Atmospheric seal and method for glow discharge analytical instrument

Abstract: This invention provides a gas atmosphere surrounding a seal between the face of a glow discharge lamp assembly (10) and the sample surface to eliminate the possibility of the introduction of atmospheric gases into the glow discharge lamp. A groove (68, 118) concentric with an O-ring seal (110) in the face of the glow discharge lamp is provided. This groove is supplied with gas slightly in excess of atmospheric pressure. This provides a gas blanket or jacket around the O-ring seal. Any leaks in the O-ring seal would draw the gas rather than atmospheric gases into the lamp. This is particularly important when a sample is being analyzed for elements which are also commonly present as atmospheric gases. The invention pertains to DC as well as RF glow discharge lamp assemblies.

Description: This invention relates generally to glow discharge methods and apparatus and, in particular, to seals for analytical instruments and to a method and apparatus for preventing atmospheric gases from contaminating glow discharge spectral analyses. Surface analysis or depth profiling of a conductive or non-conductive sample on a glow discharge spectroscope is finding wide acceptance. This is due in substantial part on the Grimm lamp design used on such instruments which provides convenient and rapid sample substitution. This is accomplished by simply placing the sample over the opening to the glow discharge lamp. The opening has an outer concentric O-ring which allows a vacuum seal to be substantially formed between the face of the Grimm lamp and the sample surface. However, because the sample surface is not always perfectly smooth, the vacuum used to retain the sample against the opening also draws atmospheric gases into the lamp. The presence of these gases can substantially alter the results of an analysis. To date, no suitable method has been proposed to prevent the Grimm lamp from aspirating atmospheric gases past the sample. EP-A-0636877 discloses a glow discharge analytical instrument having a non-conductive isolator for supporting a sample to be analysed by radio frequency energy induced glow discharge techniques. EP-A-0312066 discloses a particle beam lithography system for forming semi conductor chips having a guard ring surrounding a seal apparatus formed by a pressurised gas at a pressure higher than ambient pressure. The ring of gas is formed by a ring of small ports to a source of pressurised gas. In a first aspect, the invention provides a method for preventing atmospheric gases from passing between a glow discharge lamp assembly and a sample placed over an opening of the glow discharge lamp assembly in which a vacuum seal is substantially formed between the sample and a face of the glow discharge lamp assembly, the method comprising providing a predetermined gas about the sample and the glow discharge lamp assembly at a pressure greater than atmospheric pressure. In a second aspect, the invention provides a glow discharge lamp assembly, comprising an anode having an aperture extending therethrough; an insulator adjacent the anode and having an aperture aligned with the aperture in the anode; a cathode adjacent the insulator on a side opposite the anode and having an aperture aligned with the apertures in the insulator and the anode, the cathode having a face surrounding the aperture extending through said cathode for receiving the sample over the aperture whereby a vacuum seal is substantially formed between the sample and the face; and a gas dispenser on the face of the cathode to provide a predetermined gas about the sample. In a third aspect, the invention provides an apparatus including a glow discharge lamp assembly for preventing atmospheric gases from passing between the glow discharge lamp assembly and a sample placed over an opening of the glow discharge lamp assembly in which a vacuum seal is substantially formed between the sample and a face of the glow discharge lamp assembly, the apparatus comprising a first passage defined in the face of the glow discharge lamp assembly proximate the region configured to receive the sample for dispensing a predetermined gas about the sample. The invention also extends to a glow lamp assembly including a sampling plate; e.g. it extends to a glow lamp assembly including a cathode plate and, additionally or alternatively, including an isolator adapted for attachment to a cathode plate of a glow lamp assembly; the sampling plate may include a gas port for dispensing a predetermined gas in the region of a sample to prevent atmospheric gas from entering a lamp assembly of which the plate may define a sampling wall. In general, the instant invention may provide a sealing curtain of gas about a sample placed over an opening of a glow discharge lamp. The gas may be provided at a pressure slightly greater than atmospheric pressure such that any leaks between the sample and the glow discharge lamp result in the aspiration of the provided gas instead of atmospheric gases which could slew the data. In one form of the invention for use on a direct current glow discharge lamp, a groove may be machined into the face of the cathode plate in a pattern concentric with the opening through which the sample is analyzed. Gas may be supplied to the groove through a second passage which extends from the cathode plate. Because the gas is provided at a pressure slightly greater than atmospheric pressure, the supplied gas provides a barrier