Patent Publication Number: US-8978896-B2

Title: Method for filtering liquid using a filter vial

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 13/670,190 filed Nov. 6, 2012 and application Ser. No. 13/411,225 filed Mar. 2, 2012 and issued as U.S. Pat. No. 8,322,539 on Dec. 4, 2012, the complete contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Filter vials have a tubular plunger with a filter at the bottom end of the plunger. The plunger fits into a mating vial containing a fluid and things suspended in the fluid. When the plunger is inserted into the vial fluid passes through the filter and into the plunger in order to separate the fluid from particles or molecules too large to pass through the filter in the bottom of the plunger. The filtered fluid can be extracted from the plunger for further use. 
     The diameter and thickness of the filters can vary greatly from filter to filter and the thickness of the filter can vary greatly even across one filter. These variations make it difficult to seal the filters so all the fluid being filtered passes through the filter, and so that fluid does not wick around the peripheral edge of the filter to contaminate the filtered fluid in the plunger. Further, as fluid is forced through the filters the filters can bow or otherwise deform and allow fluid to bypass the filter as it enters the body of the plunger. Many current assemblies use ultrasonically welded assemblies. Even if the filters are ultrasonically welded to the adjacent walls of the assemblies that still leaves plasticizers that may contaminate the fluids placed in the vials during use. 
     Moreover, current filter vials leave a slight gap between the bottom of the plunger and the bottom of the vial, leaving a portion of the sample unfiltered and unusable for future testing or processing. While the volume of this unfiltered sample may be small in absolute terms, it may represent a sufficiently large portion of the sample that its absence can be important. There is thus a need for a filter vial that avoids the loss of fluid samples and processes more of those samples. 
     BRIEF SUMMARY 
     A filter vial for separating biological, chemical or other fluids has a cylindrical sidewall with an open top and a closed bottom. A frustoconical protrusion extends upwards from the middle of the bottom to form an annular recess in the bottom of the vial. A tubular plunger has an open bottom end to which is fastened an annular cup having an outer sidewall that is sized to fit into and seal against the vial&#39;s sidewall. The annular cap has an inner sidewall that holds a filter over an opening in the bottom of the plunger and forms a shaped cavity leading to that filter. The space between the cap&#39;s inner and outer sidewalls mates with a tubular end of the plunger. The annular cup on the end of the plunger fits snugly into the annular recess in the vial as the protrusion fits into the shaped recess so as to force fluid from the vial past the protrusion, through the filter and into the plunger. 
     In more detail, there is provided a filter vial and plunger assembly for filtering fluids in which the plunger is a tubular plunger having sidewalls defining an internal fluid cavity. The plunger has a top and bottom ends with a plunger opening in the bottom end of the plunger. An annular cap is connected to the bottom end of the plunger. The annular cap has concentric inner and outer sidewalls with the outer sidewall connected to an outer sidewall of the plunger and the inner sidewall defining a shaped cavity in fluid communication with the plunger opening. The cap&#39;s outer sidewall has an outwardly extending first sealing surface at or by its bottom. A filter is interposed between the cap&#39;s inner sidewall and the plunger opening to filter fluid passing through the opening. The plunger fits into a vial that has an open top and closed bottom joined by cylindrical sidewalls of sufficient diameter to form a fluid tight seal with the first sealing surface. The vial has a protrusion extending from the center of its bottom toward the top. The annular the bottom of the cap and the bottom of the vial having complementary shapes, including complementary shapes of the protrusion and shaped cavity, to direct fluid from the vial through the shaped cavity during use. Optionally, shallow channels may be formed in the bottom of the vial and in an exterior surface of the protrusion with the channels orientated to carry fluid toward the top of the protrusion. Preferably, but optionally, the protrusion is frustoconical. 
