Multipoint Field Ionization Source

Abstract

A field ionizing source for ionizing components of a gas sample includes a multipoint array of conductive needle-like elements on a porous substrate. A grid is disposed in close proximity to the needle-like elements so that an ionizing electric field is generated between individual needle-like elements and the grid when a potential difference is established between the grid and array. The gas sample is introduced into the ionizing electric field zone by passing it through the back of the porous substrate and into the array between the needle-like elements so that substantially all of the gas is exposed to the ionizing electric field. Ions so produced are formed into an ion beam by a focusing electrode structure and directed into a spectrum analyzer and components of the gas sample are determined.

Description

BACKGROUND OF THE INVENTION

The phenomenon of ionization plays a significant role in many scientific instruments and experiments, e.g., in ionization gauges and mass spectrometers. In mass spectrometry, an unknown material under investigation is ionized prior to injection into the analyzer or mass-separator section of the mass spectrometer. Ionization is usually produced by electron impact with the unknown material, utilizing a suitable electron source such as a thermionic emitter. However, electron impact with molecules not only ionizes them, but also tends to fragment them into two or more species, so that the mass spectrum, obtained by this ionization method, may show the presence of the daughter species but little or nothing of the parent species. Moreover, if any of the daughter species is the same as, or has a mass-to-charge ratio approximately equal to, another species originally present in the unknown material, then the mass spectrum obtained can be difficult or impossible to interpret correctly regarding the original constitutents of the unknown material.

In some applications where mass spectrometry is used to monitor or control other processes, e.g., the preparation of photoemissive surfaces, the use of a thermionic emitter for ionization is disadvantageous because the heat or light from the emitter tends to disturb the process. The use of a cold nonluminous ionizer in such applications constitutes a significant improvement. Field ionization, a phenomenon in which molecules entering a region of very high electric field (10.sup.8 to 10.sup.9 V/cm) are ionized by extraction of electrons by the field, causes substantially less fragmentation than electron-impact ionization. Also, this phenomenon does not require or involve the generation of light or heat.

In order to reduce to a practical level the voltage required for producing the required high fields, sharp blades, needles or points are used as field ionizing electrodes, a counterelectrode is spaced from the needle-like structures and a voltage is applied so that the counterelectrode is negative relative to the blades or needle-like structures. However, even with the use of sharp points, if the counterelectrode is spaced a macroscopic distance from the blades or points, e.g., of the order of millimeters, the voltages required for the field ionization are of the order of tens of kilovolts.

Ionization efficiency of prior art field ionizers of the single blade or single needle-like structure is low since the effective region where ionization takes place is confined to the small volume in the pg,3 immediate vicinity of the apex of the sharp point or blade edge, so that the rate of ion production for a given pressure of material to be analyzed is much lower for field ionization than for electron-impact ionization. Another reason is that the field-produced ions attain velocities equivalent to the voltage applied between ionizer and counterelectrode and the ions are impelled away from the ionizer over a wide range of angles, so that only a small fraction of the ions is collimated into a beam suitable for injection into the analyzer of the mass spectrometer without employing complex ion-optical lenses.

Parallel operation of many needle-like members to provide a correspondingly large ionization volume is feasible, but the problems of formation of the parallel structures and providing ion-optical collimation are formidable. For example, ion-optical collimation is practical only if emission energies of the ions can be kept small, which necessitates spacings between the ionizer and counterelectrode of the order of microns with the ionizer point having a tip radius of a fraction of a micron, e.g., 0.1 micron. Also, it is desirable to space the needle-like structures as close together as possible without incurring significant decrease of the field at each point by the presence of its neighbors.

Many of the problems involved in the construction of arrays of the fine needle-like structures and the problems thought to be inherent in parallel operation of such structures have been solved by structures and methods of producing the structures as disclosed in U.S. Pat. No. 3,453,478, entitled "Needle-Type Electron Source," issued July 1, 1969, and U.S. Pat. No. 3,497,929, entitled "Method of Making a Needle-Type Electron Source," issued Mar. 3, 1970, to Kenneth R. Shoulders and Louis N. Heynick. Further refinements are illustrated and described in U.S. Pat. No. 3,665,241, entitled "Field Ionizer and Field Emission Cathode Structures and Methods of Production," issued May 23, 1972, to Charles A. Spindt and Louis N. Heynick. The subject matter of these patents is specifically incorporated by reference.

