US2790080A - Mass spectrometer - Google Patents

Description

April 23, 1957 w. H. /wELLs MAss sPEcTRoMETER 2 Sheets-Sheet 1 Filed Nov. 16, 1953 April 23, 1957 w. HY WELLS 2,790,080

f MASS SPECTROMETER Filed NOV. 16', 1955 2 Sheets-Sheet 2 if o 9 Q E ll..

K3 Q Q Q Q Q \0 R I Afro/WSV Y sin Mass sPECTnoMEIEn WilliamH. Wells, Detroit, Mich., assignor to B endix Aviation Corporation, Detroit Mich.a corporation of Deia wage Application November-"16,1053, Serial No. 3?2,180 i Claims. (C l. Z50-:1419) This invention relates to mass s pectrorneters and more particularly to ,mass spectrometers forprovidingan enhanced resolution between ions of d iierent mass.

In some mass spectrometers, a pluralityof ions are formed from the molecnles of different gases and vapors in an unknown mixture. Aftera considerable number of'ions haveV been formed, a force is imposed upon the ions to accelerate the ions in a pulse -from/their place of retention. Since the ions of relativelylight mass receive agreater acceleration than the ions of heavier mass during application er the'force, the ions of light mass have a'highenvelocity. than the. ions of heavier mass at the time of removal of "the force. Asuaresult, the ions of relatively light mass travel throngha particular distance before theions otheavier mass. v By ,1r1easuring the relative times at which` the ions of diierent ymass are detect'eithe masses of the dierentlgases in the unlgnown mixture can nbe determined. Ideally, optimum indications would beobtained'if all the ions .of a'particular mass reachedfa detector at, s u bstan'tially the Vsame instantof time. kThis does not occur lin'nas's spectrometersno'w in use 'for arnurnber of reasons(V One reason is thationsofa particular mass are located at difierentvpositions within lthefirrspaeeofretention before a force isapplied to Yaccelerate theions towards a detector. `As a result, ions of the 'same mass thermore, thermal and other `energy'in the ions imparts ited States Parent 2,790,080 Patented. 23, .1957

of the above character for applying tothe ionsvanaccel- `erating forcehaving a magnitude .Whichmcreases' 1n `frelation to time to enhance the resolution .between ions of different mass.

'Astill furtherobject is to provide4 a -rnass-:spectrometer -ofthe above character for minimizing errorsin. resolution which result from the diierent-positioning and trandorn motion of individual ions -before they-are'subjected to v an accelerating force.

mass and of determining the massesof thediiferentions.

a mildern/motion wth@ iam betet@ the' appliation swf 'ayf S0 'that Some' das et? given mass may be moyingtowards the'detector at the time the forceisirnposed "and other ions'of'the saine mass may be moving away from the detector at lthat instant. 'ylfhus, ,the tliermal and Vother energyi in thev ions also prevents all the im. f a fficlilaf mass frsm being ,Similltaaeusly aetectedin mass's'pectrcrrleters now in ilse?.

This invention provides a mass spectrometer `in which errors resulting from'd iiferenc'es in the positioning and random motion Aof individual ions are minimized. rlhe errors are minimizedmby disposing 'a detector at Aa pre,- determined distance from' anion source and `by apply,- ing to the ions an accelerating force having a magnitnde which increases in V relation tfo time. "The application of such a force c auses'an enlianoed resolution to obtained An object of this invention is to provide a rnass spectrometer for distinguishing between ions of dil'ferent mass by'v measuring `thetimerequired'for to travel through" a predetermined distance.

lOthenobjectsand advantages willfbe apparentfromfa detailed description of the invention zand fromthe appended drawingsand` claims.

ilarnent lll madefrom a suitable .material `such :as -tung sten is adapted tczemit electrons when heated. AnneleetrolelZ is disposed at -a relatively short distance `such as l@ millimeter from-theutip of .thela-ment l0. vThe electrode 12 `is provided withf a. vertical slot 14, "the .median position ,of which is at substantially .the v,Same

horizontal ,level as` Athe lilament. l0. l

in electrode 16 is disposed -in substantially parallel relationship to the electrode l 2.and at arelativel'yshort distance s uch as 2 millimeters from ,the electrode. The

electrode 16 has a slot l corresponding substantially in. shap e and position ,to theslot le. A.col1ector.-20 is disposed at are latively great .distance .such as .10 vcentimeters from theielectrode l16 and in substantiallyparallel. yrelationship .to vthe electrode.

