US3390264A - Ion source and accelerator assembly for a time-of-flight mass spectrometer - Google Patents

Description

June 25, 1968 R. s. GOHLKE ET AL 3,390,264

ION SOURCE AND ACCELERATOR ASSEMBLY FOR A TIME-OF-FLIGHT MASS SPECTROMETER 5 Sheets-Sheet 2 Filed Dec. 4, 1964 INVENTORS. Ro/ana 5. oh/A'e Frank/in J. Kar/e Fq. 10 BY A13 1 June 25, 1968 R. s. GOHLKE ET AL 3,390,264

ION SOURCE AND ACCELERATOR ASSEMBLY FOR A TIME-OF-FLIGHT MASS SPECTROMETER 5 Sheets-Sheet Filed Dec. 4, 1964 WQN QWN NWN QQN fin United States Patent 3,390,264 ION SOURCE AND ACCELERATOR ASSEMBLY FOR A TIME-OF-FLIGHT MASS SPECTROMETER Roland S. Gohlke, Ashland, and Franklin .I. Karle, Natick,

Mass., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Dec. 4, 1964, Ser. No. 415,897 9 Claims. (Cl. 25041.9)

ABSTRACT OF THE DISCLOSURE This invention relates to electrostatic time-of-flight mass spectrometer apparatus in which a ribbon of electrons is brought to sharp focus along the longitudinal axis of the ion source, ion acceleration and flight tube assembly. The ions produced by the collision of the electron beam with the sample in the ion source are accelerated by means of substantially uniform accelerating fields through collimating slits in the ion source, ion accelerator, and flight tube and impinge on a detector which usually is an electron multiplier device.

This invention relates to an improved mass analyzer and particularly to an electrostatic time-of-flight mass spectrometer having improved sensitivity and resolution.

Time of flight mass spectrometers of the prior art types have suffered from one or more of the following problems: they have been very expensive; have been bulky and not especially adaptable for quick changes in the type of analytical work in which they were used; had less resolution than was desirable for many proposed uses; were less sensitive than was desired, or took too much down time whenever repairs or modifications were made in connection with the instrument.

Accordingly, a principal object of the present invention is to provide an improved time-of-flight mass spectrometer.

Another object of this invention is to provide an improved, more economical to manufacture, time-of-flight mass spectrometer.

A further object of this invention is to provide an improved, easier to operate and maintain time-of-flight mass spectrometer.

Yet another object of this invention is to provide a time-of-fiight mass spectrometer which has an improved electron gun assembly.

A still further object of this invention is to provide an improved ion source for use in a time-of-flight mass spectrometer.

An ancillary object of this invention is to provide an improved ion acceleration assembly for use in a time-offlight mass spectrometer.

An additional object of this invention is to provide an improved more compact time-of-flight mass spectrometer.

Yet another additional object of this invention is to provide a time-of-flight mass spectrometer having improved resolution.

A subordinate object of this invention is to provide an improved method of electronically extracting ions from an ion source.

In accordance with this invention, there is provided electrostatic time-of-flight mass spectrometer apparatus in which a ribbon of electrons is brought to sharp focus along the longitudinal axis of the ion source, ion acceleration and flight tube assembly. The ions produced by the collision of the electron beam with the sample in the ion source are accelerated by means of substantially uniform accelerating fields through collimating slits in the ion source, ion accelerator, and flight tube and impinge on a detector which usually is an electron multiplier device.

