US2768304A

US2768304A - Mass spectrometer

Info

Publication number: US2768304A
Application number: US251352A
Authority: US
Inventors: William H Wells; William C Wiley
Original assignee: Bendix Aviation Corp
Current assignee: Bendix Aviation Corp
Priority date: 1951-10-15
Filing date: 1951-10-15
Publication date: 1956-10-23
Anticipated expiration: 1973-10-23
Also published as: GB728172A

Description

0d 23, 1956 W. H. WELLS E1' AL 2,768,304

MAss sPEcTRoMETER 2 Sheets-Sheet 2 Filed Oct. 15, 1951 V1.5 wfu. MMM mw .1 /.MM Mm MU. /b/ MW United States yPatent O masses MAss rsrrzecrrwrvnsma `William H. Wells and William C. Wiley, Detroit, Mich.,

assignors to Bendix Aviation Corporation, Detroit, Mich., a corporation of Delaware This invention relates to mass spectrometersand more particularly to mass spectrometers for, and methods of, distinguishing between the masses of dilerent ions `b 'y measuring their times of flight. The invention ,is especially adapted to provide a relatively sharp diiere'ntiation between zions of different masses.

vIn some mass spectrometers now in use, pulses of ions are formed from the different molecules of gas in an unknown mixture and are subjected to va predetermined Iforce so as tobe accelerated through a predetermineddistance. The ,ions of relatively light mass are Vgiven a greateracceleration by the predetermined vforce than the ions .-of heavy mass and are collected atthe end ofthe ,predetermined distance before the Aions of heavy mass. Thus, by measuring the times required for diierent ions `in t-he pulse 'to travel through the predetermined distance, the massesof the ions can be determined.

Difficulties in measurements arise in thespectrometers now in use andprevent the measurements fromkbeingdas l z;1c c t 1rate andmeaningfulas might bedesired. For example, isinceuthe nite Width ofthe pulse may pause Ividual ions of a given massto be located at d iterent positions in the pulse before the,ionsareaecelerated wards the collectorplate, all ions of the givenA mass arenot c oiiected at the same time. vIn addition, thethermaLand other energy of theions imparts a random motionto the ions before the application of any eXternalforce 1so that somedons of -a given mass are moving towards the Ycollector plate at the `time that the predetermined f orceis .imposed and otherions of the same mass aremoving away from the collector plate4 at that instant. Thus, thermaland other Ienergy ofthe ,ions als-O prevents all ions of agiven in ass from, being, simultaneously rcoilected in spectrometers now in use. v l I This invention provides la mass spectrometer in which errors resulting from the different positioning and random motion of individual ions are minimized. YThe errors are minimized by the .utilization .of an accelerating gijidhaving an extended width andby the dispositionof the collector plate at va predetermined vdistance f-ronlfthe. starting apoint of the ions.

vAn object of this invention is to provide a mass spectrometerffor distinguishing between ions of different mass by measuring the time required for the ions :to travel through a predetermined distance.

Another object is to provide a mass spectrometer yof ,the above character forproviding a relativelyv sharp dif- Iierentiation between ions of diierent mass.

.A -furtherobjectis to provide amass spectrometer of the above character utilizing 4an acceleratinggrid having an ,extendedwidth so as to improve the,res ponse char ac teri stics .ofy the spectrometer. Y

@Stillanother object is to, provide amass; spectrometer of the above character having a collector plate disposed Vv.at a.predetermined distance from the ,grid toimprove the response characteristics of the, spectrometer.

`Astill Vfurthenobject isto .providemethodsof thefabove 2,768,304 Patented Oct. 23, 1956 rice character for providing a relatively sharp differentiation betweenions of different mass.

Qther objects and advantages will be apparent from a detailed Vdescription of the invention and from the appended drawings and claims.

In the drawings:

Figure l is a view illustrating somewhat schematically the mechanical and electricalv features constituting one embodiment of the invention, vthe mechanical features Ybeing'shown in perspective and some of the electrical features being shown in block form;

vFigure 2 is an enlarged sectional view of the grid structure'constituting a part of the embodiment shown in Figure 1, the sec-tion being taken substantially on the axis of the structure; and i Figure 3 4is a schematic perspective view illustrating somewhat schematicallyanother embodiment of the inventon.