A highly refined and practical field ionization source is provided using the teachings of the above patents. A bare-point structure is provided in which a regular array of closely spaced metallic points of controlled geometry is provided over the surface of a conductive substrate and a screen-like counterelectrode is placed in close proximity to the points of the needle-like elements with apertures in the screen-like counterelectrode in register with the points. A field ionization structure is provided by making the counterelectrode of the arrangement just described negative relative to the substrate electrode and providing the proper electrode counterelectrode spacing as well as ratio of such space to the distance between electrode points. The sample to be ionized is introduced in the area of the needle-like structures.

While the field ionization just described is highly efficient relative to prior art field ionization sources, it is still desirable to increase the ratio of sample particles ionized to sample particles introduced, i.e., increase the efficiency of ionization. This is especially critical in the many applications where the total sample of the material to be analyzed is very small.

SUMMARY OF THE INVENTION

In carrying out the present invention, a field ionization source for ionizing components of a gas sample is provided by utilizing a highly regular array of closely spaced needle-like points of controlled geometry on the surface of a porous substrate. An electric field is provided in the area of needle-like elements by disposing a conducting grid in close proximity of the needle-like elements with apertures in the grid in register with the points of the needle-like elements and providing a potential difference between the grid and needle-like elements. The gas sample to be ionized is introduced into the ionizing electric field zone by passing it through the porous substrate and into the ionizing electric field around the needle-like elements so that substantially all of the gas passes in close proximity to the ionizing electric field, thus increasing the probability of ionization.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objectives and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged perspective view showing a bare-point array utilized in the field ionization source of one embodiment of the invention;

FIG. 2 is an enlarged fragmentary prospective view of a bare-point array using a different substrate arrangement (different from the one illustrated in FIG. 1) for one embodiment of the field ionization source;

FIG. 3 is a central, vertical, sectional view of a field ionization source structure utilizing the bare-point array of FIG. 1; and

FIG. 4 is a diagrammatic cross-sectional view of a mass spectrometer utilizing the field ionization source.

DESCRIPTION OF PREFERRED EMBODIMENTS

A form of the basic bare-point array 10 used as a field ionization source is illustrated in FIG. 1. The bare-point array structure 10 includes a supporting disk-like substrate 11 and an array of electric field-forming needle-like bare points 12 formed thereon. Since an important consideration in the design of the source is to provide a way to bring the sample to be ionized directly into the ionizing electric field for most efficient ionization, the substrate is made porous so that the sample can be brought in through the back of the substrate and up along the surface of the field-forming needle-like elements 12. As illustrated, the bare points are pyramidal but may be of other conical shapes. The substrate 11 in this embodiment is of porous sintered tungsten which is conductive and, therefore, along with the array of needle-like points 12 formed thereon, constitutes a conductive electrode. In one embodiment, the tungsten substrate 11 is 65 percent porous and the needle-like points 12 cover a circular area 1.5 millimeters in diameter and are spaced 0.0025 centimeter apart (center-to-center).

For some applications the permeability of the porous tungsten substrate 11 may not be sufficient or it may not have the characteristics desired. For example, though tungsten is considered inert, it may, in fact, chemically affect particular samples; therefore, it may be preferable to use a semiconductive or an insulative substrate. One such embodiment is illustrated in FIG. 2. The substrate 14 illustrated in FIG. 2 constitutes a glass disk 16 which has a myriad of open channels 18 all of the way through the disk. A practical disk substrate 16 for the array is about 1.5 millimeters in diameter with about 60 percent of the disk comprising straight, open channels.

Thus, it is seen that the substrate is highly porous and shows little resistance to gas flow-through. In order to provide a conductive electrode, the upper surface of the substrate 14 is provided with a layer of conductive or semiconductive material 20 which does not cover (does not close) the open channels 18 in the substrate 14 and the array of needle-like elements 22 which are used to form the ionizing electric field are laid down on the conductive coat 20 on lands defined by the substrate surface area between the open channels 18. The array of needle-like elements 22 and 12 in the embodiments of both FIGS. 1 and 2 may be resistive, semiconductive, insulative or composite materials and their surfaces overcoated or otherwise treated to obtain the desired characteristics just as taught in the patents previously referred to. Further, the arrays of needle-like elements may be formed as described in those patents.

