US2774882A

US2774882A - Mass spectrometer

Info

Publication number: US2774882A
Application number: US361699A
Authority: US
Inventors: William H Wells
Original assignee: Bendix Aviation Corp
Current assignee: Bendix Aviation Corp
Priority date: 1953-06-15
Filing date: 1953-06-15
Publication date: 1956-12-18
Anticipated expiration: 1973-12-18

Description

Dec. 18, 1956 w. H. WELLS 2,774,882

MASS SPECTROMETER Filed June l5, 1953 S Sheets-Sheet l INVENTOR. W/LL/AM H. was

WR- W ATTORNEY 1956 w. H. WELLS 2,774,882

MASS SPECTROMETER Filed June 15, 195-3 3 Sheets-Sheet 2 IN V EN TOR. W/LL/AM ll. WELLS Dec. 18, 1956 w. H. w|-:| s 2,774,882

MASS SPECTROMETEIR I Filed June 15; 1953 3 sheets shee't 3 IN V EN TOR.

W/LL MM H. WELLS- WK. m

United States Patent MASS SPECTROMETER William H..Wells, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit,,Mich., acorporation of Delaware Application June 15, 1953, Serial No. 361,699

9 Claims. (Cl. 25041.9)

This invention relates to apparatus for determining the masses of different molecules in an unknown mixture. More particularly, the invention relates to apparatus for separating ions of different mass linearly in space by the use of electrostatic techniques so as to provide a determination of the masses of the different ions. The invention also relates to amethod of separatingions in accordance with their mass.

Apparatus now exists for measuring the masses of different ions by subjecting the ions to a magnetic field which causes the ions to deviate from a linear path.

Since the amount of deviation that is produced on each,

ion is directly dependent upon the mass of the ion, the ions of different mass become separated as they'travel' through a relatively great distance. By measuring the amount'of deviation of each group ofions, the mass of the ions in the group can be determined.

Mass spectrometers which employ a magnet toproduce a spatial dispersion of the ions are not entirely desirable for several reasons. Since a relatively large magnet must' be used, the mass spectrometers are fairly heavyand bulky and relatively expensive. dispersion of theions'does not have a linear relationship; with respect to the mass of the ions so that special circuits or additional computation is required to determine the ions of different mass in an unknown mixture. Since the mass spectrometers operate on steady streams ofions, D. C. amplifiers must be used to amplify the output signals. such amplifiers do not have as sensitive response as A. C. amplifiers.

This invention provides a mass spectrometer-foreliminating the above disadvantages. The mass spectrometer includes apparatus for applying a force on a pulse of ions for a predetermined period oftime and in a first direction so as to impose a constant momentum on the ions. for imposing an electrostatic field on the pulse of ions in a second direction substantially perpendicular to the first direction so as to produce a linear separation of the ions in the seconddirection on the basis of their mass. The ions are detected by their displacement in the second direction after they have traveled through a relatively great distance in the first direction. Since the spatial dispersion of the ions in the second direction is linearly related to the masses of the different ions, a relatively simple determination of the ion masses can be made. The simplicity of the determination is also enhanced since A. C. amplifiers can be employed which are resonant at the repetitionrate for producing the ion pulses.

An object of this invention is to provide apparatus'for determining the masses of the different ions in an unknown mixture by imposing a movement of the ions in a first direction and by subjecting the ions to a displacement dependent upon their mass in a second direction transverse to the first direction.-

Another object is to provide apparatus of the above Furthermore, the spatial The use of D. C. amplifiers is notdesirable since The mass spectrometer also includes apparatus ice character which is relatively inexpensive and compact because of the utilization of an electrostatic field. to produce a separation of the ions in the second direction on the basis of their mass.

A further object is to provide apparatus of the above character for utilizing pulses of ions to obtain a determination of the masses of the different ions in an unknown mixture and for utilizing A. C. amplifiers to obtain a sensitive and reliable indication of the different ion masses. I

Still another object is to provide apparatus of the above character for displacing each of the ions in a pulse in the second direction, through a distance linearly related to the mass of the ion so that a. simple determination of the ion mass can be made over a relatively wide range of ion masses.