     There is also provided a filter vial apparatus having a tubular vial with a closed bottom, an open top and a sidewall defining a cylindrical interior with a diameter D located along a longitudinal axis of the vial. The closed bottom has a protrusion extending toward the open top and the vial bottom and protrusion define an annular recess about the vial&#39;s longitudinal axis of the vial with the protrusion at the center of the annular recess. The apparatus also includes a tubular plunger assembly having a longitudinal axis. The plunger assembly includes a plunger with opposing top and bottom ends joined by a sidewall to define a fluid holding cavity in the plunger. The plunger&#39;s bottom end has an outer annular surface and an inner annular surface axially offset toward the top end from the outer annular surface. The inner annular surface is generally orthogonal to the assembly longitudinal axis and is joined to the first annular surface by a recess sidewall to form a recess that encircles the assembly&#39;s longitudinal axis. The second annular surface defines a recess opening that is in fluid communication with the fluid holding cavity of the plunger. The assembly also has an annular cap with inner and outer concentric sidewalls defining an annular recess into which the outer annular surface on the bottom of the plunger is placed. The cap&#39;s outer sidewall extends over the outer surface of the bottom of the plunger while the cap&#39;s inner sidewall is placed adjacent the sidewall of the recess in the end of the plunger. The inner sidewall of the cap defines a shaped cavity configured to snugly receive the protrusion on the vial&#39;s bottom. The shaped cavity has an upper end in fluid communication with the recess opening in the plunger. Further, an outer surface of the outer sidewall of the cap has a diameter sufficiently larger than diameter D to form a fluid tight seal when the plunger assembly is inserted into the vial yet allowing movement of the annular cap to the bottom of the vial. The assembly also includes a filter interposed between the inner annular surface in the plunger and the inner wall of the annular cap. Further, the annular cap on the bottom of the plunger assembly is configured to fit into the annular recess at the bottom of the vial. 
     In further variations of this filter vial apparatus, the annular cap on the bottom of the plunger conforms in size and shape to the annular recess in the bottom of the vial and forms a snug fit with the bottom and protrusion and a snug fit or a slight interference fit with the vial sidewall. Further, the protrusion and shaped cavity may both be frustoconical with substantially the same dimensions and larger at the bottom than toward the top. Still further, shallow channels may optionally be formed in the bottom of the vial and the sides of the protrusion in order to channel fluid through the channels to the filter. Also, the filter vial assembly may have a releasable end cap on the top end of the plunger, with the end cap configured to provide a fluid seal to the fluid holding cavity of the plunger. Still further, the above filter vial apparatus may optionally have a cap flange on the upper end of the outer sidewall of the cap with the cap flange extending outward from the cap sidewall to provide the outer surface of the outer sidewall of the cap that forms the fluid tight seal with the sidewall of the vial. 
     The apparatus may also include an outward facing recess in the outer surface of the tubular plunger with that recess located adjacent the bottom end of the plunger and with the cap flange located in that outward facing recess. The plunger may have an outwardly extending plunger flange at or adjacent to the bottom end of the outward facing recess. The outer sidewall of the cap and outward facing recess may be located so the outer sidewall of the cap extends over the plunger flange and into that outward facing recess, with the outer diameter of the outer wall of the cap at the location of the flange having a diameter larger than diameter D to form a fluid tight fit with the sidewall of the vial during use. 
     The filter vial apparatus may advantageously have the recess sidewall in a convex shape and curved toward the longitudinal axis of the plunger, with the inner sidewall of the annular cap having an outer surface is concave and conforms to the shape of the recess sidewall to nest with the convex surface on the recess sidewall. Further, the vial may advantageously be configured to hold about 10 or fewer micro liters of fluid to be forced through the filter. Advantageously, the bottom of the annular cap may abut the bottom of the vial and less than about 10 micro liters of fluid has passed through the filter into the fluid holding cavity of the plunger. Also, the apparatus may include a frit or filter support on the upper side of the filter and abutting the filter. Preferably, the inner and outer sidewalls of the annular cap each engage a portion of the plunger to hold the annular cap onto the bottom end of the plunger. 