The disk-shaped glass substrates are available commercially from Bendix Corporation and Varian Associates and are used for other purposes. The plate consists of microscopic hollow glass channels fused into a disk-shaped array. The disk is practically obtainable in sizes ranging up to 25 millimeters in diameter (a diameter about equivalent to that of a quarter), about 5 millimeters thick (about one third the thickness of a quarter), and contains approximately 1,670,000 open channels. Thus, about 60 percent of the disk is comprised of open channels. Such disks are shown and described in the catalog entitled "Application for Microchannel Plates," published by Varian, Palo Alto Tube Division. The surface of the microchannel plates, including the interior of the channels as described in the catalog, is semiconductive and the plates described are entirely suitable as substrates for the field-forming arrays for the present field ionization source. Other practical insulative substrates are made of sintered glass and porous alumina.

A setting for the bare-point array 10 of FIG. 1 is illustrated in FIG. 3. The field ionization source 30 illustrated in FIG. 3 is designed specifically to provide a convenient way to direct the flow of a gas sample through the back of the porous sintered tungsten substrate 11 and up around the electrical field-forming needle-like elements 12. The field ionization source 30, illustrated, also provides means for providing electrical connections to elements of the source in order to generate the ionizing field. The gas flow channel 32 to the back of the sintered plug 11 is identified by a series of arrows which extend directly through the device. Electrical connections must be made to the sintered tungsten plug 11 and a field-forming grid structure 34 spaced from and parallel to the plane of the ionizing points on the needle-like field ionizer 12.

Consider first the physical mounting structure for the multipoint ionizing array 10 which also serves as the conductive electrical connection thereto. The source 30 is constructed so that it is easily removable in order to be able to replace a damaged or inactive multipoint array 10. The sintered tungsten plug 11 of the array 10 is mounted at one end (right in the figure) of a tubular conductive array support member 36 as by brazing or welding. In order to provide a means for removably mounting the support tube 36 to a supporting conductive washer or collar 38, the tubular member 36 is threaded internally at its opposite (left) end (at 37) so as to receive the external threads of a bolt 40 which extends up through a centrally located threaded aperture 39 in the support collar 38 and also threads into the internal threads 37 of the support tube 36. The collar 38 is also provided with a recess 41 to receive the head of the securing bolt 40. In order to provide a clear open channel for gas flow to the back of the sintered tungsten plug 11, a bore or channel 42 is provided through the securing bolt 40. The structure thus far described is rigidly held together as a unit, is electrically conductive so that the electrical connections can be made to the multipoint field ionizer array 10, and has a clear opening to the back of the multipoint array so that a gas sample may be introduced.

In order to secure the multipoint field ionizer assembly, just described, to the instrument in which it is to be used, here a mass spectrometer, the instrument itself is provided with a highly conductive heat sink and ion source support 46 which is illustrated as a heavy cylindrical copper bar 46 provided with a centrally located longitudinal aperture, that forms part of the gas flow channel 32, and external threads around the upper end. The field ionizer assembly is then secured to the rod-like heat sink 46 by an internally threaded conductive cap 48 that threads on to the outer end (threads 47) of the heat sink 46 and has an inwardly extending annular lip 49 that fits snugly into an annular recess 50 around the outer periphery of the collar 38 of the field ionizer assembly. In order to provide a good electrical and thermal conduction between the heat sink 46 and the field ionizer assembly mounted thereon, washer 52 of relatively soft, highly conductive material is provided between the upper surface of the heat sink 46 and the lower surface of the field ionization source supporting washer 38. Gold is a highly acceptable material for washer 52.