.backing plate 22 `is positioned between theelectrode 5er-1d the collector 20 and in substantially perpendicular tol the `collector 20. An electrode 24 is disposed in substantially .parallel relationship to the backing plate .and at a moderate distance such as 5 centimeters from the plate. The electrode 24 is positioned in front .of the imaginary line disclosed above and is provided vwith `a ylioirizorsta,l,slot 26- f A pair of. slats 28 made from suitable insulating material extend from the top and bottom of the'backing plate 22. and are suitably connected to the electrode 2* i0 ifQrm acompartment with these members. A horizontal slot 3 0 is provided in the bottom slat Zitat a posit'on diretly below the imaginary line disclosed above, A conduit 32j communicates at one yend with the slot 30 and vat the `other end with areceptacle 34 adapted to hold the molecules of different gases an tinknown in ixture. Y

.A detector such asa collector 36 is positioned at a 'relativelyA great distance such as 8A centimeters 4from lthe electrode 2li. The distance between the collector 36 and the electrode 24 should be substantially twice the distance between the imaginary line and the electrode. 24 for reasons which -will be disclosed in detail hereinafter. An indicator such as an'osclloscope 38 is connected to the collectory 36 to indicate the relative times at which ionsof' different mass are detected',

A direct voltage of positive polarity is applied to the electrode 12 through a resistance 40 from a suitable power supply 42. The collectors 2li and 36 also have slightly positive voltage applied to them through suitable resistances 44 and 46, respectively, from the power supply 42. These voltages are applied to the collectors 20 and 36 so that the collectors will attract electrons secondarily emitted from them upon impingernent of charged particles. The filament 10 and the backing plate 22 are connected to grounded resistances 4S and 59, respectively, and the electrodes 16 and 24 are directly grounded.

Negative pulses of voltage are applied to the lament 10 and the electrode 12 through coupling capacitances 52 and 54 from a suitable pulse forming circuit 56. The output terminal of a Waveform generator 58 is connected to the backing plate 22 through a suitable coupling capacitance 60. The waveform generator 58 is adapted 'to introduce to the backing plate 22 a positive voltage pulse having an amplitude which increases in relation to time. For this purpose, the waveform generator 58 may include any one of a number of circuits, the construction and operation of which are known to persons skilled in the art. For example, the circuit disclosed in U. S. Patent No. 2,729,815, to Gilman B. Andrews et al. can -be conveniently adapted to provide a voltage pulse having an increasing amplitude by superimposing a sawtooth voltage upon a square wave voltage.

A trapezoidal waveform generator as fully disclosed on page 297 of Waveforms, volume 19 of the Radiation Laboratory Series published by Massachusetts institute of Technology, can also be used. The increase in the amplitude of the voltage pulse applied to the backing plate 22 need not necessarily be linear with respect to time but may be of any desired shape. For example, circuits disclosed on pages 301 to 312, inclusive, of the above-mentioned publication may be utilized as part of the waveform generator 58 to provide voltage pulses having amplitudes which increase hyperbolically or parathe filament 10 are attracted to the electrode 12 because of the positive voltage on the electrode relative to the voltage on the filament. The electrons are decelerated in the `region between the electrodes 12 and 16 since the electrode 16 is at substantially the same potential as the filament 10. This prevents electrons from travelling through the region between the backing plateV 22 and the electrode 24 with suiiicient energy to ionize molecules of gas and vapor introduced into the region from the receptacle 34'.