3,390,264 Patented June 25, 1968 The invention, as well as additional objects and advantages thereof, will best be understood in connection with the accompanying drawings, in which:

FIGURE 1 is a side elevational and block diagram view of mass spectrometry apparatus in accordance with this invention;

FIGURE 1A is an end elevational view of the spectrometer tube shown in FIGURE 1;

FIGURE 2 is an end elevational view, partly in section, of an electron source in accordance with this invention;

FIGURE- 3 is a side elevational view of the electron source shown in FIGURE 2;

FIGURE 4 is a plan view of the electron source shown in FIGURE 2;

FIGURE 5 is a side elevational view, partly in section, of an ion source and flight tube assembly in accordance with this invention;

FIGURE 6 is an enlarged fragmentary cross-sectional view of a tubular element of the ion source, showing resistive coating on its wall surface and an electrically conductive end surface coating;

FIGURE 7 is a sectional view taken along the line 7-7 of FIGURE 5;

FIGURE 8 is a sectional view taken along the line 8-8 of FIGURE 5;

FIGURE 9 is a sectional view taken along the line 9-9 of FIGURE 5;

FIGURE 10 is a sectional view taken along the line 10-10 of FIGURE 5; and

FIGURE 11 is a sectional view taken along the line 11-11 of FIGURE 9.

Referring to FIGURE 1 and FIGURE 1A, there is shown mass spectrometer apparatus 10 in accordance with this invention which comprises an evacuated housing 12 composed of a plurality of sections which as illustrated, an input end section 14 which contains an electron source and ion source, a body section 16 which contains a flight tube, and an output end section 18 which contains a detector. Electrical and vacuum system connections to the various parts of the apparatus are made through headers in the various flanges 20, 22, 24, 26 for example.

A vacuum system 28, for example, is coupled to the housing 12 through the flange 24.

A power supply 36 is coupled to electron source and ion source electronic circuitry 30 through the cable 32. A clock generator 34 is coupled to the power supply 36 by means of the cable 38, to the electron source and ion source electronic circuitry 30 through the cable 40 and to the readout device 42, which may be an oscilloscope or chart recorder, for example, through the cable 44.

The detector is coupled through the header in the flange 26 and the cable 46 to the readout device 42.

The power supply is coupled, via the cables B, C, and D, to the electron source (header in flange 22), the detector (header in flange 26), and the ion source (header in flange 20) respectively.

Referring now to FIGURES 2, 3 and 4 it may be seen that the electron gun assembly of this invention, indicated generally by the numeral 50, comprises a block-like body member 52 having a generally rectangular configuration except near one end 54 which is rounded oil to be semicircular.

The member 50 has a pair of outwardly extending flanges 56, 58 at its lower or non-rounded end 60. The member 50 is coupled to a suitable stem assembly 62 which is part of the flange 22 and is adapted to be vacuum sealed to the housing part 14 of the mass spectrometer 10 by means of the screws 64, 66, for example. The stem 62, which usually (but not necessarily) is made of metal, has a plurality of pin connector elements 68 extending therefrom on the side of the stem which faces the exterior 3 of the mass spectrometer housing. The pin connector elements 68 are, if the stem is made of an electrically conductive material, insulated therefrom and from one an other.

A bore 70 is disposed adjacent to the rounded end 54 of the body member 52, extending completely through the member 52. The bore 70 is perpendicular to the wall 72 and in axial alignment with the outer surface 74 of the member 52.

The bore 70 has a counter-bore 75, 76 at each end.

An annulus 78, 80 is provided which is made of an electrically insulating material of good thermal conductivity, such as synthetic ruby, for example, and has an outer diameter such that one of the annuli may be press-fitted into each of the counter-bores 75, 76. A slot 82, usually having parallel sides, extends between the inner diameter and outer diameter of each annulus 78 or 80. The width of the slot 82 is equal to or greater than the width of the slot 84 which extends across the top of the round-ed end 54 of the body member 62. The slot 82 and the slot 84 are axially aligned with respect to the bore 70.

Arotted tubular member 86 having an electrically conductive inner wall surface and a generally C-shaped transverse cross-sectional configuration is disposed between the annuli 78 and 80, the outer diam-eter of the tubular member 86 being such with respect to the inner diameter of the annuli that it may be press-fitted between the annuli. The width of the slot 88 i the member 86, which is a beam focusing electrode, is less than or equal to the width of the slot 82 in the annuli 78 or 80.

