US2772365A - Mass spectrometer - Google Patents

Description

Nov. 27, 1956 Filed Dec. 20, 1954 RECORDER C. B. SHELMAN MASS SPECTROMETER 2 Sheets-Sheet l c. B. SHELMAN BY v H i n Nov. 27, 1956 c. B. SHELMAN MASS SPECTROMETER 2 Sheets-Sheet 2 Filed Dec. 20, 1954 AGCELERATING POTENTIAL ACCELERATING POTENTIAL N v. NE W mw 0 1N..B a: A c .b 2 G H H Y B d 2 m United States Patent MASS SPECTROMETER Cecil B. Shelman, Fort Worth, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware Application December 20, 1954,Serial No. 476,521

8 Claims. (Cl. 250-413) This invention relates to mass spectrometry. In one specific aspect it :relates to a method of and apparatus for compensating a mass spectrometer for variations in accelerating potentials.

In recent years mass spectrometers have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of extreme sensitivity and accuracy. At the present time applications are being found for the use of mass spectrometers in process monitoring and control. Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their masses to determine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electron bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by negative electrical potentials applied thereto. Such potentials impart equal kinetic energizes to ions having like charges such that ions of different masses have different velocities after passingthrough the electrical field and consequently have different momenta.

In United States Patent 2,535,032 there is :disclosed a mass spectrometer which is provided with two sets of three equally spaced accelerating grids. Direct potentials are applied to the outer two grids and a radio frequency potential is applied between the center grid and the two outer grids of each set. Ions which enter the space between the first two grids in proper phase are accelerated through the fields between the first and second grids and the second and third grids. The ions subsequently pass through a field-free drift space and enter the second group of accelerating grids. The spacing between the grids, the frequency of the accelerating radio frequency voltage and the magnitudes of the accelerating potentials are such that ions of predetermined mass receive sufficient energy to overcome a potential barrier and impinge upon a collector plate. The mass spectrometer of the present invention is an improvement over the mass spectrometer disclosed in Patent 2,535,032. Five sets of accelerating electrodes are employed to provide four drift spaces. This greatly improves the resolving power of the spectrometer. The radio frequency potential applied to the accelerating grids is modulated by the output of a square wave audio oscillator so that the output signal generated by the selected ions impinging upon the collector plate is modulated at the audio frequency. An alternating signal of this form can be measured much more accurately than can a direct current signal.

The velocity selection types of mass spectrometers to which this invention is particularly applicable are capable of detecting ions of selected masses in a precise manner. A peaked output signal is provided for the individual mass in focus. However, if the magnitudes of the accelerating potentials vary even slightly from the proper values, the

measured signal.

magnitude of the output signal decreases rapidly. For this reason it is diflicult to monitor a gas sample for a selected mass over a long period of time withouthaving the output signal fluctuate dueto changes in the acceleratingpotentials. Even small changes of magnitudesof the accelerating potentials seriously limit the accuracy of the In accordance with the present invention, the accelerating potential applied to at least oneof the grids in "the tube is modulated by an alternatingsignal of small magnitude, forexarnple, a sixty cycle signal of approximately two volt peak-to-peak amplitude. While this modulating potential decreases the selectivity of the instrument to some extent, it makes the instrument less sensitive to accelerating voltage fluctuations so that a given mass peak can be :monitored continuously even though the magnitude of the accelerating potential may vary slightly.

Accordingly, it is an object of this invention to provide an improved mass spectrometer which is capable of continuously monitoring selectedion masses.

Another object is to provide a mass spectrometer of the velocity selection type wherein an alternating potential modulates the accelerating potential of the spectrometer to compensate for variations in magnitude of the accelerating potential so that a constant output signal is obtained.

A further object is to provide a method. of compensating a mass spectrometer for fluctuations in the accelerating potentials.

Other objects, advantages and features of the invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure 1 is a schematic representation. of a mass spectrometer incorporating the modulation feature of this invention; and

Figures 2a and 2b are graphical representations of operating features of the mass spectrometer of Figure 1.

