US2508163A - Control scheme for mass spectrometers - Google Patents

Description

y 1950 J. A. HIPPLE, JR 2,508,163

CONTROL SCHEME FOR MASS SPECTROMETERS Original Filed Nov. 30, 1944 To Pump Gus Chamber WITNESSES: INVENTOR JglsnAHipple, Jr.

ATTORNEY Patented May 16, 1950 CONTROL SCHEME FOR MASS SPECTROMETERS John A. Hippie, Jr., Chevy Chase, Md., assiznor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Original application November 30, 1944, Serial No. 565,821. Divided and this application June 3, 1949, Serial No. 96,845

3 Claims. (01. 250-413) The present invention relates to a device for analyzing the composition of matter, and it has particular relation to an arrangement which employs a mass spectrometer to continuously analyze a gas for a plurality of different constituents. This application is a division of my application Serial No. 565,821 filed November 30, 1944 for a Control scheme and assigned to the Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania.

The mass spectrograph or mass spectrometer, as it is also known, is widely used for separating particles having a particular mass or atomic weight from a gaseous composition. There are many types of this instrument, a number of which are described along with the principles of operation in an article by Walker Bleakney, entitled the Mass spectrograph and its uses, published on pages 12 to 23, inclusive, of the February, 1936 issue of the American Physics Teacher. Briefly stated, the mass spectrometer ionizes the particles of material to be analyzed and subjects them to the action of a combination of electrostatic or electromagnetic fields, or both, to cause only those particles having a particular ratio of mass to electric charge to impinge on a current collecting member. The current received by the collector may then be amplified and analyzed by various known methods. Since it is possible by adjusting the ionizing potential to a relatively low value to prevent any substantial number of particles from receiving a multiple charge, the device may be made to effectively segregate particles according to the mass alone instead of the ratio m/e, where m is the mass or atomic weight of the particle and e is its charge.

The mass spectrometer in many cases offers advantages over chemical methods of analysis. It is thus suitable for a number of applications such as the analysis of soil gas for petroleum pros pecting or of annealing furnace gases. In these applications it is desirable to provide an arrangement which will furnish a substantially continuous analyzation of a gas for a plurality of different constituents. As an example in the case of furnace gases, it may be required that the methane, moisture, oxygen and carbon monoxide content be kept within predetermined limits.

A feature of my present invention is the provision of means for rapidly comparing the output currents respectively indicating the relative amounts of two different ions in a gas undergolnganalysis. This is done by arrangements for alternately impressing ion-accelerating voltages of two difierent magnitudes corresponding to the respective ion-masses and simultaneously interposing in the grid circuit of an output amplifier two voltages just capable respectively of neutralizing the efiects of the ion-current on the amplifier control electrode.

It is, accordingly, an object of the present invention to provide a novel and improved device for analyzing compositions of matten-v Another object of the invention is to provide a novel device for continuously analyzing ages which is subject to change in composition for a plurality of different constituents.

Still another object of my invention is to provide an arrangement for rapidly comparing the values of the ion-current representing the respective quantities of ions of different masses.

other Objects a advantages of the invention will appear from the following detailed description, taken in conjunction with the accompany ing drawing, in which the single figure is a schematic diagram with parts in section of apparatus embodying certain features of my invention.

Referring to the figure of the drawing, the apparatus includes a curved tube or vessel 2 which may be of round cross section disposed within an electromagnet (not shown) which produces a magnetic field normal to the plane of the figure. Preferably the tube axis follows a curve of constant radius. The vessel 2 is preferably of glass, in which case it is provided with a grounded. metallic lining or coating 5 which may be of the. type described in my Patent No. 2,354,122 for a Vacuum tube. If desired, however, the vessel; may be made entirely of metal in which case no.- separate lining is required. The particular struc-. ture of the above-mentioned electromagnet forms; no part of the present invention since any device suitable for providing a uniform magnetic field; throughout the tube perpendicular to the plane, of its curved portion might be used. By way of example, it may be of the type described in my copending United States patent application Serial No. 296,607, filed September 26, 1939, which ma tured into Patent No. 2,265,041 on December 2, 1941. A chamber 6, which may be an annealing furnace, communicates by means of a tube or pipe 8 with the interior of the vessel 2 through an opening I 0 at an end thereof. The tube 8 may include a capillary for allowing a small continuous sample of the atmosphere in the chamber 6 to enter the tube. The tube is evacuated through an opening at approximately the center thereof by means of a pump.