Upon the imposition of negative pulses of voltage on the filament 10 and the electrode 12, the voltage on the electrode 12 becomes negative with respect to the voltage on the electrode 16. This causes electrons travelling through the slot 14 to be further accelerated in .the region between the electrodes 12 and 16. Because of the acceleration imparted to the electrons lin the region between the electrodes 12 and 16, the electrons travel into the region between the backing plate 22 and the electrode 24 with Vsufficient energy to ionize molecules of the gases and vapors in the region. Most of the ions which are produced have a unitary positive charge.

The ions which are produced by the electron stream are retained in the electron stream because of their opn posite charges relative to that of the stream. The pulse forming circuit 56 is set to apply the pulses to the filament 10 and the electrode 12 for a particular period of :time to allow a considerable number of ions to be produced for retention in the stream. When the pulses are cut ot, the ions `are available for easy withdrawal upon the applica# tion of a voltage pulse to the backing plate 22 from the waveform generator 5S. The waveform generator 53 is set to apply the pulse to the backing plate 22 at a particular vtime after the pulses applied to the filament 10 and the electrode 12 are cut olf.

Upon the application of voltage pulses to the backing plate 22 from the waveform generator' 53, an electric iield is produced between the plate and the electrode 24 to withdraw the ions towards the slot 26. The ions of light mass will have a greater acceleration imparted to them than the ions of heavier mass. This causes the ions of light mass to attain a greater velocity upon reaching the slot 26 than the velocity attained by the ions of heavier mass. After travelling through the slot 26 in the electrode 24, the ions travel to the collector 36 with substantially the same velocity as they had upon reaching the slot 26. This results from the fact that substantially no electric field is produced in the region between the electrode 24 and the collector 36 since the electrode is at ground potential and the collector is at a potential only slightly above ground. During their travel through the region or drift space between the electrode 24 and the collector 36, the ions become materially separated on the basis of their mass because of the differences in the velocities imparted to them on the basis of their mass. By measuring the-relative times at which different groups of ions reach the collector 36, the masses of the ions in each group can -be determined.

It has been found that errors in measurement result in spectrometers now in use because of the diierent positioning of individual ions in the ionization region. For example, as illustrated in Figure 2 an ion 100 of a given mass may be located spatially behind the imaginery line disclosed above and an ion 102 of the same mass may be located in front of the imaginery line. Since the ion 100 has to ltravel through a lgreater distance to the electrode 24 than the ion 102, the ion 100 is subjected to the electrical eld between the backing plate 22 and the electrode 24 for a longer period than the ion 102. As a result the ion 100 attains a greater velocity upon reaching the slot 26 than does the ion 102. Even though the ion 100 will have a greater velocity than the ion 102, the ion 100 may arrive at the collector plate at a later time than the ion 102 because the drift space between the electrode 24 and the collector 36 may not be of a suicient length to permit the ion 100 to catch up to the ion 102 which initially had an advantage in its spatial position. In this way, individual ions of a given mass may arrive at the collector plate before other ions of the same mass to cloud the measurements which are obtained. Errors resulting from the different positioning of the ions can be minimized if the collector plate is placed at a proper distance from the electrode 24 to permit the ions behind the imaginary line to overtake the ions in front of the line. This distance has been found by analysis to be approximately twice the distance between the imaginery line and the electrode 24, as fully disclosed in co-pending application Serial No. 251,352 filed October 15, 1951, by me and William C. Wiley. By placing the collector 36 at this distance from the electrode 24, optimum focusing is provided to compensate for the different positioning of individual ions of a given mass.

