METALLIC CLAD CAPILLARY TUBING

Abstract: Metallic clad glass capillary (10) suitable for use in gas chromatography applications. A metallic coating or jacket (12) is formed on the glass capillary (11) by coating the glass capillary with a metal or alloy as the capillary is drawn from the furnace. The metal or alloy employed is one that is substantially chemically inert with respect to the material comprising the glass capillary at the deposition temperature during coating of the metal or alloy onto the glass capillary and forms a hermetic seal around the outer surface of the glass cladding. The metallic coating prevents chemical or mechanical damage to the glass surface to thereby maintain or preserve the nascent strength of the glass capillary. A plastic coating (13) is optionally provided for additional protection of the metal surface.

Description: METALLIC CLAD CAPILLARY TUBING CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to Serial No, filed (PD-80025) and Serial No. filed (PD-79299), which disclose metallic claddings on glass fibers for optical waveguides. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to silica capillary columns for gas chromatography, and, in particular, to metallic clad silica capillaries. 2. Description' of the Prior Art In the field of analytical chemistry, gas chroma- tography (GC) is the most widely used and the most rapidly growing analytical technique. Although the field of gas chro atography is more properly subdivided into specific categories, the gas-liquid chromatography or gas-liquid partition chromatography, as it is also called, is mainly responsible for the tremendous development of the field as a whole. Usefulness of this technique is closely tied to the proper selection of a column, or partition column, in which the separation of sample components takes place. The most useful of these columns is the Golay or open tubular column. Typically, these columns are capillary tubing with a bore between 0.2 and 0.5 mm (0.008 to 0.02 in.). In some cases, the bore may range OMPI between 0.1 mm (for fast analysis) to 2 mm (for semi- preparative work) . The length of these capillaries tubes is on the order of 15.to 100 m (about 50 to 300 ft). Preparation of these capillary columns requires consideration of uniform inner diameter, good temperature resistance, good mechanical strength and non-reactivity with respect to the sample and the stationary phase (inner coating). Typically, stainless steel, copper and glass are all common capillary materials. However, because of inertness requirements, copper is the least used material. High purity glasses, commonly found in optical fiber work, are an excellent candidate for cheap, disposable column manufacture. These same columns, e.g. about 0.25 r.m ID x 100 , of high purity silica may also be used in conjunction with mass spectrometers (MS) to analyze small amounts of com¬ plicated mixtures that can be fractionated satisfactorily only with this technique. However, in either the GC or GC-MS systems, the pro- blem of placing a long length, e.g., about 50 to 300 ft, of tubing in a constant elevated temperature environment places two constraints on the tube. The first constraint relates to stress. In order to place a long length of a tube having an ID of 0.25 to 0.5 mm in a small space, the tube might be coiled in a 2.5 cm diameter space. This would place the tube OD (typically 0.3 to 1.0 mm) under an effective tensile stress of between 120,000 to 400,000 psi. The ID of the tube is at a stress be¬ tween 100,000 to 200,000 psi. To keep the fibe from failing due to mechanical abrasion, it is necessary to protect the outside of this tube. (Mechanical abrasion of the inside of the tube is, of course, neglected.) While organic coatings may provide this service, they would burn or melt off at the elevated temperatures often required in GC work (e.g., 300°C) . In addition, organic coatings do not protect the fiber from the second O PI constraint, which is generally referred to as subcritical crack growth, or static fatigue. Here, a surface flaw placed under stress (by bending, for example) will start to grow in an atmosphere containing water or other mono- valent ion. This growth is time dependent and will continue to failure. SUMMARY OF THE INVENTION In accordance with the invention, the nascent strength of a glass capillary tube is maintained or pre¬ served by coating the glass capillary with at least one layer of a metal or alloy that (a) is substantially chemically inert with respect to the material comprising the glass capillary at the deposition temperature during the time of coating the metal or alloy onto the glass cap¬ illary tubing and (b) forms a hermetic seal around the glass capillary tubing. The mechanical properties of the glass, capillary tubing are better preserved with the metal and alloy coatings disclosed herein, as compared with other coatings, whether metallic or non-metallic. Use of the metallic coating provides a hermetic seal around the outside of the capillary v/hile providing mechanical protection at elevated temperatures. This coating protects the capillary circumference from the deleterious effects of moisture. The capillary bore, of