US2450462A

US2450462A - Mass spectrometry

Info

Publication number: US2450462A
Application number: US508923A
Authority: US
Inventors: Harold W Washburn
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1943-11-04
Filing date: 1943-11-04
Publication date: 1948-10-05
Anticipated expiration: 1965-10-05

Description

Oct. 5, 1948. w, WASHBURN 2,450,462

Mass SPECTROMETRY Filed Nov. 4. 1943 YINVYENTOR. #12040 /I Ill/515201? A T T OFNEKS Patented Oct. 5, 1948 MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application November 4, 1943, Serial No. 508,923

This invention is concerned with mass spectrometry and provides improvements that are particularly useful in the analysi of mixtures containing organic compounds such for example as hydrocarbons, which crack under the conditions to which they are subjected in the ionization chamber of 1a massspectrometer.

This application is a continuation-in-part of my co-pending application Serial No. 451,664, filed July 20, 1942, now Patent No. 2,387,785, October 30, 1945.

4 Claims. (Cl. 25041.9)

I chamber.

Basically a mass spectrometer-is an apparatus for producing and sorting ions, and comprises an ionization chamber, an analyzer and a collector. Material to be analyzed, for example a gas mixture, is introduced into the ionization chamber from a sample chamber at relatively low pres sure. In the ionization chamber, the molecules of the mixture, or some of them, are bombarded With ionizing particles, say electrons, and converted into ions. With the aid of one or more electric fields,- the ions are propelled out of the ionization chamber into the analyzer, where they are separated into a plurality of ion beams according to their mass-to-charge ratio, i. e. their specific mass. The several ion beams are discharged separately at the collector, and the current given up in each case is measured with a galvanometer or the like to produce a mass spectrum.

A thermionic emitting element, for example a heated filament, is convenient mechanism for generating an electron beam with which to bombard the molecules in the ionization chamber, but the presence of the heated filament in the ionization chamber itself is disadvantageous in many instances, particularly when the mixture to be ionized contains compounds, such as hydrocarbons, which tend to crack. In such cases the presence of the electron beam source in the ionization chamber introduces irregularities, errors, and inconsistencies in the spectrum, .so that qualitative and quantitative analysesof the mixture are difiicult. I have found that the accuracy of the mass spectrum in such cases is increased if the heated element employed for producing the electron beam is enclosed in a separate chamber connected to the ionization chamber by a conduit (aperture) through which the electron beam is projected, means being provided for retarding the entry of gas from the separate chamber into the ionization chamber and for keeping the amount of gas so entering the ionization chamber small compared with the amount of gas entering the ionization chamber from the sample In this way, the entry into the ionization chamber of products of thermal cracking formed in the separate chamber is inhibited,and the consistency and reproducibility of the spectrum is improved, apparently because in these circumstances and'with proper conditions of temperature and pressure in the ionization chamber, electronic bombardment of the molecules of the mixture results in more regular cracking patterns. Whatever be the explanation, the fact remains that improved results are obtained if the electron beam source (say a filament) is disposed in a separate chamber which communicates with the ionization chamber through a conduit or aperture that serves as an avenue for the entry of the electron beam to the ionization chamber, the pressure in the separate chamber being low (say of the order of 10 mm. Hg) and preferably lower than the pressure in the ionization chamber.

The proper relationships of pressure in the filament chamber (electron beam source) and ionization chamber can-be obtained by (a) mak ing the pumping speed between the filament chamber and the evacuating apparatus high in comparison with the pumping speed between filament chamber and the ionization chamber and by (12) making the conduit between the two chambers very small.

Expressed in another Way PzPy Pz+Px should be large in comparison with Where Py is the pumping speed of the conduit directly connecting the filament (electron emitting) chamber with the ionization chamber;

P2 is the pumping speed of the conduit directly connecting the filament chamber with the region adjoining the ionization chamber (Fig. 1)

Pat is the pumping speed of the conduit directly connecting the ionization chamber to the region adjoining that chamber, and

Pw i the pumping speed from the region adjoining the ionization chamber to the pump.

A further beneficial result can be obtained by placing the filament at a point in the filament chamber that is remotefrom the conduit through which the electron beam (originating at the filament) enters the ionization chamber, thereby reducing the direct evaporation of cracked gas from the filament.

I believe that the benefits accruing to the practice of my invention when employing a filament (metallic or oxide coated) a an electron source, are attributable at least in part to the fact that the emitting characteristics of the surface of the filament are aifected by the presence of gas and that the presence of gas may have both long term and temporary effects. It is desirable to minimize the first and essential to minimize the secnd in order to obtain reproducibility, and the maintenance of a low pressure in the region of the filament is a step in this direction.