     In another embodiment, there is provided a filter vial apparatus having a longitudinal axis that includes a vial, a tubular plunger and an annular cap. The vial comprises a cylindrical walled vial with a closed bottom and an open top. The vial has a centrally located protrusion on the bottom that extends toward the top to form an annular recess at the bottom of the vial. The plunger comprises a hollow, tubular plunger that extends into the vial. The plunger has an annular bottom with an axially facing recess therein ending at an annular wall encircling and defining a recess opening in fluid communication with a cavity inside of the tubular plunger. The annular cap has inner and outer sidewalls placed over the annular bottom of the plunger with the outer sidewall on an outer sidewall of the plunger. The outer sidewall has an outer surface a first portion of which extends outward from the longitudinal axis a distance sufficient to form a first sealing surface that forms a fluid tight seal with the cylindrical wall of the vial when the plunger is advanced into the vial a distance sufficient to engage the first portion with the vial&#39;s wall during use of the assembly. The inner sidewall of the cap fits into the axially facing recess and extends a distance sufficient to hold a filter against the annular wall of the recess and to also place the filter in fluid communication with the shaped cavity formed by the inner sidewall of the annular cap. The annular cap is configured to fit into the annular recess of the vial with the vial&#39;s protrusion snuggly fitting into the shaped cavity. 
     In further variations of this filter vial assembly, the first sealing surface comprises a flange at the bottom end of the plunger forcing the outer sidewall of the annular cap outward against the vial&#39;s sidewall. Preferably, the sealing surface is located at a bottom end of the plunger and annular cap. Further, the assembly may have a second sealing surface on the outer sidewall of the annular cap located upward of the first sealing surface. Advantageously the protrusion is frusto-conical in shape and the inner wall of the cap has an inward facing surface that is frusto-conical in shape. Further, the axial facing recess in the end of the plunger may have a sidewall that is convex in shape and the cap may have an outward facing surface on the inner sidewall that is configured to mate with the convex sidewall. Still further, channels in the bottom of the vial may be placed in fluid communication with channels in the surface of the protrusion in order to carry fluid from the container bottom toward the filter during use of the filter vial assembly. 
     There is also provided a tubular plunger assembly for filtering fluids, for use with various vials. The tubular plunger has at least an open bottom with a fluid holding cavity in the plunger. The plunger has a bottom end with an inner and an outer annular surface which not only encircle and are orthogonal to a longitudinal axis of the plunger but are separated a distance along that by a recess sidewall that encircles and faces the longitudinal axis to define a plunger recess in the bottom end of the plunger. The inner annular surface encircles and defines an opening to the recess which opening is in fluid communication with the fluid holding cavity of the plunger. The plunger assembly has an annular cap with a cap bottom from which extends inner and outer concentric sidewalls configured to fit over the bottom end of the plunger. The outer sidewall fits over an outer surface of the bottom end of the plunger. The inner and outer sidewalls receive the outer annular surface therebetween. The inner sidewall has an outer surface abutting the sidewall of the plunger recess. The inner surface of the inner sidewall defines a shaped cavity in fluid communication with the opening to the recess. The inner sidewall has a top end. A filter is clamped between that top end of the inner sidewall and the inner annular surface. 
     In further variations, the tubular plunger assembly has a portion of the outer surface of the outer sidewall of the annular cap, at or by the bottom of the annular cap, extending outwardly a distance sufficient to form a first sealing surface that provides a fluid tight seal against a filter vial when the cap is inserted into the vial during use of the plunger assembly. The first sealing surface is advantageously formed by a flange on the end of the plunger which forces a portion of the cap&#39;s sidewall outward. The plunger assembly may also include a frit or support interposed between the filter and the first annular surface. Further, the assembly may have a sealing surface facing along and encircling the longitudinal axis and formed on at least one of the first and second annular surfaces. Also, the recess sidewall may have a convexly curved shape in which case the outer surface of the inner sidewall of the cap preferably has a mating concave shape. The shaped cavity is preferably frustoconical in shape. Advantageously, the tubular plunger has a sidewall that is thicker at the location of the second annular surface. Preferably, the plunger assembly may have a removable closure configured to releasably form a fluid tight seal at the top end of the plunger. 