course, remains uncoated or is coated with a suitable stationary phase, since it must be free from all but the compounds to be analyzed. These compounds generally do not contain any of the onovalent free radicals that contribute to static fatigue. BRIEF DESCRIPTION OF THE DRAWING The Figure is a perspective view, partly in section, showing on an enlarged scale the details of the metallic clad silica capillary tube of the invention. DETAILED DESCRIPTION OF THE INVENTION There is shown in the Figure (not to scale) a metallic clad capillary tube 10 in accordance with the invention. Metallic clad capillary tube 10 comprises a glass tube or capillary 11 and at least one metallic coating or jacket 12 concentrically surrounding the glass capillary 11. Glass capillary 11 is preferably high- purity Si0 2 or doped silica. However, the composition of the glass capillary is not otherwise critical, so long as it remains inert during use in gas chromatography analysis. The metallic jacket 12 comprises a metal or alloy that is substantially chemically inert with respect to the glass capillary 11. That is, the metal or alloy is one that does not react chemically with the glassy material at the deposition temperature during the time of coating the metal or alloy onto the glass capillary. The metallic coating, as more fully described below, is preferably applied by passing the capilllary through a molten pool of the metal or alloy. In view of the velocity with which a capillary passes through a molten pool of the metal or alloy being coated on the capillary (about 1 to 10 ft/sec, usually about 3 ft/sec) , the melting point of the metal or alloy may be higher than the softening point of Siθ2« In particular, the melting poing of the metal or alloy to be coated may range as high as about 2,300°C, so long as the capillary itself does not experience a temperature greater than its softening point. In determining whether a particular metal or alloy is stable under the conditions described above, a com¬ parison of the free energy of formation of the corre¬ sponding oxide at the deposition temperature of the metal or alloy with the free energy of formation of silica at the same temperature is necessary. The free energy of formation of- an oxide of the metal or alloy must be less negative than the free enerαv of formation of silica at OMP the deposition temperature of the metal or alloy; otherwise, such metal or alloy would have a higher affinity for oxygen than silicon, and thus would react with Si0 2 . However, if the kinetics of oxide formation are sufficiently sluggish, use of metals and alloys having a free energy of formation of the corresponding oxide somewhat more negative than the free energy of formation of silica may be employed. An example of such a metal is aluminum and its corresponding alloys, such as aluminum- bismuth (Al-0.1-0.2 at% Bi) . Although it is known that aluminum reacts with silica, this reaction is sufficiently slow in the application of chromatography capillaries so as to permit its use in this application. Metal elements suitable in the practice of the invention additionally include vanadium, chromium, manganese, iron, cobalt, nickel, copper, arsenic, strontium, zirconium, niobium, rhodium, palladium, tellurium, barium, iridiu , platinum, thallium, tin, lead, zinc and cadmium. Alloys suitable in the practice of the invention include alloys containing these elements in combination with each other or combination with other elements, whether metallic or non-metallic, so long as the alloys remain ductile at ordinary temperatures of use. Examples of such other alloying metal elements include magnesium, while examples of such other alloying non-metal elements include antimony, bismuth, boron, carbon, phos¬ phorus and silicon. Such alloys of metal elements with additional elements comprise a major portion (i.e., greater than 50% by weight) of the metal elements and a minor portion (less than 50%) of additional elements. Due to factors related to toxicity, expense, ease of handling and other factors, elements such as manganese, arsenic, strontium, rhodium, tellurium, barium, iridium, platinum and thallium, which are otherwise suitable, are not likely to find use except as alloying elements and hence are not preferred as elemental metal coatings. ( ' OMPI Additional metallic coatings (not shown) may be applied to the metallic coating 12 above and may comprise any metal or alloy; such metal or alloy is not subject to the same constraints as the first metallic cladding. Thus, the additional cladding may be of the same composi¬ tion as the first in order to cure localized defects or to build up the thickness. Alternatively, the second clad¬ ding may be of a different metal or alloy which otherwise might or might not react with the underlying glass capil- lary. Accordingly, a hybrid structure may be fabricated having mechanical properties that are different than either metallic cladding alone. For example, the first metallic cladding may comprise tin which does not react with silica and the second metallic cladding may comprise aluminum. Another combination is a steel (high yield point) on tin (low yield point). Additional combinations include tin on