A threaded bore 90, extends through the side wall of the body member 52 at or near its rounded end 54.

An electrically insulating bushing 92 engages and extends through the bore 90. An electrical lead 94 extends through the bushing 92 and is electrically connected to the conductive inner surface of the member 86. Actually, the member 86 is usually made of metal, such as copper, for example.

Referring especially to FIGURE 4, as well as to FIG- URES 2 and 3, it may be seen that the rounded end 54 of the body member is flattened over at least a part of its surface so that, at the flattened part, the thickness of the end wall is only a few thousandths of an inch (.002 inch is commonly used). The surface 96 of the flattened part of the end 54 is substantially parallel with respect to the surface of the end part 60 of the body member 52. The length of the flattened surface 96 (as measured along the slot 84), is about of the length of the slot 84. The surfaces 98, 100, each beginning at an end of the flat surface 96, is beveled upwardly at an angle of approximately 45 degrees with respect to an endwise extension of the flat surface 96.

A pair of filament mount support flanges 102, 104 extend outwardly from the wall surfaces 72, 73 of the body member 52 intermediate of the ends 54, 60. The flanges 102, 104 are rigidly coupled to the body member and may, if desired, be an integral part of the block member 52, as shown.

Each of the flanges 102, 104 has a bore 106, 108 extending therethrough. The axis of each of the bores 106, 108 is parallel with each other and with the wall surfaces 72, 73. An electrically insulating bushing 110, 112 having an internally threaded bore 114, 116 therein is press-fitted into each of the bores 106, 108 in the flanges 102, 104.

An electron source filament support element 118 or 120 having a threaded end 122 or 124 and a slotted, thinned spring- like end 126 or 128 is coupled to each of the threaded bores 114, 116, the slotted thinned ends being so aligned that the bottom of the slots in the ends 126, 128 is below the longitudinal axis of the bore 70 by a distance approximating one half the diameter of the wirelike electron source (filament) 130.

The filanmntary electron source 130 illustrated is a tungsten wire having stop means disposed intermediate of its ends 132, 134. While the stop means may be a knot in the wire, it is often easier, from a mechanical construction standpoint, to spot weld a small metal element to the tungsten wire at an appropriately spaced distance along the wire. The space between the stop means should be such that the spring-like ends of the electron source support elements holds the wire firmly in tension.

If the filament 130, the slotted tubular member 86, and the slot 88 in the rounded end 54 are properly made and aligned, a single plane should pass midway between the slot 84 and slot 88, and pass all along the length of the filament 130, while the filament is equidistant from any point on the inner surface of the focusing electrode 86 (measured perpendicularly).

Referring now to FIGURES 5 to 10, there is shown an ion source assembly, indicated generally by the numeral 150, comprising a first electrode element 152 (see also FIGURE 8) which is a disc-like element having a diameter which is several times its thickness and has an annular flange 154 disposed on one side thereof concentrically with respect to the center of the disc-like electrode element. An array of small diameter bores 156 extend axially through flange 154 of the electrode element 152.

A rod-like metal element 160 extends from the center of the side of the electrode element 152 which is opposite the side having the flange 154 therein.

The electrode element 152 and the rod 160 may be an integral structure or the rod 160 may be secured to the electrode 152 as by a fusion coupling, for example, or other suitable rigid coupling means.

The end 162 of the rod 160 which is remote from the electrode element 152 is rigidly coupled, as by a weld, for example, to a disc-like base element 164 which has an array of terminal pins 166 extending therethrough and insulated therefrom. The pins 166, are, of course, electrically insulated from the base element 164. The base element 164 is adapted to be coupled in a gas-tight sealing relationship with the housing section 14 of the mass spectrometer 10.