A still further object is to provide a method of producing a spatial dispersion. of the ions of different mass so as to provide a determination of the masses of the ions.

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawings and claims.

In the drawings:

Figure 1 is a view, partly in perspective and partly in block form, illustrating one embodiment of the invention;

Figure 2 is a schematic view illustrating the operation of'the apparatus shown in Figure 1;

Figure 3 is a view, partly in block form and partly in perspective, of'a modification of the apparatus shown in Figure 1 to performcertain additional functions over the apparatus shown in Figure 1; and

Figure 4 is a schematic view illustrating the operation ofthe apparatus shown in Figure 3.

In the embodiment of' the invention shownin Figure l, a wedge-shaped filament 10. made from. asuitable material such as tungsten is provided. An electrode 12 is disposed at a relatively short distance, such as two millimeters, fromthe filament 10. and is provided with a horizontal' slot 14 at substantially the same horizontal levelas the filament 10;

An electrode 16 is disposed in parallel relationship to the electrode 12 and at a relatively short distance, such.

as two. millimeters, from the electrode 12. The electrode 16' is providedwith a slot 18' corresponding substantially. in shape and, position to the slot 14. A collector 20 is positioned at a relatively great distance, such as twov centimeters, from the electrode 16 and in substantially parallel'relationship to the electrode.

A backing plate 22 is disposed between the electrode Top andbottom slats 28 made from a suitable insulat-- ing materialextend between the backing plate22 and the electrode 24. A horizontal slot 30 is provided in the bottom slat 28 in the direction of and directly below the flow of electrons. A conduit 32 communicates at one end with the slot 30 andat the other end with a receptacle 34-adapted to hold' the molecules of the different gases and vapors in an unknown mixture.

A n'electrode 36made from a suitable wire mesh is preferably positioned in front of the baffle 24" at a relatively short distance such as a few millimeters in front of the bafile; The electrode 36' has a' suitably curved" configuration to provide a-focussing actionon the ions traveling through the slot 26 and the electrode. A pair of

plates

40 and 42 are positioned in front of the electrode 36 in substantially parallel relationship to each other and in substantially perpendicular relationship to the backing plate 22 and the bafile 24. The

plates

40 and 42 are separated from each other by a sufiicient vertical distance to permit a vertical deflection of the ions as they travel through the region between the plates. A bafile 44 is disposed at a relatively great distance such as ten centimeters in front of the electrode 36. The bafie 44 is provided with a relatively narrow slot 46 so that only ions of a particular mass can pass through the slot. A collector plate 48 is positioned at a relatively short distance such as a few millimeters in front of the slot 46 to receive the ions passing through the slot. A. C. amplifiers 50 are connected to the collector 48 to amplify the signals produced by the collector upon the impingement of ions on the collector. An indicator 52 such as a meter or an oscilloscope is connected to the amplifiers 50 to provide an indication of the passage of ions to the collector 48.

The electrode 12 normally has a positive potential applied to it from a suitable power supply 56 through a resistance 58. The collector 20 has a slightly positive voltage applied to it from the power supply 56 through a suitable resistance 60 to attract back to it electrons secondarily emitted from it by the impingement of the electron stream flowing from the filament 10. The filament and the backing plate 22 are connected to grounded resistances 64 and 66, respectively, so as to be substantially at ground potential in the steady state operation. The electrodes 16 and 36 are directly grounded.

Steady state voltages are also applied from the power supply 56 through resistances 68 and 70 to the

plates

40 and 42, respectively. The voltage applied to the plate 40 may be less than that applied to the plate 42 for reasons which will be disclosed in detail hereinafter. When ions of relatively light mass are being detected, potentials in the order of +50 and 50 volts may be directly applied to the

plates

46 and 42. Since the ions enter into the region between the

plates

40 and 42 at an intermediate position in the vertical direction, they are subjeced initially to ground potential. This causes the ions to be subjected to the same potential as that applied on the ions by the electrode 36. In this way, the ions are not subjected to sudden variations in the electrical field. Relatively low potentials are applied to the bafile 44 and the collector 48 from the power supply 56 through suitable resistances 72 and 74, respectively. The voltages applied to the baffle 44 and the collector 48 are dependent upon their positioning in the vertical direction relative to the plates 49 and 42. For example, when +50 and 50 volts are applied to the

plates

40 and 42, respectively and the slot 46 is disposed above the intermediate position between the

plates

40 and 42, voltages of approximately 10 or volts may be applied to the bafl'le 44 and the collector 48. In this way, the baflle 44 and the collector 48 provide a minimum amount of electrical disturbance on the electrical field produced between the

plates

40 and 42.

