US2570158A

US2570158A - Method and apparatus for separating charged particles of different mass-to-charge ratios

Info

Publication number: US2570158A
Application number: US198870A
Authority: US
Inventors: Paul O Schissel
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1950-12-02
Filing date: 1950-12-02
Publication date: 1951-10-02
Anticipated expiration: 1968-10-02
Also published as: NL86953C; FR1049592A; CH306176A; DE882769C; GB698850A

Description

1951 P. o. SCHISSEL 2,570,158

' METHOD AND APPARATUS FOR SEPARATING CHARGED PARTICLES I OF DIFFERENT MASS-TO-CHARGE RATIOS Filed Dec. 2, 1950 5 Sheets-Sheet 1 Fig.2.

AMPLIFIER Figi.

by)Q/ M H is Attorney.

Oct. 2, 1951 P. o. SCHISSEL METHOD AND APPARATUS FOR SEPARATING CHARGED PARTICLES OF DIFFERENT MASS-TO-CHARGE RATIOS 5 Sheets-Sheet 2 Filed D80 2, 1950 m w W UH ml 0 MM n a w Inventor: Paul O. Schissel, by )QJ% His Attofiney.

1951 P. '0. SCHISSEL 2,570,158

METHOD AND APPARATUS FOR SEPARATING CHARGED PARTICLES OF DIFFERENT MASS-TO-CHARGE RATIOS Filed Dec. 2, 1950 5 Sheets-Sheet 5 Fig. 4.

AMPLIFIER 67 RECORDER Invent or: aLU (lschissel,

His avne y.

1951 P. o. SCHISSEL 1 2,570,158

' METHOD AND APPARATUS FOR SEPARATING- CHARGED PARTICLES OF DIFFERENT MASS-TO-CHARGE RATIOS Filed Dec. 2, 1950 5 Sheets-Sheet 4' Fig.7.

AMPLIFIER RECORDER Inventor-z Pgul schi'ssel,

H is Attorney.

Oct. 1951 P. o. SCHI SSEL 2,570,158

METHOD AND APPARATUS FOR SEPARATING CHARGED PARTICLES OF DIFFERENT MASS-TO-CHARGE RATIOS Filed Dec. 2, 1950 5 Sheets-Sheet 5 Fig.8.

OSCILLA TOR 38 RECORDER Inventor: Paul O. Schissel,

by His Attorney.

Patented Oct. 2, 1951 METHOD AND APPARATUS FOR SEPARAT- ING CHARGED PARTICLES OF DIFFERENT MASS-TO-CHARGE RATIOS Paul 0. Schissel, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 2, 1950, Serial No. 198,870

This invention relates to mass spectrometry and, in particular, to novel methods and apparatus for separating ions according to their massto-charge ratios.

One form of mass spectrometer apparatus employed heretofore comprises means for accelerating a plurality of ions having heterogeneous mass-to-charge ratios through an electrostatic field into a direct current magnetic field wherein they are separated into a plurality of spatially distributed ion beams, each of which is homogeneous with respect to mass-to-charge ratio. In order to secure desired high resolution with such ap aratus, however, it is necessary to employ collimating slits and apertured focusing plates which, while they serve their purpose, tend to restrict the amount of output current which may be obtained from a given ion beam. Ac-

cordingly, it is a principal object of the present invention to provide a method and apparatus for separating ions according to their mass-tocharge ratios with which it is unnecessary to arrange beam constricting structure along the ion paths to obtain high resolution.

Another form of mass spectrometer a aratus involves the successive application of a plurality of radio frequency and direct current electric fields whereby separation of ions is accomplished by permitting only ions having a given mass-tocharge ratio to pass completely through the succession of fields. This apparatus, however, is not easily adaptable to the production of high beam currents inasmuch as the ultimate passage of ions through the successive fields depends not only upon the mass-to-charge ratio, but also upon the time of origination of the ions with respect to the phase of the radio frequency field. Therefore, it is another principal object of this invention to provide a mass spectrometer method and apparatus in which the separation of ions having a given mass-to-charge ratio does not depend upon the time of their origination.