I have observed that in some cases a gas has a differential effect upon the emitting characteristics of a metallic filament surface, 1. e. different portions of the surface are affected clifierently, the result being that the change in density of electrons at various points across the beam is not proportional to electron density at the respective points. The positive ions which are recorded are selected from only a small portion of the beam and in the circumstances indicated, tend to be an unrepresentative sample. The practice of the invention tends to improve the representativeness of the sample by reducing the differential emitting effect at the filament.

Experiment has shown that the amount of gas getting back into the ionization chamber after having been in contact with the filament should not exceed 1% of the gas sample, i. e. that introduced into the ionization chamber from a sample bottle or other outside source. Moreover, the amount of gas which is permitted to return to the ionization chamber after contact with the filament shields or other hot metal in the filament chamber should be less than 2% of the gas sample.

These and other features of my invention will be more thoroughly understood in the light of the following detailed description, taken in conjunction with the accompanying drawings in which:

Fig. 1 is a fragmentary diagram of a mass spectrometer adapted to the practice of my invention;

Figs. 2 and 3 are longitudinal sections, taken at right angles to each other, through a preferred form of spectrometer head constructed in accordance with the invention.

Referring now to Fig. 1, it will be observed that the mass spectrometer comprises an envelope Ii) of glass, or the like, adapted to be maintained during operation at low pressure by evacuation of gas through a conduit I I b means of vacuum pumps (not shown). Within the envelope there is disposed an ionization chamber I2 connected at one end to a conduit I3 through which a sample of gas to be analyzed may be admitted. The other or outlet end of the ionization chamber is connected to an analyzer tube I4 through slits S1, S2, respectively, in two propelling electrodes I5, I6 disposed in series. The analyzer tube is semi-circular, as is the envelope, and has an exit slit IT at its outlet. An ion collector or target I8 is disposed within the envelope immediately adjacent the exit slit from the analyzer tube. The collector is connected to appropriate amplifier and recording means IS.

A pusher electrode 28 is disposed within the ionization chamber in line with the slits S1, S2. An electron gun 2|, for example an electrically heated filament and an appropriate electrode, is disposed in a chamber 22 which communicates with the ionization chamber at one side thereof through an aperture S3. A beam or stream 23 of electrons is projected by the electron gun through the aperture S3 into the ionization chamher at right angles to a line connecting the pusher electrode 20 and the slits S1, S2. The chamber 22 within which the electron gun is mounted communicates with the envelope through a slit S4.

The analyzer tube may be evacuated in part through the ion exit slit H as well as through

auxiliary openings

25, 25A, in the wall of the analyzer.

In the operation of the apparatus illustrated, a gas sample to be analyzed is introduced into the ionization chamber through the inlet conduit and the molecules of this gas sample are there bombarded by electrons of the beam. In this way, molecules of the gas sample become ionized and the resulting ions are expelled from the ionization chamber into the analyzer chamber under the influence of a pusher potential maintained between the electrode 20 and the electrode I5 and an additiona1 accelerating potential maintained between the electrode I5 and the electrode I6. In this way, a heterogeneous beam of ions is propelled from the ionization chamber into the analyzer tube. A stron magnetic field produced by an electromagnet (not shown) causes the heterogeneous ion beam entering the analyzer tube to curve and to be separated into a plurality of diverging ion beams according to the specific mass of the ions.

By varying the potentials which propel the ions into and through the analyzer tube or by varying the strength of the magnetic field or both, the radii of curvature of the several ion beams in the tube may be altered, so that the diverging beams are caused to sweep successively over the exit slit I? and impinge on the ion collector I8. The currents thus collected from the diverging ion beams are amplified and separately recorded and constitute the mass spectrum of the material undergoing analysis.

As indicated hereinbefore, the thermionic electron emitting element (i. e. the electron gun 2 I) tends to bring about pyrolysis of hydrocarbons and other organic chemicals which come in contact with the filament. Accordingly, pursuant to my invention, the thermionic electron source is enclosed in a separate chamber 22 connected to the ionization chamber through the conduit S3. Th electron beam is projected through this 0011 duit and means are provided to inhibit the passage of products of thermal cracking formed in the neighborhood of the electron emitting element into the ionization region by maintaining the pressure in the ionization chamber greater than that in the electron emission chamber.

Since in the instant case the entire apparatus is evacuated by a common vacuum pump through the conduit I I, the maintenance of a higher pressure in the ionization chamber than in the electron emission chamber requires that the gas flow resistance of the passages for evacuating the electron emission chamber shall be less than the gas flow resistance of the passages for evacuating the ionization chamber. With this relationship of gas flow resistance, a relatively high gas pres sure may be built up in the ionization chamber. This permits the attainment of high sensitivity with the instrument. At the same time irregularities in mass spectra due to products cracked in the neighborhood of the electron emitting means are, for practical purposes, eliminated.