to atmospheric gases entering the glow discharge lamp. In another form of the invention for use on a radio frequency glow discharge lamp, a similar groove may be provided on a conductive member in a concentric pattern about an opening through which the sample is analyzed. The groove preferably is provided with gas through a second passage in fluid communication with the gas supply. The RF conductive member can be adapted to be received on the cathode plate of the DC discharge lamp by a nonconductive intermediate member. In preferred constructions, a gas atmosphere is provided surrounding a seal between the face a glow discharge lamp assembly and a sample surface in order to eliminate the possibility of the introduction of atmospheric gases into the glow discharge lamp. A groove is preferably provided concentric with an O-ring seal in a sampling face of the glow discharge lamp. The groove is preferably supplied with a gas slightly in excess of atmospheric pressure. Thus, a gas blanket or jacket is provided around the O-ring seal. Any leaks in the O-ring seal would draw this gas rather than atmospheric gases into the lamp. This is highly advantageous, particularly when a sample is being analysised for elements which are also commonly present as atmospheric gases. The invention pertains to DC as well as RF glow discharge lamp assemblies. One advantage provided by the sealing system and process is more accurate data, particularly when the sample under analysis contains elements present in the atmosphere. Another advantage is that atmospheric gases can be excluded or shielded less expensively than by a sealed cabinet providing a rarefied environment. The sealing system and process of preferred embodiments can also be inexpensively retrofitted to existing glow discharge lamp systems. The retrofit involves simply the replacement of the cathode plate and a connector to the gas supply line. The present invention may be carried out in various ways and two preferred glow discharge and atmospheric gas sealing apparatuses and methods will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a fragmentary, oblique view of a Grimm lamp in a glow discharge spectroscope; Fig. 2 is an exploded view of the glow discharge lamp shown in Fig. 1; Fig. 3 is an elevation view of the face of a preferred cathode of a glow discharge lamp in accordance with an embodiment of the invention; Figs. 4, 5, and 6 are sectional views of the cathode shown in Fig. 3, taken along lines IV-IV, V-V, and VI-VI, respectively; and Fig. 7 is a sectional view illustrating a preferred RF glow discharge lamp in accordance with another embodiment of the invention. Fig. 1 is an illustration of one embodiment of a Grimm lamp 10 as it is used in a glow discharge spectroscope 12. In general, the Grimm lamp 10 is disposed at one end of a lens block 14 which is in optical communication with an analyzer portion of a spectroscope. Depending from the lens block 14 and configured to engage the sample over the port of the cathode is a reamer assembly 16. The reamer assembly 16 is supported in front of the Grimm lamp 10 on a boom 18 such that an air actuator 20 may reciprocate a reamer 22 back and forth with respect to the Grimm lamp 10. The reamer 22 assists in retaining a sample and cleans deposits from an anode orifice of the lamp after each sampling. Also coupled to the Grimm lamp 10 are cooling lines 30 to dissipate heat in the lamp produced as a result of glow discharge as well as a vacuum line 24, both well-known in the art. As shown in Fig. 2, the Grimm lamp 10 includes a cathode plate 32 separated from an anode plate 34 of generally like dimension by an insulator assembly 36. A second insulator assembly 38 separates the anode plate 34 from the lens block 14. Each insulator assembly 36 and 38 includes an outer ring 40 having an outer diameter substantially equal to those of the cathode and anode plates 32 and 34, respectively. The inside diameter of each ring 40 concentrically receives an O-ring seal 42 disposed about the circumference of insulator disk 44. With the outer ring 40, O-ring seal 42, and insulator disk 44 concentrically arranged, the O-ring 42 is compressed slightly between the insulator disk outside diameter and the inside diameter of the outer ring such that when each insulator assembly 36, 38, is sandwiched between the cathode and anode plates 32 and 34, or the anode plate 34 and the lens block 14, an air-tight seal is formed between the two plates 32, 34 or the anode plate 34 and the lens block 14. Shown at the bottom of Fig. 2 is an anode insert 46 having a sleeve portion 48 and a threaded base 50. The anode insert 46 is threaded to anode plate 34 such that the sleeve portion 48 extends concentrically through the central orifices or ports 52 in anode plate 34, port 54 in insulator disk 44, and substantially into port 56 extending through cathode plate 32. As is conventional in the art, a ceramic insulator shroud or sleeve (not shown) may be disposed between anode sleeve 48 and port 56. Figs. 3-6 illustrate one preferred cathode plate for use in a preferred apparatus and method in accordance with the present invention. For the purposes of example only, the following description will be with particular reference to a cathode plate used in direct current (DC) glow discharge lamps. Application of the invention on a radio frequency (RF) glow discharge lamp will be specifically addressed below. It is preferred that cathode plate 32 be machined or otherwise formed from