According to one important aspect of the invention, which is more fully described hereinafter, ions having heterogeneous mass-to-charge ratios are accelerated by means of an alternating or R. F. electric field against the force of an electrostatic or direct current electric field having a linear space distribution. Thereupon,

the ions having a given massto-charge ratio,

properly correlated with the frequency of the alternating or R. F. field, execute simple harmonic motion to and fro through and in phase with the alternating or R. F. electric field whereby they become separated in space from ions having 20 Claims. (Cl. 250-413) other mass-to-charge ratios. The ions thus separated are then collected and measured to determine the amount present in the sample being studied.

The features of the invention desired to be protected are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following specification taken in connection with the accompanying drawings in which Fig. 1 is a graph illustrating the direct current voltage distribution which obtains in devices constructed in accordance with the present invention; Fig. 2 is a sectional View presenting one embodiment of the invention; Fig, 3 is a sectional view illustrating a second embodiment of the invention; Fig. 4 represents in section a third embodiment of the invention; Fig. 5 is a. second graph illustrating direct cur-- rent voltage distribution pertinent to an explanation of the invention; Fig. 6 is a graph illustrative of operational characteristics pertinent to an adequate explanation of portions of the invention; and Figs. 7 and 8 illustrate in sectional manner respectively fourth and fifth embodiments of the invention.

If it is assumed that a parabolically distributed voltage, as indicated by the solid line I in Fig. 1 wherein the voltage V is plotted as a function of distance X, is arranged to have its apex 2 positioned upon the y-axis or median plane 3, then, since by definition the electrostatic or direct current electric field is the negative of the first derivative with respect to a: of the voltage, a linear electrostatic field increasing in both directions from the plane 3 will be created. If it is also assumed that a confined alternating electric or R. Fifield is arranged to have a component parallel to the direction of the x-axis at the apex 2 of the voltage or potential distribution curve I, ions injected into the alternating or R. F. field will continuously extract energy therefrom, providing the period of oscillation of the ions about the alternating or R. F. field is equal to the period of the field. The requirements of ion mass-tocharge ratio and R. F. field frequency may be derived as follows:

From Newtons laws of motion,

where m is the mass of the ion, :0 is the displacement of the ion from the y-axis, e is the charge upon the ion, s is the strength of the electrostatic field, and V is the voltage creating the field 3 Since it has been stipulated that V has a parabolic space distribution,

V(w) Kac (2) where K is a proportionality constant. Differentiating Equation 2 to obtain the first partial derivative with respect to ac and substituting in Equation 1, we have Integrating Equation 3, We obtain a:(t) =A sinl: -ZKt+C] 4 where A and C are constants.

But it has been specified that, to obtain continuous acceleration of the ions to and fro through the R. F. field, the period of the ions, Ti, must be equal to the period of the R. F. field, Tar, therefore, from Equation 4 we may state Ti=21rx -=TRF And from Equation 5, it follows that m K Trap (6) energy to overcome the opposing force of the electrostatic field. Despite the fact that the resonant ions travel difierent distances from the median plane up the potential bowl each time they traverse th R. F. field, they always have the same period and remain in phase-with the R. F. field. Non-resonant ions of other mass-tocharge ratios will also have a constant period,

but their period will not be equal to the period ,of the R. F.'field. Hence, non-resonant ions will fail to remain in phase with the R. F. field whereby their acceleration will be limited in space.

Referring now to Fig. 2, there is shown in crosssection a mass spectrometer according to the invention which comprises an evacuable envelope It having re-entrant seal portions H and i2 disposed respectively at opposite ends thereof. Opposed cup-shaped collector electrodes is and M are supported within envelope 50 by means of rigid leads l5 and It hermetically introduced through seal portions H and !2 respectively.