In the steady state operation, the electrons emitted by the filament 10 are attracted towards the electrode 12 because of the positive potential on the electrode with respect to the potential on the filament. However, the electrons are decelerated back to substantially their original energy level in the region between the electrodes 12 and 16 since the electrode 16 is at substantially the same potential as the filament 10. This prevents any electrons from entering into the region between the backing plate 22 and the electrode 24 with a sufficient energy to produce an ionization of gas molecules introduced into the region.

When negative pulses of voltage of substantially equal magnitude are applied from a pulse forming circuit 76 through coupling capacitances 78 and 80 to the filament 10 and the electrode 12, respectively, the voltage on the electrode 12 becomes negative with respect to the voltage on the electrode 16. This causes the electrons passing through the slot 14 to be attracted towards the electrode 16 and to pass through the slot 18 with considerable energy. These electrons then travel through the region between the backing plate 22 and the electrode 24 to the collector 20. During their travel through the region between the backing plate 22 and the electrode 24, the electrons impinge on molecules of gas and vapor introduced into the region from the receptacle 34. This causes the molecules to be ionized into electrons and positive ions, most of which have a unitary charge.

The ions produced from molecules of gas and vapor are retained within the electron stream since they have an opposite charge to that produced by the electrons in the stream. When a sufliciently large number of ions have been produced to saturate the stream, the electron stream is interrupted by cutting olf the pulses of voltage on the filament 10 and the electrode 12. By interrupting the electron stream, the ions are made available for easy withdrawal upon the imposition of a positive voltage pulse on the backing plate 22.

The voltage pulse is applied to the backing plate 22 through a suitable coupling capacitance 82 from the circuit 76. When the pulse of positive polarity is applied to the backing plate 22, the voltage on the backing plate 22 becomes greater than the voltage on the electrode 36. This causes the ions to be repelled by the backing plate 22 towards the electrode 24. The pulse is applied to the backing plate 22 for a relatively short time such as one or two microseconds and is cut off before any of the ions reach the electrode 36. In this way, all of the ions in each pulse are provided with a substantially constant momentum.

After the ions have been accelerated in a pulse from their place of retention, they pass through the slot 26 to have their height reduced to a finite and relatively small value. The ions then move towards the electrode 36. The electrode 36 is suitably shaped to provide an optimum focussing action on the ions of a particular mass. This focussing action provides a compensation for diflerences in movement resulting from thermal and other energy in the ions. The electrode 36 also provides a compensation for difierences in the vertical displacement of individual ions, such differences resulting from the finite height of the slot 26.

Upon the movement of the ions past the electrode 36, the ions enter into the electrostatic field produced between the

capacitive plates

40 and 42. This field is in a direction perpendicular to that in which the ions are moving and is substantially constant in magnitude at different positions. The field operates upon the ions to produce a vertical displacement of the ions such that the vertical displacement is directly related to the mass of the ions. This is illustrated in Figure 2 by a plurality of broken lines 90, 92 and 94 to indicate the movement of ions of different mass.

After passing through the region between the

plates

40 and 42, the ions reach the bafile 44. The ions which pass through the slot 46 in the bafiies have a mass which is dependent upon the positioning of the slot 46 and upon the momentum imparted to the ions between the plate 22 and the electrode 36 and upon the electric field imposed upon the ions between the

plates

46 and 42. By varying the position of the slot 46, the momentum imparted to the ions or the electrostatic field imposed upon the ions, ions of any particular mass may be made to pass through the slot 46 These ions are received by the collector 48 so as to produce signals on the indicator 52.

The operation of the mass spectrometer disclosed above may be clearly seen by the following mathematical '5 analysis. Because of the application of an accelerating force on the ions in the'region between the backing plate 22 and the electrode 24,

Where F=the force on'each ion; m=the mass of the ion; and

2 Z-g: the acceleration imparted to the ion in the direction gerpendicular to the backing plate 22 and the electrode Since each ion has the force F applied to it for a time t,

where =the velocity of the ion at the end of the time t; and

c =a constant.