A tubular element 168 having a circular transverse cross-sectional configuration and an outer and inner diameter such that its end is closely but slidably in the flange 154 in the electrode element 152, extends from the grooved side of the element 152. The length of the tubular element 168 is a minor fraction of its diameter. The tubular element 168 has an electrically conductive pyrolitically deposited coating 170 on it inner wall surface (see FIGURE 6 for details) and a coating 172 of electrically conductive metallized paint (a silver compound is commonly used) at its end.

The tubular element 168 has diametrically oppositely disposed slots 174, 176 (see FIGURE 9, especially) which lie along a plane parallel with the ends of the element 168. The length and width of the slots 174, 176 are such that the ribbon electron beam emanating from the electron gun 50 may pass therethrough without impinging on the tubular element 168.

As seen in FIGURES 9 and 11 a wire-like electrode 178 is disposed adjacent to but spaced from the outer wall of the tubular element 168 in axial alignment with the slots 174, 176, and serves as an electron trap. A rigid electrical lead 180 holds the trap electrode 178 in position and is coupled (by means not shown) to one of the pins 166 in the header 24.

A metal annular member 182 is provided which has a circular groove 184 or 186 in each side surface. The outer diameter of the member 182 is approximately the same as the outer diameter of the element 152. The inner diameter of the member 182 is slightly less than the diameter of the tubular element 168. A disc 188 having a slot 190 therein is fixedly coupled to the member 182 by means of screws 192, the disc 188 spanning the open inner part of the member 182.

The ends of the tubular element 168 lie in the groove 186 and adjacent to the flange 154 (end 196 in groove 186).

A tubular element 194, like the tubular element 168 except that it has no slots (as 174, 176, for example) and has greater length, has its end 198 fitted into the groove 184 (the end 196 of element 28 is fitted into the groove 186). The coating 199 on the inner wall of the element 194 is essentially the same as the coating 170 on the section 168, as shown in FIGURES and 6, for example. The elements 168 and 194 may be made of glass, for example, although other insulating materials may be used.

A metal annular member 200, which is physically identical to the annular member 182, is coupled to the end of the tubular element 194 which is remote from the memher 182. A disc 202 having a slit 204 therein is coupled to the member 200, the disc 202 being similar in form to the disc 188.

A tubular element 205, shorter in length than the length of the tubular element 194, but otherwise identical in physical form to the element 194, has one end fitted into the groove in the annular member 200 which corresponds to the groove 184 in the member 182. The element 205 has a conductive coating 206 on its inner wall surface and conductive coating on its endsurfaces as do the elements 168 and 194.

Referring now to FIGURE 7 as well as to FIGURE 5, it may be seen that the structure of the annular member 208 is the same as that of the annular member 182. However, instead of a disc having a slot in it being coupled across the open central part of the annulus, an annular shaped insulating bushing 210 having an L-shaped transverse cross-sectional configuration is coupled to one side of the member 208 by means of screws 212 made of insulating material. Deflection plates 214, 216, each of which is rectangular in configuration, are rigidly connected to support plates 218, 220. The deflection plates are generally perpendicular with respect to the support plates. The support plates are secured to but insulated from the annular member 208 by being disposed against the insulating bushing 210 and held in place by the insulating screws 212.

The deflection plates 214, 216, are each displaced an equal distance from the longitudinal axis of the annular member 208 and are wider than the thickness of the member 208.

A tubular element 222, shorter than the tubular element 205, has an end telescoped within the groove 223 of member 208. Like with the element 168, 194, a conductive coating or surface is on the inner wall of the element 212.

The other end of the tubular element 222 is telescoped within the grooved end 224 of a metal annular member 226 whose surface facing the element 208 corresponds in configuration to the surface of the element 200 which faces the element 208. Thus, a disc 226 having a slot 228 therein is secured to the element 226, the slot 228 being axially aligned with the slots 204 and 190.

A flight tube assembly, indicated generally by the numeral 230, has a flanged base 232 and an elongated tube 234 of metal, such as stainless steel or copper, for example. The inner diameter of the tube 234 is approximately the same as the inner diameter of the annular members 182, 200, 208 and 226, for example.