Thus, it is seen that a clear gas flow channel 32 is provided through the heat sink 46, the supporting bolt 40 and tubular multipoint field ionizer array support member 36 to the back of the porous sintered tungsten plug 11. Further, all of the parts thus described are conductive of both heat and electrical current. Thus a common path conductive of both heat and electricity is provided for the multipoint field ionizing array 10. Since a potential difference must be established between the grid 34 of the ionization source and the needle-like points 12 of the array 10, a tubular insulating grid support member 54 is mounted on the upper surface of the field ionization assembly support collar 38 so that it surrounds the support tube 36 of the multipoint array 10 and is concentrically spaced therefrom. The length of the insulating tubing member 54 is such that the grid 34 is spaced above the plane of the ionizing points of the needle-like elements 12 by the appropriate distance.

In the source illustrated here, the grid 34 is a mesh of 400 lines per centimeter, is spaced about 0.0125 centimeter above the points with the openings therein positioned in register with the points of the needle-like elements 12. The grid 34 is firmly secured in place on the upper end of the insulating tube 54 by a plug-like stainless steel cap 56 which is open in the center to allow egress of ions formed by the source. Retaining cap 56 is provided with appropriate threaded screw recesses 58 extending through its upper surface and screw recess 59 which extends through its outer periphery. Set screws 60 are provided in the screw recesses to 58 and 59 to engage the upper end and outer periphery, respectively, of the supporting insulating tube 54 to hold the cap 56 firmly in position.

The insulating tube 54 is firmly secured to the upper surface of the support collar 38 of the field ionization source by a clamp arrangement. Specifically, an annular retaining groove or recess 51 is provided around the periphery of the grid support member 54 near the end opposite the grid 34. A hold down collar or washer 53 is positioned in the retaining groove 51. The grid assembly hold down arrangement is completed by hold down bolts 55 that pass through apertures in the hold down collar 53 and are threaded into internally threaded apertures 35 provided in the upper surface (toward the grid 34) of the source supporting washer 38. Thus, the grid 34 is properly positioned relative to the multipoint elements 12 and held firmly in place.

The field ionization source 30 is shown, diagrammatically, in a mass spectrometer (the setting where it is used) in FIG. 4. Since the mass spectrometer, aside from the novel ion source and the novel combination, is conventional, it is illustrated only diagrammatically and not in great detail. A vacuum-tight envelope 60 is provided to enclose the entire mass spectrometer. The multipoint field ion source is illustrated in one end immediately followed by conventional focusing electrodes 62 which collimate the ions developed by the multipoint ion source 30 into a stream and direct the stream into a conventional quadrupole analyzer section 64. A conventional ion collector 77 is provided at the opposite end of the mass spectrometer to receive transmitted ions. As is usual, a vacuum pump 68 is connected to maintain the proper vacuum in the spectrometer envelope 60.

For operation in the intended manner, the gas flow channel 32 from the back of the porous tungsten plug 11 of the multipoint field ionizer array 10 may be traced back through a supply line 70 to a source of sample gas 72. The gas flow entry channel 32 is provided with an on-off valve 74 so that the channel may be closed entirely and also a variable leak valve 76 for fine control of the sample admitted for ionization. For normal field ionization operation, a point to grid potential difference of 3,645 volts is applied. The quadrupole section used has pole pieces 1.6 centimeters in diameter and 22 centimeters long. The multipoint source 30 is maintained at +45 volts and the extraction grid 34 kept at -3,600 volts. The ions leaving the grid are decelerated and focused by a single aperture electron optical lens 62 with a potential of about -400 volts. The field produced ions enter quadrupole ionizer section 64 through a 5 millimeter aperture with a net energy of 45 ev. Using toluene as sample gas, an ionization efficiency of about 1 in 3,000 and a transmision efficiency of 1 in 330 is obtained. Thus, for every million sample molecules of toluene, one is ionized, massed analyzed, and detected.

In order to obtain a comparison of results between ionization using the multipoint field ion source 30, as just described, and using the same source bringing the sample into the area of the multiple points from the side or top rather than through the porous sintered tungsten plug 11, a second supply line 80 is connected to the sample source 72 just prior to an on-off valve 82 in the regular gas flow channel 32. The second supply line 80 is connected to provide entry of sample gas into the mass spectrometer envelope 60 along the side in the region of the multipoint array 10. The auxiliary supply line 80 is also provided with an on-off valve 84 to allow the line to be opened and closed. By closing on-off valve 82 in the normal supply line 70 and opening the normally closed valve 84 in auxiliary supply line 80, the sample effluent from variable leak valve 74 is diverted directly into the spectrometer envelope 60. The signal obtained with the same sample flow using the channel through the sintered tungsten array support and the sample flow to the side of the array is 550 times as strong, thus showing the improvement provided by the present invention over the next best known field ionization source.

