Abstract:
A chromatography column with filters having minimal volume. A stamped screen disk is coated with fluorocarbon polymer on both sides, leaving a center area open. The fluorocarbon polymer coating serves as a gasket, providing reliable sealing for pressure up to approximately 10000 psi. The chromatography column comprises a fitting having an internal cavity. A fluorocarbon polymer coated screen is placed within the cavity. A threaded capillary tubing is screwed inside the cavity. At the end of the capillary lies a compression screw solvent tubing connection.

Description:
FIELD OF THE INVENTION 
     The present invention relates to chromatographic columns, especially capillary columns. 
     BACKGROUND OF THE INVENTION 
     Liquid chromatography systems are used to carry out chemical separations. A typical liquid chromatography system consists of the following major components: a pump, an injector, a column, and a detector. The pump compels a mobile phase, for example, a solution, through a fluid path comprising an injector, column and a detector. The injector permits the introduction of samples into the fluid stream above the column. The column contains a packed bed of media. The media is normally porous and relatively inert. Compounds in the sample will exhibit a characteristic affinity to the media. That is, some compounds exhibit high affinity and some compounds exhibit low affinity. As a result, as the compounds are carried through the media, the compounds separate into bands which elute or come off the column at different times. These bands are detected by the detector. 
     The media within a column is held in place by filters or frits. The filters or frits are secured in the column by end caps. The volume of the column&#39;s filters or frits greatly affects the performance of the chromatography system. It is desirable to minimize volume of the chromatograph system. Once removed from the media and the influence of affinity thereto, compounds tend to redistribute in solution. The smaller the volume of a column&#39;s filters or frits, the better the column performs. Small volume columns tend to use filters, as opposed to frits. In a capillary column, which volume is measured in microliters, the volume of the filters is especially important. 
     There are numerous manufacturers offering filter assemblies for chromatographic columns (Upchurch Scientific, Isolation Technologies, Optimize Technologies, Merck, Alltech and others). The smallest thickness of all available filters is 0.75 micron. It would be advantageous to have a thinner filter because its volume would be smaller. 
     Existing powder metallurgy technology cannot provide parts thinner than 0.25 micron. Filters and frits of that thickness cannot be reliably pressed into thin plastic rings typically used to seal and retain the frits and filters in the column. 
     SUMMARY OF THE INVENTION 
     The present invention provides a chromatography column with filters or frits having minimal volume. One embodiment of the present invention is directed to a chromatography column for use with a chromatographic pump having a solute conduit. The column comprises a cylindrical tube having a cylindrical wall having a interior surface, an exterior surface, a first face and a second face. The interior surface defines a chamber. The first and second faces are between the interior and exterior surfaces, and with the interior surface, define two openings to said chamber. At least one of the first and second faces has a circular planar surface for receiving a filter element. The circular planar surface has an inside diameter and an exterior diameter. A filter element is received on the flat planar surface of the face. The filter element has a screen having a top surface, a bottom surface, and a diameter greater than the diameter of the inside diameter of the circular planar surface. The screen has a coating of a plastic material. The plastic material is positioned on the screen in a circular ring defining an area substantially free of the plastic material in the center of the screen. The plastic material engages the flat planar surface in sealing engagement upon compression upon the top surface. The column further comprises at least one end-fitting assembly at one of the faces. The end fitting assembly has a fitting body having a tube opening for receiving the exterior wall of the tube. The tube opening has a lip extending radially inward to form a retaining surface. The retaining surface receives the filter element and engages the coating in sealing relationship upon compression. The lip defines a lip opening having a diameter greater than the diameter of the solute conduit, for receiving the solute conduit in sealing engagement with the coating of the filter element upon compression. The end fitting assembly receives a solute conduit and places the chamber in fluid communication with a chromatographic solute through the filter element. 
     As used herein, the term filter element means a filter or frit. Preferably, the filter element is formed of stainless steel screen, sintered stainless steel frit, or membrane. Preferably, the coating is fluorocarbon polymer. Fluorocarbon polymers such as PTFE, FEP or PVDF are sold by a variety of vendors. A preferred fluorocarbon polymer is sold under the trademark “TEFLON” (Du Pont). 
     The chromatography column filter element of the present invention provides several benefits over conventional columns filters and frits. The chromatography column equipped with this filter element has minimal dead volume. The properties of the fluorocarbon surfaces create a sealing gasket that dispenses with the need for additional sealing rings. It does not require precisely machined parts. The chromatography column can be reliably sealed. One embodiment of the filter element is a stamped screen disk which is coated with a fluorocarbon polymer on both sides, leaving a center area open. The coating serves as a gasket, providing reliable seal. 
