US2769911A

US2769911A - Mass spectrometer for analysing substances or indicating a small amount of a determined substance

Info

Publication number: US2769911A
Application number: US350577A
Authority: US
Inventors: Warmoltz Nicolaas
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1952-05-06
Filing date: 1953-04-23
Publication date: 1956-11-06
Anticipated expiration: 1973-11-06
Also published as: FR1085804A

Description

Nov. 6, 1956 N. WARMOLTZ 2,769,911 MASS SPECTROMETER FOR ANALYSING SUBSTANCES OR INDICATING A SMALL AMOUNT OF A DETERMINED SUBSTANCE Filed April 23, 1955 ill/l y IN VEN TOR N ICOLAAS WARMOLTZ AGENT Unite States Patent MASS SPECTROMETER FOR ANALYSING SUB- STANCES OR INDICATING A SMALL AMOUNT OF A DETERMINED SUBSTANCE Nicolaas Warmoltz, Eindhoven,

Hartford National Bank and ford, Conn., as trustee Netherlands, assignor to Trust Company, Hart- This invention relates to mass spectrometers for analysing substances or indicating a small amount of a determined substance.

It is known that mass spectrometers are very suitable for separating particles of dilterent molecular weight. In this case the molecules of a gas are ionised, whereby, as a rule, separation occurs, whereafter the ions are subjected, for example, to the combined action of an electric field and a magnetic field. In the electric field the ions acquire a certain velocity and in the magnetic field they are deflected from their path and this more strongly, according as the mass of the ions is smaller and their load is higher. As a rule, said types of mass spectrometer is constituted by a semicircular tube, of which one extremity contains an ionisation'device and the other extremity contains the members required for separating the ions according to the mass. Said members are constituted inter alia by a screen comprising a narrow slit, oehind which a collector is arranged. In most mass spectrometers of known type, the collector is connected to an amplifier, of which the output circuit acts upon an indicator instrument, for example, a meter or an oscillograph.

The invention relates more particularly to the device cator system.

In this case it is preferable that as a rule, is plate-shaped and which ity of emitting secondary electrons or coating the collector with such material.

larger portion of the electrons is used for the actuation of the Geiger-Muller counter.

According to a further ple beryllium, or it velocity,

meter thick, as is known per se.

2,769,911 Patented Nov. 6, 1956 feature of the invention, for this purpose a magnetic field is produced in the vicinity of the collector, which field is so directed that a large proportion of the emerging electrons, after traversing the accelerating electric field, strikes the window of the Geiger-Muller counter.

In order that the invention may be readily carried into elfect, it will now be described with reference to the accompanying drawing showing, by way of example, two embodiments thereof.

' The figures only show that portion of the mass spectrograph which is essential to the invention. The ionisation device and the part in which the ions are separated are thus omitted.

In Fig. 1, reference numeral of the constituent of a the beam is band-shaped havmg its largest dimension at The ions finally impinge upon the collector 4, losing thereon their charge. For this purpose the collector is connected to a point having a potential substantially equal to that of the electrode 3.

When the ions impinge upon the collector 4, they release therefrom electrons 10, which have a comparatively low speed of emergence, viz. of the order of a few volts. In order to increase the number of emerging electrons, the collector 4 may either consist of material having a high capacity of emitting secondary electrons, for exam may be coated with such material.

According to the invention, provision is made of a plurality of apertured electrodes 5, 6 and 7, which have positive potentials, preferably progressing, such that the released electrons are sufiiciently accelerated, so that their after having passed the last electrode 7, has become such that the electrons of a Geiger-Muller counter 8. Said window may be constituted by a mica plate of some thousandths of a milli- The discharges brought about in the Geiger-Muller counter by the incoming electrons are counted by means of a device 9. They may be counted individually, but it is also possible to measure the mean electron current. A comparatively small amount of ions striking the collector per unit-time alreadyprovides an indicati small concentration of a determined gas.

The assembly, except the device 9, may be arranged in the glass envelope or earthed metal envelope 11 for the tube of the mass spectrometer. If the envelope is of metal, the electrode 2 is preferably connected to the envelope in a conductive manner. The electrode 3 may have a negative potential of about 15 kilovolts relatively to earth. The potential of the electrode 7 may be 15 kilovolts positive or higher relatively to the collector 4.

