US2868986A - Ion resonance mass spectrometer - Google Patents

Description

Jan. 13, 1959 K. P. LANNEAU ET AL I 2,368,986

ION RESONANCE MASS SPECTROMETER Filed June 8, 1954 24 I I I I H n I H y n 3 I I lrr 2 f 2O 5 28 (5; 327 2| '6 '3 IO 29L \/I 22 I? u I 23 I I, I ["P 3?; 36 37 35/ 3s T0 AMPLIFIER FIG. I

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TO AMPLIFIER KEITH P LANNEAU LLOYD H LANE INQVENTORS ranged parallel one to the rates Patent fiice 2,868,986 ION RESONANCE MASS SPECTROMETER Keith P. Lanneau and Lloyd assignors to Esso Research a corporation of Delaware Application June 8, 1954, Serial No. 435,160 Claims. (Cl. 250-41.9)

H. Lane, Baton Rouge, La., and Engineering Company,

This invention relates to mass spectrometry and more particularly relates to radio frequency ion resonance mass spectrometry. Still more particularly, this invention relates to a method and means for applying voltage potentials to plates of a radio frequency ion resonance mass spectrometer.

The application of mass spectrometry in the industrial field has become increasingly more important in recent possible accurate and rapid analyses of the components of gaseous mixtures. For example, mass spectrometers have been successfully utilized for continuous process monitoring, routine gas analysis, leak detection, and trace constituent analysis. One particular type of mass spectrometer. is the radio frequency ion resonance mass spectrometer. of mass spectrometer has been described in the literature, such as in a the Omegatron. The voltage plates are normally arvoltage plates provided with a central opening, which opening defines the ionization chamber and the analyzer section of the mass spectrometer. The magnetic field, which may be produced by a permanent magnet or electromagnet, is aligned with the electron beam so that the magnetic field acts perpendicularly to the electrical field.

By varying the frequency of the radio frequency voltother with all but the end The Measurement of e/m by Cyclotron Resonance 2 age impressed on the radio frequency voltage plates, it is possible to measure the amounts of ions of diiferent mass/charge (m/e) ratios. In general, for a given frequency electric field and a given strength magnetic field, only ions of one particular m/e will impinge upon the ion target, and these particular ions are known as resonant ions. The ion resonance principle is describedin detail in the article in Physical Review of June 1,. 1951, entitlgd Y H. Sommer, H. A. Thomas and J. A. Hippie. More specifically only the ions formed in the ionization chamber which are resonant will continue to be accelerated in a spiral path of increasing diameter so as to impinge upon the ion target. The non-resonant ions, on the other hand, will oscillate in the analyzer section due to the effect of the electrical and magnetic fields but will never be-accelerated sufiiciently to impinge upon the ion target. These non-resonant ions periodically spiral to a maximum radius or distance short of the ion target and then spiral back to the center of the analyzer section or tube as this section is sometimes called. It is essential that both a radio frequency (R. F.) electric field and a magnetic field be employed in this type of accomplish these results.

Features such as resolving power, sensitivity, linearity, cracking stability, background interference, etc., are important in analytical mass spectrometers. In particular, the present invention is concerned with-the resolving power, sensitivity, stability and linearity of ion resonance mass spectrometers or Omegatrons. The resolving power of a mass spectrometer is a measure of its ability to separate ions of a particular m/e ratio from ions of other m/e ratios. The sensitivity of a mass spectrometer is a measure of its ability to collect on the ion target a high concentration of ions of a particular m/e ratio and to reject others. Linearity is a measure of the ability of a mass spectrometer to produce an ion current whose magnitude is directly proportional to the concentration of resonant ions which are formed by the electron bombardment of sample gas molecules. For an analytical instrument it is further required that ions of a given m/ e ratio be formed in direct proportion to the molecular concentration of each stability be virtually constant.

