Abstract:
A device adapted to hold and compress a granular medium is disclosed. The device includes a body having an opening. A channel is positioned in fluid communication with the opening. A flexible substrate is attached to the body overlying the channel. The substrate and the channel cooperate to form a conduit in fluid communication with the opening. A compression member is mounted on the body. The compression member is engageable with the substrate. Engagement of the substrate by the compression member compresses the granular medium within the conduit. A plug blocks the opening and retains the granular medium within the conduit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    High performance liquid chromatography (HPLC) is a process by which one or more compounds from a chemical mixture may be separated and identified. A transport liquid, for example, a solvent, is pumped under high pressure through a column of packing medium, and a sample of the chemical mixture to be analyzed is injected into the column. As the sample passes through the column with the liquid, the different compounds, each one having a different affinity for the packing medium, move through the column at different speeds. Those compounds having greater affinity for the packing medium 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 the column. 
         [0002]    The transport liquid with the separated compounds exits the column and passes through a detector, which identifies the molecules, for example by spectrophotometric absorbance measurements. A two dimensional plot of the detector measurements against elution time or volume, known as a chromatogram, may be made, and from the chromatogram the compounds may be identified. 
         [0003]    For each compound, the chromatogram displays a separate curve or “peak”. Effective separation of the compounds by the column is advantageous because it provides for measurements yielding well defined peaks having sharp maxima inflection points and narrow base widths, allowing excellent resolution and reliable identification of the mixture constituents. Broad peaks, caused by poor column performance, are undesirable as they may allow minor components of the mixture to be masked by major components and go unidentified. 
         [0004]    An HPLC column typically comprises a stainless steel tube having a bore containing a packing medium comprising, for example, silane derivatized silica spheres having a diameter between 0.5 to 50 microns, or 1-10 microns or even 1-7 microns. The medium is packed under pressure in highly uniform layers which ensure a uniform flow of the transport liquid and the sample through the column to promote effective separation of the sample constituents. The packing medium is contained within the bore by porous plugs, known as “frits”, positioned at opposite ends of the tube. The porous frits allow the transport liquid and the chemical sample to pass while retaining the packing medium within the bore. 
         [0005]    The packing medium is slightly compressible. When the column is operated at pressures higher than about  400  bar, the packing medium experiences a volume reduction and a void forms at the high pressure end of the column. The void within the column permits mixing of the transport liquid and the chemical sample. This leads to poor column performance evidenced by broadening of the peaks. It would be advantageous to provide a column which can be operated at pressures above 400 bar without suffering a degradation of performance. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention concerns a device adapted to hold and compress a granular medium. The device comprises a body having an opening therein. A channel is positioned within the body in fluid communication with the opening. The channel is adapted to hold the granular medium. A flexible substrate is attached to the body overlying the channel. The substrate cooperates with the channel to form a fluid tight conduit in fluid communication with the opening. A compression member is mounted on the body. The compression member is engageable with the substrate. Engagement of the compression member with the substrate compresses the granular medium within the conduit. A plug blocks the opening for retaining the granular medium within the conduit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an exploded view of a column embodiment according to the invention; 
           [0008]      FIG. 2  is a longitudinal sectional view of the column embodiment shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a cross-sectional view taken at line  3 - 3  of  FIG. 2 ; 
           [0010]      FIG. 4  is a perspective view of another embodiment of a column according to the invention; 
           [0011]      FIG. 5  is a cross-sectional view of the embodiment shown in  FIG. 4 ; 
           [0012]      FIG. 6  is a cross-sectional view of another embodiment of a column; 
           [0013]      FIG. 7  is a longitudinal sectional view of the embodiment shown in  FIG. 4 ; 
           [0014]      FIG. 8  is a perspective view of another embodiment of a column according to the invention; 
           [0015]      FIG. 9  is an exploded perspective view of another embodiment of a column according to the invention; and 
           [0016]      FIG. 10  is a perspective view of a component of the column shown in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]      FIG. 1  shows an exploded view of a column embodiment  10  according to the invention. Column  10  includes a first body portion  12 , being elongated and having a semi-circular cross section  14  with a surface  16  in which a channel  18  is positioned. A flexible substrate  20  is positioned on surface  16  overlying channel  18 , the substrate and the channel cooperating to form a conduit  22  extending lengthwise along the body portion  12  as best shown in  FIG. 2 . 