A preferr d form of instrument head, including anfionization chamber and an electron emittin chamber, is illustrated inFigs. 2 :and3, which are longitudinal sections through the head taken at right. angles to .each other. As shown in these figures, the apparatus comprises a cylindrical head 30 attached to the end of the analyzer tube [4 mounted within the envelope I, which is adapted to be evacuated through the pumping line H. The end of the head opposite the analyzer tube is connected to the gas inlet conduit 13;

During operation, the envelope (and the head and analyzer tube which it encloses') are maintained at'relatively low pressure, the pressure in the ionization chamber being greater than that in the filament chamber or'analyzer tube. By way'of example, the analyzer tube and the filament' chamber may be maintained at 10* mm. Hg or less While the pressure in the ionization chamber is of the order of 10- or 10- mm. Hg.

To consider the head in greater detail, it will be observed that it comprises a block 35 which is a thickwalled cylinder having a circular bore that-defines an ionization chamber 36. A quartz disc 31 is securedto the inlet end of the block. A pair of

conductive pusher segments

38, 39 project through the quartz disc into the bore. A quartz plate 40 is clamped between the two pusher segments and serves to insulate them from each other and at the same time to separate the space which they enclose into a pair of parallel passages or channels 4 I, 42.

A pair of

jaws

46, 41 form the other end of the ionization chamber and are fastened to the other end of the block. They have knife edges which are spaced slightly from each other to define a first slit S1 that bisects the cylindrical head and is parallel to a gap between the ends P1, P2 of the pusher segments. A Pyrex spacer ring 50 is disposed immediately adjacent the first pair of jaws and carries a second pair of jaws 5|, 52 matching the first pair with a slit S2 therebetween that is parallel to the slit S1. The second pair of jaws is separated fromthe analyzer tube by a conductive ring 54 and a head mounting flange 55 which is fastened to the end of the anlyzer tube, and-is electrically connected to the second pair of jaws, to the analyzer tube and to ground.

The pusher segments are secured to the block by means of a pusher clamp 51, which covers a Pyrexpusher locking ring 58 and is screwed to the block through the quartz disc.

The block, the first pair of jaws, the Pyrex insulating, ring, the second pair of jaws, the conductive spacer and the head mounting flange are held together with quartz links 60. Each link has an eye that is positioned over a link stud screw Bl that projects from the side of the block. The other end ofthe ring carries a second eye which is secured to the head mounting flange by means of spring link clips52.

One side of the block is cutaway to forma space 65 within which an electron gun 66 is mounted. The space in which the gun is mounted is enclosed on one end by the quartz disc, on the inside by the wall of the block, and on the other end and outside by an L-shaped shield 61 of metal. This shield is spaced from the quartz block by an evacuation gap or slot 68.

The electron gun comprises a filament '10 adapted to be heated by electric current and partly enclosed by a box-shaped metallic shield H. At the level of the filament there is a bore 1'2 through the wall of the block into the ioniza This bore carries an apertured: barrel or insert 13. An electron propelling election chamber.

' block.

In line with the barrel of the electron gun and on the opposite side of the block is a bore 801 through which electrons of the beam pass to an electron catcher 8| that is disposed within a second bore 82 at right angles to the bore '80.

As indicated hereinbefore, the apparatus is so constructedthat the gap between the pusher segments, the axis of the electron beam, and the axes of the slits S1 and S2 are all parallel to each other and also parallel to lines of force of a magnetic field in which the assembly is disposed.

Considered from an electrical standpoint, the block and the first pair of jaws that define the slit Sr represent a first-ion accelerating electrode. The jaws forming thesecond slit comprise a second ion accelerating electrode, whereas the two pusher segments are pusher electrodes. An electrical potential is impressed between the pusher electrodes and the first accelerating electrode,

and the second electrical potential is impressed between thefirst and second accelerating electrodes;

The voltages of the filament, of the electron accelerating electrode, of the pusher electrodes,

and of the first ion accelerating electrode may be controlled independently. In normal operation when analyzing for positive ions, all of the above are maintained at high potentials with respect to thesecond -ion accelerating electrode which, as

indicated hereinbefore, is at ground.

A thermo-couple 90 is attached to the side block for measuring the block temperature.

A gas mixture to be analyzed is introduced into the ionization chamber through the channels between the pusher segments. Within the ionization chamber the molecules of' gas are bombarded by the electron beam and some of them are ionized. The resulting ions are pushed through the slit Si by the pusher potential and further accelerated by additional potential through the slit S2 and thence into the analyzer tube. Thus, a heterogeneous'ion beam is propelled into the analyzer tube and is there separated into a plurality of homogeneous ion beams which are collected separately to form the mass spectrum.

Some of the gas introduced into the ionization chamber is not ionized. This gas for the most partflows through the slit S1 and thence through ports (not shown) in the jaws 5|, 52 into the analyzer tube. From the analyzer tube this gas escapes into the envelope through ports 92 in the wall of the analyzer tube. From this point the gas is evacuated through the pumping line H.