The present invention provides a method and means for improving these qualities of a radio frequency ion resonance mass spectrometer. This is accomplished by employing a particular non-linear and asymmetrical R. F. field rather than the linear R. F. fields now employed in these types of mass spectrometers wherein the: amplitude of the radio frequency voltage varies linearly from voltage plate to voltage plate. A substantial improvement in the resolving power, sensitivity, stability and linearity of an ion resonance mass spectrometer is effected by the present invention; In addition, the present invention is concerned with the application of direct current voltages to the voltage plates in combination with the radio frequency voltage impressedon the plates. The present inwith the non-linear asymmetrical R. F. electric field of resolving power, sensitivity, stability and linearity of the radio frequency ion resonance mass spectrometer.

The discussion regarding the present invention will be apparent from a reading of the specification, which may be more clearly understood by reference to the drawings in which:

Fig. 1 is a diagrammatic cross-sectional view of a mass spectrometer in order to I present invention is conventional radio frequency ion resonance mass spec trometer, taken perpendicular to the electron beam;

Fig. 2 is a diagrammatic cross-sectional view of a radio frequency ion resonance mass spectrometer showing. a particular embodiment of; the present invention for producing-a non-linear: asymmetrical electric field, and

Fig. 3. isv a diagrammatic cross-sectional view of a radio frequency ion resonance mass spectrometer, similar toFig. 2, showing another embodiment of the presentinvention which incorporatesa combination of a non-linear radio. frequency electric field and a D. C. voltage field.

Referringnow to the drawings, Fig. 1 illustrates a con ventional, radio frequency ion resonance mass spectrometer. The mass spectrometer is provided with oscillator 10,for: producing an R. F. electric field. Oscillator 19 is connected: to.ground 12 andto plate 24 through coupling condenser 1-1; Reference character 13 designates a resistance divider network-which comprises a series of equal resistances. 14, 15; 16; 17, and 18. Divider networlc 13 isprovided'with electric taps19, 2th, 21, 22, 23, and 24. for. connection to. the radio frequency voltage platestofthe mass spectrometer. The mass spectrometer is provided with end voltageplates 24 and and a series. of voltage gradingplates 26, 27,- 28, and 22 which are provided with central openings soas to provide a zone for the analyzing section of the mass spectrometer. The voltage platesare provided with electric taps 3t 31, 32, 33, 34, and 35 for electrical connection to the electric taps 'of-divider network 13. Thus, electric tap is connectedato-electrictap 19; electric tap 31 is connected to electric tap20; electric tap 32 is connected to electric tap- 21', electric tap- 33 is connected to electric tap 22; electric tap 34 is connected to electric tap 23, and electric tap is connected to electric tap 24. In this particular arrangement, the R. F. electric field is linearly distributed fromplate 24 to voltage plate 25 through grading plates 26 to 29. This results'in a linear and uniform R. F. field in a region enclosed between the end plates and cutouts of the intermediate plates.

Electron beam 36 is directed perpendicularly to the plane of- Fig. 1 and through the center of the ionization chamber which, in this type of mass spectrometer, also serves as the analyzer section of the mass spectrometer. The resonant ions follow the spiral path indicated by the dotted lines in the analyzer section of the mass spectrometer and impinge upon ion target 37. The ion current produced by the impingement of the resonant ions on ion target 37 is generally sent to an ion current amplifier and then to a recording device. Divider network 13 and voltage plate 25 are connected to ground 38. The foregoing, as illustrated in Fig. 1, is conventional in radio frequency ion resonance mass spectrometers.

Fig. 2 illustrates an embodiment of the presentinvention which utilizesa non-linear asymmetrical R. F. electric field; that is asymmetrical as related to the center of the tube. In this particular form of the present invention, oscillator lt} which is employed to produce an R. F. electrical Voltage is connected to ground 12 and electric contact 34 of voltage grading plate'29-through coupling condenser 11. Voltage plates 24 and 25 and grading plates 26;, 27, and28 are connected by means of electric. taps 30, 35, 31, 32, and 33, respectively, to ground 38, such that all voltage plates except voltage grading plate 29 areat ground potential. Thus only voltage grading piate 29 is coupled by circuit component to the R. F. voltageproduced by oscillator 10. Although the action of the non linear electrical field on the ions which are forrrrcdgbyelectron beam 36 and which aredirected to ion target 37 is not fully understood, the result of the to substantially increase the resolving power, sensitivity, linearity and stability of the mass spectrometer. It will be noted that the divider network 13 shown in Fig. l is not utilized in the form'of invention shown in Fig. 2.