         [0018]    With reference again to  FIG. 1 , a second body portion  24  is attached to the first body portion  12  overlying the substrate  20 . The first and second body portions  12  and  24  may be attached to one another using fasteners  26  which compress the substrate  20  between the body portions and form a fluid tight seal between the substrate and the first body portion  12 , effectively sealing the conduit  22  along the length of the column. An inlet  28  for conduit  22  is located at one end of the column, and an outlet  30  is located at the opposite end. The ends of the column have screw threads  32  that adapt the column to receive fittings  34  which allow it to be connected to capillary tubes  36  (see  FIG. 2 ) of a chromatograph (not shown). Porous plugs  38  are positioned at the inlet  28  and outlet  30 . The plugs may be, for example, sintered metal frits or wire mesh screens that retain packing medium  40  within the conduit  22  but allow fluid, such as transport liquid for liquid chromatography, to pass through the column. 
         [0019]    As shown in  FIG. 1 , the second body portion  24  has an opening, for example, in the form of a slot  42 . Slot  42  extends lengthwise along column  10  and is aligned with channel  18  as shown in  FIG. 3 . Slot  42  extends through the second body portion  24  and provides access to the substrate  20 . A compression member, for example, in the form of an elongated finger  44 , is positioned within slot  42 . Finger  44  is movable within the slot  42  to engage the substrate and apply lateral compression to it. The substrate is flexible and deformable, thus allowing pressure exerted by the finger to laterally compress the packing medium  40  within the conduit  22 . As illustrated in  FIGS. 1 and 2 , finger  44  is mounted within slot  42  so as to pivot around an axis  46  oriented substantially perpendicular to the column  10 . An actuator, for example, an adjustment screw  48 , engages the end of finger  44  opposite its pivot axis  46 . The adjustment screw  48  also engages a threaded hole  50  located in the second body portion  24 . Rotation of the screw pivots the finger about axis  46  and allows for fine adjustment of the lateral compression exerted by the finger on the substrate  20  for control of the packing medium compression. Other types of actuators are also feasible including adjustable clamping bands, toggle joints and the like. 
         [0020]    The various components of column  10  may be fabricated from stainless steel, including the substrate  20 . The substrate is designed thin enough so as to be flexible and deformable and transmit the laterally applied compression to the packing medium within the conduit, yet strong enough so as to withstand the internal pressure within conduit and the compressive force exerted by the finger without failing. Steel substrate thicknesses between 0.1 mm and 5 mm are considered feasible, as well as thicknesses between 0.2 mm and 5 mm, or even between 0.2 and 1 mm. 
         [0021]    The column  10  is assembled by positioning a porous plug at the outlet, attaching one of the fittings  34  over the outlet, and attaching another fitting  34  over the inlet without the second porous plug  38  at the inlet. The column is then connected to a device for packing the column which loads packing material into the conduit  22  under pressure. Packing pressures between 1 and 1500 bar, or 10 and 800 bar, or even 100 and 500 bar are used to load the packing medium. This operation deforms the substrate  20 , which bulges outwardly in response to the pressure. Pressure on the column is relieved and the inlet fitting is then removed, the porous plug  38  is positioned overlying the inlet so as to retain the packing medium within the conduit and the fitting  34  is then reattached. 
         [0022]    During operation within a liquid chromatograph, the column  10  may be subjected to pressures as great as 90% of the packing pressure. This may cause the packing medium to compress and form a void at the inlet  28  which will permit mixing of the sample being analyzed and lead to poor column performance. This effect may be indicated by a broadening of the chromatogram peaks. To compensate for the increased operational pressure, adjustment screw  48  may be tightened to force the finger  44  through slot  42  and into lateral engagement with substrate  20 , deforming the substrate and compressing the packing medium  40  within the conduit  22 . Compression of the packing medium forces it into any voids that may have formed as a result of the increased operational pressure, thus eliminating zones within the conduit where mixing may occur. This will be observed by an increase in column performance as evidenced by sharpening of peaks on the chromatogram. The packing material may also be pre-compressed as a preventive measure. This increases the life of the columns. 