solid copper alloy stock, most preferably containing beryrillium such that the metal will be harder than pure copper. Most preferably, copper alloy T172 is used. The dimensions of the cathode plate can vary depending upon the specific application although it is contemplated that the diameter may range between 75 and 150 mm (3 and 6 inches), and have a thickness ranging between 5 and 25 mm (¼ and 1 inch). Preferably, the diameter of the cathode plate is approximately 115 mm (4.5 inches) and has a maximum thickness of about 13 mm (0.5 inch). Port 56 extending through cathode plate 32 is preferably located so as to be aligned with all the other ports or passages to the lens block 14 described above. It is also preferred that two or more equidistant radially spaced holes 62 extend through the plate 32 to accept resin or other non-conductive bolts 64 which extend through and interconnect the anode plate 34 and the intervening insulator assemblies 36 and 38 in tight relationship. Also machined or otherwise formed in surface 60 of the cathode plate 32 is an O-ring seat 66 adjacent to and concentric about port 56 (Figs. 4 and 6). Seat 66 is configured to receive an O-ring seal (not shown), such as made from silicon, to provide as good a seal with a specimen or sample as possible when a vacuum is applied. Surrounding port 56 and concentric therewith is a circular passage or channel 68 having a depth generally equal to its width, most preferably about 1.5 mm (0.06 inch) deep and about 1.5 mm (0.06 inch) wide. Extending from one or more points in the bottom of the channel 68 in a direction generally parallel to port 56 is at least one passage 70. Passage 70, in turn, terminates in one end of a second passage 72 oriented generally at a right angle to passage 70 and extending radially outward to a circumferential surface 74 of the plate 32. An inlet tube 76 preferably has one end received and sealed in passage 72 with the opposite end extending from the cathode plate where it can be coupled to a hose to transfer a gas, such as argon, therethrough. In the embodiment shown, it is contemplated that passage 74 extends through plate 32 generally parallel to surface 60 between a pair of passages 78 and 80 (Fig. 5) interconnected at one end by a third passage 82 for circulating a coolant through the plate to absorb heat generated during the glow discharge process. Hose fittings such as 84 may each have an end received in a respective passage 78, 80 to permit coupling to a coolant hose, such as one in the group 30 mentioned above. Referring to Figs. 4 through 6, surface 60 of the cathode plate 32 is contoured or stepped to define concentric regions or areas. It is preferred that the highest plateau or circular area 86 lies immediately adjacent and encircles port 56, having its outer diameter defined by circular channel 68. The successive circular areas or plateaus 88, 90 are each reduced in elevation from the immediately adjacent inner plateau by approximately 2.5 mm (0.10 inch) and 5 mm (0.20 inch) and most preferably about 3.8 mm (0.15 inch). The instant invention is also applicable to RF glow discharge spectrometers for analysing non-conductive samples, such as disclosed in US-A-5408315 Fig. 7 illustrates one embodiment of an interface assembly 100, including a non-conductive isolator plate 102 which is attached to the front face 60 of the cathode plate 32. Insulator 102 can be made of a suitable non-conductive material, such as ceramic, and is approximately 75 mm (3 inches) in diameter and about 10 mm (3/8 inch) thick. The insulator has a central aperture 104 which is bounded by a raised edge 106. A circular channel or groove 108 is defined in the raised edge 106 and receives an O-ring seal 110. A preferably circular electrical contact member 112 surrounds the raised portion of the isolator and is received within a recess 114 formed in the face of the isolator. The face 116 of contact member 112 projects outwardly approximately 0.13 mm (0.005 inch) beyond the raised portion 106 to provide a uniform electrical contact with the face of the sample. While a circular electrical contact member is preferred, the contact member 112 can have other configurations so long as it provides a substantially uniform contact with the glow discharge region. Defined in the surface 116 of the contact member 112 and concentric about aperture 104 is a circular channel 118. Extending into the contact member 112 from the bottom of channel 118 at one or more points is a passage 122 oriented substantially perpendicular to passage 120 and extending generally radially away from aperture 104 out of a circumferential surface 124 of the contact member 112. A tube or pipe 126, preferably made from TEFLON™ or similar high-temperature material, has one end received in passage 122 and the other end connected to a gas supply, such as argon, not shown. The outer face 116 of contact member 112 is preferably contoured in at least two plateaus, each having a generally circular shape and concentric about aperture 104. Plateau 130 constitutes the highest relief with respect to isolator 102 and immediately surrounds the raised edge portion 106. Plateau 130 extends from the raised edge portion 106 outwardly to the circular channel 118. A second lower plateau 132 extends from channel 118 outwardly to the peripheral edge 124. The plateau transition at channel 118 acts as a nozzle and permits the outward radial flow of a gas introduced into channel 118 through pipe 126 and passages 120 and 122, as