A fine mesh metal screen 236 is disposed across the output end 238 of the flight tube 234. A 90 mesh nickel screen has been successfully used. Screens having from 50 lines per inch up to the limit where transmission,

through the mesh becomes too small are practical. The fine screen prevents any substantial penetration of the drift tube by external fields.

Insulating spacer screws 240 made of nylon, for example, are disposed in array around the periphery of the flight tube 234 intermediate of the ends of the tube 234. The length of the flight tube is approximately 40 centimeters. The ion source assembly 150 and the flight tube assembly 230 are held together to form a unitary structure by means of a plurality of bolts 242 made of insulating material which extend from the member 152 along the peripheral part of the ion source assembly and the periphery of the base 232 of the flight tube assembly.

The ion source and flight tube assembly is inserted in the housing 12 of the spectrometer apparatus by removing the flange 20 to which the disc-like base element 164 is coupled, and sliding the assemblies into the housing tube. The flight tube assembly and the ion source assembly are supported within and insulated from the housing 12 by means of the spacer screws 240 and the disc-like header element 164 which is secured to the flange 20.

The detector used in this apparatus may be any of a number of conventional detectors used for this purpose, an electron multiplier type of detector being commonly used.

The electron source assembly, coupled to the flange 22 on the housing 12, is inserted into the housing perpendicularly with respect to the longitudinal axis of the ion source assembly. The slot 84 of the electron source is axially aligned with respect to the slots 174, 176 in the element 128.

In operation the potential on the electron source focusing electrode 86 is adjusted to cause the electron beam emanating from the electron source to come as nearly as practical to a line focus in axial and planar alignment with the slots 190, 204, 228 in the ion source assembly.

The sample material to be analyzed may be introduced into the space defined by the members 152, 182 and element 168, known as the ion generation chamber, through a suitable port, such as a septum 244 in the flange 246 which is coupled to the housing section 14. The septum 244 is aligned with a small bore 248 in the member 168, and sample may be inserted by means of a hollow needle, small tube, or other means known to those skilled in the art.

During operation, voltages derived from the electron source and ion source electronic circuitry are repetitively applied to the block-like body member 52 (and thus across the slot 84) of the electron source and to the member 182 of the ion source while the remainder of the ion source is held under a constant accelerating field. l

The manner of applying the repetitive voltages to the ion generating region and the remainder of the ion source assembly is shown in simplified form in FIGURE 5 by means of the voltage dividing resistor 248 which actually represents the resistance of the resistive coating on the inner wall surfaces of the tubular elements 168, 194, 205 and 222, for example. For the sake of simplicity, the leads 250, 252, which are connected to the deflecting plates 214, 216 (by fused connection to the metal parts 218, 220, for example) and which provide some focusing of the ions passing from the ion source through the slit 228 are shown as being connected to the voltage divider resistor 248 rather than to an external voltage source.

When a suitable voltage is applied across the slot 84 of the electron source which permits the electron beam to pass into the ion source, the switch 254, which is coupled to the resistor 248 at the junction 256 and to the metal member 182, is opened, providing an electrical field in the ion accelerating region which urges ions formed as the electron beam impinges on the sample in the region to be accelerated towards and through the slot 190.

As indicated by the connections 256, and 262 along the voltage divider, the ions are subjected to accelerating fields (which are uniform in each section of the ion source because of the conductive resistive coating on the inner wall surfaces of the elements 194, 205 and 212) before entering the drift tube 234 which is at the same potential along its length as the potential on the member 226.

The purpose of the electrode 178 is to be a trap for electrons which pass through the ion generation region of the ion source from the electron gun (through the slits 174, 176 which are about wide). The electrode 178 also provides a convenient means by which electron beam current may be measured.

After each group of ions are urged down the ion source and into the flight tube, they separate in their passage in accordance with their mass as is well known in the art of time-of-flight mass spectrometry and successively impinge on the detector, are amplified, and are displayed on a readout device on a time base and amplitude of received signal scale. The readout device and the application of voltages to the electron beam and ion source are synchronized by means of the clock generator, as is well known in the art.