         [0023]    Columns according to the invention as described above, when used as analytical columns, may have an inner diameter between 1 mm and 25 mm, or 2 mm and 10 mm, or even 2 mm and 5 mm. The column length may be between 10 mm and 1000 mm, or 20 mm and 250 mm, or even 30 mm and 250 mm. When used as preparatory columns, the columns may have an inner diameter between 9.4 mm and 200 mm, or 9.4 mm and 100 mm, or even 20 mm and 50 mm. The column length may be between 100 mm and 2000 mm, or 150 mm and 1000 mm, or even 250 mm and 1000 mm. When used as guard columns, the columns may have an inner diameter between 1 mm and 200 mm, or 2 mm and 50 mm, or even 2 mm and 25 mm. The column length may be between 10 mm and 100 mm, or 10 mm and 50 mm, or even 10 mm and 20 mm. 
         [0024]    For the embodiments described above, the practical lower limit for the inner diameter is about 1 mm. Other embodiments according to the invention, described below, provide practical columns having inner diameters between about 1 mm and as small as  25  microns. One such embodiment  52  is illustrated in  FIG. 4 . Column  52  comprises a body  54 , in this example in the form of a relatively thick plate  56 . As shown in  FIGS. 5 and 6 , a channel  58  is positioned in a surface  60  of the body. A flexible substrate  62 , generally significantly thinner than the body, is attached to the body  54 . The body and substrate may be metal, such as stainless steel, and attachment of the two components may be effected by methods such as diffusion welding as described in U.S. Pat. No. 3,530,568, hereby incorporated by reference. Channel  58  may be formed by machining, laser cutting, or etching. 
         [0025]    Flexible substrate  62  has a portion  64  overlying the channel. Together, substrate portion  64  and channel  58  cooperate to define a conduit  66  adapted to contain packing medium for chromatographic analysis. Substrate portion  64  may be curved, as shown in  FIG. 5 , or it may be flat as shown in  FIG. 6 . In the example embodiment shown in  FIG. 5 , the substrate portion  64  has a radius of curvature equal to that of the channel  58  so as to provide a conduit  66  having a circular cross section. 
         [0026]    As shown in  FIG. 7 , two fittings  70  and  72  are in fluid communication with conduit  66  and allow the column  52  to be connected to a chromatograph (not shown). The fittings are in spaced relation at opposite ends of the conduit. Fitting  70  comprises a threaded male component  73  welded to either the body or the substrate (in this example the body  54 ) and having a bore  74  in fluid communication with the conduit  66 . Fitting  70  is received within one end of a threaded female-female sleeve  76 . A porous plug  78 , for example, a frit or a screen, is positioned within the sleeve  76 . The plug allows transport liquid to flow through the fitting and the column but prevents packing medium from exiting the column. The other end of the threaded female-female sleeve receives another fitting  80  for connection to an external component, for example a pump for loading the column with packing medium, or the chromatograph. Fitting  72  is substantially the same as fitting  70  and need not be described in any detail. Note that for loading a column, a female-female sleeve not having a plug  78  (see  FIG. 7 , fitting  72  for example) would be employed at the inlet end of the conduit  66 , while the sleeve at the outlet end will have the plug to retain the packing medium. Upon completion of column packing, a female-female sleeve having a plug will replace the plugless version. Either fitting  70  or  72  may serve as the inlet or the outlet. 