Along the median plane, indicated by the broken line ll, there is positioned a source of electrons 18 which may comprise a thermionically emissive filament 59, supported by

conductive leads

20 and 2| hermetically introduced through 'a sealing boss 22 suitably located upon the'peapparatus wherein undesirable photoelectric cf;-

A hollow cylindrical fects may be created. Electrons emanating from heated filament I9 are accelerated, as indicated by the dotted lines 25, through an aperture 26 within an annular member 21, which may be termed an electron trap. Trap 2'! is supported by means of a rigid conductive lead 28, hermetically introduced through sealing boss 22 and connected as shown to a source of direct current 29 whereby accelerating potential is applied between trap 21 and filament l9.

To produce an R. F. or alternating electric field having a component parallel to the common axis of collector electrodes l3 and I4, there are provided opposed, cup-shaped, annular electrodes 30 and 3| having apertures 32 and 33 positioned respectively therein. Electrodes 30 and 3| are supported respectively by means of rigid conductive leads 34 and 35 between which is connected the secondary winding 36 of a transformer 37. Alternating or R. F. voltag may be supplied to electrodes 33 and EH through transformer 31 by means of an oscillator 38 connected to the primary winding 39 of transformer 31. Oscil-- lator 38 preferably has a'variable frequency output, the reason for which will be more fully appreciated in the light of the following discussion.

An electrostatic or direct current electric field may be produced between collector electrodes [3 and [4 by means of a plurality of annular grading rings 49-43 and 44-41. All the grading rings 4941, along with electrodes 30 and 3|, may be insulatingly supported in coaxial alignment by means of spacer members 48 and may be positioned as a unit within envelope ID by means of bolts 49, spring fingers 5i] and nuts 5|. Direct current potential may be applied to the grading rings it-41 and electrodes 30 and 3|, which are interconnected by resistors 52--59, by means of a rigid conductive lead 60, hermetically introduced through sealing boss 22 and connected to the positive terminal of a source of direct cur-- rent voltage indicated conventionally by battery 6!. The negative terminal of battery 6! is connected, as shown, to the center tap 62 upon sec ondary winding 36 of transformer 31. As will now be appreciated, resistors 52- and '5659 constitute potential dividers which, if the resistances have the proper values, will provide a parabolic voltage distribution increasing positively in both directions from median plan ll between terminal grading rings 43 and 41. Terminal grading rings 43 and 41 are interconnected by lead '63 to assure that the extremities of the parabolically distributed voltage are at the same potential.

In the operation of the device of Fig. 2, if it is assumed that envelope H] has been evacuated through a pump lead 64 and a suitable gas sample or ionizable medium introduced therein in a manner well known to those skilled in the art, it will be realized that a plurality of ions are formed by impact when electrons, emanating from filament I9 and traveling within electron trap 21, strike the gas molecules. The ions thus generated are accelerated to the left or to the right from the median plane, indicated by line ll, depending upon whether electrodes 30 and 31' are respectively positive'and negative or vice'versa at the time of their generation. The ions move under the impetus of the kinetic energy imparted to them by the R. F. field toward either collector V electrode l3 or 54 against the force of the linear electrostatic field heretofore mentioned until all their kinetic energy has been transformed into potential energy, whereupon they come to rest and travel in the opposite direction under the impetus of the electrostatic field. If, however, a particular ion has a mass-to-charge ratio such that the above-stated Equation 6 is satisfied, it

will execute simple harmonic motion and arrive back in the region of influence of the R. F. field exactly one-half cycle later when the polarity of the R. F. field has reversed. Consequently, such an ion receives another increment of kinetic energy from the R. F. field in the opposite direction and travels even further toward the opposite collector electrode against the force of the electrostatic field. This process, with the particular resonant ion executing simple harmonic motion to and fro through the R. F. field and re maining in phase therewith, continues until the ion has extracted enough energy from the R. F. field to overcome the potential barrier of the electrostatic field.

When the ions having the correct or resonant mass as above described receive enough energy from the R. F. field to overcome the force of the opposing linearly distributed electrostatic field, they emerge from the aperture in

terminal grading ring

43 or 41, as the case may be, with substantially zero kinetic energy or velocity and impinge upon collector electrode l3 or I 4. As shown, collector electrodes l3 and [4 are connected in parallel such that the total ion current thus generated flows through resistor 65 to the ground connection indicated at 65'. The voltage generated by the ion current flowing through resistor 65 may be supplied to an amplifier 66 and thence to a recorder 61 whereby a permanent record of the concentration of such ions may be obtained. To insure that ions emerging from either

terminal grading ring

43 or 41 reach the respective collector electrodes, a small negative potential may be provided therebetween, as is indicated by the connection of battery 68 in circuit with resistor 65.

