Abstract:
A seal forming a fluid tight connection between a gas chromatography column and a sample inlet assembly is disclosed. The seal is formed by a metal injection molding process. The seal has a first surface adapted for sealing with the sample inlet assembly and a second surface adapted for sealing with the column. The seal has an aperture extending between the first and second surfaces. A method of sealing a connection between a gas chromatography sample inlet assembly and a gas chromatography column is also disclosed. The method includes providing a seal as described above, compressing the first surface of the seal against an end of the inlet assembly, positioning the column in fluid communication with the aperture, and engaging the column with the second surface.

Description:
BACKGROUND  
       [0001]     Gas chromatography is a process by which one or more compounds from a chemical mixture may be separated and identified. A carrier gas, for example, an inert gas such as nitrogen or helium, flows through a tube known as a column. Large columns may have inner diameters between about 3 mm and about 8 mm and lengths between about 1 meter and about 3 meters. Capillary columns may have inner diameters between about 0.05 mm and about 1 mm and may be 100 meters or more in length. The large column may be packed with an inert packing medium coated with an active substance that interacts with compounds in the chemical mixture being analyzed. Capillary columns are preferably coated on their inner surface with the active substance.  
         [0002]     A sample of the chemical mixture to be analyzed is injected into the column. As the sample is swept through the column with the carrier gas, the different compounds, each one having a different affinity for the active substance lining the column or coating the packing medium, move through the column at different speeds. Those compounds having greater affinity for the active substance move more slowly through the column than those having less affinity, and this speed differential results in the compounds being separated from one another as they pass through and exit the column.  
         [0003]     The carrier gas with the separated compounds exits the column and passes through a detector, which identifies the molecules. Various types of detectors may be used, including a thermal conductivity detector, a flame ionization detector, electron capture detector, flame photometric detector, photo-ionization detector and a Hall electrolytic conductivity detector. A two dimensional plot of the detector measurements against elution time or volume, known as a chromatogram, may be made, and from the chromatogram or the digital representation thereof the compounds may be identified.  
         [0004]     Injection of the sample chemical mixture into the column is effected using a sample inlet assembly. The sample inlet assembly has an injection port that receives a syringe for injecting the sample into the inlet assembly. The inlet assembly is connected to the column with a seal that provides a fluid tight joint between the relatively large diameter of the inlet assembly and the small diameter of the capillary column.  
       SUMMARY  
       [0005]     The invention concerns a seal forming a fluid tight connection between a gas chromatography column and a sample inlet assembly. The sample inlet assembly comprises a conduit. The seal comprises a plate formed from metal powder using a metal injection molding process. The plate has a first surface on one side adapted for sealing engagement with the conduit. The plate has a second surface on an opposite side adapted for sealing engagement with the column. An aperture extends through the plate between the first and second surfaces, the aperture being positioned to provide fluid communication between the column and the sample inlet assembly.  
         [0006]     The invention also includes a method of sealing a connection between a gas chromatography sample inlet assembly and a gas chromatography column. The inlet assembly has a conduit. The column has a ferrule. The method comprises:  
         [0007]     providing a seal as described above made from metal powder using a metal injection molding process;  
         [0008]     compressing the first surface of the seal against an end of the conduit;  
         [0009]     inserting the column within the aperture; and  
         [0010]     compressing the ferrule against the second surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a longitudinal sectional view of a sample inlet assembly shown with a gas chromatograph;  
         [0012]      FIG. 2  is a view of a portion of the sample inlet assembly of  FIG. 1  shown on an enlarged scale;  
         [0013]      FIG. 3  is a perspective view of one side of a seal used with the sample inlet assembly of  FIG. 2 ;  
         [0014]      FIG. 4  is a perspective view of the opposite side of the seal shown in  FIG. 3 ;  
         [0015]      FIG. 5  is a view of a portion of an alternate embodiment of the sample inlet assembly shown on an enlarged scale; and  
         [0016]      FIG. 6  is a perspective view of an alternate embodiment of a seal used with the sample inlet assembly of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  shows a gas chromatograph apparatus  10  having a column  12  comprising a capillary tube  14  lined with an active substance for separating constituent compounds from a gas mixture. Capillary tube  14  has an outlet  16  connected with a detector  18 , for example, a thermal conductivity detector, a flame ionization detector, electron capture detector, flame photometric detector, photo-ionization detector, a Hall electrolytic conductivity detector or other detectors used in gas chromatography. Capillary tube  14  has an inlet  20  connected to a sample inlet assembly  22 . Sample inlet assembly  22  has a sample injection port  24  which is adapted to receive a syringe  26  containing the gas sample to be analyzed. The sample inlet assembly  22  is also connected to a source of carrier gas  28 , which may contain, for example, nitrogen or helium under pressure.  
         [0018]     The sample inlet assembly  22  comprises a conduit  29  having a tubular outer shell  30 , preferably made of stainless steel. Outer shell  30  has a longitudinal bore  32  in which a liner  34  is positioned. Liner  34  is preferably glass or other inert material and has a longitudinal bore  36 . Preferably liner  34  has a smaller outer diameter than the inner diameter of shell  30  thereby creating an annular space  38  lengthwise between the liner and the shell. A vent port  40  is positioned within shell  30  and is in fluid communication with space  38 .  
         [0019]     As shown in  FIGS. 2 and 3 , a fluid tight connection of the capillary tube inlet  20  to the sample inlet assembly  22  is effected using a seal  42 . Seal  42  preferably comprises a plate  44  having a first surface  46  on one side adapted for sealing engagement with an end  48  of shell  30 . Plate  44  is compressed against the end of the shell preferably by a threaded nut  50  that mounts on the end of the shell and engages compatible threads thereon. It is understood that plate  44  need not be flat, but should have a shape that accommodates whatever opposing surface it is to seal against.  
