Mass Spectrometer Method And Apparatus Employing High Energy Metastable Ions To Generate Sample Ions

Abstract

Metastable particles of high kinetic energy are produced by accelerating a stream of charged rare gas molecules, and contacting the accelerated stream of charged rare gas molecules, thereby producing metastable high kinetic energy particles which are directed perpendicularly into contact with a target beam of molecules moving at thermal velocities, the resulting ions being analyzed by a mass spectrometer.

Description

BACKGROUND OF THE INVENTION

Heretofore, metastable particles have been utilized in certain ionizing reactions referred to as Penning ionization. This involves the inelastic collision between a neutral electronically excited particle, i.e., a metastable particle, and another atom or polyatomic molecule. The result is production of a positively charged molecule or atom (hereinafter generically referred to as a particle) together with liberation of a free electron. Such reactions are useful in the analysis of gas streams, as by a mass spectrometer.

The utilization of such reactions has been limited by the fact that the metastable particles have only thermal velocities (energies of less than 0.2 electron volt) and, hence, substantially no kinetic energy was transferred to the target particles in the ionizing collisions. It has been proposed to produce particles of higher energy by effusing alkali metal atoms, such as potassium atoms, from an oven. A charge transfer reaction between rare gas ions and the alkali metal atoms was utilized to produce beams of metastable particles of high kinetic energy. A rather complicated apparatus is required for this purpose, and it is difficult to maintain the alkali metal vapor at a constant pressure of the order of 10.sup.-.sup.4 torr.

BRIEF STATEMENT OF THE INVENTION

We have discovered that a high energy beam can be readily produced without the use of alkali metals requiring an oven for effusion of the particles. In accordance with our invention, molecules of a rare gas are accelerated by an alternating electric field and passed into a chamber containing neutral rare gas particles at low pressure. The resulting collisions produce a beam of highly energetic rare gas particles which is collimated and passed into a vacuum vessel. There, it intersects, at right angles, a target beam of particles moving at thermal velocities which is effused into the vacuum vessel.

The resulting collisions produce ions which are directed into the analyzer of a mass spectrometer through suitable electronic focusing elements, the analyzer of course, being located within the vacuum vessel.

The use of the high kinetic energy, i.e., "hot," beam of metastable particles results in mass cracking patterns significantly different from those obtained with thermal metastable particles. For example, C.sup.+ and CH.sup.+ ions were produced where methane was utilized as the target beam whereas only CH.sub.2.sup.+, CH.sub.3.sup.+ and CH.sub.4.sup.+ were observed when the metastable particles had thermal velocities. Accordingly, Penning ionization investigations as a function of the transfer energy are no longer limited by the electronic energy levels of the metastable atoms and molecules. Also, translational and electronic energy transfer processes can be studied simultaneously. The method and apparatus of the invention can also be used advantageously for providing an ion source for mass spectrometers.

DETAILED DESCRIPTION OF THE INVENTION

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

The FIGURE is a schematic perspective diagram of an analyzer constructed in accordance with the invention.

Referring now to the drawing in detail, a rare gas stream 10 is introduced from a source, not shown, into a tube 11 of insulating material, such as Pyrex. The term, rare gas, has its usual meaning and includes all the members of the inert gas series of the periodic table, i.e., helium, neon, argon, krypton, xenon and radon. Other inert or rare gases which can be produced by nuclear reactions are within the scope of the invention.

The tube 11 is surrounded by a core 12 upon which two metal rings 13 and 14 are disposed. An alternating electric field is applied to the rings 13, 14 by a radio frequency generator 15. This field accelerates the rare gas molecules, and they pass axially through the tube 11 and an aperture 16 to a chamber 17 containing neutral rare gas molecules at low pressure.

The aperture 16 is formed in a plate 18 constituting a part of a vacuum chamber diagrammatically indicated by reference numeral 19. A conduit 20 leads from this chamber to a vacuum pump, now shown, whereby a low pressure is maintained in the chamber 19, typically of the order of 50 to 5,000 microns of mercury, i.e., 5.times. 10.sup.-.sup.2 to 5 millimeters of mercury. The chamber 17 has a conduit 21 connected to a vacuum pump, not shown, whereby the inert rare gas molecules in the chamber are maintained at a low pressure, typically of the order of 10.sup.-.sup.1 to 10.sup.-.sup.3 millimeters of mercury.

In the chamber 17, the energetic particles produced in the chamber 19, which have considerable kinetic energy, for example, of the order of 10 to 50 electron volts, undergo charge exchange with the neutral molecules of gas contained within the chamber 17, and thus produce a beam of metastable particles of high kinetic energy.

The kinetic energy of these molecules can be varied by changing the direction of the electric field vector produced by the generator 15. Maximum kinetic energy is obtained when the vector is parallel to the effusion direction, i.e., coaxial with the tube 11, and the kinetic energy is a minimum when the vector is perpendicular to that axis.

It is a feature of the invention that the chamber 17 may contain a pair of spaced electrodes 22 and 23. When connected to a suitable current source, not shown, the electrodes constitute electrostatic deflectors which remove ions and electrons from the beam of hot metastable molecules passing axially of the chamber 17.

The beam of hot metastable rare gas molecules is collimated by the aperture 16 and by an aperture 24 in axial alignment therewith. Suitable dimensions for securing proper collimation of the beam are a diameter of 1 millimeter for the aperture 16 and a diameter of 2 to 4 millimeters for the aperture 24.