     There is also provided a filter vial kit that includes any of the above described plunger assemblies as well as a filter vial. The filter vial may have an open top and a closed bottom with a protrusion on the center of the bottom. The protrusion extends toward the top a distance about the same as a height of the inner sidewall of the annular cap measured along the longitudinal axis. The protrusion conforms in shape to the shaped cavity in the cap. The filter vial may also have an internal diameter slightly smaller than the outer diameter of at least a portion of the outer sidewall of the cap on the bottom of the plunger to form the fluid tight seal with the first sealing surface. 
     In further variations the filter vial kit may include a removable cap and a volume of less than about 30 micro liters. Also, the interior bottom of the vial and protrusion may be configured to conform to the exterior shape of the bottom of the cap and shaped recess. Advantageously, but optionally, the bottom of the filter vial may have shallow channels formed therein which channels are in fluid communication with shallow channels in the surface of the protrusion. The channels are orientated to carry fluid to the top of the protrusion during use. 
     Another filter vial kit is also provided in which includes a plunger, a filter, an annular cap and a vial. The plunger comprises a hollow tubular plunger having top and bottom ends with an opening at a bottom end of the plunger. The filter extends across the opening in the bottom end of the plunger. The annular cap is fastened to the bottom end of the plunger. An outer sidewall of the cap extends along an outer surface of the bottom end of the plunger with a portion of that outer sidewall extending outward a predetermined diameter to form a first sealing surface. The annular cap has an inner sidewall forming a shaped cavity in fluid communication with the opening in the bottom end of the plunger. The vial has an open top and a closed bottom with the bottom having a central protrusion extending toward the open top of the vial. The vial also has a cylindrical sidewall that is sufficiently smaller in diameter than the predetermined diameter to form a fluid tight seal between the vial and annular cap. The bottom of the annular cap and the shaped recess are configured to mate with the shape of the vial&#39;s bottom and the protrusion so as to force fluid in the vial through the shaped cavity and filter. 
     In further variations of this filter vial kit the bottom of the vial has shallow channels in fluid communication with shallow channels in the surface of the protrusion so fluid is forced through the channels toward the filter as the annular cap fits snuggly against the vial&#39;s bottom and its protrusion. Further, the inner sidewall of the cap may push the filter against a portion of the plunger to hold the filter in place. Preferably, the shaped cavity is frustoconical and the protrusion has a mating frustoconical shape with shallow channels in the surface of the protrusion to carry fluid to the top of the protrusion. 
     An improved filter vial is also provided. The improved vial includes a plastic filter vial having an open top and a closed bottom connected by a cylindrical sidewall. The bottom has a protrusion extending a short distance along a longitudinal axis of the vial toward the top. The filter vial has a volume of about 30 micro liters or less and a sidewall thickness of about 0.050 inches or less. Optionally, the protrusion is frustoconical and extends for a distance of less than about 0.2 inches. Optionally, the bottom of the vial has shallow channels therein that are in fluid communication with shallow channels in an outer surface of the protrusion and orientated to carry fluid through the channels toward the top of the protrusion. The channels are preferably radially orientated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
         FIG. 1A  is a side view of a filter vial having a protrusion or boss in its bottom; 
         FIG. 1B  is a top view of the filter vial of  FIG. 1A ; 
         FIG. 1C  is a cross-sectional view of the filter vial of  FIG. 1A  taken along  1 C- 1 C of  FIG. 1B ; 
         FIG. 2A  is a side view of a plunger for use with the filter vial of  FIG. 1A ; 
         FIG. 2B  is a cross-sectional view of the plunger of  FIG. 2A  taken along Section  2 B- 2 B of  FIG. 2A ; 