Referring now to the drawing in detail. and to Figure 1 in particular, there is shown a mass spectrometer tube 10 comprising a glass envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior of tube 10 through a conduit 11. A sample of the gas to be analyzed is supplied by a conduit 12 to an ionization chamber 13 within tube 10. An electron emitting filament 14 is disposed in one end of tube 10. Theend terminals of filamerit 14 are connected to the respective end terminalsof a source of potential 15 which supplies heater current to filament 14. A point on filament 14 is: connected to a negative potential terminal 16. The electrons emitted from filament 14 are accelerated toward apair of spaced ion permeable grids l3 and 19 that are maintained at ground potential. Grids 18 and 19 define the ionization chamber 13 wherein the gas molecules introduced into tube 1 through conduit 12 are subjected to electron bombardment to produce positive ions. The positive ions formed in chamber 13 are accelerated toward a pair of spaced focusing grids 20 and 21 which are connected to the contactors of respective potentiometers 22 and 23. First end. terminals of potentiometers 22 and 23 are connected to a negative potential terminal 24, and second end terminals of these potentiometers are connected to ground. The positive ions pass through grids 26 and 21 toward a collector plate 26 which is mounted in the opposite end of tube 10.

The ions accelerated by grids 20 and 21 pass through a first set of three equally spaced grids 30, 31 and 32. Grid 30 is connected to a point of negative potential on a potential dividing network 33. One end terminal of network 33 is connected to a negative potential terminal 34. Terminal 34 is connected to ground through net work 33 which. comprises the secondary winding 35 of a.

3 transformer 36 that is connected in series with resistors 37, 38, 39, 40 and 41. Grid 30 is connected to the junction between resistors 37 and 38. Grid 31 is connected to one output terminal of a radio frequency oscillator 43, the second output terminal of oscillator 43 being grounded.

A second set of equally spaced grids 30a, 31a and 32a is positioned in tube in spaced relation with the first set of grids; a third set of equally spaced grids 30b, 31b and 32b is positioned in tube 10 in spaced relation with the second set of grids; a fourth set of equally spaced grids 30c, 31c and 32:: is positioned in tube 10 in spaced relation with the third set of grids; and a fifth set of equally spaced grids 30d, 31d and 32d is positioned in tube 10 in spaced relation with the fourth set of grids.

Grid 30 is connected to grid 30d through four series connected resistors 45, 46, 47 and 48 which have approximately equal ohmic values. Grids 32 and 30a are connected to one another and to the junction between resistors 45 and 46. Grids 32a and 30b are connected to one another and to the junction between resistors 46 and 47. Grids 32b and 300 are connected to one another and to the junction between resistors 47 and 48. Grids 32c and 32d are connected to one another and to grid 30d. Grid 32d is also connected to the junction between resistors 39 and 40 on potential dividing network 33. Grids 31a, 31b, 31c and 31d are connected to grid 31 which is connected to the first output terminal of oscillator 43. An inductance coil 50 is connected between the first output terminal of oscillator 43 and the junction between resistors 46 and 47.

A set of three grids 52, 53 and 54 is positioned between grid 32d and collector plate 26. Grids 52, 53 and 54 are connected to one another and to a positive potential terminal 55. A set of two grids 57 and 58 is positioned between grid 54 and collector plate 26. Grids 57 and 58 are connected to one another and to a negative potential terminal 60. Collector plate 26 is connected to one input terminal of a tuned amplifier 62, the second input terminal of which is connected to ground. The output terminals of amplifier 62 are connected to the input terminals of a rectifier 63, and the output terminals of rectifier 63 are connected to the input terminals of a voltage recorder 64.