         [0027]    As shown in  FIG. 5 , column  52  further includes a compression member  82  overlying the substrate  62 . Compression member  82  may be a plate  84  and is adjustably movable toward and away from the substrate  62  to apply compression laterally against the substrate. In this example, the movement of plate  84  is effected by threaded fasteners  86  that attach the plate to the body  54 . A projection  88  extends from the compression member  82 . The projection is aligned with the channel  58  and positioned so as to engage the substrate portion  64  along its length. When the compression member is moved toward the substrate  62 , i.e., when fasteners  86  are tightened advancing plate  84  toward body  54 , the projection  88  forcibly engages the substrate portion  64  to apply pressure laterally. The substrate portion, being relatively flexible, is deflected, causing a reduction in the volume of conduit  66  and eliminating any voids within the column caused by regions of lower packing medium density where mixing may occur. Deflections of about 0.12 mm to 0.5 mm along the length of the conduit are expected to improve column performance. For substrate portions  64  having a curvature as shown in  FIG. 5 , it is advantageous to provide a contact surface  90  on the end of projection  88  that has a complementary curvature adapted to receive and support the substrate portion  64 . 
         [0028]    The body  54  may have a length between 5 mm and 1200 mm, or 5 mm and 300 mm, or even 5 mm and 200 mm. Widths between 5 mm and 100 mm, or 5 mm and 40 mm, or even 5 mm and 10 mm are feasible for many practical applications. The thickness of body  52  may range between 1 mm and 25 mm, or 1 mm and 10 mm, or even 1 mm and 5 mm, while the thickness of the more flexible substrate  62  may range between 0.1 mm and 5 mm, or 0.2 mm and 5 mm, or even 0.2 mm and 1 mm. Projection  88  may be, for example between 0.1 mm and 2 mm high, and the channel may have a radius of between 12 microns and 2.5 mm, or 25 microns and 1 mm, or even 25 microns and 500 microns. The effective column length, measured as the length of the conduit  66 , may range between 2 mm and 1000 mm, or 2 mm and 250 mm, or even 2 mm and 150 mm. The longer lengths may be achieved despite the fact that the body and substrate are physically shorter that the effective column length because the conduit can follow a non-straight path, for example, a serpentine path, a sinusoid, a spiral or other profile. As shown in  FIG. 8 , a single body  54  may have more than one conduit  66  and hence provide multiple columns in a single unit. 
         [0029]      FIG. 9  shows another column embodiment  92  according to the invention. In column  92  there is no substantial difference between the body and the substrate, and the column may be thought to comprise two flexible substrates  94  and  96  attached to one another in facing relation. The substrates may be metal such as stainless steel, and are diffusion welded to one another. The thickness of the substrates may range between 0.1 mm and 5 mm, or 0.2 mm and 5 mm, or even 0.2 mm and 1 mm. The length of the substrates may range between 2 mm and 1200 mm, or 5 mm and 300 mm, or even 5 mm and 200 mm. One or both substrates have a channel  98  positioned in their facing surfaces, the channels, when two are present, being aligned and cooperating to form a conduit  100 . The channels  98  may have any shape, for example, straight, as shown in  FIG. 9 , or non-straight, as shown in  FIG. 10 . The diameter of the channels may range between 25 microns and 5000 microns, or 50 microns and 2000 microns, or even 0.05 mm and 1 mm. 
         [0030]    Conduit  100  contains the packing medium for chromatographic analysis. Support surfaces  104  and  106  are provided at each end of the substrates for the mounting of fittings  108  and  110 . The support surfaces may be formed by bending the ends of each substrate  94  and  96  transversely to the substrate. 
         [0031]    The fittings  108  and  110  are similar to those described above and provide fluid communication between the conduit  100  and external components, such as the chromatograph. The fittings may be welded to the support surfaces and are adapted to receive standard fittings as are commonly used with chromatographic equipment. 
         [0032]    First and second compression members  112  and  114  are positioned in facing relation overlying the substrates  94  and  96 . The compression members may be formed by metal plates attached to each other by threaded fasteners  116 . The fasteners allow the compression members to be drawn toward each other upon tightening so as to compress the substrates laterally between them. One or both compression members may have a projection  118  aligned with the conduit  100  defined between the substrates  94  and  96  by the channels  98 . Upon tightening of the fasteners  116  the projection or projections engage the substrate or substrates over the conduit and deflect the substrates to reduce the volume of the conduit and thereby eliminate any voids that could allow in transport liquid and sample mixing and result in poor column performance.