US2798956A - Ion resonance mass spectrometer - Google Patents

Description

y 1957 K. P. LANNEAU ET AL 2,798,956

ION RESONANCE MASS SPECTROMETER Filed June 9, 1954 RA IO FR U CY g ig i DIFFERENTIAL FOR SS FEED BACK AMPLIFIER A? SPECTROMETER I2 i II II I RECTIFIER I I MASS SPECTROMETER AMPLIFIER RECORDER FIG.I 2O

RADIO A s IIO FREQUENCY MASS Q O OSCILLATOR SPECTRO- METER RECORDER FOR MASS METER AMPUHER SPECTROMETER 38 Ix IIx IO POSITION A POSITION B POSITION C I I I I I I I I KEITH P LANNEAU INVENTORS LLOYD H. LANE 33\ 34 Ian WW ATTORNEY RELAY COIL V SUPPLY FIG nite States Patent ION RESONANCE MASS SfECTROMETER Keith P. Lanneau and Lloyd H. Lane, Baton Rouge, La, assignors to Esso Research and Engineering Company, a corporation of Delaware Application June 9, 1954, Serial No. 435,524

4 Claims. (Cl. 250-413) This invention relates to mass spectrometry and more particularly relates to the resolving power and sensitivity of radio frequency ion resonance mass spectrometers. Still more particularly, this invention concerns a method and means for adjusting the amplitude of the radio frequency voltage of a radio frequency ion resonance mass spectrometer to provide optimum resolution and sensitivity.

The present invention concerns an improved form of ion resonance mass spectrometer of a type which has been described in the literature, such as in a paper presented at the American Chemical Society Symposium on Process Instrumentation, Chicago, Illinois, September 9, 1953, entitled An Ion Resonance Mass Spectrometer for Industrial Application by W. A. Morgan, G. Jernakoff and K. P. Lanneau. In the ion resonance mass spectrometer, the ionization chamber serves not only as a Zone for ionizing the gaseous sample but also serves as the analyzer section of the mass spectrometer. This is accomplished by impressing a radio frequency electric field by means of electric voltage plates across the ionization chamber perpendicular to the electron beam, and a magnetic field across the ionization chamber perpendicular to the electric field. The combination of the electric and magnetic fields tends to move the ions formed in the ionization chamber in a spiral path in the plane of the electric field. For a particular set of conditions, namely, for a given electric and magnetic field, only ions of a particular m./e. ratio will continue to be accelerated in a spiral path of increasing diameter. These ions have natural frequencies which correspond to the frequency of the applied radio frequency electric field, and are called resonant ions. Located at a fixed distance from the center of the ionization chamber in the plane of the ion path is an ion target upon which the resonant ions impinge. resonant ions, will oscillate in the analyzer section due to the effect of the electric and magnetic fields but will never be accelerated sufi'iciently 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. The ion resonance principle is described in detail in the literature such as in the article in Physical Review of June 1, 1951, entitled The measurement of e./rn. by cyclotron resonance by H. A. Sommer, H. A. Thomas and J. A. Hipple.

The quantity and the m./e. ratio of ions striking the ion target is an indication of the composition of the gaseous sample. By varying the frequency of the electric field while maintaining a constant magnetic field, and/ or by varying the strength of the magnetic field while maintaining the frequency of the applied alternating electric field constant, ions having different natural frequencies, and hence different m./ e. ratios can be collected. The quantities of ions of different m./e. ratios thus col- The other ions, which are termed nonice lected can be used as a measurement or indication of the composition of the gaseous sample. This measurement may be accomplished for example by determining the current produced by the ions striking the ion target. Because the ion current developed in the mass spectrometer is very small, it is normally amplified, converted to a voltage, and the amplified voltage is then fed to a recorder.