In `the embodiment of the invention shown in Figure 1, a cathode`10 made from a wedge-shaped strip of a suitable material such as tungsten is provided. A control grid y1 2 is disposed relatively close to the cathode 1t) and isprovided with a vertical slot 14 whose median position lies in substantially the s ame horizontal plane A'as the cathode. An accelerating vgrid 1 6 is positioned relatiyelyclose to the control grid 12 on vone side and to a shield grid 18 .on the other Vside and is substantially in alignment with both grids. Vertical slots l2 0 an d2 2 corresponding substantially in shapeva'nd position to the Slot 14 impro-vided in the grids .16 ,and 18, respectively- A vertically disposed backing plate 24 is provided to the rear of .the slots 1 4, 20 and.2 2 and s ubstantiallyrin parallel `with the dow of electrons from ythe cathode 10 through the slots. A non-conductive container 2 6 hav- 1ng .-a relatively great axial length and a relatively small width extends in a horizontal plane lfrom the plate .24.

The container L26 preferably has'a circular crosssection in a direction perpendicular to its axisbut may have other sectional contigurations, suchasa squarecross-sec- Ytion.

' vBores 28 4areprovided inthe container 2 6 substantially in alignment with the median positions of the Vslots 14, 2.0V and The bores 28 serve to provide a passageway for the flow of electrons .transversely through the container 26 vto a collector plateA 29 positioned on'the far side ofthe container substantially in parallel with the ,grids 12,16 and 1 8. A grounded conductive sleeve 3 0 "having holes corresponding in position to the bores 2 8 ts on the container 26. T he sleeve l30 eliminates any eiects trom ,electrical charges which result when electrons from the'electron stream Virnpinge on theeontainer instead of passing through the bores. vI-lolesl are also madein the Vbot-tom ofthe container zoand the sleeve @substantially in vthesarne plane as the vbores 2,8 tofprovide forthe introduction of gasthrough a conduit 32. The conduit in turn extends from a receptacle 34 adapted to yhold molecules of the ditierent gases constituting an unknownmixture. I

'A ,plurality 'of conductive grids 3 6 made from a suitably meshed material extend at spaced yintervals across the b ore in the container 26 substantially in perpendicular relationship fto the'axis of rthe container. The grids sive grids. The grid furthest removed from the plate 24 may be grounded.

A collector plate 42 is positioned substantially parallel to the plate 24 and grids 36 at a distance from the end of the container 26 approximately twice as great as the length of the container itself. A time indicator 44 such as an oscilloscope, is connected to the collector plate 42 to indicate the relative times at which the ions of different mass arrive at the collector plate. Since the collector 42 actually serves as an ion detector, other types of detectors may be used.

In the steady-state condition, the control grid 12 and the accelerating grid 16 have positive voltages applied to them from suitable output terminals of a direct power supply 46, and the

collector plates

29 and 42 have slightly positive voltages applied to them from other terminals of the power supply. The cathode 10, the shield grid 18, the backing plate 24 and the grids 36 are substantially at ground in the steady-state condition.

Because of the positive voltage on the grid 12 relative to the voltage on the cathode, electrons emitted by the cathode are accelerated towards the grid in the steadystate condition. The electrons are not accelerated beyond the grid 12, since the grid 16 has approximately the same positive voltage as the grid 12, and the grid 1S is substantially at ground. As a result, any electrons which pass through the bores 28 in the container 26 do not have sulicient energy to ionize molecules of gas introduced into the container through the conduit 32.

At predetermined times, negative voltage pulses of substantially equal magnitude are applied to the cathode and the control grid 12 from the pulse forming circuit 40. Since the grid 12 is still more positive than the cathode 10, the electrons emitted by the cathode continue to be attracted towards the grid. The electrons are further considerably accelerated in the region between the grids 12 and 16 because of the positive voltage which exists on the grid 16 relative to the voltage on the grid 12 during the pulses. The additional acceleration imparted to the electrons by the grid 16 causes the electrons to travel through the bore 28 into the container 26 with suicient energy to ionize some of the gas molecules introduced into the container.

