for the phase sensitive detector 74.

In operation, a chemical reaction is initiated in the chamber 12. Typical reactions are the burning of hydrogen in oxygen, and methane in oxygen. A sample of the gas generated by the reaction flows through the orifice 14. The molecular beam thus formed is modu lated by the mechanical chopper 18 at a frequency determined by the characteristics of the chopper and the system for driving the chopper. The central portion of the modulated beam passes through the orice 26 into chamber 28 and thence through orifice 30 into the chamber 40 in the ion source 38. In the ion chamber, ions result from the interaction of a beam of electrons from the filament 41 and the molecules entering the chamber, the electron beam being formed and controlled by a collimating magnetic field and an electrostatic eld in a manner well known in the art. A transverse electric iield provided in the ion source expels the ions from the source into a conventional mass analyzer 54. By varying the strength of the magnetic teld associated with the analyzer, ions of different mass can be caused to pass through the slit S6 into chamber 58. The gate 60 permits the selection of either the electrometer amplifier 64 or the electron multiplier detector 70 for detecting the total ion current passing through the slit S6. The iselected one of these devices is connected through a switch 72 to the phase sensitive detector, thereby supplying a signal including7 background and the ion current which it is desired to measure'to the phase sensitive detector. The reference signal for the phase sensitive detector is provided by the oscillator employed to drive the chopper. The phase relationships are adjusted so that the background is rejected, only that part of the unknown signal due to the ion current which is in phase with the vibrations oi' the chopper being detected.

It is clear therefore that the signal to noise ratio is much greater in the present mass lspectrometer than in prior spectrometers. The background due to reacted molecules, molecules that have collided with the walls of the chamber, and other sources discussed above, is substantially eliminated by this invention.

Experiments have been conducted to determine the degree of background discrimination actually realized by this technique. An argon beam from the llame chamber which gave a detector signal of three millivolts, corresponding to about 400 ions per second, was observed while a background pressure of argon was developed in the ion source by admitting argon directly. The modulated beam could be detected in the presence of a 40 volt D. C. background signal so that a discrimination factor of approximately 104 was obtained. In the case where the background arises from scattered beam molecules, including decomposition products formed at the filament, the collimation of the molecular beam introduces an additional discrimination estimated to be a factor of 10 because the molecular density in the beam s about 10 times the scattered molecular density in the apparatus. Other experiments were performed with gases such as hydrogen and oxygen which are known to react on the filament to produce hydrogen atoms and carbon monoxide, respectively, to see if pyrolysis products would be observed with the chopped beam method. No indication of decomposition products was observed.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A mass `spectrometer for analyzing a gas, comprising means for forming a beam of molecules of the gas, means for modulatingsaid molecular beam, means for forming ions from said molecular beam, a mass analyzer through which the ions thus formed are passed, and means for detecting that part of the ion current passed through said analyzer which is snychronized with the modulation of said molecular beam.

2. A mass spectrometer for detecting atoms and radicals formed in chemical reactions, comprising a reaction chamber, means for extracting a beam of molecules of the gas formed in the reaction chamber from said chamber, means adjacent the reaction chamber for modulating said molecular beam, means for forming ions from the modulated molecular beam, a mass analyzer `through which the ions thus formed are passed, and means for detecting that part of the ion current passed through the analyzer which is synchronized with the modulation of said molecular beam.

3. A mass spectrometer for detecting atoms and radicals formed in a chemical reaction, comprising a reaction chamber having a sampling oriiice in one wall thereof, said orifice emitting a beam 'of molecules of the gas formed in said chamber, means adjacent said reaction chamber for periodically varying the concentration lof molecules in said beam, means to which said modulated beam is supplied for forming ions from said beam, a mass analyzer through which said ions are passed, and means for detecting that part of the ion current passed through said analyzer which is in synchronization with the periodic variations of concentration of said beam.

4. A mass spectrometer for detecting atoms and radicals formed in a chemical reaction, comprising a reaction chamber, said reaction chamber having an orifice for emitting a collimated beam of molecules of the gas in the reaction chamber, means positioned adjacent said reaction chamber for periodically interrupting said molecular beam, means for energizing said last named means, means for forming ions from said molecular beam, a mass analyzer through which said ions are passed, and a phase sensitive detector to which a signal representing the ion current passed through said mass analyzer is supplied, the reference signal for said phase sensitive detector being in phase with the periodic interruptions of said molecular beam, whereby only that part of the ion current which is synchronized with said interruptions is detected.

5. A mass spectrometer for detecting atoms and radicals in a chemical reaction, comprising a reaction chamber having an orice for emitting a collimated beam of molecules of the gas formed in said chamber, means positioned adjacent said chamber for periodically interrupting said beam of molecules, an amplitude stabilized oscillator for energizing said interrupting means, means for forming ions from the molecular beam, a mass analyzer through which said ions are passed, means for detecting the ion current passed through said analyzer, and a phase sensitive detector to which a signal representing the ion current passedthrough said analyzer is fed, the reference signal for said phase sensitive detector vbeing supplied from the amplitude stabilized oscillator whereby only that part of the ion current which is in synchronism with the interruptions of said molecular beam is detected.

References Cited in the tile of this patent UNITED STATES PATENTS 2,221,467 Bleakney Nov. 12, 1940 2,457,162 Langmuir Dec. 28, 1948 2,694,151 Berry Nov. 9. 1954