copper, tungsten on tin, copper on tin, copper on zinc, tin on nickel and aluminum on copper. Since the free energy of formation values are not always available, especially for alloys, it is sufficient evidence that the tensile strength of the metallic clad capillary be substantially not less than that of the unclad capillary in determining the suitability of a particular metal or alloy. The strength comparison should be made at the same strain rates; these rates should be fast enough to limit static fatigue in the range of interest. Further, the unclad capillary used for comparison should be tested before it contacts any other solid. Failures in the holding fixture for the strain test are ignored in the comparison; such failures are not considered part of the original sample population, since the failure is clearly associated with the testing fixture. The metallic claddings may be applied by a variety of methods, including the passage of the fiber through a molten pool, as described below, vapor plating, vacuum deposition, sputtering, electro-deposition and the like. In any event, the coating is applied to the glass capillary during the drawing operation immediately after the capillary emerges from the furnace. Apparatus well- known for coating glass fibers with metals may be suit¬ ably employed. Importantly, the coating is applied before the capillary has a chance to be abraded by the take-up drum onto which the coated capillary is spooled, and even before the capillary cools to the point where ambient moisture can react with its surface. Preferably, at least one of the metallic layers, and most preferably the first metallic layer, is applied by passing the glass capillary through a coating cup which contains the molten metal or-.alloy, to be coated _onto the capillary at a temperature slightly above the melting point of the metal or alloy. The cup has a small hole in its bottom large enough to pass the glass capillary but sufficiently small so that surface tension of the molten metal or.alloy prevents it from running out. As the glass capillary passes through the cup, a thin layer of metal or alloy freezes onto the surface of the glass capillary. This method rapidly forms thick metallic coatings having characteristics similar to those of the bulk metal or alloy. Proper conditions for the formation of a strong, adherent metallic layer on the glass fiber surface require that the temperature of the metal-containing bath through which the glass capillary is passed be slightly greater than the melting point of the metal or alloy, while the temperature of the glass capillary be somewhat below this melting point. Further, in the case of alloys, the alloys must evidence solubility in the liquid state in order to avoid segregation of phases in these alloys during cooling . OMPI The metallic cladding provides a tight, permanent and durable hermetic seal about the glass fiber. Deter¬ mination of the adequacy of hermetic sealing is made by preparing a plot of log (stress) versus log (time) or a plot of log (strain rate) versus log (mean failure stress). As is well-known, a slope of substantially zero implies a hermetic seal. The thickness of the metallic layer is controlled by adjusting the capillary drawing rate and the metal deposition parameters such as, for example, in the case of molten metal coating, the temperature differential between the capillary and the metal-containing bath. Typically, the total thickness of the metallic cladding 12 (including subsequent metallic claddings) lies in the range of about 5 to 200 urn and preferably lies in the range of about 20 to 50 μm. The maximum thickness is restricted by a requirement to not impair the flexibility of the capillary, whereas the minimum thickness is set by a requirement to achieve adequate strength and hermetic sealing. The resulting glass capillary 10 evidences a reten¬ tion of mechanical strength of the nascent glass capillary to a greater extent than evidenced by glass capillaries clad with other materials. Further, the metallic clad capillary of the invention will not fail due to static fatigue if it is used at less than about 3/4 of its ori¬ ginal tensile strength, regardless of the absolute value of that tensile strength. The high pristine ultimate strength of the glass capillary material is approximately 2 x 10° psi and is therefore more than adequate to achieve a final desired level in excess of 50,000 psi. The metallic layer or jacket 12 provides good mechanical protection and a hermetic seal against contam¬ ination. Additional mechanical protection, galvanic pro- tection and electrical insulation can be achieved as needed by applying a plastic overcoat 13 outside of the metallic jacket 12. For example, as little as 10 to 25 urn of a polyvinyl formate coating is useful in preserving the integrity of the metallic cladding in an electrolytic cell with stainless steel electrodes and salt water. Other plastic coatings may also be used. The metallic clad glass capillaries are capable of operating continuously for at least a year up to about 2/3 T m (°K), where T m is the melting point of the metal or alloy, wi.th no more than about 100% damage (i.e., no more than about 50% loss of strength).