will be described in greater detail below. On the opposite face of isolator 102 is a circular groove or channel 134 in which an O-ring 136 is positioned for forming a gas-tight seal with the plateau 88 on the face of the cathode plate 32. Near the central portion of the isolator is a raised, circular ridge 138 which fits into the circular channel 68, defined above, for aligning the aperture 104 of isolator 102 with the aperture 56 in the cathode plate 32. The raised ridge 138 on the back face of isolator 102 is also designed to fit into and interlock with the circular channel 68 in the face of the cathode plate 32. In both of the embodiments described above, Figs. 4 and 7 illustrate that passages 70 and 120, respectively, extend generally perpendicularly from passage 72, 122, respectively, into the channel 68 and 118. In the instance where only a single passage is provided into the channel, it may be preferred to evenly distribute the gas therein. To this end, it is contemplated that the passages leading into the channel may be inclined such that the gas enters the channel in a single direction. With the channels inclined, the gas will flow in a circular pattern around the channels and provide a more even distribution of the gas between the lamp and the sample. It is further contemplated that a plurality of inlet passages may be provided instead of the circular channel described above. It is possible that a number of inlet passages may be machined into the surface of the lamp assembly and interconnected by one or more passages below the coolant passages, provided the cathode has sufficient thickness. In operating the DC glow discharge source, as shown in Figs. 1-6, a DC potential is applied between anode plate 34 and cathode plate 32. The ceramic sleeve described above surrounding the anode sleeve 46 shields the anode sleeve extending through a portion of the aperture 56 within the cathode plate 32. A glow discharge formed between the anode and the exposed portion of the sample will generate ions of an inert gas which bombard the face of the sample. The bombardment will excite atoms of the sample. As electrons and the excited atoms of the sample drop back to a lower energy state, the glow discharge emitted will change characteristic indicative of the composition of the elements in the sample. Because the seal between the sample and the O-ring within the seat 66 is not always complete, the vacuum drawn on the interior of the spectrometer may cause atmospheric gases to leak past the seal and enter into the discharge chamber. The atoms of the atmospheric gas are also excited by the ion bombardment and can change the glow characteristic of the discharge, making it appear that these elements are present in the sample. To prevent this contamination, the inert gas flooding the glow discharge chamber is also pumped through inlet tube 76 at a pressure slightly greater than atmospheric pressure so as to flow radially outward from channel 68 adjacent the sample. Thus, any tendency of the atmospheric gases to migrate to the aperture 56 will be replaced by the inert gas flowing from channel 68. Thus, in effect, the gas flowing from channel 68 provides a curtain to prevent the atmospheric gases from entering the chamber. In other words, a gas seal is provided about the sample. With respect to the RF discharge source shown in Fig. 7, the sample principle is applied. With the cathode plate 32 and the isolator 102 assembled as shown in Fig. 7, a non-conductive sample is pressed against the O-ring 110 and then the interior of the glow discharge apparatus is evacuated. The atmospheric pressure pushing against the sample will hold it in place against the electrical contact surface of member 112. At this point, the anode and cathode are at a zero DC potential. A source of RF energy of approximately 30 watts at 13.56 MHz is then applied to the conductive member 112 and into the face of the sample contacting surface 130. The glow discharge will then form between the anode insert or sleeve and the sample. During the glow discharge process, materials sputter from the face of the sample. In order to remove this material, a vacuum pump (not shown) is connected through a valve to a conduit which leads to the interior of the glow discharge cell in the sample face. By pumping out approximately 50 ml per minute of the inert gas, the sputtered material would be drawn away from the sample face enabling a fresh sample surface to be continually presented for analysis. As mentioned above, the glow discharge is generated in an area between the sample face and the anode sleeve. The analysis is carried out with a suitable spectrometer which is focused to look down through the bore of the anode sleeve at the glow discharge region. It could be said that the spectrometer is bore-sited onto the glow discharge with the anode insert or sleeve providing the bore. Again, to prevent the passage of atmospheric gases past the sample in contact with surface 130 and O-rings 110, the inert gas flooding the glow discharge region is also pumped through pipe 126 into passages 122 and 120 so as to flow radially outward from channel 118 about the sample at a pressure greater than atmospheric pressure. By flooding the atmosphere immediately adjacent the sample with the same inert gases contained in the glow discharge region, atmospheric gases are prevented from being drawn past the sample and into the chamber. Any gas drawn into the chamber will be the gas flowing out through channel 118.

