GB2217628A - Chromatographic tube - Google Patents

Abstract

A tube assembly, e.g. a chromatographic separation column, comprises an inner tube (3) of fused silica, an outer sheathing (1) of a form retaining coilable material (e.g. stainless steel), and an intermediate protective coating (2) for the silica. The coating (2) is adhered to both the tube (3) and the sheathing (1).

Description

TUBING ASSEMBLY The present invention relates to a tubing assembly intended particularly, but not exclusively, for use as a chromatographic separation column.

It is already known to used fused silica tubing for chromatograph columns. Such tubing generally has an inner diameter or bore of 50 microns to 5mm (typically 530 microns) and a wall thickness of 100 microns to lmm (typically 100 microns).

The tubing is produced from fused silica at about 20000C which is drawn and solidified into the final capillary form. The tubing cools rapidly and unless the surface is protected microfissures will begin to form in the tube walls. These microfissures lead to failure of the tube during its end use application. In order to retard this deterioration process, the tube is coated during the cooling stage (generally at about 1500C) with a protective coating (eg about 100 microns thick).

When employed as a chromatography column, the silica tube is used as a long coiled length which may heated during the chromatographic separation. It is thus necessary that the protective coating is flexible (to permit the tube to be coiled) and also to have a degree of heat resistance (to permit heating during a chromatographic separation process). Generally the coating employed is a polyimide, eg Du Pont PI2550.

There are however a number of disadvantages associated with the polyimide coated tubes. In particular, the tubing retains its inherent springiness and it is necessary for the coil to be held in a wire frame to retain its form.

Additionally temperatures above about 3500C in the chromatograph oven cause deterioration of the protectively coated tubes thus limiting the separation for which the tubing may be used.

It is an object of the present invention to obviate or mitigate the abovementioned disadvantages.

According to a first aspect of the present invention there is provided a tube assembly comprising an inner tube of fused silica, an outer sheathing of a form retaining coilable material, and an intermediate protective coating for the silica between said tube and said sheathing, said coating being adhered to both the tube and the sheathing.

According to a second aspect of the present invention there is provided a method of producing a tube assembly as defined in the preceding paragraph comprising passing fused silica tube provided with said protective coating through tubular sheathing having an inner diameter greater than the outer diameter of the prptectively coated silica tube, either reducing the diameter of the sheathing to bring its inner walls into contact with the protective coating or introducing extra protective coating material into the space between the sheathing and protectively coated tube, and heating the assembly to cause the protective coating to adhere to the sheathing.

According to a third aspect of the present invention there is provided a gas chromatograph column which comprises a coil of a tubing assembly as defined above.

The protective coating is preferably a high temperature resin, eg a silicone resin or more preferably, a polyimide.

The tubing assembly of the invention is particularly useful as a chromotography column. The form retaining coilable material allows the capillary tubing assembly to be coiled into the desired form and to retain this form without the need for a wire frame as required for chromatography columns which simply have an outer coating of polyimide.

Additionally, the form retaining sheathing acts as a barrier to prevent exposure of the protective coating to an atmospheric oxygen which allows the coil to be heated, during chromatographic separation, to higher temperatures and for a silica coil provided only with a polyimide coating. Such higher temperatures may in certain instances permit higher boiling point separations.

For preference the outer sheathing is a metal tube (preferably of stainless steel) with a wall thickness which permits coiling of the tube, e.g. 1 to 4mm. The use of a metal tube as the sheathing has the advantage (additional to those specified above) tha-t standard compression couplings (as currently available for chromatographic apparatus) may be used for providing the gas tight connections at the end of the column. This is not the case where the prior art silica tubing simply coated with polyimide where gas tight fittings, although possible, are inconvenient.

An improved axial temperature distribution along the column is also obtained with the use of the metal sheathing. A further advantage of metal tubing is that it makes possible the use of low voltage resistance heating techniques.

Tubing assemblies in accordance with the invention may be prepared from commercially available protectively coated fused silica tubing. Such tubing may have the bore diameters and wall thicknesses indicated above. The protectively coated silica tube is inserted into a tubular sheathing (preferably of metal) having an inner diameter greater (e.g. by 0.2 to 0.5mm) than that of the protectively coated tube.