An ion resonance mass spectrometer may be employed to determine all of the components of a gaseous sample by a continuous scan of the mass spectrum or it may be employed to selectively measure only several of the components of the gaseous sample. If it is desired to analyze a gaseous sample for all of its constituents by a continuous scan, normally the magnetic field is maintained constant while the frequency of the radio frequency voltage is uniformly varied over a range which will include all of the ions of the sample. If, on the other hand, only several of the constituents of the gaseous sample are to be analyzed, only a few preselected frequencies will be chosen so that only ions of m./e. ratios corresponding to these frequencies will actually be measured. Each type of analysis requires a particular method and apparatus to accomplish the desired analysis. In an analysis of the entire sample, the tuning condenser of the mass spectrometer oscillator may be uniformly varied by mechanical means such as a motor. On the other hand, in an analysis wherein only ions of several m./e. ratios are to be measured, a number of fixed condensers may be successively employed. However, in this latter type of analysis, it will be apparent that it is essential that the proper frequency be selected so that a correct reading for the ions of each m./ e. ratio may be successfully determined.

A number of difficulties have been encountered in measuring ions of different m./e ratios in either of the two types of analyses described above. More specifically, it has been found by applicants that the sensitivity and the resolving power of an ion resonance mass spectrometer are dependent upon the particular m./e. ratio measured. The term resolving power of a mass spectrometer as used herein refers to the instruments ability to separate ions of a given m./e. ratio from ions having m./e. ratios immediately above and below the given m./e. ratio, and the term sensitivity of a mass spectrometer as used herein refers to the instruments ability to collect on the ion target a high concentration of ions of a given m./ e. ratio and to reject others. More particularly it has been found by applicants that when varying the frequency of the mass spectrometer to thereby select ions of diiferent m./e ratios, the resolving power of the instrument decreases at high m./e ratios and therefore in accordance with this invention the amplitude of the radio frequency voltage is varied inversely with m./e ratio to thereby prevent this decrease in resolution. In addition, it has been found by applicants that when measuring ions of any given m./e. ratio, this sensitivity of the mass spectrometer is dependent upon the amplitude of the radio frequency voltage employed and therefore in accordance with this invention the amplitude of the radio frequency voltage is varied to thereby also provide optimum sensitivity throughout the analyses.

Thus, to obtain optimum operation of an ion resonance mass spectrometer, it has been found that it is necessary to consider not only the frequency of the radio frequency voltage but also the amplitude of the radio frequency voltage when measuring ions of any given m/e. ratio. The present invention provides a method and means for varying the amplitude of the radio frequency voltage of an ion resonance mass spectrometer when varying the frequency of the radio frequency voltage in either a continuous scan of the entire mass spectrum or a sequential selection of ions of several individual m./e. ratios.

An object of the present invention is to provide a method and means for obtaining the optimum resolving power with an ion resonance mass spectrometer'for ions of any given m./e. ratios.

A further object of the present invention is to provide a method and means for obtaining maximum sensitivity as well as adequate resolution with an ion resonance mass spectrometer when measuring ions of any givenrn/e. ratios. 1

Further objects of the present invention will be apparent from a reading of the specification and the drawings in which Fig. 1 is a partial block diagram of one embodiment of the present invention adapted for a continuous scan of a mass spectrum and Fig. 2 is a partial block diagram of another embodiment of the present invention adapted for a sequential selection of ions of several different m./e. ratios.

Referring now to Fig. 1, reference character designates a radio frequency ion resonance mass spectrometer, and reference character 11 designates an oscillator adapted to vary the frequency of the radio frequency voltage of mass spectrometer 10. The radio frequency voltage is impressed across end voltage plate 111, intermediate voltage grading plates 112, 113, 114, 120 and end voltage plate 115 of mass spectrometer 10 through resistance divider network 116, end voltage plate 115 being connected to ground 117. The intermediate voltage grading plates 112, 113 and 114 and 120 are cut out in their central portion to provide chamber 118 which serves as the ionization chamber as well as the analyzer section. Extending through end voltage plate 115 into chamber 118 is ion target 119 upon which the resonant ions impinge to produce an ion current. Oscillator 11 is provided with a tuning condenser 12 which is mechanically connected to synchronous motor 13. In this particular embodiment of the invention, motor 13 uniformly adjusts condenser 12 so as to provide a continuous scan of the mass spectrum of a gaseous sample being analyzed in mass spectrometer 10. The ion currents obtained from ions of the various m./e. ratios in mass spectrometer 10 are converted to voltages which are amplified by mass spectrometer amplifier 14, and the amplified voltages which are a measure of the quantities of their corresponding ions are registered by recorder 15. The foregoing is conventional in ion resonance mass spectrometry.