As previously disclosed, 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 as illustrated in Figure 2, an ion 104 of a given mass may have an initial velocity Vo towards thebacking plate 22 and an ion 106 of the same mass may have an initial velocity of Vo towards the electrode 24, Obviously, the ion 106 will reach the co1- lector ssbefore the-,ipa 10'4 uppcnfapplieauon 'fak pulse t' thel backing 22. is true' beeanse'the ron 10ft-win travel towards Ythe backingplate a' certain distanc'efd" before it is stopped by the electrical'iield. 'lfhen the Aion 10`4wil1 reverse its' direction of' flight 'tow'ards'the elec frode 24- Y,

When the ion 104 reaches its .initial startingposition after having'htraveled through. 'the d's ancejffdf the direction' of the electrode 24v it wil'laaquifre' v'e'locity Vo which is the same as the initial' velocity Vo of ythe ion 106. Therefore, the ion 1 04 will attainthe same velocity that the ion 106 had attained upon reaching electrode 24. Since the ion 104 travels from theelectrode'2'6 t the collector 36 at substantially the same velocity as the ion 10s,Y the-ien' rsa" can nvrsvenkthe'icn 106 which had an initial velocity Vo towards the electrode 24. As a'result, the ion 106 will reach the `collector 36 before the ion 104. In this way, individual ions-of a given mass may arrive at the collector atV different" Vtimes and affect the sharpness and accuracy of the' measurements obtained.

if the voltage pulses applied Vto the backing plate 22 have a constant amplitude such as +300 volts', and the distance between the backing plate and the electrode 24 is centimeters, the voltage gradient between the plate and the electrode will remain constant at -60 volts per centimeter, during the period of application of the pulses. With such a constant voltage gradient, ions having different thermal and other energy such as the ion 104 and the ion 106 can never arrive at the collector 36 at the same time for the reasons disclosed-above.

This invention provides apparatus for minimizing the errors resulting from thermal and other energy in the ions. As previously disclosed, the voltage pulses applied to the backing plate 22 from the waveform generator 58 have an amplitude which increases in apredetermined manner with respect to time. For example, the volt-age pulses applied may have an amplitude which increases in a parabolic curve from an initial value of +300 volts to a value of +450 volts at the end of the pulse. Such a voltage pulse is illustrated in Figure 3A. With the application of such a pulse,'the voltage gradient between the backing plate 22 and the electrode 24 will' increase parabolically in a negative direction from -60 volts per centimeter to -90 volts per centimeter as shown in Figure 3B.

Since the voltage gradient increases in a l negative direction with respect to time, individual ions of a given mass will be subjected to different amounts of force depending upon the time at Iwhich the ions reach the electrode 24. For example, the ion 106 may rea-ch the electrode 24 at point 108 of the voltage gradient curve and the ion 104 may reach the electrode at a later time such as point 110 of the voltage gradient curve. Since the Voltage gradient at point 110 is higher than the gradient at point 103, a greater acceleration will be imparted to the ion 104- than to the ion 106. As a result, the ion 104 will pass from the slot 26 in the electrode 24 into the drift space between the electrode and the collector' 36, with a greater velocity than the velocity ofthe ion 106. lf the greater velocityimparted to the ion 104 is of -a suicient magnitude, the ion 104 will overtake the ion 106 at substantially the time that the ion 106 arrives at the collector 36. Therefore the ion 104 and the ion 106 will impinge upon the collector V36 at substantially the n in their place of retention before application of an accelerating force. The resolution` is further enhanced by applying an accelerating field which changes with respect The enhanced resolution is obtained Y 6 tork time 'toI iiupartV diiferent'- velocities'v Aind'ividual ions of agiven 'mass' so asA to minimize an "resulting from the thermal andv other energies in jthe 1o It should be recognized that the acceleratirig-fforelei'may -be varied in other ways than by increasingthe amplitude of the voltage pulses applied to thefbac'kjingfplatelZZ'. For example, a' voltage pulse havinga constantjampli# rude may `be appli'ed'to the backing plate 22 and"a-"'v'olt'n pulse having a'decreasing kamplitude may'beapplied' to the electrode 24. Furthern'iore, the curve of hangefin the amplitude of the pulse can be 'varied'in any' desired manner to provide enhanced` resolutions over* ai Wide range of different masses. It should also be; recognized that an increasing accelerating forcel 'to compensate lfor thermal and other enengies of io'ns'ir'nayv'be 'utili ed; in mass spectrometers having* yaA pluralityy of va eleratirig regions such as is fully' 1disclosed vin U. S. Patent'No'. 2,685,035 to william c. whey. Y p y, j ,p y