Within the tube Zat the end which commu nicates with the vessel 6 is a filament or cathode;

I4 disposed immediately in front of a slot I6 in a right angle shield I 8 which, in turn, defines an ionizing chamber. A collecting or conducting plate (not shown) is provided near the slot 55 for the purpose of collecting and neutralizing excess electrons. Disposed within the tube 2 immediately above the ionizing chamber are a pair of ion accelerating plates 22 and 24 having aligned narrow vertical slots at 25 and 26, respectively.

Within the vessel at the other endis a baffle plate 28 having a narrow slot formed thereinat 30. The center line a of a tube between the slot 26 in plate 24 and slot 30 inplate 20'is circular and preferably covers 90 degrees of arc although my invention is not limited to such an arc. It will be noted that particles passing-through the slots 25, 25 in the accelerating-piatesareinitially accelerated tangentially to this center line.

The filament or cathode I4 is connected for energization by a battery 32, while another battery 34 is connected to one of the filament terminals and to the shield I8 to'maintain the latter member at a positive potential with respect to the filament. The accelerating plate 22 is maintained at a potential intermediate that of the filament and the shield I by means of a conductor 36 extending from an intermediate point on a resistor 30 which is connected in shunt with the battery 34.

Means for impressing an accelerating voltage on the ion-accelerating plate 24 comprises a double throw switch member SI having the plate 24 connected to its hinge-terminal and having its other two terminals respectively connected to sliding contacts 92 and 93 on a potentiometer 94 which is energized by a voltage source 95 of relatively high voltage of which the positive terminal is connected to the plate'lS. The plate 24 is grounded as indicated at 44.

The collector plate 54 of the tube 2 is connected to the control electrode of an amplifier tube 96 which may, of course, symbolize an amplifier of any number of stages well knownin the art as suitable for amplifying small direct currents. The cathode of the amplifier 96 is grounded and its output circuit is connected to a current indicating instrument 91 in series with a voltage source 98 of which the negative terminal is connected to thesaid cathode.

The collector electrode 54 is likewise connected through a grid resistor 09 to the hinged terminal of a second double throw switch I0! having its moving member tied in to move synchronously with the moving member of the switch 9L One of the remaining terminals of the switch IOI is connected to the common terminal of a pair of serially connected resistors I 02, I03 which, are connected in multiple with another resistor I04 having a sliding contact I04 connected to the remaining terminal of the switch IOI. One terminal of the resistor Is is grounded whilethe other terminal is connected through a variable resistor I to the positive terminal of a constant voltage source I00 of which the other terminal is grounded. It may be desirable, in certain instances to replace the switches 9| and IOI with electronic circuit interrupters of types well-.- known in the art for altering the circuits in the same way, but more speedily, than mechanical switches like 3| and it; can.

The arrangement just described makes it possible to rapidly compare the relative quantities of two ions in the gas present in .the chamber below the plate. 22 in tube 2. In order todo. this the switches 9| and IM are thrown to connect.

ill

'tube 2. Ions of this particular mass then deposit their charges on the collector electrode 54 and these flow through the grid resistor 99, the resistor I02 to ground. This impresses a voltage on the control electrode of the amplifier 96 and changes the value of the current fiowing in its anode circuit through the instrument 91. By adjusting'the value of the resistor I05, a voltage equal and opposite to the IR drop in resistor 99 can be interposed on resistor I02 in series with the grid resistor 99 thereby neutralizing the effect of the ion-current flowing from electrode 54 on the control electrode of tube 96, and bringingthe reading of the instrument Q'I'back to zero.

The switches 9%, Mil are then thrown to their other terminal connecting the sliding 'contac't92 to the plate 24 and connecting the resistorSS to the operating contact on resistor I04. The sliding contact 92 is adjusted to such a value that the ion which it is desired to compare with the standard ion on which measurements have just been made describes a pathfrom the opening 26 through the opening 30 to the collector electrode 54 in tube 2. The incidence of this ion on electrode 54 will cause current to flow through resistor'99 and the portion of resistor I04 above the sliding contact I04 to ground This will impress a voltage on the control electrode of amplifier'96 which will, in the general case, be different from that impressed thereof when the movable member of switch I 0| made contact with resistor I03. on the resistor I 04 may now be adjusted to. such a position as will bring the current flow through the instrument 01 back'to its original value. The relative amounts on the two ions on which measurements have thus been made will then correspond to'the position of the sliding contact I04. The quantities of the constituents corresponding to such positions can be calculated from observed values of the resistors 99, I02, I03, I04, I05 and battery I06 by a simple application of the well-known laws governing current flow in electric circuits; they can be determined by making measurements on gas mixtures made up from known relative amounts of ion of two different masses. In this way a scale calibrated directly in ratio of the quantities of an ion which it is desired to measure to the quantity of a standard ion can be placed adjacent the sliding contact on resistor I04.