Some of the gas flows through the barrel of the electron gun into the region of the filament.

From this point the gas (either unchanged or in a cracked condition) is in large part withdrawn through the evacuation gap or slot 68 into the envelope and thence through the pumping line.

There are cracks in the box-shaped filament shield which permit gases to flow from the region of the filament through the exit slot 68.

As indicated previously, it is desirable from the standpoint of accuracy and reproducibility of the mass spectrometer to make sure that the passage of particles other than electrons into the ionization chamber through the electron gun barrel is inhibited by making the pressure in the space enclosing the electron gun less than the pressure in the ionization chamber itself.

The pressure differential between the ionization chamber and the electron gun chamber is maintained in the apparatus of Figs. 2 and 3 by assuring that the avenue of escape of gases from the electron gun chamber into the pumping line, offers less resistance than does the avenue of the escape of gases from the ionization chamber, this being accomplished by properly proportioning the pumping speeds or resistances along the respective avenues, as described hereinbefore.

I claim:

1. In a mass spectrometer having an ionization chamber, a sample chamber, means for introducing a material to be analyzed into the ionization chamber from the sample chamber, means for shooting an electron beam against molecules of the material in the ionization chamber, an analyzer connected to the ionization chamber through a restricted passage, and means for expelling ions formed from the material from the ionization chamber into the analyzer, the combination which comprises an electron producer chamber, means disposed in that chamber for producing an electron beam, a conduit connected between the electron producer chamber and the ionization chamber for admitting the beam to the latter, said conduit acting to restrict the flow of gas from the ionization chamber to the electron producer chamber, and a pump for evacuating the electron producer chamber, the ionization chamher and the analyzer, said pump being connected to the electron producer chamber and the analyzer so that gas is drawn from the ionization chamber into both the electron producer chamber and the analyzer.

2. In a mass spectrometer having an ionization chamber, means for introducing a material to be analyzed into the ionization chamber, means for shooting an electron beam against molecules of the material in the ionization chamber, an analyzer connected to the ionization chamber, and means for expelling ions formed from the material from the ionization chamber into the analyzer, the combination which comprises an electron chamber, means disposed in that chamber for producing an electron beam, a conduit connected between the electron producer chamber and the ionization chamber for admit-ting the electron beam to the latter, means for directly evacuating the electron producer chamber and the analyzer, and a restriction in the conduit between the electron producer chamber and the ionization chamber and a restriction in the means for expelling ions into the analyzer for retarding the flow of gas from the ionization chamber to the electron producer chamber and the analyzer, the arrangement being such that the ionization chamber is evacuated through the electron producer chamber and the analyzer.

3. In a mass spectrometer having an ionization chamber, means for introducing a material to be analyzed into the ionization chamber, means for shooting an electron beam against molecules of the material in the ionization chamber, an analyzer connected to the ionization chamber, and means for expelling ion-s formed from the material from the ionization chamber into the analyzer, the combination which comprises an electron producer chamber, means disposed in that chamber for producing an electron beam, a conduit connected between the electron producer chamber and the ionization chamber for admitting the electron beam to the latter, and a pump connected to evacuate both the electron chamber and the analyzer, a second conduit between the ionization chamber and the analyzer for allowing ions to pass in to the analyzer, both of said conduits having a restricted opening so that when the pump is operating, the evacuation of the electron producer chamber and the analyzer draws gas from the ionization chamber and causes it to be at a high-er pressure than the analyzer and the electron producer chamber.

4. In a mass spectrometer having an ionization chamber, means for admitting a material to be analyzed into the ionization chamber, an analyzer connected to the ionization chamber, and means for expelling ions formed from the material from the ionization chamber into the analyzer, the combination which comprises an electron source, a restricted passage disposed between the electron source and the ionization chamber and through which a beam of electrons from the source is shot into the chamber, a second restricted passage between the ionization. chamber and the analyzer through which the ions are expelled, a pump connected to the analyzer for evacuating it and also connected to the region of the electron source, so that gas is drawn from the ionization chamber simultaneously through both restricted passages. HAROLD W. XNASHBURN.

REFERENCES CITED The following references are of record in the file of this patent:

Technical publication A Mass Spectrum Analysis of the Products'of Ionization by Electron Impact in Nitrogen, Acetylene, Nitric Oxide, Cyanogen and Carbon Monoxide, by John 'I'. Tate, 'P. T. Smith and A. L. Vaughan. Physical Review, Vol. 48, Sept. 15, 1935, pages 525-531.

Technical publication A Mass Spectrometer for Routine Isotope Abundance Measurements, by Alfred 0. Nier, Review of Scientific Instruments, vol. 11, July 1940, pages 212-216.