Fig. 2 shows a modification of the device for counting secondary electrons. Here the electrons emerging from the collector 4 are subject to the action of a magnetic field and an electric field, of which the lines of force are at right angles to one another. The electrons emerge from the collector in different directions, the spatial division of the velocity approximately following a cosine regularity, such as shown in Fig. 3. The magnetic lines of force are at right angles to the plane of the drawing so as to be substantially parallel to the surface of collector 4. The electric field is produced by the electrodes 4, 12 and 14. The electrons emerging at different angles traverse cycloidal paths 15 between the electrodes 12 and 14, it being possible by suitable choice of the strength of the magnetic field and of the potentials of the electrodes 12 and 14 that the greater portion of the electrons or at least a very large portion is focused at the window of the Geiger-Miiller counter 8. The'elect-rode 12 is apertured at this area. The electrons may be further accelerated by means of an electrode 13, which may have apositive potential of afew kilovolts-relatively to the surroundings 16 designates a scree'iiirig' cap for electrode 13, which 'is connected in an electrically conductive manner to the electrode 12. v I

Asia rule, the electrode '14 will have apositive potential of some hundreds of volts with respect to electrode 12. The collector 4 may be connected in a conductive manner to the electrode 12. Each of said electrodes preferably has a negative potential which is approximately equal to that of the electrode 3, for example from to kilovolts.

Simila'rly' as in the device shown in Fig. 1, the various electrodes and the counter 8' are arranged in the envelope for the tube of the spectrometer, which envelopeis not shown here. As an alternative, the Geiger-Miiller counter may be arranged under the plate 12 at the area at which the electron paths closely approach one another. The devices described are very suitable as leak detectors, that is to say for finding leaks in vacuum tubes. In this case the interior of the tube under investigation is connected to the interior of the spectrometer tube. The tube under examination is surrounded by a determined gas which may be indicated by mean's'of the mass spectrometer, even in extremely small concentration, if there is a leak" in the tube.

The Geiger-Miiller counter may be substituted by a crystal counter which is constituted by a crystal which becomes fluorescent or conducting under the action of electrons and a device for measuring small amounts of light, for example, a photo-sensitive member with amplification or a counter of current pulses.

What is claimed is:

1. In a mass spectrometer having means producing a beam of ions, a collector in the path of said beam adapted to-produce electrons when struck by said beam, electrostatic accelerating means disposed in the vicinity of said collector for accelerating said electrons, radiation detecting and counting means responsive to incident electrons fordetecting the same and disposed to receive said accelerated electrons, and indicating means coupled to'said detecting means. i

2. In a spectrometer as claimed in claim 1 wherein the detecting means is a Geiger-Miller counter,

3. In a spectrometer as claimed in claim 1 wherein the detectingmeans includes a scintillation counter.

4. In a spectrometer as claimed in claim 1 wherein the collector is mounted in an inclined position with respect to the path of the beam, said collector inclining toward said electrostatic accelerating means.

5. In .a spectrometer as claimed in claim 1 wherein at least-the surface of the collector facing said ion-producing means consists of a material exhibiting good secondary-electron-emitting properties.

6. In a mass spectrometer having means producing a beam of ions, a collector in the path of said beam adapted to produce electrons when struck thereby, electrostatic accelerating means including at least one perforated plate maintained at a positive potential disposed in the vicinity of said collector outside of the path of the ion beam for accelerating and directing said electrons, a Geiger-Miiller counter positioned to receive said accelerated electrons and produce electrical signals corresponding in intensity to that of the electrons, and indicating means coupled to said counter.

7. In a mass spectrometer as claimed in claim 6 wherein means are provided produc'ing a magnetic field in the vicinity of said collector for focussing the electrons onto the counter.

8. In a mass spectrometer as claimed in claim 7 wherein the magnetic lines of force are substantially at right angles to theelectrostatic lines whereby the electrons describe a cycloidal path before being detected by the counter.

References Cited in the file of this patent UNITED STATES PATENTS 2,422,264 Seaman June 17, 1947 2,543,859 Long Mar. 6, 1951 2,599,166 Dempster June 3, 1952 2,625,653 Wouter's Jan. 13, 1953 2,642,535 Schroeder June 16, 1953 2,664,515 Smith Dec. 29, 1953 2,720,593 Richards et al. Oct. 11, 1955 OTHER REFERENCES I The Detection of SinglePositive Ions, Electrons and Photons by a Secondary Electron Multiplier by James Allen, Physical Review, vol. 55, May 15, 1939, pages 966,972,