But 01:0, since the ions may be considered at rest before the application of the force F. Furthermore Ft=a constant since the same force is applied on all of the ions for substantially the same period of time. Therefore,

Upon the application of a force F1 upon the ions in the direction perpendicular to the

plates

40 and 42,

d y F1 where =the acceleration of the ion in the direction perpendieular to the

plates

40 and 42.

F =the force exerted upon the ion by the electrostatic field produced between the

plates

40 and 42.

Integrating,

where t1=the time required for the ions to travel in the x direction through the region between the plates .40 and i2; and

c2=-a constant.

But 02:0, since the ions have no velocity in the y direction as they enter into the region between the plates 4-0 and 42. This causes Q F t -m Integrating again,

1 t 2 =my+ s where K2=a constant.

As may be seen from the above mathematical analysis, the displacement produced by the electrostatic field between the

plates

40 and 42 is linearly related to the mass of the ions. This is advantageous in that the baffle 44 can be easily adjusted in position to dispose the slot 46 for the reception of ions of a particular mass.- The transverse displacement of the ions of different mass is accomplished simply and inexpensively by the utilization of an electrostatic field and without any requirement for using heavy and expensive magnets; Furthermore, by utilizing pulses of ions at a substantially constant repetition rate, the ions of a particular mass are received at the repetition rate. Because of this, A. C. amplifiers can be utilized to amplify the received signals and a sensitive response can be obtained by the indicator 52.

It should be appreciated that other apparatus than the filament it}, the electrodes 12 and 16 and the collector 20 can be utilized to produce ions. Furthermore, other apparatus than the backing plate 22 and the electrode 36 can be utilized to impart a constant momentum to the ions in a pulse. It should also be appreciated that different embodiments of the pulse forming circuit 76 may be utilized. For example, a circuit is disclosed in co-pending application Serial No. 288,104, filed by William C. Wiley and Macon H. Miller, for producing a pair of pulses separated by a relatively short time from each other. Such a circuit may also be built in accordance with the principles outlined on pages 223 to 238, inclusive, of volume 20 entitled Electronic Time Measurements of the Radiation Laboratory Series prepared by the Massachusetts Institute of Technology. Equipment such as Model 902 of the Double Pulse Generator manufactured by the Berkeley Scientific Company of Richmond, California, may also be used to produce a pair of pulses having a variable time separation relative to each other.

The embodiment shown in Figure 3 is similar to the embodiment shown in Figure 1 except that the backing plate 22, the baffie 24 and the electrode 36 are tilted at an angle relative to the

plates

40 and 42. This causes the ions in each pulse to have a momentum imparted to them in a vertical direction. The momentum imparted to each ion is substantially the same as that imparted to the other ions and can be adjusted to any value by varying the angle at which the plate 22 and the electrode 24 are tilted. By adjusting the momentum imparted to the ions in each pulse in a vertical direction, ions of any mass that may be desired may be made to pass through the slot 46 for reception by the collector 45 3.

known mixture can be easily determined.

I The following mathematical analysis relates to the operation of the embodiment shown in Figure 3. As in theembodiment shown in Figure 1, the ions have a constant momentum imparted to them in the x direction. This causes Equation 13 to apply such that ti Kim' (13) As previously disclosed in Equation 5,

y m Ftrc (16) Because of the constant momentum imparted to the ions in the y direction before their movement into the electrostatic field This causes 2 2 m Ftl m (18) Integrating Equation 18,

E i my 2 +m +5 Since the zero position of the y axis can be chosen at the position that the ions enters into the electrostatic field between the plates 40 and 4-2,

In accordance with Equation 20, the displacement in the y direction for different ions varies linearly with the mass of the ions.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other appli cations which Will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for providing pulses of ions, means for imposing a substantially constant momentum on the ions in a first direction, a pair of plates positioned in substantially parallel relationship to each other, means for imposing a voltage difference onthe plates soas to produce an electric field for separating the ions in the transverse direction on the basis of their mass, and means for determining the relative disposition of the ions of different mass in the transverse direction after their travel through a relatively great distance in a direction substantially parallel to the plates so as to provide an indication of the difierent ion masses.