Since it has been found by the applicants that it is desirable to vary the amplitude of the radio frequency voltage inversely with the m./ e. ratio to thereby maintain the resolving power of mass spectrometer 10 at an optimum value, this particular embodiment of the invention provides a method and means for accomplishing this throughout the entire analysis. Motor 13 is thus mechanically linked to potentiometer 16 so that during the course of the analysis, the potential generated by potentiometer 16 is also varied at the same time the frequency of the radio frequency voltage is varied by condenser 12. Potentiometer 16 is connected into circuit 17, through which a direct current flows from battery 18 which is connected to ground 20. Motor 13 varies the position of potentiometer tap 16a on resistance 19 of potentiometer 16 so as to produce a varying potential from potentiometer 16.

Thus, for each position of condenser 12, there is a corresponding direct current reference potential generated by potentiometer 16. This reference potential is fed to differential feedback amplifier 21 wherein the output of potentiometer 16 is compared with the amplitude of the radio frequency voltage of mass spectrometer 10. This latter feature is accomplished by connecting oscillator 11 to primary grid 121 of cathode follower 23. The radio frequency output from cathode follower 23 is fed to rectifier 22 as well as to mass spectrometer 10. The radio frequency output of cathode follower 23 is rectified in rectifier 22 and is then fed to differential feedback amplifier 21 which compares the rectified radio frequency output from cathode follower 23 with the reference potential generated by potentiometer 16. Any difference between these two potentials is amplified and fed back as a corrective control signal to secondary control grid 122 of cathode follower 23. The present invention in this manner provides a method and means for maintaining an optimum value of radio frequency amplitude for each frequency selected for mass spectrometer 10.

While it is generally preferred to vary the amplitude of the radio frequency voltage inversely with frequency, it will be understood that the optimum relationship for certain spectrometers may require that 16 be other than a linear potentiometer. In the preferred embodiment of this invention, the circuit constants of oscillator 11 are arranged such that a rotation of condenser 12 will span a frequency covering the mass range desired. Also potentiometer 16 is preferably of the continuous 360 rotation type. In an alternative arrangement, capacitor 12 could be replaced with a potentiometer which could be mechanically ganged with potentiometer 16. This form of potentiometer would serve to provide a varying D. C. current to an R. F. oscillator of the current controlled type. f

Referring now to Fig. 2, reference character 10 designates a radio frequency ion resonance mass spectrometer identical with the mass spectrometer of Fig. 1. The ion current from mass spectrometer 10 is converted to a voltage and amplified by mass spectrometer amplifier 14 and then the amplified voltage is sent to recorder 15. The form of the present invention shown in Fig. 2 is employed in a type of analysis wherein a sequential selection of ions of several m./e. ratios is made. The frequency of the radio frequency voltage of mass spectrometer 10 is changed by oscillator 11-a by selecting different pairs of condensers for ions of each m./e. ratio desired to be measured. Oscillator 11-a is provided with condenser banks 35, 36, and 37 in Fig. 2. However, it will be understood that the present invention is not to be considered as being limited to solely three banks of condensers as it may be utilized with any number of such banks. The sequential selection of condenser banks is accomplished as follows. Motor 30 having shaft 31 is continuously operated throughout the analysis. Connected to shaft 31 is cam 32 which, every 360 of revolution, closes the circuit fed by relay coil supply 33. When this occurs, energy is developed in stepper relay 34 so that it mechanically connects in successive banks of condensers. In this manner, ions of different m./e. ratios are selected by properly choosing the capacitances of the various condenser banks. If the capacitances are properly preselected then the reading obtained for ions of each m./e. ratio will be correct and the instrument will detect the top of each mass peak. The term mass peak as used herein refers to the maximum ion current which can be detected when measuring ions of any particular m./e. ratio.

