US2672559A - Mass spectrometry - Google Patents

Description

March 16, 1954 P. s. GOODWIN MAss SPECTROMETRY Filed Jan. 19, 1951 PAUL GOODWIN AT TORNE Y Patented Mar. 16, 1954 UNITED .STATES 'i OFF ICE MAss SPECTROMETRY Application January 19, 1951, Serial No. 206,777

-8 Claims. (Cl. 250-41.9)

This invention is concerned wthmass ,spectrometry and has :particularly to do Hwith the ion collection system of a mass spectrometer. .'Ihe invention adapts the spectrometer lin `Which 'it is incorporated to be used both VAfor -mixtureanalysis with automatic attenuation and isotope ratio analysis,the changeover'being carried out Without physicallyclisturbing the collection system.

The principal of mass vspectrometry is lin general one of ionizinga sample -to be analyzed, as by anelectron beam, segregating ions in accordance with their mass-to-charge ratio by inducing' spatial separation thereof, and .selectively -discharging as at a collector electrode, ions of a given 1mass-to-charge ratio. A"The current developed by discharge of va group or -beam of ions of the same mass-to-charge -ratio is proportional to the partial pressure of the 'particles in the -original sample from which these particular ions 'Were derived. Hence, a method is afforded Afor calculating the concentrationofthese particles-ortheir parent molecules in the sample.

Inanalyzinga sample for morethan one component, lthe segregated ion beams Aconstituting a part orall of the mass-spectrum ofthe particular sample are successively focused on the collector electrode so that a plurality vof separate discharge currents are obtained, each being proportional to .the number of ions .in a particular beam. The `mass spectrum is scanned in thisffashion by varying one or more of the parameters aectingthe spatial separation `of the ions of differing `massto-charge ratio. In some instances electricrscanyning is preferred, in :which case propelling potentials are varied to effectuate sweep of .the-ion beams across a so-called fresolving slit giving access to the collectorelectrode.

In other instances magnetic scanning is preferred, in which case the transverse magnetic `field established across the spectrometer is varied in intensitytn bring about the same sweep of the ion beams lacross the resolving slit.

heights are determined by the amplifier-recorder sensitivity aswell as'-by-thenumbe1-o ions-of a 1' given mass-to-charge ratio represented by the given peak.

Because ofthe wide differences in the number oi ions Which may be encountered in dilerent beams as a result of large concentration spreadsinthe original sample, it is necessary when using a single channel recorder to provide means forv'arying the sensitivity of the recorder so as -to adapt it to this Wide variation in ion abundance. If such provision is not made, vthe less `abundant ions of a given spectrum willnot develop a peak of suflicient height on a record having a full scale sensitivity such as to accommodate the most abundant ions. This principle has been recognized in the prior art and various so-called ianticipator circuits have been suggested iforanticipating the ion abundance in'succeeding beams of ythe spectrum and suitably-adjusting thesensitivity of the recorder in advance of each separate peak.

in general these anticipator lcircuits employ .a double collector or target system. r"he 'tvvoion targets are 'so arrangedtha't a so-calledfauxiliary or anticipator target receives the full ioncurrent signal in advance of the collector targetjthe resovlution of the instrument being 'such that the auxiliary or anticicator targetsenses on-a particular ion beam ybefore this beam strikes the collector target. The anticipator signal isampliiied and fed to a calculating device which selects the optimum attenuation range to be used in recording the data when the same ion beam is 'focused on the main collector. The informationas to the proper attenuation range is delivered from the .calculator to the main -recording channel fat a Apre-selected time intermediate the lrecording of succeeding peaks. Such anticipator or attenuating circuits are described vin my co-pending ap;- plications Serial No. 32,337, Ifiled March 1,9, :1949, vand Serial No. 182,691, filed September l, 1950.