From the aperture 24, the collimated beam passes into a vacuum chamber 25 supplied with a conduit 26 leading to a vacuum pump, not shown. There, it intersects a target beam of particles discharged from a source 27 in a path perpendicular to that of the collimated beam. The source 27 may advantageously comprise a series of fused gas capillaries 100 microns in diameter and 2.5 millimeters long. The target beam, which can be composed of a hydrocarbon gas, for example, methane, accordingly effuses from the source 27 at thermal velocities.

The intersection of the target beam with the beam of metastable particles produces ions which, by virtue of their electric charge, can be guided to an analyzer 28 disposed in the vessel 25. The analyzer 28 forms a part of a mass spectrometer, now shown, which is preferably of the quadrupole type, i.e., one that uses an electric field rather than a magnetic field to deflect the ions.

The ions produced by the collision of the two beams are guided to the analyzer 28 by an electric field existing between an electrode 29 and the analyzer 28. To this end, a current source 30 is connected between the electrode 29 and analyzer 28 to produce a field vector mutually perpendicular to the hot metastable beam and the target beam. Suitable guiding and focusing electrodes 31, 32 and 33 are provided to direct the ions into the analyzer.

The analyzer and mass spectrometer are operated, in well understood fashion, to analyze the proportions of ions of various mass numbers in the particles directed into the analyzer 28. By this means, mass spectra are obtained showing the cracking patterns resulting from contact of the hot metastable particles with the target stream.

In a preferred form of operation, helium, neon or argon is injected into the tube 11, and accelerated by the field produced by the generator 15 to an energy well above 0.2 electron volt, for example, 39 electron volts. The resulting particles contact the rare gas molecules in the chamber 17 which, preferably and advantageously, are of the same composition as the gas fed to the tube 11. The particles undergo charge exchange with the neutral molecules in the chamber 17 and produce a beam of hot metastable particles which is collimated by the apertures 16, 24 an passes into the vessel 25. Here, the beam of hot metastable molecules contacts perpendicularly the target beam of particles to be analyzed which flow at thermal velocity from the source 27. Ions are produced by the resulting collisions and these are directed into the analyzer 28 of the mass spectrometer, and the mass spectrum thereof is determined.

In contacting a methane target stream with hot metastable helium particles, ions of mass numbers 12 and 13 were produced and appeared in the mass spectrum whereas, with metastable helium particles at thermal velocities, only ions of mass numbers 14, 15 and 16 were produced.

In another case, hot metastable atoms of helium, neon and argon were contacted with a target beam consisting of a mixture of helium, neon and argon. The following table shows the ion abundances in the stream of ions analyzed, these being normalized to equal concentrations of target beam components.

Metastable Ion Abundance, % Atom Helium Neon Argon __________________________________________________________________________ Helium 0.21 2.3 97.5 Neon 1.5 3.7 94.8 Argon 1.1 5.0 93.9 __________________________________________________________________________

The method and apparatus of the invention are also useful for obtaining the mass spectra of various hydrocarbons, such as butene-1, isobutylene, n-heptane, isobutane and 2,2,4-trimethylpentane. When bombarded with hot mestable particles of helium, neon, argon and krypton, different cracking patterns containing ions of lower mass numbers are produced than where the target material is contacted with metastable particles having thermal velocities. Accordingly, by the method and apparatus of the invention, we are able to substantially extend the study of Penning ionization phenomena. It is an additional feature of the invention that the same apparatus can be utilized for the study of beams of thermal metastable particles simply by adjusting the field direction of the generator 15 or by changing the concentration of rare gas in the chamber 17. Finally, in many cases, our invention is useful in providing ion sources for mass spectroscopic analyses.

Claims

We claim:

1. A method for detecting the mass of components in a gas sample which comprises impressing an alternating electric field upon a stream of rare gas ions to increase the kinetic energy thereof, contacting the resultant gas stream with neutral rare gas to produce a high energy beam of metastable rare gas ions directing said beam perpendicularly into contact with a target beam of said sample gas moving at thermal velocities, applying an electrical field to the zone of contact to deflect ions produced by such contact, and detecting the mass of the deflected ions.

2. The method of claim 1 wherein the pressure of the rare gas subjected to the alternating electric field is 5.times.10.sup.-.sup.2 to 5 millimeters of mercury and the pressure of the neutral rare gas is 10.sup.-.sup.1 to 10.sup.-.sup.3 millimeters of mercury.

3. The method of claim 1 wherein the rare gas ions are selected from the group consisting of helium, neon, argon, krypton, xenon and radon, and the target beam is a hydrocarbon gas.

4. Apparatus for investigating the mass of components in a gas stream which comprises, in combination, a tube of insulating material, means for evacuating said tube, a source of rare gas ions connected to one end of said tube, a pair of electrodes disposed around said tube, a radio frequency generator connected to said electrodes, a chamber containing rare gas molecules at low pressure, a vacuum vessel, said chamber having an aperture therein communicating with the other end of said tube and a larger aperture communicating with said vessel, said apertures being coaxial with said tube, a capillary source in said vessel arranged to effuse a gas sample thereinto perpendicular to the extended axis of said tube, a mass spectrometer having an analyzer disposed in said vessel, and means for directing ions produced in said vessel into said analyzer.

5. In the apparatus of claim 4, a pair of deflecting electrodes in said chamber, and means for applying an electric potential to said electrodes to deflect ions and electrons away from said larger aperture.

6. The apparatus of claim 5 wherein the capillary source is an array of fused glass capillaries coaxially arranged to discharge said gas sample into said vacuum vessel.

7. In the apparatus of claim 6, means for varying the direction of the electric field vector produced by said electrodes.

8. The apparatus of claim 7 wherein the mass spectrometer is of the quadrupole type.