Claims: CLAIMS What is Claimed is; 1. A flexible glass capillary for gas chromato¬ graphy, characterized in that the nascent strength of said glass capillary is substantially preserved without substantially interfering with the flexibility thereof by providing said glass capillary with at least one coating of a metal or alloy that (a) is substantially chemically inert with respect to the material comprising said glass capillary at the deposition temperature during coating of said metal or alloy onto said glass fiber? and (b) forms a hermetic seal on the outer sur¬ face of said glass capillary. 2. The capillary of Claim 1 in which said metal comprises an element selected from the group consisting of vanadium, chromium, iron, cobalt, nickel, copper, zirconium, niobium, palladium, aluminum, tin, lead, zinc and cadmium. 3. The capillary of Claim 2 in which said metal comprises an element selected from the group consisting of iron, cobalt, nickel, copper, aluminum, tin and zinc. 4. The capillary of Claim 1 in which a major portion of said alloy comprises at least one element selected from the group consisting of vanadium, chromium, iron, cobalt, nickel, copper, zirconium, niobium, palladium, aluminum, tin, zinc, lead and cadmium. 5. The capillary of Claim 4 in which said alloy includes at least one additional element selected from the group consisting of manganese, arsenic, strontium, rhodium, tellurium, barium, iridium, platinum, thallium, antimony, bismuth, boron, carbon, phosphorus and silicon. 6. The capillary of Claim 5 in which said addi¬ tional element is at least one selected from the group consisting of manganese, arsenic, antimony, bismuth, boron, carbon, phosphorus and silicon. 7. The capillary of Claim 1 in which the total thickness of said at least one coating ranges from about 5 to 200 μ . 8. The capillary of Claim 7 in which said total thickness ranges from about 20 to 50 μm. 9. The capillary of Claim 1 in which said alloy evidences continuous solubility in the liquid state. 10. The capillary of Claim 1 additional comprising a plastic coating formed on said coating of metal or alloy. 11. A process for substantially preserving the nascent strength of a glass capillary employed in gas chromatography, characterized that said process com¬ prises forming at least one coating on said glass cap- illary with a metal or alloy that (a) is substantially chemically inert with respect to the material comprising said glass capillary at the deposition temperature during coating of said metal or alloy onto said glass capillary; and (b) forms a hermetic seal around the outer sur¬ face of said capillary. OMPI 12. The process of Claim 11 in which said metal comprises an element selected from the group consisting of vanadium, chromium, iron, cobalt, nickel, copper, zirconium, niobium, palladium, aluminum, tin, zinc, lead and cadmium. 13. The process of Claim 12 in which said metal comprises an element selected from the group consisting of iron, cobalt, nickel, copper, aluminum, tin and zinc. 14. The process of Claim 11 in which a major portion of said alloy comprises at least one element selected from the group consisting of vanadium, chromium, iron, cobalt, nickel, copper, zirconium, niobium, palladium, aluminum, tin, zinc, lead and cadmium. 15. The process of Claim 14 in which said alloy includes at least one additional element selected from the group consisting of manganese, arsenic, strontium, rhodium, tellurium, barium, iridium, platinum, thallium, antimony, bismuth, boron, carbon, phosphorus and silicon. 16. The process of Claim 15 in which said additional element is at least one selected from the group consisting of manganese, arsenic, antimony, bismuth, boron, carbon, - phosporus and silicon. 17. The process of Claim 11 in which said at least one coating is formed to a total thickness ranging from about 5 to 200 μm. 18. The process of Claim 17 in which said total thickness ranges from about 2C to 50 μm. IS. The process of Clai : 11 in which said alloy evidences continuous solubility in the liquid state. O PI 20. The process of Claim 11 further comprising forming a plastic coating on said coating of metal or alloy. 21. The process of Claim 11 in which said glass capillary is coated by passage at least once through a molten pool of said metal or alloy following drawing said glass capillary.

Inventors: WYSOCKI JOSEPH A
Applicants: HUGHES AIRCRAFT CO
Application year: 1981
Main classification: F16L9
IPC codes: B01D15, C03C17, F16L9, G01N30