Mass spectrometers have also r'been adapted vto measurement of isotope ratios again by inclusion of a pair of collector electrodes. In this application one of the two electrodes is Areferred to asa low current electrode and at this electrode thelion lbeam derived from molecules vof a low abundance :isotope is discharged. Theother electrode `is referred to as ythe high current electrode andthe ion beam ofthemore abundant isotope'is focused land discharged at this electrode. In the majority of instances the less abundant isotopes are 'o'f higher mass and hence the low current or main target electrode must lie on a larger radi-ustha'n the high cur-rent electrode irrespective of direction ofvbeam sweep inthe instrument. For-example, consider the analysis of carbon dioxide to determine the relative abundance of the C.33 and C12 isotopes. C12 being the most abundant isotope, the C0244 ions will be focused on the high current or auxiliary collector and the C0245 ions will be focused on the low current or main collector.

In isotope ratio analysis it is generally desired to measure the ratio of the ion currents rather than the absolute concentrations of the ccmponents. Accordingly, the ampliiication and recording circuit connected to the two collector electrodes is frequently arranged as a null balance network wherein the current developed at one collector is effectively divided by the other to give a recorded value representative of the indicated ratio. A network oi this character is illustrated and described in some detail in U. S. Patent 2,456,426 issued to Alfred O. C. Nier on December 14, 1948, and in co-pending application Serial No. 104,030, iiled by Robert L. Sink on July 11, 1949.

Either in automatic attenuation or isotope ratio application a mass spectrometer with a main target or collecting electrode and an auxiliary electrode is required with each electrode being connected to separate amplification means. However, the same two electrodes are not adaptable both to attenuated mixture analysis and isotope measurement for the reasons (1) different resolution is required in each instance, and (2) for most eiicient operation the auxiliary electrode should be on opposite sides of the main collector in the two circumstances, interchangeability of function between the main electrode and the auxiliary electrode being impossible because of the problem of resolving power.

For automatic attenuation the resolution required is that necessary to distinguish between adjacent masses at the upper end of the mass range bearing in mind that spatial separation of ion beams of one mass unit difference is inversely proportional to the mass. This requirement insures that only a single ion beam will fully strike either collector electrodeat a given time. For isotope ratio measurements the ideal resolving power is that which will resolve adjacent masses only at the lowest extremity of the expectable mass range. A higher degree of resolution would preclude measurement of hydrogen-deuterium ratios for example. Hence a dual collector system for isotope ratio measurements is not satisfactory for automatic attenuation application. This lack of interchangeability is totally independent of any diierences in external circuitry required for the two operations, it being otherwise a relatively simple matter to interconnect the two collector amplifiers to alternately develop a voltage ratio or to vary recorder sensitivity responsive to the output of one of the amplifiers` As to the orientation of the auxiliary collector in each instance, this problem is affected by the general practice of scanning spectra from low mass to high mass. This practice is dictated by two considerations. One of these is the relatively more rapid depletion of low mass components of a sample because of the molecular' ow characteristics of the system and the other is the difficulty of arriving at the proper initial accelerating voltages to focus the highest mass of a given sample when that mass is not known. Both of these factors therefore, dictate the practice of scanning from low mass to highmass. For this reason the anticipator or auxiliary target must be located at a greater distance from the center of curvature than the main collector for automatic attenuation and at a lesser distance for isotope measurement, bearing in mind that the greater the mass the larger the radius of curvature at a given accelerating voltage and that the most abundant isotope of a given element is generally that of lowest mass or more specically of lower mass than the rare isotope which is of interest.

At the present time the conversion of a mass spectrometer from one mode of operation to the other, i. e. from mixture analysis with automatic attenuation to isotope ratio analysis, is impractical in requiring removal of one collector system and replacement of another for the several reasons given above. When the whole collector system has to be thus replaced to change the mode of operation, the vacuum system must be broken down and then re-established. Such a process is sufliciently costly in both time and money to be substantially prohibitive on a periodic basis.