It will be evident that by providing the switches 9i and I0! with additional contacts connected respectively to additional sliders like 92 and I04, it will be possible to compare the relative amounts of additional ions with the standard ion.

While I have described the slider I04 on pctentiometer I04, as manually adjusted, it will be evident to those skilled in the art that conventional instruments which automatically adjust sliding contacts to a null or balanced position can be substituted for manual operation. Thus, for instance, the N ucromax controller of the Leeds & Northrup Company comprises an arrangement in which a motor-driven sliding contact is at: tached to a pen or other marking device which indicates on a moving chart the position of the The sliding contact slider along a potentiometer. The motor drive is energized by departure from zero of the electrical quantity being recorded and is caused to move in such a direction that the slider reduces the electrical quantity to zero again, whereupon further movement of the motor and slider ceases. In the present instance, current in instrument 9! when switch IOI energizes it from slider I04 would be the electrical quantity used to energize the motor drive and the pen on the instrument would be displaced across the chart until that current was reduced to zero. Thereafter, displacement of the pen would cease. The position of the slider I04 when the movement ceased would be evident from inspection of the chart and would furnish the same information for estimate of the relative amounts of the compared ion and the standard ion as the position of the slider I04 has already been described as doing under the manual adjustment outlined above.

The resistor I05 has been described as being manually varied to bring the reading of instrument 9! to zero with the switch IOI connecting resistor 99 to resistor I02. It will be evident to those skilled in the art that conventional instruments which vary a resistor until current in a given circuit falls to zero can be employed instead of manual operation to adjust the variable resistor I05. In this instrument, the current flow through ammeter 91 would be the current controllin the motor-drive oi the resistor and the variable resistor of the instrument would be substituted for the resistor I05.

From the foregoing description it will appear that the present invention provides a convenient and efficient means for analyzing a, continuous gas sample for a plurality of constituents. The apparatus will be found of particular value for commercial analysis because no constant attention is required. In fact, it may be employed along with automatic controls for maintaining the gas composition constant, if desired.

While the invention has been described with reference to a particular embodiment thereof, it will be evident that various modifications may be made without departing from its spirit and scope. It is, therefore, intended that the invention be limited only by the appended claims.

I claim as my inventionz' 1. In combination with a mass spectrometer having an accelerating electrode and a collector electrode, means for impressing a first accelerating voltage on said ion-accelerating electrode and for simultaneously connecting said collector electrode to a first bias voltage, means for disconnecting said accelerating electrode from said first ion-accelerating voltage means and connecting it to a second accelerating voltage and for simultaneously disconnecting said collector electrode from said first bias voltage and connecting it to a second bias voltage.

2. In combination with a mass spectrometer tube having an ion-accelerating electrode and a collector electrode, a potentiometer having a first sliding contact and a second sliding contact, a first switching means for successively connecting said sliding contacts to impress ion-accelerating voltages on said ion-accelerating electrode, a voltage source in series with a first resistor and a net-work comprising two parallel-connected resistors, a second switching means for successively connecting an intermediate point on one of said resistors and a sliding contact on the other of said parallel-connected resistors to said collector electrode.

3. In combination with a mass spectrometer tube having an ion-accelerating electrode and a collector electrode, a potentiometer having a first sliding contact and a second sliding contact, a first switching means for successively connecting said sliding contacts to impress ion-accelerating voltages on said ion-accelerating electrode, a voltage source in series with a first resistor and a net-work comprising two parallel-connected resistors, a second switching means for successively connecting an intermediate point on one of said resistors and a sliding contact on the other of said parallel-connected resistors to said collector electrode, an amplifier having a control electrode circuit connected to said collector electrode, and a current indicating device in its output circuit.

JOHN A. HlPPLE, JR.

No references cited.

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