     Preferably, the fitting body and cylindrical tube have cooperating threads to provide compression of said coating. 
     Preferably, the lip opening of the fitting body has a cylindrical section proximal to the filter element for receiving the solute conduit, a conical section expanding from the cylindrical section for receiving a cooperating conical section of a sleeve assembly, and a cylindrical section. The end fitting assembly further comprises a sleeve assembly comprising a ferrule and a compression screw. The ferrule and compression screw have axial openings for receiving the solute conduit. The ferrule and compression screw are configured and arranged to cooperate with the conical and cylindrical sections of the fitting body. The conical section of the fitting body compresses the ferrule and solute conduit as the ferrule is received in the conical section of the fitting body. The ferrule engages the conical section of the fitting body to seal the solute conduit against the ferrule and the ferrule against the end-fitting body. 
     Preferably, the compression screw further comprises a cylindrical section to cooperate the cylindrical section of the fitting body. The fitting body cylindrical section and the compression screw cylindrical section have cooperating threads to provide compression on the ferrule. The ferrule engages the solute conduit to compel the conduit against the filter element in sealing engagement with the coating. 
     Typical chromatography columns are packed with a solid phase media, such as particles of silica oxide, titanium oxide, zirconium oxide, carbon, hydrocarbon polymeric material, and combinations thereof. The filter element serves to contain such material. Preferably there are at least two filter elements and end fitting assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments, taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a column filter according to the present invention. 
     FIG. 2 a  shows a capillary chromatography column according to the present invention. 
     FIG. 2 b  is an enlargement of a portion A of FIG. 2 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring in detail to the drawings, the chromatographic filter element  11  of the present invention is shown in perspective in FIG.  1 . It comprises a circular screen  13  that is coated on both a bottom surface  15  and a top surface  17  with a plastic material  19 . A preferred plastic material  19  is a fluorocarbon polymer. Fluorocarbon polymers are known to be relatively chemically inert. Fluorocarbon polymers such as PTFE, FEP or PVDF are sold by a variety of vendors. A preferred fluorocarbon polymer is sold under the trademark “TEFLON” (Du Pont). 
     The coating is applied or removed leaving a center area  21  open for fluid passage. The circular screen  13  is coated with a fluorocarbon polymer to form a ring of plastic material or gasket that allows reliable sealing when the filter element  11  is placed within the end fittings  23 , as described hereinafter with reference to FIGS. 2 a  and  2   b.    
     Circular screen  13 , as shown in FIG. 1, in the illustrative embodiment, is fabricated from a stainless steel screen that is capable of trapping particles as small as approximately 2 microns. The screen  13  is a stamped circular section of stainless steel screen having a thickness of between 90 to 140 microns. 
     In an alternative embodiment a sintered stainless steel frit can be used; and, other materials known in the art may be utilized, provided that the material is inert to the sample substance and solvents utilized. In the event the coating  19  is placed on the entire screen  13 , a circular area  21  of the screen  13  can be formed by removing the coating  19  with a carbon dioxide laser. In the alternative, a mask [not shown] can be placed on the screen  13  prior to applying the coating  19 . After the coating  19  is applied, the mask is removed to expose area  21 . 
     Referring now to FIGS. 2 a  and  2   b,  one embodiment of the present invention is directed to a chromatography column, generally designated by the numeral  25 . The column  25  comprises a cylindrical tube  29  having a cylindrical wall  33  having a interior surface  33   a,  an exterior surface  33   b,  a first face  35  and a second face  37 . 
     The interior surface  33   a  defines a chamber  39 . The first and second faces  35  and  37  are between the interior and exterior surfaces  33   a  and  33   b  respectively. And, such faces  35  and  37 , with the interior surface  33   a,  define two openings to the chamber  39 . Chamber  39  is typically packed with a solid phase media [not shown for purposes of clarity]. This media can take any number of forms known in the art. Typical chromatography columns are packed with particles of silica oxide, titanium oxide, zirconium oxide, carbon, hydrocarbon polymeric material, and combinations thereof. The filter element  11  serves to contain such material. Preferably there are at least two filter elements  11 . 
     At least one of the first and second faces  35  and  37  has a circular planar surface  41  for receiving a filter element  11 . The circular planar surface  41  has an inside diameter and an exterior diameter. 
     Turning now to FIG. 2 b  in particular, a filter element  11  is received on the flat planar surface  41  of the face  35 . The filter element  11  has a top surface  15 , a bottom surface  17 , and a diameter greater than the inside diameter of the circular planar surface  41 . The filter element  11  has a coating of a plastic material  19 . The plastic material  19  is positioned on the screen  13  in a circular ring, defining an area  21  substantially free of the plastic material  19 , in the center of said screen  13 . The plastic material  19  engages the flat planar surface  41  in sealing engagement upon compression upon said top surface  15 . 