         [0020]     As shown in  FIGS. 2 and 4 , plate  44  has a second surface  52  on an opposite side from the first surface  46 . Second surface  52  is adapted for sealing engagement with the column  12 . In this example, the capillary tube  14  comprising column  12  has a ferrule  54  attached proximate to outlet  16 . Second surface  52  is shaped and sized to receive the ferrule and effect a fluid tight connection when the ferrule  54  is compressed against the second surface  52 . Compression of the ferrule against the second surface is effected using a threaded nut  56  that engages a nipple  58  that extends from the nut  50  used to compress plate  44  against end  48  of shell  30 . An aperture  60  extends through plate  44  between the first and second surfaces. Aperture  60  receives the capillary tube  14  and allows it to pass through the plate and into the liner bore  36 .  
         [0021]     As best shown in  FIG. 3 , a depression  62 , in this example shown as a groove, is positioned in the first surface  46  of plate  44 . Depression  62  is dimensioned and positioned so that it extends between the bore  36  of liner  34  and the space  38  between the liner and the shell. The depression provides fluid communication between the liner bore  36  and the space  38 . Depression  62  may have other shapes and configurations, and is not limited to the groove embodiment illustrated here.  
         [0022]     In an alternate embodiment, shown in  FIGS. 5 and 6 , a projection  64 , in this example shown as a rib, is positioned on the first surface  46  of plate  44 . Projection  64  extends outwardly from the first surface and engages the end  66  of liner  34  to create a gas space  68  between the liner and the plate  44  that provides fluid communication between the liner bore and the space  38  between the liner  34  and the shell  30 . Although shown as a rib in the example embodiment, the projection could have other forms and shapes as well, and is not limited to a rib. The opposite side of plate  44  shown in  FIG. 6  is substantially identical to that shown in  FIG. 4 .  
         [0023]     Gas flow through the gas chromatograph apparatus  10  is described with reference to  FIGS. 1, 2  and  5 . Carrier gas flows from the source  28  and enters the sample inlet assembly  22  where it flows down the bore  36  of liner  34 . The sample gas mixture to be analyzed is injected into the bore  36  through injection port  24  using syringe  26 . The sample gas mixture is swept along with the carrier gas through the inlet assembly. A first portion of the sample gas mixture and the carrier gas enter the inlet  20  of the capillary tube  14  which comprises the column  12 . The constituent compounds of the sample gas mixture are separated from one another as they travel through the column  12  and exit the column one after another through the outlet  16  which is connected to the detector  18  where the analysis is performed. A second portion of the gas bypasses the capillary tube inlet  20 , and flows further through the liner bore  36  and along depression  62  in plate  44 , best illustrated in  FIG. 2 . Depression  62  is in fluid communication with space  38 , allowing the second gas portion to flow upwardly between the shell  30  and the liner  34  and outwardly through the vent port  40  in the shell. Alternately, the second gas portion bypasses the capillary tube inlet  20 , flows further through the liner bore  36  and into the gas space  68  between the liner end  66  and the plate  44  created by the projection  64 , shown in  FIG. 5 . Gas space  68  provides fluid communication between the liner bore  36  and the space  38  between the liner and the shell, allowing the second gas portion to flow upwardly through the space  38  and exit at the vent port  40 .  
         [0024]     Plate  44  may range in size between about 0.1 and about 0.6 inches in diameter and is made using metal injection molding. In this process, micron sized particles of metal are mixed with a thermoplastic binder. The mixture is heated to a molten state and injected into a mold. Upon curing, the molded part is subjected to a debinding process whereby the thermoplastic binder is removed. Debinding may be effected by heating, use of chemical solvents or a capillary process. After debinding, the part comprises predominantly micron sized metal particles which are then sintered at temperatures above 2400 degrees F. to drive off any remaining binder and create metallurgical bonds joining the particles together. Unlike a machined surface, the surface formed by metal injection molding comprises a surface having randomly oriented irregularities which are not conducive to forming paths permitting leakage. This makes the metal injected molded part advantageous for use as a seal. The part may then be polished or lapped if necessary to obtain a desired surface finish.  
         [0025]     For sealing the sample inlet assembly a surface finish having no irregularities larger than about 0.4 microns deep is advantageous. The part may then be coated to provide an inert surface that does not react with the sample compounds being analyzed, as this may adversely affect column performance. The seal is preferably made of stainless steel which may be coated with nickel, nickel alloys as well as stainless steel alloys to improve the inert quality of the surface. The nickel also acts as a bed for receiving other metal coatings, such as gold or tantalum, which further increase the chemical inertness of the part by filling surface voids and thereby reducing the surface area. Metal coating may be by vacuum deposition, sputter, or electroplating techniques. Non-metal coatings such as silica, for example in the form of silicon dioxide, may also be used to coat the seal.  
         [0026]     The use of metal injection molding to make seals for sample inlet columns may provide one or more or other various advantages over machined parts. For example, expensive and time-consuming machining steps may be eliminated from the manufacturing process. Machined parts must also be heat treated to the annealed condition so that the part will readily deform and create a fluid tight seal when compressed against the end of the shell. This heat treating procedure can be avoided in embodiments of the present invention since metal injection molded parts emerge from the sintering process in the annealed state. It is advantageous that the seals have a hardness between about 60 and about 80 on the Rockwell B scale so that they are deformable to achieve a fluid tight seal.