In some instances it may be found desirable to discourage premature impingement of ions upon the various electrodes comprising the device of the invention. Consequently, solenoidal windings 69 and may be positioned about envelope Ill to provide a paraxial magnetic field which tends come the force of the opposing electrostatic field and to reach one of the collector electrodes. However, a more general expression of the relationship stated in Equation 6 is where n is an odd integer. This means that, in addition to the ions having the fundamental mass-to-charge ratio (where n=l), ions having harmonics of the fundamental etc.)

will be resonant or will be continuously in phase with the R, F. field and receive energy therefrom during each traversal thereof. Upon first consideration, this appears to be a disadvantage but, as a practical matter, such harmonics are very seldom found coincidentally, particularly where ions given by the following relation.

l".- 1 K "dm=2 E (8) where E is the peak of value of the R. F. voltage of oscillator 38. If the voltage of battery 61 is selected at 1000 volts, the peak voltage of oscillator 38 chosen to be 1 volt, the frequency of oscillator 38 set at 0.07 megacycle, and the device constructed such that the distance between terminal grading rings 43 and 41 is centimeters; then ions having an atomic mass number of about 100 will be collected upon collectors i3 and M. In such event theresolution, as defined in Equation 8, is of the order of 1600.

With particular reference now to Fig. 3, wherein numerals used hereinbefore are utilized to designate like elements, there is shown an embodiment of the invention in which the aforementioned electrostatic or direct current electric field having a linear space distribution may be obtained without the use of grading rings. As illustrated, opposed bent-surface electrodes 15 and I6 serve as collector electrodes and also as a means of establishing the desired linearly distributed electrostatic field. Electrode I5 is supported by a stud 15 sealed into envelope l0 while electrode 16 is supported by lead 60 as shown.

It may be proved that, if

electrodes

15 and 16 have bent surfaces which correspond in shape to that of hyperboloids of revolution, then with the indicated exterior circuit connections the desired parabolically distributed voltage or linearly distributed electrostatic field will exist between the electrodes. Consequently, if it is so desired,

electrodes

15 and 16, shaped as hyperboloids of revolution, may be employed to replace the plurality of grading rings and interconnecting resistors illustrated in Fig. 2. It should be observed, however, that the collimating solenoids .69 and 10 are more nearly essential when electrodes l5 and 16 are utilized, because vertical components of the electrostatic field, existing between the electrodes except along the axis thereof, tend to withdraw the ions from the field.

Referring now to Fig. 4 wherein parts corresponding to those shown in Fig. 2 are designated by like numerals, there is shown a modification of the invention in which the R. F. or alternating electric field, rather than being generated along the median plane I! between collector electrodes l3 and I4, is generated in series with the electrostatic or direct current electric field existing between terminal grading rings 43 and 41. As shown, the secondary winding 36 of transformer 31, to which oscillator 38 is connected, is now connected through battery 68 to the positive terminal of battery 6| whereby the potentials of the voltage dividers comprising resistances 52-59 vary in accordance with the alternating or R. F. voltage of oscillator 38. To assure maintenance of the linear electrostatic field distribution adjacent median plane I! with these altered connections, electrodes and 3| are connected together by means of shorting bar 80 and electrode 3| is joined with lead through resistor 8|.

An understanding of the effect of the modification illustrated in Fig. 4 may be obtained from Fig. 5 wherein Fig. l is duplicated. Added curves 82 and 83 illustrate the voltage distribution be- 7 tween terminal grading rings 43 and 41 at two separate instants of time when the R. F. field supplied by oscillator 38 is other than zero.