It is to be clearly understood that the R. F. electric voltage may be impressed on any one of the voltage gradingv plates 26, 27, 28, or 2% and is not restricted solely to. voltage grading plate 29. It will also be understood that the form of the present invention illustrated in Fig. 2 is not restricted to four voltage gradingplates as shown but may be employed with more or less voltage grading plates in accordance with the present invention. It is preferred however to couple the R. F. voltage from oscillator 10 through a circuit component to one of the plates near or adjacent to the end plate through which the ion target is inserted. It will further be understood that other means for developing non-linear asymmetrical fields can be utilized in this invention.

Referring now to Fig. 3, it will be seen that the nonlinear electric field shown in Fig. 2 is combined with a unique application of D. C. voltage to the plates. Oscillator 10 is employed to produce a radio frequency voltage and is connected to ground 12 and electrical tap 34 of voltage grading plate 29 through coupling condenser 11- in the same way as in Fig. 2.

It will be noted that the circuit shown provides for essentially the same R. F. fields developed by the circuit of. Fig. 2. In this case capacitors 43 and 44 are of sufficient magnitude to provide a virtual short circuit for R. F. voltages of the frequencies with which we are concerned. Therefore, as in Fig. 2, the R. F. voltage-is supplied from the oscillator 1t) to plate 29 through capacitor 11 and tap34, and all other plate elements are at essentially R. F. ground potential.

Fig. 3 difiers from Fig. 2 only in the respect that certainD. C. voltages have been applied in combination with the same R. F. circuit.

Voltage sources 46, 41, and 42 may be variable sources of D. C. potential in the range of 0.001-3 volts suchas would be derived from batteries and low resistance divider networks. These voltage sources have a common positive side, which is at ground potential. The negative side of D. C. voltage source 41 is applied to plate 24 at tap 30 andis isolated from the other plates bycapacitor 43. The negative side of DC. voltage source 42 is applied tograding plates 26 and 28 at taps 31 spectively. Capacitor 44 serves as an R. F. short to ground across D. C. voltage source 42. Plates 2,7 and 25 are directly connected to ground.

As previously mentioned R. F. oscillator 10is coupled to plate29 at tap 34 through capacitor 11, Also the negative side of D. C. voltagesource 40 is coupled.

to plate 29'at tap 34 through resistor 45. Resistor 45 is of sufficient magnitude to prevent impedance loading of'oscillator 10.

If R: F. potential is applied to a plate. other than 29, then D. C. voltage source 40 may also beapplied to the same plate as the R. F. Itis preferable. however that th is plate be near target 37. D. C. source, 41 is preferably applied to terminal plate 24.. in any case. D. C. voltage source 42 might be applied to,a.pair. of plates other than 26 and 28,- but in any case such pair of plates should be in thecentral part of the tube and should lie on either side ofv a, grading plate near the center of the tube and the electron beam.

It is essentialthat thepolaritybf the D. C. voltages developed by D. C. sources40, 41, and 42 be as indicated in Fig. 3. The magnitudes .of the three potentials developed by these batteries are simultaneously adjusted to give-optimum sensitivity, linearity and resolution for the mass spectrometer. This will-also result in the best stability. For different m/e ratios, R; F. voltages, elec-v tron beam intensities, and operating pressures, there may be different optimum values for theDa C. voltages, but it is possible to selectgan optimum set of D. C. voltages which produce excellent results .over an extended range in the mass spectrometer.- ltwill be understood'that theap'plication of D. C.

and 33 revoltagesto the R." Fivoltagefield may be employed with a linear R. F. voltage fieldor any form of non-linear asymmetrical R. field with desirable results.

A comparison was made on operating characteristics of (I) a linear R. F. field ion resonance mass spectrometer substantially as shown in Fig. 1 with (II) 5 the ion mass spectrometer of this invention. utilizing a non-linear asymmetrical R. F. field substantially as shown in Fig. 3. In both cases optimum D. C. potentials were applied to the plates. The results are as F. voltage follows: N

I II Results Linear Non-Linear R. F. Field R. F. Field N Maximum ion mass (m/e) satisfactorily resolved 43 100 Ion current magnitude at H'OAHIO Mass eak, Amp 10- 10- ity Fair Good Synthesis Typical Analysis: Percent Percent4 In each case various operating parameters of the tube were adjusted to provide maximum reproducibility and linearity.