     The column  25  further comprises at least one end-fitting assembly  51 , and preferably two. Each end fitting assembly is positioned at one of the faces  35  and  37 . The end fitting assembly  51  has a fitting body  53  having a tube opening  57  for receiving the exterior wall  33   b  of the tube  33 . The tube opening  57  has a lip  59  extending radially inward to form a retaining surface  61 . The retaining surface  61  receives the filter element  11  and engages the coating  19  in sealing relationship upon compression. The lip  59  defines a lip opening  63  having a diameter greater than the diameter of the solute conduit  27 , for receiving the solute conduit  27  in sealing engagement with the filter element  11  on the coating  14  upon compression. The one end fitting  51  receives a solute conduit  27  and places the chamber  39  in fluid communication with a chromatographic solute through the filter element  11 . 
     The chromatography column filter of the present invention provides several benefits over conventional columns filters. The chromatography column equipped with this filter has minimal dead volume. The properties of the fluorocarbon surfaces create a sealing gasket that dispenses with the need for additional sealing rings. It does not require precisely machined parts. The chromatography column can be reliably sealed. The fluorocarbon coating serves as a gasket, providing reliable sealing for pressure up to approximately 10,000 psi. 
     Preferably, the fitting body  53  and cylindrical tube  25  have cooperating threads  71  to provide compression of said coating  19 . In the alternative, the tube  25  can be fitted into the fitting body  53 , compressing the coating  19 , and secured in place by gluing, welding, brazing and crimping. 
     Preferably, the lip opening  63  of the fitting body  53  has a cylindrical section  65  proximal to the filter element  11  for receiving the solute conduit  27 , a conical section  75  expanding from the cylindrical section  65  for receiving a cooperating conical section of a sleeve assembly, and end cylindrical section  77 . The end fitting assembly further comprises a sleeve assembly  81  comprising a compression screw  87  and a ferrule  89 . 
     Compression screw  87  has an axial opening  83  for receiving the solute conduit  27 . Compression screw  87  has a flat section  85  configured and arranged to cooperate with ferrule  89 . The flat section  85  of the compression screw  87  compresses the ferrule  89  as the ferrule  89  engages the conical section  75  of the fitting body  53 . The ferrule  89  compresses the solute conduit  27  to seal the solute conduit  27  against the ferrule  89  and the ferrule  89  against the fitting body  53 . 
     Preferably, the compression screw  89  further comprises a cylindrical section  87  and the fitting body has a cylindrical section  91 . The fitting body cylindrical section  91  and the sleeve cylindrical section  87  have cooperating threads  93  to provide compression on the sleeve body  87 . The sleeve body  87  engages the solute conduit  27  to compel the conduit against the filter element  11  in sealing engagement with the coating  19 . 
     In the alternative, the sleeve assembly  87  can be forced against the fitting body  53  and fixed by gluing, welding, brazing and crimping. 
     Embodiments of the present invention feature a chromatography column having a small dead volume. That is the distance between the solute conduit and the filter element  11  is minimal. The filter element of the present invention can withstand pressures of approximately 10,000 psi within the chromatographic apparatus. The properties of the coating  19  creates a sealing gasket that dispenses with the need for additional sealing rings within the apparatus and thus decreases the need for additional components in the form of sealing structure. The incorporation of the sealing structure and the filtering structure into a singular component provides the user with a more efficient column with fewer precision dimensioned parts. 
     As disclosed herein, the invention provides a new method and apparatus in the form of a sealed chromatography column that integrates the sealing structure within the structure of the filter assembly. This design allows for a greater reliability of the sealing of the above columns and maximizes the performance of the column. While the sealing arrangement integrated within the filter of the present invention can be used in virtually any size chromatography column and for virtually any column pressure up to approximately 10,000 psi, it is particularly well suited to capillary applications. Additionally, the simplicity of the design of the above filter with integrated sealing greatly enhances the reliability and performance of the column. 
     Although the filter material described in the illustrative embodiment herein is a stainless steel screen it should be appreciated that other filters could be implemented such as sintered stainless steel frit, or the like. Similarly, rather than a screen, filtering could be effected by making the filtering component from a membrane material that possesses sufficient strength containing pores specific to the needed size. 
     The foregoing has been a description of an illustrative embodiment of the present invention. While several illustrative details have been set forth, such are only for the purpose of explaining the present invention. Various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.