The positive ions formed within chamber 13 are accelerated toward collector plate 26 by the negative potentials applied to grids and 21. During one half cycle of the output signal from oscillator 43, the electrical field between grids 30 and 31 is of such phase that ions entering this field are accelerated. Those ions which enter this field at a particular time receive maximum energy. During the next half cycle of the output signal from oscillator 43, the field between grids 31 and 32 is reversed and is of such phase that the ions are further accelerated. The ions then drift through the field-free space between grids 32 and 30a. The masses of the individual ions determine the accelerations initially imparted thereto and, ac-

I velocity greater than a predetermined value are able to pass through the potential barrier to impinge upon collector plate 26. The purpose of negative grids 57 and 58 is to suppress electrons which may be formed by the ions impinging upon plate 26.

The ions impinging upon collector plate 26 create a flow of current in the input circuit of tuned amplifier 62, the magnitude of which is proportional to the number of ions impinging upon collector plate 26 per unit time. Because the output of oscillator 43 is modulated at the frequency of audio oscillator 44, one thousand cycles per second, for example, the current flow in the input circuit of amplifier 62 is also modulated at this frequency. Amplifier 62 is tuned to pass only signals of the frequency of oscillator 44. Oscillator 44 preferably provides output signals of square wave form.

A negative accelerating potential is applied to grid 30 from voltage dividing network 33. Potentials of lesser negative magnitude are applied to grids 31, 32, 30a, 31a, 32a. 30b, 31b. 32b, 30c, 31c, 32c, 30d, 31d and 32d from the voltage dividing network comprising resistors 45, 46, 47 and 48. This latter network is energized by a portion of the voltage drop across network 33. The potentials applied to these last-mentioned grids are referred to as step-back potentials. The magnitudes of these step-back potentials are adjusted so that the ions remain in proper phase while passing through the drift spaces to receive maximum energy from the radio frequency potentials. Potential dividing network 33 is adjustable such that the magnitudes of the accelerating potentials and the step-back potentials can be adjusted selectively. Resistors 37 and 41 are adjustable and mechanically coupled to one another to be varied in unison. An increase in the resistance of resistor 37 results in a corresponding decrease in the resistance of resistor 41 and vice versa. This adjustment varies the magnitude of the accelerating potential applied to grid 30. Resistors 39 and 40 are also variable and mechanically coupled to one another in a corresponding manner. Adjustment of these latter two resistors varies the magnitude of the step-back potentials without changing the accelerating potential.

In Figure 2a there is shown a graphical representation of the magnitude of the output signal applied to recorder 64. This signal is plotted as a function of the accelerating potential applied to grid 30. This curve represents the output signal in the absence of a signal being applied to transformer 36 from voltage source 71. It can be seen that the peaks in the curve at negative accelerating potentials of 205, 196 and 187, for example, are very sharp. The mass spectrometer thus possesses a high degree of sensitivity. However, this high sensitivity has certain disadvantages when the instrument is employed for continuous monitoring of a sample stream todetermine selected masses. If the accelerating potential should vary even slightly from the correct value, it can be seen that the output signal decreases rapidly. It is thus diflicult to determine whether the particular ion being detected is present in the tube in smaller quantities or whether the accelerating potential has varied slightly.

To overcome this difficulty, the primary winding 70 of transformer 36 is energized from the source of alternating potential 71. This potential can conveniently be obtained from a conventional sixty cycle power source. The magnitude of the potential is adjusted such that a small voltage of approximately two volts peak-to-peak, for example, is induced in the secondary winding 35. It is important that the frequency of voltage source 71 be different from the frequency of audio oscillator 44 so that the former component does not appear in the output signal transmitted by amplifier 62. It is further desirable that one of these alternating signals not be a harmonic of the other. When the alternating signal from voltage source 71 is superimposed upon the accelerating potential, the mass spectrometer tube is operated such that this potential periodically is varied from the value necessary to focus a particular ion mass sharply. This decreases the amplitude 'of the output signal for a particular mass periodically so that the rectified output signal is reduced in magnitude as shown in Figure 2b. The mass peaks are no longer as sharp as they were in the absence of this modulating potential. However, it is no longer necessary that the accelerating potentials be maintained at precisely the correct values to monitor a given mass peak. The accelerating potential can thus deviate small amounts without substantially changing the amplitude of the output signal for a given mass. Therefore, the sensitivity of the instrument is decreased slightly in accordance with this invention to make the instrument less sensitive to minor fluctuations of the accelerating potentials.