I have now developed a collection system which is suitable for both types of operation and which is adapted to be connected to associated amplification and recording channels of the types described in the aforementioned co-pending applications for carrying on either type of analysis. My invention contemplates in a mass spectrometer having an ion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer chamber and for sorting the ions in the analyzer chamber. The combination comprising a barrier electrode and iirst, second and third collector electrodes arranged in successively spaced relationship adjacent the outlet end of the analyzer chamber, the barrier electrode having an aperture therein forming a iirst resolving slit, the rst collector electrode having a narrower aperture in oficenter alignment with the barrier aperture and forming a second resolving slit, the second collector electrode being positioned to mask a portion of the aperture in the rst collector and forming therewith a third effective resolving slit, the third collector electrode being positioned to receive ions passing through this third resolving slit, and amplification means for amplifying the ion discharge signals at the several collector electrodes.

The feature which makes the present invention possible is the independent determination of the resolving power of the instrument with respect to isotope ratio and automatic attenuation application. The iirst collector electrode, above referred to, is connected to the auxiliary amplifier for mixture analysis, the resolving power being determined by the dimensions of the barrier aperture and the extent to which the aperture is masked by this electrode. The second collector electrode is connected in this circumstance at a reference potential substantially equal to that of the first and third collector electrodes acting as a shield. For isotope ratio measurements the reverse connection of the rst and second collector electrodes is employed and resolution in this instance is a function of theA size of the second resolving slit formed by the aperture in the first collector electrode and the extent to which this aperture is masked by the second collector. As a consequence, the resolving power of the mass spectrOmCter may be predetermined and maybe independently set-dependinguuponithefuseto which the spectrometer is to be put.

The invention `willlbe more .clearly understood by reference to the laccompanyingidrawing Awhich shows diagrammatically c. conventional 180"2 mass spectrometer provided with a collection system in vaccordance with the invention.

`Referring to the drawing, the mass spectrometer there shown comprises an ion Vsource Iift, analyzer chamber I2 and a collector system T4, all enclosed within an envelope Sift. Vrlfhe `envelope may be evacuated through a `conduit Jl connected to evacuating means (not shown). Communicating apertures l-S between the analyzer chamber and -envelope enable evacuation of the 4analyzer chamber and ion vsource through the envelope. In some Ainstruments the .envelope Ii `is omitted by making the tube l2 vacuum tightand'byextending it to enclose thecollection system.

The ion source is provided with -an :inletconduit for receiving a sample of gas 'to ibeanalyzed, the -gas being ionized in the ion Isource by an electron beam '22 developed at a source not shown. The ions are propelled Zfrom the ionization chamber under the inuence of l-a potential established between a pusher electrode 2'4, a rst accelerating electrode 2,5 and a Aterminal accelerating-electrode 25, the last two-electrodes being apertured to pass and collimatethe ion beam. Under the inuence of this potential established betweenthe electrodes and the source of ion formation, ions are propelled from :the point of formation at the electron beam as va thin heterogeneous ribbon or beam B into the analyzer atube, where Aunder lthe in'fiuence ofthe magnetic eld, above mentioned, the heterogeneous beam B separates into a plurality of beams B1, B2, B3, each consisting of ions of a given mass-to-charge ratio differing from the mass-.to-charge ratio of kthe ions in the other beams.

A collector .system I4, in accordance with the invention includes a barrier 2li `which .may form the end of the .analyzer chamber as shown -or which may be spaced therefrom. The barrier has an aperture 28A through which ions are projected from the analyzer tube and forming a rst resolving slit. The spectra may be scanned by sweeping the `several vbeams vacross the aperture 2BA either by Variation ofthe propelling potentials in the ion source or Vby `variation of the magnetic held.

A circuit is shown diagrammatically for supplying the necessary'potentials to the ion'source electrodes and for Varying these potentials lto scan the ion spectra. This circuit includes a battery or voltage source 3e, a slidewire :resistor 32 connected across the source and through a tapand switch 34 across a capacitor 35. A second slidewire resistor 26 is connected across vthe capacitor 35 with one end `of this resistor' :being at the potential of the negative side of the source 3U, Vin this case ground. Pusher electrode 24 is connected toward the positive end of slidewire 36 through a slider 33, the terminal accelerating electrode 26 is connectedto the grounded end of the resistor 35 and the :iirst propelling electrode is .connected through a slider 40 to an 'inter-- mediate point on the resistor 36.