However, because of the difficulties encountered in focusing directly on the tops of mass peaks, this embodiment of the invention is arranged to select frequencies at the base of individual mass peaks and then to scan over each suchmass peak. This is accomplished by means of condenser bank 38, which is at all times connected into the circuit of oscillator 11-a. Condenser bank 38 is provided with a pair of condensers which are of the continuous rotation type and have a magnitude just large enough to cause a scan across the mass peak. As motor 30 rotates, the capacitance of condenser bank 38 is uniformly varied during a rotation of 360 of shaft 31 of motor 30. In this way, motor 30 operates continuously causing. not only a stepwise selection of ions of different m./e. ratios but also each m./e. ratio is scanned because of the action of condenser bank 38.

Oscillator 11-a is connected to mass spectrometer through resistance divider network 42, which is made up of a number of fixed resistances such as resistance 39, 40, and 41. The tap on divider network 42 is mechanically linked to stepper relay 34 such that each time a difierent condenser bank is connected to oscillator 11-a, a different resistance is thrown into the circuit connecting oscillator 11-a with mass spectrometer 10. The resistances in divider network 42 are preselected to provide an optimum amplitude of the radio frequency voltage for mass spectrometer 10 for each selection of frequency from oscillator 11-a. In this way the resolving power and sensitivity of mass spectrometer 10 are maintained at an optimum value for each mass peak selected.

Mass spectrometer amplifier 14 is connected to recorder 15 through divider network 46 which has a number of different preselected resistances such as resistance 43, 44, and 45. Divider network 46 is mechanically linked to stepper relay 34 similar to the mechanical connection of stepper relay 34 to divider network 42. Divider network 46 provides a means for varying the recording sensitivity of recorder 15 to provide an optimum recording sensitivity for each m./e. ratio individually scanned.

In Fig. 2, the invention is shown operating in position B, such that condenser bank 36 is being employed to tune oscillator 11-a, potentiometer 42 is connected between resistances 40 and 41, and divider network 46 is connected between resistances 44 and 45. The resistances of divider network 42 and 46 are predetermined to provide optimum resolving power of mass spectrometer 10 and the optimum recording sensitivity of recorder 15 for the frequency obtained with condenser bank 36. In the operation of this embodiment of the present invention, the initial reading on recorder 15 is accomplished when position A is utilized. Upon a 360 rotation of shaft .31 of motor 30, the present invention shifts to position B, and after another 360 revolution of shaft 31, the present invention shifts to position C. Similarly other positions could be obtained by employing additional condenser banks for oscillator 11-a and additional resistances in divider networks 42 and 46. Divider networks 42 and 46, shown as series networks, could be separate parallel type networks to provide maximum flexibility of selection of the radio frequency voltage amplitude and recording sensitivity for each mass to be scanned.

It will be understood that it is possible to employ the corrective control signal system shown in Fig. l in Fig. 2. That is, divider network 42 in Fig. 2 could be used in a manner similar to potentiometer 16 in Fig. 1 to provide an R. F. amplitude signal for an automatic amplitude control circuit as in Fig. 1.

Just as in Fig. l where it was possible to replace capacitor 12 with a potentiometer to operate a current controlled type of oscillator, in Fig. 2 it is similarly possible to replace condenser banks 35, 36, 37, and 38 with variable resistances to operate a current controlled R. F. oscillator. The advantage of this particular embodiment is that the frequency controls can be more conveniently remotely located from the oscillator than can the capacitors.

EXAMPLE The following example of the invention is presented in order (1) to more specifically set forth the invention, and (2) to illustrate the advantages to be obtained therewith in regard to the resolution and sensitivity of an R. F. ion resonance mass spectrometer. However, it will be understood that this example is not to be considered as limiting the invention in any way to the specific values set forth therein.