Claims: A method for preventing atmospheric gases from passing between a glow discharge lamp assembly (10) and a sample placed over an opening of the glow discharge lamp assembly, in which a vacuum seal is substantially formed between the sample and a face of the glow discharge lamp assembly, the method comprising providing a predetermined gas about the sample and the glow discharge lamp assembly at a pressure greater than atmospheric pressure. A method as claimed in claim 1 which includes flowing the gas from a first passage (68, 118) formed in the face of the glow discharge lamp assembly on which the sample is sealed. A method as claimed in claim 2 in which the flowing of the gas comprises the step of forcing the gas through at least one second passage (72, 122) in fluid communication with the first passage. A method as claimed in claim 3 which includes introducing the gas from the second passage into the first passage at a plurality of entry points. A method as claimed in claim 2 or claim 3 or claim 4 which includes forming the first passage in a pattern around said sample. A method as claimed in any one of claims 2 to 5 which includes introducing the gas into the first passage in a direction to move the gas in a single direction in said first passage. A method as claimed in any one of claims 1 to 6 which includes providing the gas to an interior of said glow discharge lamp assembly. A glow discharge lamp assembly (10), comprising an anode (34) having an aperture (52) extending therethrough; an insulator (44) adjacent the anode and having an aperture (54) aligned with the aperture in the anode; a cathode (32) adjacent the insulator on a side opposite the anode and having an aperture aligned with the apertures in the insulator and the anode, the cathode having a face surrounding the aperture extending through said cathode for receiving the sample over the aperture whereby a vacuum seal is substantially formed between the sample and the face; and a gas dispenser (68, 102, 118) on the face of the cathode to provide a predetermined gas about the sample. An assembly as claimed in claim 8 in which the gas dispenser includes a first passage (68, 118) defined in the face of the cathode and configured to dispense a gas. An assembly as claimed in claim 8 or claim 9 in which the gas dispenser includes: an isolator (102) disposed on the cathode face and having an aperture aligned with the aperture in the cathode; a conductive member (112) disposed on the isolator and concentric about the aperture in the isolator; and a first passage (118) defined in the conductive member for dispensing a gas about a sample. An assembly as claimed in claim 9 or claim 10 in which the said first passage includes: a channel (68, 118) formed in the face of the cathode; and a second passage (72, 122) in fluid communication with the channel and a supply of gas. An assembly as claimed in claim 9 or claim 10 or claim 11 in which the said second passage intersects the channel at an angle other than 90 degrees. An apparatus including a glow discharge lamp assembly for preventing atmospheric gases from passing between the glow discharge lamp assembly and a sample placed over an opening of the glow discharge lamp assembly in which a vacuum seal is substantially formed between the sample and the face of the flow discharge lamp assembly, the apparatus comprising a first passage (68, 118) defined in the face of the glow discharge lamp assembly proximate the region configured to receive the sample for dispensing a predetermined gas about the sample. An apparatus as claimed in claim 13 wherein said first passage includes a channel formed, preferably a ring, in said face of the glow discharge lamp assembly. An apparatus as claimed in claim 13 or claim 14 wherein said first passage includes a plurality of passages defined in the face of said glow discharge lamp assembly. An apparatus as claimed in any one of claims 13 to 15 in which said first passage is formed in a cathode plate of said glow discharge lamp assembly. An apparatus as claimed in any one of claims 13 to 15 in which said first passage is formed in an isolator disposed at said face of said glow discharge lamp.

Inventors: MARSHALL KIM A, MITCHELL JOEL CARLTON, MARSHALL, KIM A., MITCHELL, JOEL CARLTON
Applicants: LECO CORP, LECO CORPORATION
Application year: 1996
Main classification: H01J61
IPC codes: H01J61, G01N21, G01J3