Curve I, of course, represents the voltage distribution when the R. F. field is Zero. Therefore, it may be considered that the R. F. voltage generated by oscillator 38 modulates the direct current voltage between terminal grading rings 43 and 41 or, in other words, that it alters the rate of change of slope of the parabolically-shaped voltage distribution and, hence, the value of the electrostatic field.

If the voltage 12 of oscillator 38 in the modification shown in Fig. "4 is defined as o=-E cos (wt-l-qi) (9) then the potential function V1 between terminal grading rings 43 and 41 may be written as where V is the direct current potential as heretofore defined and R is a constant. It may now be shown that the equation of motion of ions formed by bombardment of electrons from filament I9 is as follows:

Equation 12 is known as Mathieus equation, and

for present purposes, the essential consideration is that for certain values of a and q resonance occurs. This means that ions having certain mass-to-charge ratios continue to gain energy from oscillator 38 and hence finally reach either collector electrode 53 or collector electrode l4, as the case may be. Ions having mass-tocharge ratios other than the resonant ones do not continue to gain energy from oscillator 38 and therefore do not reach the collector. electrodes.

In Fig. 6, there is shown a stability chart for Mathieu functions of integral order wherein q is plotted upon the ac-axis while a is, plotted on the y-axis. Now, if Equation 13 is divided by Equation 14, the following is obtained:

2V q q And, if V and E are both held constant by maintaining the voltage of battery El and oscillator 38 at constant values, then the equation of a straight line having a slope Fig. 6 will continue to gain energy from oscillator 38 and ultimately reach either collector electrode l3 or I4. Those ions having mass-tocharge ratios such that a and q have values falling within the unshaded regions of Fig. 6, marked stable, will not continue to gain energy and hence will not reach the collector electrodes. Consequently, to obtain the best discrimination between ions having heterogeneous mass-to-charge ratios, values of V and E should be selected so that operation of the device will occur along a line such as line 85. Since line traverses essentially only one unstable region, only ions having mass-to-charge ratios such as to cause a" and q to fall within this region will receive energy continuously from the R. F.

field. In addition, the operatin line, such as line 85, should be kept as near the vertical axis of Fig. 6 as possible in order that ions having mass-to-charge ratios immediately adjacent the desired resonant mass-to-charge ratio will not also receive continuous acceleration.

As will appear from the above, it is preferred that the voltage of battery 6i and the voltage of oscillator 38 in the device of Fig. 4 be maintained constant at a desired value. To obtain a mass spectrum, the frequency of oscillator 38 is varied. With frequency as the independent variable, the following equation may be emplcyed for the determination of the particular ion mass-to-charge ratio being collected upon electrodes l3 and I4:

ewe

and ii are so selected that the operating point of the device of Fig. 4 falls within a region of instability, as discussed in connection with Fig. 6, then, when the frequenc of oscillator 38 is changed to another value, ions having a mass-tocharge ratio as determined by Equation 16 will be collected upon electrodes I3 and M.

In Fig. 7, there is shown a modification of the invention wherein structure similar to that disclosed heretofore in Fig. 3 is utilized in connection with circuitry similar to that disclosed in Fig. 4. As will be observed from the employment of identical reference characters, used hereinbefore in the designation of similar elements, amplifier input resistor 65 is connected to the secondary winding 38 of transformer 31 which, in turn, is connected to battery 5|.

Bent surface electrodes

15 and 16 have the shape of hyperboloids of revolution to obtain the desired electrostatic or direct current field distribution therebetween as discussed in connection with Fig. 3. The equation of motion of ions generated within envelope I0 is determined by Equation 12 in a manner similar to that discussed heretofore in connection with Fig. 4.