         FIG. 2C  is an enlarged portion of taken along  2 C- 2 C of  FIG. 2B ; 
         FIG. 3A  is a side view of a cup for use with the plunger of  FIG. 2A ; 
         FIG. 3B  is a sectional view taken along  3 B- 3 B of  FIG. 3C ; 
         FIG. 3C  is a top view of the cup of  FIG. 3A ; 
         FIG. 4A  is a side view of the cup of  FIG. 3A  on the plunger of  FIG. 2A ; 
         FIG. 4B  is a sectional view taken along section  4 B- 4 B of  FIG. 4A ; 
         FIG. 4C  is an enlarged sectional view of the lower end of  FIG. 4B ; 
         FIG. 5A  is a sectional view of the plunger of  FIG. 2A  with a cup of  FIG. 3A  inserted into a vial of  FIG. 1A , before filtering fluid in the vial; 
         FIG. 5B  is a sectional view of the plunger of  FIG. 5A  with the plunger forcing some fluid into the plunger; 
         FIG. 5C  is a sectional view of the plunger of  FIG. 5A  with the plunger abutting the bottom of the vial and all the filtered fluid in the plunger; 
         FIG. 6A  is a partial perspective view of a further embodiment of the vial of  FIGS. 1A-1B  having a frusto-conical protrusion; 
         FIG. 6B  is a bottom view of a cap having a shaped recess that mates with the annular space and protrusion of  FIG. 6A ; 
         FIG. 7A  is a partial perspective view of a further embodiment of the vial of  FIGS. 1A-1B  showing a protrusion with a four sided cross-sectional shape; 
         FIG. 7B  is a bottom view of a cap having a shaped recess that mates with the annular space and protrusion of  FIG. 7A ; 
         FIG. 8A  is a partial perspective view of a further embodiment of the vial of  FIGS. 1A-1B  showing a protrusion with a three-sided cross-sectional shape; 
         FIG. 8B  is a bottom view of a cap having a shaped recess that mates with the annular space and protrusion of  FIG. 8A ; 
         FIG. 9A  is a partial perspective view of a further embodiment of the vial of  FIGS. 1A-1B  showing a protrusion with a four sided cross-sectional shape in a vial with a four sided cross-sectional shape; 
         FIG. 9B  is a bottom view of a cap having a shaped recess that mates with the annular space and protrusion of  FIG. 9A ; 
         FIG. 10A  is a partial perspective view of a further embodiment of the vial of  FIGS. 1A-1B  showing a protrusion with a three-sided cross-sectional shape; 
         FIG. 10B  is a bottom view of a cap having a shaped recess that mates with the annular space and protrusion of  FIG. 10A ; 
         FIG. 11  is an exploded cross-sectional view of a lower portion of a plunger, filters, filter supports or frits, and a cap. 
     
    
    
     DETAILED DESCRIPTION 
     Briefly described with reference to  FIGS. 1A-1C  and  5 A- 5 C, a tubular filter vial  10  has a closed bottom  12  with a boss or protrusion  14  located at the middle of the bottom. The protrusion  14  extends into the cavity formed cylindrical wall  16  of the vial forming an annular space between the protrusion  14  and the wall  16  of the vial  10 . Shallow channels  15  are formed in the bottom  12  and sides of the protrusion  14 . When a fluid sample is placed in the vial  10  the fluid surrounds the protrusion  14 . A tubular plunger  18  has a first and outer, annular bottom end  20  over which fits an annular cap  50 , with the bottom of annular cap  50  shaped to fill the annular space surrounding projection  14  and fit snugly over the protrusion  14  so as to force all of the fluid in the vial  10  and annular space surrounding projection  14  through a filter  70  in the bottom of the plunger and into the inside of the tubular plunger  18  where the filtered fluid may be removed for further use. The shallow channels  15  in the bottom  12  and sides of projection  14  allow any residual fluid to be forced from the vial through filter  70  and into the plunger  18 . The annular bottom end  20  of the plunger  18  is preferably orthogonal to the longitudinal axis  27  of the plunger, but since the end  20  may take various shapes it need not be a flat, annular surface. A removable end cap  23  ( FIG. 5A-5C ) on the upper end  22  of the plunger provides a closed container for the filtered fluid. The end cap  23  is removed to access the fluid in the tubular plunger or a needle is inserted through membrane  25  to access the filtered fluid. The end cap  23  may have various configurations and may attach to the end  22  of the plunger  18  various ways, including use of threads  24 . 