Because of their positive charge, the ions produced by the collision between electrons and gas molecules are retained within the negative field created by the stream of electrons owing towards the collector plate 29. By retaining the ions within the electron stream, a pulse of ions having a relatively narrow width is obtained. The electron stream is relatively narrow because of the collimating action provided by the slots 14, 20 and 22 and the collimating action which may be provided by a suitable magnetic eld (not shown).

In addition to producing an ion pulse having a relatively narrow width, the relatively large charge of the negative eld causes the number of ions forming the pulse to be considerably increased over the number of ions comprising a pulse in spectrometers now in use. The creation of an ion pulse having a relatively narrow width and a relatively large number of ions produces an increase in the sensitivity with which the mass spectrometer responds to ions of different mass. The formation of the electron stream and the retention of ions within the stream are disclosed in detail in co-pending application Serial No. 221,554, filed April 18, 1951, by Ian H. Mc Laren and William C. Wiley, now Patent No. 2,732,500.

The ions retained within the negative field are produced during the application of the negative pulses on the cathode 10 and grid 12. At approximately the same time as the negative pulses on the cathode 10 and grid 12 are discontinued or at a slightly later time, positive pulses are applied to the backing plate 24 and' the grids 36. Equipment for producing a pair 4of pulses separated from each other by a relatively short time is. well known to persons skilled in the art and may be either purchased or built. For example, the Berkeley Scientific Company of Richmond, California, manufactures equipment for producing a pair of pulses which may be separated from each other by a variable period of time dependent upon the adjustment of a knob. This equipment is designated by Berkeley as Model 902 of its double pulse generator. If it is not desired to purchase equipment which provides a pair of pulses separated from each other by a variable period of time, such equipment may be built in accordance with principles outlined on pages 223 to 238, inclusive, of volume 20 entitled Electronic Time Measurements of the Radiation Laboratory Series prepared by the Massachusetts Institute of Technology.

The voltage pulses on the plate 24 and the grid 36 cause the ions to be accelerated through the container 26 towards the collector 42. Since the ions of relatively light mass are accelerated more by the predetermined forces applied by the grids 36 than the ions of heavy mass, they reach the collector plate 42 before the ions of heavy mass. By measuringV the times at whichthe diierent ions reach the collector plate, the masses of the ions can be determined.

We have found that, in spite of the narrow Width of the ion pulse, errors in measurement result in spectrometers now in use from the different disposition of individual ions in the ionization region. For example, some ions of a given mass may be located spatially at the back of the pulse and other ions of the same mass may be positioned at the front of the pulse. This causes individual ions of a given mass to arrive at the collector plate before other ions of the same mass and clouds the measurements which are obtained.

Errors in measurement also result in spectrometers now in use from the random motion imparted to the ions by the thermal and other energy of the ions before the application of any external forces. For example, some ions of a given mass may be moving towards the collector plate at the instant that a voltage pulse is applied to the ion accelerating grid, and other ions may be moving away from the collector plate at that instant.

We have also found that the errors resulting from the different positioning and the random motion of individual ions can be minimized if the collector plate is placed at a predetermined distance from the accelerating grid. This distance has been found by analysis to be approximately twice the distance between the grid and the ion pulse before the pulse is accelerated towards the grid when the ion pulse has a relatively narrow dimension in the direction of the grids 36. For increased lengths of the ion pulse, the position of focus is given as Q=the position of optimum ion focus;

S1=the distance from the last grid 36 to the end of the ion pulse nearest the backing plate 24;

S2=the distance from the last grid 36 to the end of the ion pulse most removed from the backing plate 24.