In one embodiment of the process, the diameter of the sheathing may then be reduced, e.g. by stretching, so that its inner walls firmly contact the protective coating. In the case of metal sheathing this reduction in diameter be reduced, e.g. by stretching, so that its inner walls firmly contact the protective coating. In the case of metal sheathing this reduction in diameter may be effected by drawing or rolling. In a second embodiment of the process extra protective coating material is introduced into the gap between the protectively coated tube and the sheathing.

It is not however essential that the extra protective coating material be introduced along the full length of the gap. It is possible, for example, for the material to be provided only at the ends of the gap, in which case the residual air in the gap does not adversley affect the protective coating. It is however also possible to purge the gap with nitrogen before the material is positioned thereby excluding air from the gap.

The invention will be further described by way of example only with reference to the accompanying drawings in which; Fig. 1 diagrammatically illustrates a first stage in the manufacture of a tubing assembly in accordance with the invention; Fig. 2 diagrammatically illustrates a second stage in the manufacture of a first embodiment of tube assembly; Fig 3 diagrammatically illustrates a second stage in the manufacture of a second embodiment of tube assembly in accordance with the invention.

Fig. 1 illustrates a first stage in the manufacture of various embodiments of tubing assembly in accordance with the invention. As shown in Fig.

1, a fused silica tube 1 (typically with an outer diameter of 0.7mm microns) has a protective coating 2 of a polyimide and is shown as being inserted in an outer tubular metal sheathing 3 (typically with an outer diameter of about 1.5mm and an inner diameter of lmm. There is thus a small air gap 4, of 0.15mm width between the tubes 1 and 3.

In the next stage of manufacture (Fig. 2), further polyimide resin 5 may be provided in the air gap 4 at each end of the assembly tubes 1 and 3 so as to close the gap. Finally, the resin 5 may be heated to effect cross-linking thereof and bond the outer tube 3 to the inner tube 1 and prevent further ingress of air into the gap 4.

The tube assembly thus produced may be converted to a chromatographic column by coiling of the tube and providing gas tight compression fitting at each end of the tube.

When the tube assembly is heated in the chromatograph oven, it is found that the residual amount of oxygen in the gap 4 does not deletoriously affect the coating or silica tube.

In an alternative method of manufacture (Fig. 3) the outer tube 3 of the arrangement shown in Fig. 1 is drawn down so that its inner surface contacts the coating 2. The drawing down may be effected by pulling the tube assembly in the direction of arrow A through rollers 6. After this drawing operation, the tube may be heated along its length so as to bond the polyimide to the inner surface of the tube 3. As previously, the tube may then be coiled and provided with compression fittings.

In an alternative method (not shown) the ends of the assembly illustrated in Fig. 1 may be gripped by hydrualic clamps and the ends of the metal tube pulled away from each other so as to effect a reduction in the diameter of the metal tube.

Claims (12)

1. A tube assembly comprising an inner tube of fused silica, an outer sheathing of a form r-etaining coilable material, and an intermediate protective coating for the silica between said tube and said sheathing, said coating being adhered to both the tube and the sheathing.

2. An assembly as claimed in claim 1 wherein the outer sheathing is a metal tube.

3. An assembly as claimed in claim 2 wherein the outer sheathing is of stainless steel.

4. An assembly as claimed in any one of claims 1 to 3 wherein the fused silica tube has an inner diameter of 50 microns to 5mm.

5. An assembly as claimed in claim 4 wherein the fused silica tube has a wall thickness of 100 microns to lmm.

6. An assembly as claimed in any one of claims 1 to 5 wherein the protective coating is a high temperature resin.

7. An assembly as claimed in any one of claims 1 to 6 wherein the protective coating is a silicone resin.

8. An assembly as claimed in any one of claims 1 to 6 wherein the protective coating is a polyimide.

9. An assembly as claimed in any one of claims 1 to 8 in the form of a coil.

10. An assembly as claimed in any one of claims 1 to 9 provided with compression couplings.

11. Chromatographic separation apparatus having a separation column which is comprised of a tubing assembly as claimed in any one of claims 1 to 10.

12. A method of producing a tubing assembly as claimed in any one of claims 1 to 8.