While particular embodiments of the invention are shown, it will be understood that the invention is not limited to the specific structures since many modifications may be made both in the material and the arrangement of elements. It is contemplated that the appended claims will cover such modifications which fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A field ionization source comprising a plate-like porous substrate pervious to flow of substantially all gases and a multiplicity of needle-like elements located on one surface of said substrate, said needle-like elements being highly uniform in space and uniformly spaced on said substrate.

2. A field ionization source, as defined in claim 1, wherein both said substrate surface on which the needle-like elements are deposited and said needle-like elements consist of conductive material.

3. A field ionization source comprising a plate-like porous substrate pervious to flow of substantially all gases and a multiplicity of needle-like elements located on one surface of said substrate, said substrate having uniformly spaced land areas thereon defined by uniformly spaced apertures therethrough which provide the porosity, said needle-like elements being highly uniform in space, uniformly spaced on said substrate, comprised of conductive material and located on the said land areas of said substrate.

4. A field ionization source, as defined in claim 3, wherein said plate-like substrate is of a substantially nonconductive material with a plate-like conductive electrode at least on one surface thereof, said plate-like electrode having aperture therein in registry with the apertures in said nonconductive substrate and said conductive needles located on said conductive electrode.

5. A field ionization source for ionizing components of sample gases, including the structure defined in claim 1, and means to introduce a sample gas to be analyzed to the surface of said plate-like substrate opposite that occupied by said needle-like elements whereby sample gas flow is provided through said porous substrate and around said needle-like elements, grid means spaced from said needle-like elements and means for applying a potential between said grid and said needle-like elements thereby to provide an electric field therebetween whereby the said gas sample is exposed to the said electric field in the area of said needle-like elements and molecules of said gas sample are thereby ionized.

6. A field ionization source for ionizing components of sample gases, including the structure defined in claim 2, and means to introduce a sample gas to be analyzed to the surface of said plate-like substrate opposite that occupied by said needle-like elements whereby sample gas flow is provided through said porous substrate and around said needle-like elements, grid means spaced from said needle-like elements and means for applying a potential between said grid and said needle-like elements thereby to provide an electric field therebetween whereby substantially all of the said gas sample is exposed to the said electric field in the area of said needle-like elements.

7. A field ionization source for ionizing components of sample gases, including the structure defined in claim 3, and means to introduce a sample gas to be analyzed to the surface of said plate-like substrate opposite that occupied by said needle-like elements whereby sample gas flow is provided through said porous substrate and around said needle-like elements, grid means spaced from said needle-like elements and means for applying a potential between said grid and said needle-like elements thereby to provide an electric field therebetween whereby the said gas sample is substantially all exposed to the said electric field in the area of said needle-like elements.

8. A field ionization source for ionizing components of sample gases, including the structure defined in claim 4, and means to introduce a sample gas to be analyzed to the surface of said plate-like substrate opposite that occupied by said needle-like elements whereby sample gas flow is provided through said porous substrate and around said needle-like elements, grid means spaced from said needle-like elements and means for applying a potential between said grid and said needle-like elements thereby to provide an electric field therebetween whereby the said gas sample is substantially all exposed to the said electric field in the area of said needle-like elements.

9. A mass spectrometer, including the structure as defined in claim 5, including means to focus said ions into an ion beam and means to analyze ions in said beams whereby constituent components of said sample gas are determined.

10. A mass spectrometer, including the structure as defined in claim 6, including means to focus said ions into an ion beam and means to analyze ions in said beams whereby constituent components of said sample gas are determined.

11. A mass spectrometer, including the structure as defined in claim 7, including means to focus said ions into an ion beam and means to analyze ions in said beams whereby constituent components of said sample gas are determined.

12. A mass spectrometer, including the structure as defined in claim 8, including means to focus said ions into an ion beam and means to analyze ions in said beams whereby constituent components of said sample gas are determined.