In general, materials used in the apparatus of the invention should not out-gas and must be non-magnetic.

In one apparatus made in accordance with this invention, the operating potential on the member 152 is ground, the potential on the member 182 is between 50 and 300 volts, the potential on the member 200 is about -3,000 volts, the potential on the plate 214 is about 3,000 volts, the potential on the plate 216 is about 3,000 volts -5%, and the potential on the member 226- and flight tube 234 is typically 3,000 volts.

The potential of between 50 and 300 volts on the member 182 is determined by setting the voltage to give the best resolution for the instrument.

The detector used may be an electron multiplier tube (with glass envelope removed) such as an RCA type 7746 or EMI type 9603, for example.

What is claimed is:

1. Ion source, accelerating and focusing apparatus for use in time-of-flight spectroscopy apparatus, comprising:

(A) an ion source chamber which is generally cylindrically in configuration, said chamber having ends made of electrically conductive material, one of said ends having a slit therein, and cylindrical side walls, said side walls being by an electrically resistive inner surface and having diametrically opposed slits extending therethrough, said diametrically opposed slits being perpendicular to said slit in the end of said chamber, means for introducing a sample into said ion source chamber,

(B) a generally cylindrical ion accelerating chamber having as its input end the end of said ion source chamber having said slitted end, its output end of electrically conductive material having a slit which is of the same dimensions and is axially aligned with its input end slit, cylindrical side walls having an electrically resistive coating covering their inner surface, and

(C) a generally cylindrical ion focusing chamber having as its input end the slitted output end of said ion accelerating chamber, an output end of electrically conductive material having a slit therein which is of similar dimensions and is axially aligned with the slit in the input end of said focusing chamber, a pair of cylindrical side wall elements, each of said side walls being covered by an electrically resistive inner wall surface, said pair of side walls being disposed in end-to-end relationship and having ion beam focusing support structure sandwiched therebetween, a pair of ion beam deflecting plates, said deflecting plates being axially aligned with respect to said slits in the input end and output end of said focusing chamber and spaced apart a distance which is greater than the width of said last men tioned slits, and

(D) means for sequentially applying a potential difference between the ends of said ion source chamber, means for applying .a potential difference between the ends of said ion accelerating chamber, means for applying electrical potentials to the ends of said ion beam focusing chamber, and means for applying electrical potentials to said deflecting plates.

2. Apparatus in accordance with claim 1, wherein the electrically resistive side wall surfaces of each of said chambers are electrically coupled to the ends of the respective chambers.

3. Apparatus in accordance with claim 1, wherein said all of said side walls are vitreous and have a resistive coating on their inner surface.

4. Apparatus in accordance with claim 1, wherein the inner diameter of each of said chambers are substantially the same.

5. Apparatus in accordance with claim 1, wherein the spacing between said deflecting plates is approximately three times the width of the slits in the output end of said focusing chamber.

6. Apparatus in accordance with claim 1, wherein said deflecting plates are disposed parallel to the longitudinal axis of said chamber.

7. Apparatus in accordance with claim 1, wherein said chambers are disposed within an evacuated housing.

8. Apparatus in accordance with claim 1, wherein said chambers are physically coupled together as a unitary structure.

9. Apparatus in accordance with claim 1, wherein a stem assembly including electrical lead feed-through elements is coupled to the end of said ion source chamber which has no slit therein.

References Cited UNITED STATES PATENTS 2,612,607 9/1952 Stephans 25041.9 2,662,184 12/1953 Berry 250-419 3,163,752 12/1964 Wahrahaftig et al. 250-419 2,231,676 2/ 1941 Muller 250-207 RALPH G. NILSON, Primary Examiner.

ARCHIE 'R. BORCHELT, Examiner.

A. L. BIRCH, Assistant Examiner.

Images