     The relative directions top and bottom or upper and lower are with respect to the parts as shown in  FIGS. 1A ,  2 A and  3 A when the drawings are held vertically. This has the top orientated away from the earth and the bottom toward the earth when the vial and plunger are held vertically in a use position. The relative directions are thus those of a person standing. Similarly, up and down or upper and lower or above and below refer to the relative positions of a person standing with the vial in a vertical orientation. The relative directions inner or inward refer to direction toward common longitudinal axis  27  of the various parts as described later. The relative directions outer or outward refer to the direction away from the common longitudinal axis  27 . Further, as used herein, a snug fit means a very close fit that allows parts to fit together when manually positioned but not an interference fit. The space between snugly fitting parts may vary from zero to a few thousandths of an inch (about 0.1 mm or less). 
     Referring to  FIGS. 2A-2C , the tubular plunger  18  has a circular cross-section with bottom and top ends  20 ,  22 , respectively. Threads  24  may be optionally placed on the outer surface of the plunger adjacent the top end  22  with the threads configured to mate with a threaded closure  23  ( FIG. 5A ) to close off the top end of the plunger, preferably by providing a fluid tight seal. The closure  23  may have an elastomeric membrane  25  through which a needle may be inserted to withdraw portions of the filtered fluid from inside the plunger  18 . An upper flange  26  extends outward from the exterior surface of the plunger  18  toward upper end  22  of plunger  18 . The upper flange  26  may various cross-sectional shapes, with a triangular cross section being preferred. The triangular cross-section is inclined to make it easy to insert the upper flange  26  into the vial  10  so the flange may seal against the interior wall  16  of the vial while resisting removal of the plunger from the vial. Thus, the flange  26  has one side extending outward from the exterior surface of the plunger  18 , and preferably in radial direction from a longitudinal axis  27  of the plunger  18 , while another side is inclined so as to intersect the longitudinal axis  27  between the flange  26  and the bottom end  20  of the plunger. A vent hole  29  may be extend through the wall of the plunger  18 , with the vent hole advantageously located close to the flange  26 , but between the flange  26  and the bottom end  20 . The vent hole should be located well above the anticipated fluid level in the plunger  18 . 
     The tubular fluid holding cavity in the plunger  18  preferably has a cylindrical portion  28  beginning at the top end  22  with a narrowing portion  30  beginning about ¼ of the length of the plunger from the bottom end  20 . The narrowing portion  30  is optional, but inclined walls forming a truncated, conical surface are preferred. The bottom end of the narrowing portion  30  opens into a larger plunger recess  32  that is formed in the bottom end of the plunger  18 . The plunger recess  32  is generally cylindrical in shape, creating a stepped configuration in the bottom portion of the plunger  18 . The step may be a sharp change in diameter or a gradual one as shown in  FIG. 11 . The plunger&#39;s annular end  20  encircles the plunger recess  32  and axis  27  so the plunger recess  32  faces along axis  27 . The plunger recess  32  extends from the bottom surface  20  toward the top of the plunger for a short distance, ending at annular wall  34 , which is generally orthogonal to the longitudinal axis  36  of the plunger and surrounds the lower end of fluid holding cavity  30 . The plunger&#39;s annular end  20  may be viewed as an outer annular surface generally aligned with or close to the outer periphery of the plunger  18 , while the annular wall  34  is an inner annular surface  34  located inward of and laterally offset from the first annular surface  20  toward the upper end  22  of the plunger. Sidewall  36  faces and encircles axis  27  and join the inner edge of the annular surface  20  to the outer edge of annular surface  34 . The first and outer annular surface  20  is on the end of a tubular projection formed by the wall of the tubular plunger  16 , while the second, inner annular surface  34  is offset therefrom and defines the bottom opening to the fluid holding cavities  28 ,  30 . 