2. A mass spectrometer, including, means for providing pulses of ions, means for imposing a force on the ions in a first direction for a relatively short period of time, rneans for imposing a force on the ions in a direction transverse to the first direction to produce a separa tion of the ions'in the transverse direction onthe basis of their mass, and means for determining the relative disposition of the ions after their travel through a predetermineddistancein the first direction so as to provide an indication of the ion masses.

3. A mass spectrometer, including, means for providing pulses of ions, meansfor imposing a constant momentum on the ions in each pulse in a first direction, means for imposing an electrostatic field on the ions in each pulse in a direction transversetot-he tfirstdire'ction to produce a separation of the ions on the basis of their mass in the transverse direction, and means for deter mining the relative disposition of the-ions of different mass in the transverse direction after the movement of the ions through a predetermined distance in the first direction so as to provide an indication of the ion masses.

4. A mass spectrometer, including, means for providing pulses of ions, means for imposing a force on the ions in a first direction for a predetermined period of time, a pair of electrostatic plates positioned to produce an electric field in a direction transverse to the first direction so as to produce a movement of the ions in the transverse direction and to produce a separation of the ions on the basis of their mass in the transverse direction, and means for determining the relative disposition of the ions in the transverse direction after their movement through a predetermined distance in the first direction so as to provide an indication of the ion masses.

5. A mass spectrometer, including, means for providing pulses of ions at a substantially constant repetition rate, means for imposing a substantially constant momentum on the ions in each pulse in a first direction, a pair of plates positioned to provide an electrostatic field in a second direction substantially perpendicular to the first direction so as to produce a separation of the ions on the basis of their mass as the ions travel in the first direction, and means for determining the relative disposition of the ions in the second direction after the travel of the ions through a relatively great distance in the first direction.

6. A mass spectrometer, including, means for providing pulses of ions at a substantially constant repetition rate, means for imposing a substantially constant momentum on the ions in each pulse in a first direction, an electrode shaped to focus the Width of the ions in each pulse in a second direction substantially perpendicular to the first direction, a pair of plates positioned to provide an electric field in the second direction so as to produce a separation of the ions on the basis of their mass as the ions travel in the first direction, and means for determining the relative disposition of the ions of different mass in the second direction after their travel through a relatively great distance so as to provide an indication of the ditterent ion masses.

7. A mass spectrometer, including, means for providing pulses of ions, means for imposing a constant momentum on the ions in each pulse in a first direction, a pair of plates positioned to provide an electrostatic field in a second direction transverse to the first direction so as to produce a displacement of the ions in the transverse direction in accordance with the masses of the ions, a battle positioned at a relatively great distance in the direction of ion movement from the pulsing means, there being an opening in the bafile to provide for the passage of ions, a detector for producing signals upon the passage of ions through the baffle, and means for providing an indication of the signals produced by the detector.

8. A mass spectrometer, including, means for providing a plurality of ions, means for subjecting the ions to a pulsed electric field to impose a constant momentum on the ions in a first direction, means for providing an electric field in a second direction substantially perpendicular to the first direction to produce, upon a movement of the ions in the first direction, a separation of the ions in the second direction on the basis of their mass, and means for determining the relative disposition of the ions in the second direction after their travel through a particular distance in the first direction to provide an indication of the difierent ion masses.

9. A mass spectrometer, including, means for providing a plurality of ions, means for subjecting the ions to a pulsed electric field to produce a movement of the ions in pulses in a first direction and to impose a substantially constant momentum on the ions, a pair of parallel plates for disposing an electric field in a second direction substantially perpendicular to the first direction to produce a separation of ions in the second direction in accordance with their mass, and means for determining the relative disposition of the ions of difierent mass in the second direction after their travel through a particular distance References Cited in the file of this patent UNITED STATES PATENTS Langmuir Mar. 6, 1945 Langmuir Dec. 28, 1948 OTHER REFERENCES The Mass-Spectrograph and its Uses, published in The American Physics Teacher, February 1936, vol. 4, pages 12-23.