'Io scan the spectrum, switch 34 is closed allowing the capacitor to build up a charge. The switch 34 is then opened andthe charge on the capacitor 35 is Aallowed-to discharge-'across the resistor `36.' The potentials -between the grounded shield 44, .the electrode '42 and shield` 44 being conventional Vand forming no vpart -of the `invention apart from their cooperative Vrelationshipfwith the rest yof .the `collection system. A rst auxiliary lelectrode 4.6 is .mounted fadjacent `the barrier "28 andl has an `aperture 46A therein narrower than and in off-center alignmentwith the aperture 28A .in the barrier and forming fa second resolving slit. VFor reasons which `will shortly become apparent, 'the aperture 46A is displaced toward the centers of curvature of the ion beams so that thefelectrode 145 masks the rst resolving slit (aperture 23A) Valong its edge vremote .from the centers of curvature. Immediately following the auxiliary electrode .lili in `sequential vrelationship is a second auxiliary electrode '48 having an -aperture 48A therein narrower'than the aperture '46A in the rst auxiliary electrode and oriented so that the electrode S43 masks the second resolving slit (aperture 46A) along its edge proximate to the centers of 'ion beam curvature. Electrode y415! need not be an aperturedelectrode as shown and may comprise simply a plate extending inwardly to maskraperture 46A as described, the `alignment of the several electrodes being vfacilitated by such construction. Moreover, both of `electrodes '46 and 48 may simply be D shaped members projecting from opposite sides vtowards 'the axis of symmetry of the collection chamber. 'In such event these velectrodes may be co-planar or may even be reversed in relative position without -fin any way aiecting the results achieved.

In preferred `practice a shield electrode 5B and a metastable suppressor 52 are disposed between the relectrode 48 and shield electrode 44. The shield electrode 50 lis at ground as is the shield "44, land the metastable suppressor electrode 52 is-connected through a tap 54 to a voltage divider 5e connected between taps 58 and 40 -of the resistor 3S. The potential of the metastable suppressor is thus at a potential 'inte-r mediate that of pusher electrode 24 and yirst accelerating electrode `25 of the ion source or, in general, at a potential approximately equal to that maintained in the region of ion formation. If the spectra is scanned by variation of the potentials in the ion source, the potential at the metastable suppressor also varies, thus necessitating the shielding provided by the grounded electrodes 44 ande!) on either side of the metastable suppressor.

The collector electrode 42 is-connected through a `lead lill to a rst'amplier 66. The two auxiliaryelectrodes 4t and 48 are connected through a double pole, double throw switch 62 to a network including a lead 3 connected to a second amplifier 68 anda reference potential `connection :e4 so that when-one of said auxiliaryelec`- trodes is connected to the second ampliier'the other is connected Ito a reference potential vand their positions may be iinterchanged by 'simple manipulation o'f the switch 62.

Inpreferred practice both the iirst and second amplifiers are of `the negative feed-back type with the main `collector electrode 42 'being at or very close to vground potential 'so vthatthe referencepotential vcan .be v,sinll-y ground .potential. However, v'this is not a limitation of the inventionprovided only that the auxiliary electrode which is not connected to the second amplier be connected at a potential substantially equal to that which it would have when used as a collector electrode. In short, the reference potential must closely approximate the potential of the electrodes when connected for amplification.