The term focussing as used herein refers to the action of bringing the ions of each mass together into relatively tight groups in the direction of their movement. As such, the term focussing refers to the action of bunching the ions of each mass in the direction of their movement so that they are collected in a minimum amount of time. Since the distance between the pulse and the grid is relatively small, the distance between the grid and collector plate is also small. This short distance pre vents ions of diterent mass from becoming sufficiently resolved in space before they reach the collector plate and causes the spectrometer to give somewhat inaccurate results.

This invention focusses the ions of each mass in the direction of their travel by disposing a detector such as the collector 42 at a position approximately twice as gees-sda fas-awayf-fidm die-fleet grld 's6-asL die distance' "'etween /t'h .electroni streamL an tlielast-VI gridi *Elievr invention y produce inatt-,rial separation Between the ions of? dittfen' massi' byf 'eiec'tively'increasing'the length of die. aeeelratinglregien. This isy accomplished by prengafplurality of* equally spaced?! grids sfi-a'nd-by appl ng voltagepulses` of; progressively' decreasing amplitude-between successivelv paies off' grids. The-ions nien move through the grid with a substantially cons-tarifa@- celeration. I Because of the substantially constant accelerationriitipartedttotheions; tlie' grids in* eliect provide an electricalg eld havingvasubstantially constant intensity over an extended width. This in turn Vcauses the .dist/ance between the endv of the lastgrid 36* and: the collector plate 4'2 to be considerably increased in order tra-obtain. adistance which Ais approximately twice the distance between the ion pulse in its stationary position and the last gridl'3'6.

BeeauSe-of the extendedlengfh on the electr-ical i'eld and3 the distant-positioning? ofthe? collectorplate 42,- the ions require at least as long:- a titne-toreach." the collector plate as in the spectrometers now in use. By placing the collector plate at the optimum distance from the last grid 36 without reducing the time of ion travel, ions of a given mass are focussed in space better than in spectrometers now in use. This in turn causes the sharpness and accuracy of the measurements obtained to be considerably increased.

Since the ultimate object is to obtain an electrical iield which is substantially constant over an extended length, the operation of the grid structure shown in Figure l can probably be enhanced by increasing the number of grids and correspondingly decreasing the distances and voltages between them. The grid structure may also be used so that rings similar to the ring 38 are used without any wire mesh.

It should be realized kthat other structures than that disclosed above may be used to form a relatively narrow ion pulse for passage through the container 26. It should be realized also that the ions may be accelerated through the container by applying pulses of negative voltage to the different grids. In this way, the ions are attracted towards successive grids rather than being repelled from one grid to the next.

A modification of the invention disclosed above is illustrated in Figure 3. In this embodiment, a container 70 formed from a thin sheath of conductive material is used. The end of the container near the collector plate is grounded and the end of the container near the backing plate is adapted to receive a pulse of positive voltage from a pulse forming circuit 52. Current ows axially along the sheath and produces a voltage which gradually decreases along the axial length of the sheath.

It should be appreciated that the plurality of grids 36 are provided to produce an electrical iield of extended length. A iield of extended length is produced so that the collector 42 can be positioned a relatively great distance from the backing plate 24.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous `other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, a grid assembly positioned between the ion-obtaining means and the ion detector to provide a substantially constant acceleration of the ions through a first region equal in distance to substantially one-half the distance between the end of the first region and the detector, and means for indicating the relative times at which the ions of different mass are detected.

2. A mass spectrometer, including, means for obtaining plnraiityef ions; an .ien detecter', a" grid assenibiy positioned between the ion-obtaining means and tle'cl`e teeferj and having extended lengrlrn iii the direction of thede'tctr equal' to' substan'tiallr 'one-half the distance between-'the'I endJT of die grid' assemblyv and; die:Iv detector, means f or producing an electric 'eld through the Yex"- tended len'g'tli ofthe" grid' assembly' to provide' a constant 'a'c'celeration on the io'ns', pand' rne'a'ns for providing v an indication of' thel relative times at whieli'dieions oridiirent mass are detected.