An ion resonance mass spectrometer, of the type described above wherein ions are accelerated in a spiral path, was utilized in several tests to measure the ions of certain mass numbers of normal butane, namely mass numbers 58, 43, 29 and 15. In Test 1 the amplitude of the R. F. voltage was maintained constant throughout at 1.4 volts; 1 in Test 2 the R. F. voltage amplitude was maintained constant at 3.6 volts; 1 and in Test 3 the R. F. voltage amplitude was automatically controlled in accordance with this invention as follows:

Volts, R. M. S. (root-mean-squared voltage, as commonly used in electrical engineering terminology).

The following results were obtained:

Test 1 Mass No 43 29 15 Peak Height 517 202 7 Resolution, Percent 100.0 100.0 100.0 Peak Shape Good Sharp Sharp Sharp Test 2 Mass No 58 43 29 15 Peak Height 1 775 333 19 Resolution, Percent 2 0 85.0 98.5 100. 0 Peak Shape Asymmet- Asymmet- Good Good ric rical Test 3 Mass No 58 43 29 15 Peak Height 1 65 590 284 17 Resolution, Percent 98. 5 99. 9 100.0 100.0 Peak Shape Good Good Good Good 1 Representsi on currents expressed as divisions on the recorder chart.

2 Represents the ratio of the height of a given peak above the valley between it and the adjacent peak, divided by the height of the given peak above the zero ion current base line, expressed on a percentage basis. The pairs of peaks involved are: 58 and 57, 43 and 42, 29 and 28, and 15 and 14.

From the results of Test 3 it will be seen that by varying the amplitude of the R. F. voltage inversely with respect to mass number, better overall operation of the R. F. ion resonance mass spectrometer is obtained. More specifically it will be noted that better overall resolution was obtained in Test 3 than in Test 2 and better overall sensitivity was obtained in Test 3 than in Test 1. Thus it will be noted that by varying the amplitude of the R. F. voltage inversely with respect to mass number, it is possible to obtain both optimum resolution and sensitivity throughout an analysis.

What is claimed is:

1. In a radio frequency ion resonance mass spectrometer including a radio frequency oscillator adapted to provide a radio frequency electric field of variable frequency, the improvement which comprises means for inversely varying the amplitude of said radio frequency electric field as said frequency of said field is varied.

2. In a radio frequency ion resonance mass spectrometer including radio frequency voltage plates and a radio frequency oscillator having a tuning condenser for changing the frequency of said voltage plates, the combination of means for producing a direct current reference voltage potential, means for varying said reference voltage potential simultaneously with a change in frequency of said voltage plates, means for comparing the amplitude of the voltage of said voltage plates with said reference voltage potential, and means for adjusting the amplitude of the voltage of said voltage plates to compare with said reference voltage potential.

3. In combination with a radio frequency ion resonance mass spectrometer including radio frequency volt age plates and a radio frequency oscillator having a tuning condenser mechanically operated by a scan motor, the

combination of a cathode follower electrically connected at its primary grid to said oscillator and electrically connected at its cathode to said voltage plates, a potentiometer electrically energized by a source of stable direct current, said potentiometer including a slideable tap mechanically operated by said scan motor, a differential feedback amplifier electrically connected at its input side to said slide tap of said potentiometer and electrically connected at its output side to a secondary control grid of said cathode follower, and a rectifier electrically connected at its input side to the cathode of said cathode follower and electrically connected at its output side to the input side of said ditferential feedback amplifier.

4. In an ion resonance mass spectrometer utilizing the combined action of crossed magnetic and radio frequency electric fields to effect separation of ions having difierent m./e. ratios and including a plurality of voltage plates to which a radio frequency voltage is applied, the improvement comprising a variable frequency oscillator having a frequency adjusting circuit, the output radio frequency voltage of said oscillator being operatively coupled to said voltage plates of said mass spectrometer, and means intermediate the output of said oscillator and said voltage plates for varying the amplitude of said output voltage of said oscillator in inverse relation to the frequency of said output voltage when operating said frequency adjusting circuit to effect the separation of ions having different m./e ratios.

No references cited.

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