These results show a decided advantage for II in terms of resolution, ion current sensitivity, stability and accuracy in a hydrocarbon gas analysis. It was found that a failure to use the D. C. voltage gives vastly inferior results in both cases.

The advantage of the asymmetrical non-linear field is believed to be due to the minimization of the space charge limitations in the operation of the tube. D. C. potentials have a similar effect and therefore optimize focusing of the resonant ion beams. It will be understood that other non-linear types of fields may also be employed to obtain desirable results in accordance with this invention.

What is claimed is:

l. A mass analyzing instrument including an analyzer chamber, means for ionizing samples of matter introduced into said chamber, means for producing a magnetic field across said analyzer chamber, means for producing a concentrated alternating electric field gradient normal to said magnetic field across substantially only an asymmetrically disposed elemental region of said analyzer chamber, said alternating electric field having a frequency of alternation corresponding to the natural frequency of ions possessing a certain mass whereby such ions may be accelerated in spiral paths, means for collecting the ions thus accelerated, and means for removing ions having an undesired natural frequency from the region of said crossed magnetic and alternating electric fields.

2. In an ion resonance linear, radio frequency electric field across said chamber which field is normal to said magnetic field, asymmetrically related to the center of said chamber and of a radio frequency at which saidselected ions substantially are resonant, and said non-selected ions substantially are non-resonant, and means for substantially removing said non-selected, non-resonant ions from said chamber.

3. An apparatus according to claim 2, wherein said improved means for accelerating said ions of characteristic mass/charge ratio in a spiral path includes a series of spaced, parallel voltage plates positioned within said analyzer chamber, said series comprising a pair of terminal end voltage plates and a plurality of grading voltage plates intermediate said end plates, a radio frequency voltage source including two terminals of which one is connected to ground, electrical coupling means connected between the other terminal of said radio frequency source and an intermediate grading voltage plate disposed asymmetrically of said series whereby to establish said asymmetrical, non-linear radio frequency field, and grounding means connected to the remaining voltage plates in said series.

4. In an ion resonance an analyzer chamber, duced into said chamber, whereby said matter is ionized to produce ions of characteristic mass/charge ratios, and

target means extended into said chamber for collecting proved means for accelerating said ions in a spiral path whereby said selected ions substantially are accelerated in said spiral path into preferential contact with said target means therefor, and non-selected ions substantially are removed from said spiral path and analyzer chamber, comprising in combination, means for producing a magnetic field across said chamber, a series of spaced, parallel voltage plates positioned Within said analyzer chamber, said series comprising a pair of terminal end voltage plates and a plurality of grading voltage plates intermediate said end plates, wherein said ion target extends into said chamber through one of said end plates, a radio frequency voltage source including two terminals of which one is connected to coupling means connected between the other terminal of said radio frequency source and a grading plate disposed asymmetrically of said series and in an elemental region of said chamber which in- Whereby to establish in said reresonant, and said non-selected ions substantially are non-resonant, and means for substantially removing said non-selected, non-resonant ions from said chamber.

5. In an ion resonance removed from said spiral path, comprising in combinasource including two nected to ground, between the other terminal of said radio frequency voltsaid magnetic electrical coupling means connected between the negative terminal of said first D. C. source and'said first voltage plate, a second source oiD. C. voltageyelecttical coupling means connected between the negative terminal of said second D. C. source and a pair of voltage plates intermediate the ends of said series, said pair of voltage plates separated one from the other by a fourth intermediate plate of said series in a central portion of said chamber and said series of voltage plates, a third source of D. C. voltage, electrical coupling .means connected between the negative terminal of said third D. C. source and the terminal end plate of said seriesjfarthest removed from said ion target, and grounding means connected to the remaining voltage platesin vsa'd se'ries which are not connected to the negative terminal of .any of said voltage sources.

References'Cited in the file of this patent UNITED STATES PATENTS 2,698,389 Jernakofi Dec. 28, 1954

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