3. A- mass spectrometer, including, means for obtaininga` pluralityw of ions; anion detector, a grid assembly positioned between the ion-obtaining means and"y 'die derector andi equal inv distance substantially to one `lia-1l? die distance between the end yof" thevv grid'as's'ernblyancf tli'e detector, means for` applying voltage* pulses' of predeterrdin'e'dmagnitudeen nie-'grid' assemblyfo lpr'odee a sub- -stantially constant acceleration of tlre ions througlr die grid assembly, arianne-ans for indicating-fthe rela-rive finies at. which t-le ions-of: different massare deter-midi.h

4. A- m'ass spectrometer, including, meansfon obtain'- ing a plurality of ions, means for providing an electric iield of substantially constant magnitude through a first region to provide a substantially constant acceleration of the ions through the region, an ion detector positioned at a distance past the end of the first region corresponding to substantially twice the distance of travel of the ions through the region, and an indicator for showing the relative times at which the ions of different mass are detected.

5. A mass spectrometer, including, means for obtaining a plurality of ions, means for providing a substantially linear and constant acceleration of the ions through a rst region, an ion detector positioned at a distance past rthe end of the iirst region corresponding to approximately twice the distance of ion acceleration through the first region, and means for indicating the relative times at which the ions of diiierent mass are detected.

6. A mass spectrometer, including, means for providing p-ulses of ions, an ion detector, a grid structure positioned between the ion-providing means and the detector, means for applying voltages to the grid structure to produce a movement of the ions in each pulse towards the detector, the detector being positioned at a distance past the end of the grid structure corresponding to approximately twice the distance of acceleration of the ions by the grid structure, and means for indicating the relative times at which the ions of different mass in each pulse are detected.

7. A mass spectrometer, including, means for obtaining a pulse of ions in a coniined region, means for providing an electric field of substantially constant magnitude through a first region to provide a substantially constant acceleration of the ions through the region, an ion detector positioned at a distance past the efnd of the iirst region corresponding to substantially twice the square root yof the distance from the end of the rst region to the remote end of the confined region multiplied by the distance from the end of the first region to the near end of the conlined region, and an indicator for determining the relative times at which the ions of dilierent mass are detected.

8. A mass spectrometer, including, means for providing pulses of ions of finite width, an ion detector, a grid structure positioned between the ion-providing means and the detector, means for applying voltages to the grid structure to produce a movement of the ions in each pulse towards the detector, the detector being positioned at a distance past the end of the grid structure corresponding to substantially twice the square root of the distance between the end of the grid structure and the remote end |of the ion pulse multiplied by the `distance between the end of the grid structure and the near end of the ion pulse, and an indicator for determining Ithe relative times at'which the ions of dilerent mass in each pulse are detected. A

9. A mass spectrometer, including, means for providing a plurality of ions, an ion detector, a plurality of grids disposed at spaced intervals between the ion-providing means and the detector, means for applying progressively more negative voltage pulses to the successive grids in the plurality to produce a substantially constant electric field throughout lthe extended region defined by the grids for movement of the ions past the grids and towards the detector, the detector being positioned at a focal distance past the end of a last grid in the plurality to detect the ions of each mass at substantially the same instant of time, and means for indicating Ithe relative times at which ions of dierent mass are detected.

10. A mass spectrometer, including, means for providing a plurality of ions, an ion detector, a plurality of grids disposed at spaced intervals between the ion-providing means and the detector to provide an electric iield of substantially uniform intensity between the ion-providing means and a last grid in the plurality upon the application of particular voltage pulses to the grids in the plurality, means for applying the particular voltage pulses to the grids in the plurality to produce a movement of the ions past'the grids and towards the detector, the detector being positioned at a distance past the last grid equal to substantially twice the distance between the ionproviding means and the last grid, and means for indicating the relative times at which ions of different mass are detected.

References Cited in the file of this patent UNITED STATES PATENTS 2,476,005 Thomas July l2, 1949 2,535,032 Bennett Dec. 26, 1950 2,582,216 Koppius Jan. 15, 1952 2,685,035 Wiley -Tilly 27, 1954 OTHER REFERENCES An Ion Velocitron, by Cameron et al., published in The Review of Scientilic Instruments, vol. 19, No, 9 of September 1948, pages 605-607.