The spacings s between individual grids of each group are equal, Whereas the spacings 2' between the sets of grids are represented by the expression:

where n is an integral number and z is the thickness of each grid, all dimensions being in inches. Oscillator 43 preferably is of radio frequency, 3.9 megacycles per second for example. Oscillator 44 has a lower frequency, 1000 cycles per second for example. The accelerating potentials of 265, 196 and 187 negative volts in Figure 2a serve to focus ions of respective masses d3, 41 and 39, for example.

While the invention has been described in conjunction with a present preferred embodiment, it should be apparent that the invention is not limited thereto.

What is claimed is:

1. In a mass spectrometer comprising an ion source, ion collecting electrode spaced from said ion source, an ion permeable first electrode positioned between said ion source and said collecting electrode, means to apply a first potential of polarity opposite the polarity of the ions being detected to said first electrode to accelerate ions toward said collecting electrode, a plurality of spaced ion permeable second electrodes positioned between said ion source and said collecting electrode, a first source of alternating potential of a first frequency, means applying said first source of alternating potential between adjacent pairs of said second electrodes, a third ion permeable electrode positioned between said second electrodes and said collecting electrode, and means to apply a second potential of the same polarity of the ions being detected to said third electrode; means to compensate said spectrometer for variations in said first potential consisting of a second source of alternating potential of a second frequency, and means to apply said second source of alternating potential to said first electrode.

2. The combination in accordance with claim 1 Wherein said first electrode is the one of said second electrodes positioned adjacent said ion source.

3. The combination in accordance with claim 1 where in the magnitude of said first potential is substantially greater than the peak amplitude of said second source of alternating potential.

4. A mass spectrometer comprising a gas impermeable envelope enclosing a source of ions, a collector plate spaced from said source of ions, a plurality of groups of grids spaced in a line between said source of ions and said plate, each of said groups comprising three grids in spaced relation with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially n-0.3183(2s+t) inches, where n is an integral number, s is the spacing between adjacent grids in each group, and t is the thickness of the grids, all of said dimensions being in inches, and a second grid positioned between said collector plate and said groups of grids; means applying steady first potentials to the end two grids in each of said groups of grids; means applying a second potential to said second grid of polarity opposite the polarity of the ions being detected; means applying a first source of alternating potential of first frequency to the center grid in each of said groups of grids; means applying a second source of alternating potential of second frequency to the end two grids in each of said groups of grids; and means connected to said collector plate to measure ions impinging thereon.

5. The combination in accordance with claim 4 wherein the magnitude of said steady potential applied to the one of said grids closest to said source of ions is substantially greater than the peak amplitude of said second source of alternating potential.

6. The combination in accordance with claim 4 further comprising a third source of alternating potential of a third frequency, means applying said third source of alternating potential to modulate said first alternating potential, and filter means in said means connected to said collector plate to transmit signals of only said third frequency.

7. In a mass spectrometer comprising a source of ions, ion detecting means, means to accelerate ions from said source toward said detecting means, and means to direct the accelerated ions of predetermined mass to said detecting means; means to compensate said spectrometer for variations in said means to accelerate ions consisting of a source of fluctuating potential, and means to apply said source of fluctuating potential to said means to accelerate ions.

8. In a mass spectrometer comprising a source of ions, ion detecting means, an ion permeable electrode positioned between said source of ions and said detecting means, and means applying a potential to said electrode to accelerate ions toward said detecting means; means to compensate said spectrometer for variations in the accelerating potential applied to said. electrode consisting of a source of fluctuating potential, and means to apply said source of fluctuating potential to said electrode,

References Cited in the file of this patent UNITED STATES PATENTS

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