In the above described modifications of the invention shown in Figs. 2 and 3, the ions should pass through the R. F. field in a time short compared with the period of the R. F. field in order to insure that they receive an increase of kinetic energy during each traversal thereof. Accordingly, electrodes 30 and 3! should be as closely spaced as possible with respect to each other commensurate with the positioning therebetween of annular trap 21. It should also be noted that, when ions are first formed by impingement of '9 the electrons emanating from filament I9 upon gas molecules, the only means for their escape from the region of trapz'l is by acceleration due to the R. F. field, and therefore, some ions may fail to receive a sufiicient net increase of kinetic energy to enable the inception of the desired to and fro movement through the R. F. field. In the modification of the invention shown in Fig. 8, these difiiculties are obviated by displacing the bombarding electron streamfrom median plane II. I

- As illustrated in Fig. 8, wherein portions hereinbefore shown and described are identified by like numerals, the requisite linearly distributed electrostatic field, or parabolically distributed voltage, is provided by means of a plurality of spaced, plate-shaped, grading rings Hill-409 and potential dividing resistors IIll-IIl connected as shown; A Source of electrons H8 is positioned adjacent median plane I1 and comprises a thermionically emissive filament H9, hollow cylindrical shield lZil and supporting leads I2I and I22. Heating current for filament I I9is supplied by a source of direct current, indicated conventionally by battery I23 connected between leads HI and I22. An electron trap I24 is supported by means of a rigid conductive lead I25 in the space between grading rings I95 and I06. Accelerating voltage may be supplied between filament H9 and trap I24'by a unidirectional voltage source I26. Trap I24 is maintained at approximatel the average potential between rings I and I06 by means of a suitable source of unidirectional voltage I21 connected between the positive terminal of battery I26 and the center-tap 62 of transformer36. As indicated, rings or electrodes Hi4 and I 05, supported by leads I28 and I29, are spaced as closely as possible with respect to each other to minimize the axial extent of the R. F. field generated therebetween by oscillator 38.

As will now be observed, ions generated by bombardment within trap I24 are at once subjected to the force of the electrostatic field whereby they are propelled through the R. F. fieldbetween rings I04 and I05 with a considerable velocity on their first traversal thereof. In addition, displacement of trap I24 and electron source IIB from median plane I1 permits the closer spacing of electrodes I04 and H15 to reduce the axial'extent of the R. F. field. It will be realized, of c'ourse,fth'at trap I24 and source II 8 maybe positioned between any of the plurality of grading rings to achieve the above described purposes. 'Also, the same expedients are equally applicable to the modifications shown in Figs. 3 and 7 wherein hyperbolically-shapedelectrodes I5 and I5 render unnecessary the employmentof grading rings;

In the embodiments of the invention utilizing grading rings and resistors constituting potential dividers to provide a linearly distributed electrostatic field, the grading rings may be evenly spaced and the desired parabolic voltage distribution obtained by selecting suitable values for the resistors. However, other arrangements having unequally spaced grading rings may be employed for this purpose, as will appear to those skilled in the art from the foregoing considerations.

While theinvention has been described with reference to particular' embodiments thereof, it will be understood that numerous changes may bemade without departing from the invention. I therefore aim in the appended claims to cover these and all such equivalent variations of application and structure as are within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A mass spectrometer comprising an evacu able envelope, a pairof electrodes mounted in' spaced apart relationship within said envelope,

means for producing in the space between said electrodes an electrostatic field having a linear space distribution, the voltage creating said field having a parabolic space distribution with the apex lying substantially upon the median plane between said electrodes and increasing from the median plane toward each of said electrodes, means for generating an alternating electric field between said electrodes, means for injecting electrons into the space between said electrodes, and means for introducing an ionizable medium into said envelope.

2. A mass spectrometer comprising an evacuable envelope, a pair of electrodes mounted in spaced apart relationship within said envelope, means for producing in the space between said electrodes an electrostatic field having a linear space distribution, the axial component of said field being substantially zero upon the median plane between said electrodes and increasing from the median plane toward each of said electrodes, means for generating an alternating electric field between said electrodes, means for directing a magnetic field along the common axis of said electrodes, means for injecting electrons into the space between said electrodes, and means for introducing an ionizable medium into said envelope.