Although this invention has been: disclosedjjarid'illustrated with reference to particular applications, the'principles involved are susceptible Vof",nunuerousV other applications which will be app-arentto'persons skilled inft'he art. The invention is, therefore, to be limited only' as indicated by the scope of the appended claims;

1. A mass spectrometer, including, a backing plate, an electrode disposed a particular distance from the backing plate, means for providing a plurality of ions in the region between the backing plate and the electrode, means for applying lbetween the backing plate and the electrode an electric held having an intensity which in creases with respect to time -to produce a movement of the ions past the electrode and to produce a separation of the ions on the basis of their mass, a detector disposed ybeyond the electrode at substantially the position of optimum focus of the ions, and means for indicating the relative times at which ions of different mass are detected.

2. A mass spectrometer, including, means for providing a iirst region, means for providing a plurality of ions in the region, means for imposing in the region an accelerating force having a magnitude which increases with respect to time to produce a movement of the ions past the end lof the region and to produce a separation of the ions on the `basis of their mass, a detector disposed past the end of the rst region at substantially the position of optimum focus of the ions, and means for indicating the relative times at which ions of diierent mass are detected.

3. A mass spectrometer, including, a backing plate, an electrode disposed at a particular distance from the backing plate, means for providing a plurality of ions in the region between the backing plate and the electrode and for retaining the ions in the region, an electrical circuit for producing between lthe plate and the electrode an electrical ield having an intensity which kincreases iny a particular relationship with respect to time to produce a movement of the ions past the electrode and to produce a separation of the ions on the basis of their mass, the electric field being operative to impart'different velocities vto individual ions ofany given mass to compensate for any differences in the initial energies possessed by the ions, a Vdetector disposed at substantially the focal position of the ions of each mass, and means for indicating the relav tive times at which the ions of different masses are detected.

4. A mass spectrometer, including, means for providing a first region, means for providing a plurality of ions in the region,-means for applying in the region a force having a non-linear relationship'through the region to produce a movement of the ions past the end of the region and to produce a separation lof the ions on the basis lof their mass, means dispo-sed past lthe end of the first region at substantially the position of optimum focus of the ions to detect the ions, and means for indicating the relative times atwhich the ions of different mass are detected.

1, A5. Avn-lassv spectrometer, including, a first electrode, av second electrode disposed at a particular distance from lthe first electrode, means for providing a plurlaity of ions in 4theregion between the first and second electrodes, means for*Y applying between the first and second electrodes an electrical field of varying intensity to produce a movement of the ions past the second electrode and to produce a separation of the ions on the basis of their mass, a detector disposed at a particular distance from the second electrode, and means for indicating the relative times at which the ions of different masses are detected.

6. A mass spectrometer, including, a first electrode, a second electrode disposed at a particular distance from the first electrode, means for providing a plural-ity of'ions in .the region between the iirst and second electrodes, means for applying )between -th'e rst and second electrodes an electric field having an intensity which varies in a linear relationship with respect to time to produce a movement of the ions past the second electrode and to produce a 7. A lmass spectrometer, including, a first electrode, a second electrode disposed at a particular distance from the irst electrode, means for providing a plurality of ions in the region between .the iirst and second electrodes, means for applying between the first and ysecond electrodes electric ield having an intensity which varies in a particular non-linear relationship with respect to time to produce a movement of the ions past the second electrode and to produce a separation of the ions on the basis of their mass, a detector disposed at substantially the position of optimum focus of the ions, and means for indi eating the rela-tive times at which the ions of different mass are detected.

A pulsed Mass Spectrometer with Time Dispersion by Wolfi and Stephens published in Review of Scientific instruments, vol. 24, No. 8 of August 1953, pgs. 616-6l7. (P. O. Library.)

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