Assuming that the spectra is scanned by sweeping the ion beams from low mass to high mass as preferred, i. e. from right to left with respect to the rst resolving slit 28A, the electrodes 46 and i8 are shown connected for mixture analysis in that the beam Bi falling on the rst auxiliary electrode 46 to the right of the aperture 66A therein and by virtue of its passage through the first resolving slit 28A, is discharged and the discharge signal is amplified in the second amplifier. It should be borne in mind that the beam Bi following beam B2 with respect to the assumed direction of beam sweep is composed of ions of higher mass than the beam B2 and that the beam Bi will succeed beam B2 in focus on the collector electrode 42. The resolving-power of the instrument with respect to beams B1 and B2 when thus connected for mixture analysis is a function of the extent to which electrode 46 masks the rst resolving slit together with the resolution aiiorded by the third resolving slit, the latter being determined by the size of aperture 43A or, in other words, by the unmasked size of aperture 46A. With respect to the center line of the analyzer tube I2, the third resolving slit and the main collector are conveniently centered, while the barrier aperture is displaced off-center toward the centers of curvature of the ion beams. In this fashion resolving power of collector 46 is higher than at collector 48, this being the proper relationship for mixture and isotope ratio analysis, respectively.

If switch S2 is thrown in the opposite direction the second auxiliary electrode 48 will be connected to the second amplier, so that the signal resulting from discharge of beam B3 at this electrode will be amplified in the secondary amplier if the same orientation of ion beams is assumed. Such connection adapts the instrument for isotope ratio measurements. The first auxiliary electrode will, in this case, be connected to reference potential, i. e. grounded in the particular system illustrated. The resolving power of the instrument as between the beams B2 and B3 is partially determined by the dimensions of the aperture 45A in the first auxiliary electrode, but principally by the extent to which this aperture is masked by electrode 48. Because of the intended use of the instrument, the resolving power with respect to beam B3 in isotope ratio measurement is considerably lower than that with respect to beam B1 in mixture analysis.

Of course in the relatively uncommon situation where the most abundant of several isotopes is that isotope having the highest mass number, the instrument can be connected as though for automatic attenuation for the isotope ratio analysis of such a mixture, providing that the resolving power of an instrument with respect to automatic attenuation is satisfactory for such isotope ratio measurement.

The apparatus of the invention is adapted for use either in isotope ratio measurements or automatic attenuation of mixture analyses without disturbing the collection system of the instrument, by the simple expedient of switching one or the other of two auxiliary collector electrodes to a secondary amplifier and the remaining one of the auxiliary electrodes to a connection placing it at a potential with respect to ground equal to that of the collector electrode, It is obviously possible to connect each of the auxiliary collector As mentioned above, the amplification and comparative circuits employed for this purpose form no part of the invention and may be of the type shown in any of the aforementioned copending applications, or may be such a combination of these types as to minimize duplication of circuit elements.

I claim:

I. In a mass spectrometer having an ion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer chamber and means for sorting ions in the analyzer chamber, the combination comprising a barrier electrode and rst, second and third collector electrodes arranged in successive spaced relation adjacent the outlet end of the analyzer chamber. the barrier electrode having an aperture therein forming a first resolving slit, the first collector electrode having an aperture therein narrower than and in off-center alignment with the aperture in the barrier electrode and forming a second resolving slit, the second collector electrode being positioned to mask` a portion of the aperture in the rst collector electrode and forming with the unmasked portion thereof a third eifective resolving slit, the third collector electrode being positioned to receive ions passing through the third resolving slit, a first amplifier connected to receive and amplify signals from the third collector electrode, and means for amplifying ion discharge signals at said first and second collector electrodes.

2. In a mass spectrometer having an ion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer chamber and means for sorting ions in the analyzer chamber, the combination comprising a barrier electrode and iirst, second and third collector electrodes arranged in successive spaced relation adjacent tlie outlet end of the analyzer chamber,

. the barrier electrode having an aperture therein forming a first resolving slit, the iirst collector electrode having an aperture therein narrower than and in o-ceriter alignment with the aperture in the barrier electrode and forming a second resolving slit, the second collector electrode being positioned to mask a portion of the aperture in the first collector electrode and forming with the unmasked portion thereof a third eifective resolving slit, the third collector electrode being positioned to receive ions passing through the third resolving slit, a first amplifier connected to receive and amplify signals from the third collector electrode, an auxiliary amplier and means for interchangeably connecting one of said rst and second collector electrodes to said amplifier and the other of said electrodes to a reference potential approximating the potential of the third collector electrode.