3. A mass spectrometer comprising an evacuable envelope, a pair of electrodes mounted in spaced apart relationship within said envelope, means for injecting electrons into the space between said electrodes, means for introducing an ionizable medium into said envelope whereby said medium may be ionized by said electrons to form ions having a plurality of mass-tocharge ratios, means for producing in the space' between said electrodes a direct current voltage having a parabolic space distribution, said parabolic voltage having its apex lying substantially upon the median plane between said electrodes and increasing positively from the median plane toward each of said electrodes, means for generating an alternating electric field between said electrodes, the combined effect of said direct current voltage and said alternating electric field being to cause resonant ions to be continuously accelerated by said alternating electric field during successive passages therethrough while nonresonant ions are accelerated to only a limited extent, and means for collecting and measuring said resonant ions.

4. A mass spectrometer comprising an evacuable envelope, a pair of electrodes mounted in spaced apart relationship within said envelope, means for injecting electrons into the space between said electrodes, means for introducing an ionizable medium into said envelope whereby said medium may be ionized by said electrons to form, ions having a plurality of mass-tocharge ratios, means for producing in the space between said electrodes a direct current voltage bolic voltage having its apex lying substantially upon the median plane between said electrodes 1 and increasing positively from the median plane toward each of said electrodes, means for genmass-to-charge ratio are accelerated to and fro by said radio frequency field against the opposing force of said parabolic potential distribution to ultimately overcome said opposing force, and means for collecting said ions having a given mass-to-charge ratio.

13. A mass spectrometer comprising an evacuable envelope, a first plurality of coaxially aligned potential grading rings disposed on one side of a plane traversing said envelope, a second plurality of coaxiall aligned potential gradin rings disposed within said envelope on the opposite side of said plane, the grading rings terminating each of said pluralities of grading rings adjacent said plane being adapted to serve also as electrodes for the establishment of a radio frequency field, means constituting a potential divider connected to said pluralities of grading rings, means for energizing said potential divider to establish along said pluralities of grading rings a parabolic potential distribution having its apex lying upon said plane and increasing positively in both directions from said plane, means for establishing a radio frequency field between said terminating grading rings serving also as electrodes, and means for generating ions within said envelope whereby ions having a given massto-charge ratio are accelerated to and fro by said radio frequency field against the opposing force of said parabolic potential distribution to overcome ultimately said opposing force, and means for collecting said ions having a given mass-tocharge ratio.

14. A mass spectrometer comprising an evacuable envelope, a pair of opposed electrodes supported within said envelope, said electrodes having the shape of hyperboloids of revolution, a source of direct current voltage connected to said electrodes to establish therebetween a parabolically distributed voltage the apex of which lies upon the median plane between said electrodes, means for generating ions having a plurality of mass-to-charge ratios between said electrodes, means for generating a radio frequency field between said electrodes and along said plane whereby ions having a given mass-to-charge ratio will be accelerated to and fro by said radio frequency field against the opposing force of said parabolicall distributed voltage to overcome ultimately said opposing force while ions having other mass-to-charge ratios will fail to overcome said opposing force, and means for collecting said ions having a given mass-to-charge ratio.

15. A mass spectrometer comprising an evacuable envelope, a first pair of spaced electrodes supported within said envelope, said electrodes having the shape of hyperboloids of revolution, a source of direct current voltage connected to said electrodes to establish therebetween a parabolically distributed voltage the apex of which lies upon the median plane between said electrodes, a second pair of spaced electrodes supported between said first pair of electrodes, said electrodes in said second pair being positioned on opposite sides of said plane between said first pair of electrodes, means for generating ions having a plurality of mass-to-charge ratios between said electrodes, and a source of radio frequency voltage connected to said second pair of electrodes for generating therebetween a radio frequency field whereby ions having a given massto-charge ratio will be accelerated to and fro by said radio frequency field against the opposing force of said parabolically distributed voltage to overcome ultimately said opposing force while ions having other mass-to-charge ratios will fail to overcome said opposing force, and means for collecting said ions having a given mass-tocharge ratio.