3. In a mass spectrometer having an ion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer chamber and means for sorting ions in the analyzer chamber, the combination comprising a barrier electrode and first, second and third collector electrodes arranged in successive spaced relation adjacent the outlet end of the analyzing chamber, the barrier electrode having an aperture therein forming a iirst resolving slit, the first collector electrode having an aperture therein narrower than and in olf-center alignment with the aperture in the barrier electrode and forming a second resolving slit, the second collector electrode being positioned to mask a portion of the aperture in the first collector electrode and forming with the unmasked portion thereof a third eiective resolving slit, the third collector electrode being positioned to receive ions passing through the third resolvingslit, a first amplifier connected to receive and amplify signals from the third collector electrode, a shield electrode disposed between the second and third collector electrodes and having an aperture to permit ion access to the third electrode, and means for interchangeably amplifying ion discharge signals at said first and second collector electrodes.

4. In a mass spectrometer having an ion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer cham- 'ber and means for causing ions of differing specific mass to following different curved paths through the analyzer chamber, the combination comprising a barrier electrode and first, second and third collector electrodes arranged in successive spaced relation adjacent the outlet end of the analyzer chamber, the barrier electrode having an aperture therein forming a rst resolving slit, the first collector electrode having an aperture therein narrower than and in olf-center alignment with the aperture in the barrier electrode and forming a second resolving slit with the first collector electrode masking a portion of the barrier aperture, the second collector electrode being positioned to mask a portion of the aperture in the iirst collector electrode and forming with the unmasked portion thereof a third eiective resolving slit, the third collector electrode being positioned to receive ions passing through the third resolving slit, a iirst amplifier connected to receive and amplify signals from the third collector electrode, and means for interchangeably amplifying ion discharge signals at said first and second collector electrodes.

5. Apparatus according to claim 4 wherein the masked portion of the aperture in the rst collector electrode is larger than the masked portion of the aperture in the barrier electrode.

6. Apparatus according to claim 4 wherein the aperture in said barrier is olf-center toward the centers of ion path curvature with respect to said third effective resolving slit.

7. in a mass spectrometer having anion source, an analyzer chamber, means for propelling ions from the ion source through the analyzer chamber and means for sorting ions in the analyzer chamber, the combination comprising a barrier electrode and first, second and third collector electrodes arranged in successive spaced relation adjacent the outlet end of the analyzer chamber, the barrier electrode having an aperture therein forming a rst resolving slit, the first collector electrode having an aperture therein narrower than and in orf-center alignment with the aperture in the barrier electrode and forming a second resolving slit, the second collector electrode being positioned to mask a portion of the aperture in the first collector electrode and forming with the unmasked portion thereof a third eiective resolving slit, the third collector electrode being positioned to receive ions passing through the third resolving slit, a first amplifier connected to receive and amplify signals from the third collector electrode, a suppressor grid disposed between said seccnd and third collector electrodes and adapted to develop a potential in opposition to ion travel' toward said third collector electrode, a separate shield electrode disposed between said compressor grid and each of said second and third collector electrodes, and means for interchangeably amplifying ion discharge signals at said rst and second collector electrodes.

8. An ion collector system for a mass spectrometer comprising a barrier electrode and iirst, second and third collector electrodes arranged in successive spaced relation in the direction of ion travel in the mass spectrometer, the barrier electrode and iirst collector electrodes having apertures therein of successively decreasing width with the first collector electrode masking a portion of the barrier aperture, the second collector electrode being aligned to mask a portion of the aperture in the rst collector electrode, the third collector electrode being aligned with the unmasked portion of the aperture in the first collector electrode and means for amplifying signals produced by ion discharge at the several collector electrodes.

PAUL S. GOODWIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,456,426 Nier et al Dec. 14, 1948 2,470,745 Schlesman May 17, 1949 2,476,005 Thomas July 12, 1949

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