16. A mass spectrometer comprising an evacuable envelope, a first pair of spaced electrodes supported within said envelope, said electrodes having the shape of hyperboloids of revolution, a source of direct current voltage connected to said electrodes to establish therebetween a parabolically distributed voltage the apex of which lies upon the median plane .between said electrodes, a second pair of spaced electrodes supported between said first pair of electrodes, said electrodes in said second pair being positioned on opposite sides of said plane between said first pair of electrodes, means for generating ions having a plurality of mass-to-charge ratios between said electrodes, and a source of radio frequency voltage connected to said second pair of electrodes for generating therebetween a radio frequency field whereb ions having a given mass-' to-charge ratio will be accelerated to and fro by said radio frequency field against the opposing force of said parabolically distribute-d voltage to overcome ultimately said opposing dorce while ions having other mass-to-charge ratios will fail to overcome said opposing force, magnetic means positioned exteriorly of said envelope for producing an axial magnetic field between said first pair of electrodes to retain said ions within the space therebetween, and means for collecting said ions having a given mass-to-charge ratio.

17. A mass spectrometer comprising an evacuable envelope, a first plurality of coaxially aligned potential grading rings disposed within said envelope on one side of a plane traversing said envelope, a second plurality of coaxially aligned potential grading rings disposed within said envelope on the opposite side of said plane, a source of direct current voltage connected to said first, and second pluralities of grading rings for es-- tablishing therealong a parabolic potential dis-- tribution having its apex lying upon said plane; and increasing positively in both directions from, said plane, a source of radio frequency voltage,- connected in circuit with said source of direct, current voltage for varying the rate of change of slope of said parabolic potential distribution,, means for generating ions Within said envelope; whereby ions having a given mass-to-charge rartio will be accelerated to and fro by said vary-- ing potential distribution to emerge ultimatelyfrom the region of influence thereof while ions: having other mass-to-charge ratios will fail to so emerge, and means for collecting said ions having a given mass-to-charge ratio,

18. A mass spectrometer comprising an evacuable envelope, a first plurality of coaxially aligned potential grading rings disposed on one side of a plane traversing said envelope, a second plurality of coaxially aligned potential grading rings disposed within said envelope on the opposite side of said plane, means constituting a potential divider connected to said pluralities of grading rings, means for energizing said potential divider to establish along said pluralities of grading rings a parabolic potential distribution having its apex lying upon said plane and increasing positively in both directions from said plane, a source of radio frequency voltage connected in circuit with said potential divider energizing means for generating ions within said envelope whereby ions having a givenmass-to-charge ratio will be accelerated to and fro by said varying potential distribution to emerge ultimately from the region of influence thereof while ions having other mass-to-charge ratios will fail to so emerge, and means for collecting said ions having a given mass-to-charge ratio.

19. A mass spectrometer comprising an evacuable envelope, a pair of opposed electrodes sup ported within said envelope, said electrodes having the shape of hyperboloids of revolution, a source of direct current voltage connected to said electrodes to establish therebetween a parabolically distributed voltage the apex of which lies upon the median plane between said eelctrodes, means for generating ions having a plurality of mass-to-charge ratios between said electrodes, 2; source of radio frequency voltage connected in circuit with said source of direct current voltage for varying with time the rate of change of slope of said parabolicall-y distributed voltage whereby ions having a given -mass.to-charge ratio will be accelerated to and fro between said electrodes until they ultimately reach'a desired axial dis placement while ions having other mass-tocharge ratios will fail to reachsuch displacement, and means for collecting said ions having a given mass-to-charge ratio.

16 20. A mass spectrometer comprising an evacii able envelope, apair of opposed electrodes supported within said envelope, said electrodes having the shape of hyperboloids of revolution, a source of direct current voltage connected to said electrodes to establish therebetween a parabolically distributed voltage the apex of which lies upon the median plane between said electrodes, means for generating ions having a plurality of mass-to-charge ratios between said electrodes, a source of radio frequency voltagecon nected with said source of direct current voltage for varying with time the rate of change of slope of said parabolically distributed voltage whereby ions having a given mass-to-charge ratio will be accelerated to and fro between said electrodes until they ultimately impinge thereupon While ions having other mass-to-charge ratios will not receive sufiicient' acceleration to reach said electrodes, and means in circuit with said electrodesfor measuring the ion current generated by the impingement thereupon of said ions having a given mass-to-charge ratio. PAUL O. SCI-IISSEL;

No references cited.