     The plunger recess  32  has sidewall  36  that may be parallel, inclined, curved or of other shape to help seal with mating parts that may be placed into the plunger recess  32  as described later. A slight curve of the sidewall  36  is believed preferred, with the sidewall being convex so the middle of the sidewall extends more toward the longitudinal axis  27  than the ends of the sidewall  36 . Thus, the plunger recess  32  has a maximum diameter of D 1  at its top and bottom and a minimum diameter D 2  therebetween. As seen in  FIGS. 2A-2C , the annular surface or wall  34  is above plunger end  20  so the annular wall  34  may be referred to as top wall  34 , of the plunger recess  32 . The convex shape of the sidewall  36  helps hold the annular cap  50  on the annular bottom of the plunger  18  as described later. 
     Advantageously, but optionally, a recess sealing member  35  ( FIG. 2C ) is formed on the annular top wall  34  and extends along axis  27  in the direction of the bottom of the plunger  18 . A seal with a triangular cross section is preferred, and preferably the seal is at the edge of the juncture of the tapered fluid holding cavity  27  and the top wall  34 . 
     An end sealing member  38  is optionally formed on the annular end surface  20 . The end sealing member  38  encircles the plunger recess  32  and is centered on axis  27 . A end sealing member  38  with a triangular cross-section is preferred, with the seal extending in the direction of the axis  27  away from the plunger&#39;s bottom end  20 . Advantageously, the end sealing member  38  is formed in about the middle of the annular end surface  20 , about half way between the plunger recess  32  and the outer circumference of the plunger&#39;s end  20 . As seen best in  FIG. 2C , the outer surface of the plunger adjoining the end surface  20  may optionally be cylindrical in shape for a short distance upward along axis  27 . In contrast, the inner surface of the plunger at the end surface  20  is preferably curved as discussed above. 
     An annular groove  40  is preferably formed in the outer surface of the plunger  18  adjacent the bottom end  20  and encircling the end of the plunger. The groove  40  may have a flat bottom generally parallel to and concentric with the axis  27  with perpendicular ends. The width of groove  40  extends a short distance along axis  27 , with the distance depending at least in part on the configuration of a cap described later. The outer side of the plunger  18  between the lower edge of the groove  40  and the end  20  is generally inclined, forming a plunger flange  42  at the bottom of the plunger. Preferably, the plunger flange  42  is inclined so it intersects axis  27  below the end  20 . Further, the largest diameter of plunger flange  42  is preferably the same as that of the cylindrical diameter of the plunger  18 . As seen best in  FIG. 2C , the plunger flange  42  preferably ends below the plunger recess  32  as measured along the axis  27 , with the groove  40  beginning toward the top of the plunger recess  32 . 
     Referring to  FIGS. 2B and 2C , the bottom end of the tubular plunger  18  has an outer wall on which is located the plunger flange  42  radially opposite inner wall  36  of the plunger recess  32 . As described later, each of these opposing surfaces on the lower end of the plunger  18  seal against a mating surface on the annular cap  50  that may be snapped into place over the lower end of the plunger. 
     Referring to  FIGS. 2-4 , the annular cap  50  is dimensioned so that it can be forced over the plunger flange  42  with an interference fit while also mating with the sidewall  36  of the plunger recess  32  to also help hold the cap onto the end of the plunger  18 . In use the annular cap  50  is located on longitudinal axis  27 . The annular cap  50  has an outer, generally cylindrical and tubular sidewall  52  and an inner generally cylindrical and tubular sidewall  54  joined at the lower ends by bottom  56 . The cap&#39;s outer sidewall  52  has an outwardly extending flange  58  at its free end. The inner sidewall  54  has an outward extending flange  60  at its free end. The outer sidewall  52  is generally of uniform thickness and is higher than inner wall  54 , preferably having the outer wall flange  58  located beyond the end of inner wall  54 . 
     The sidewalls  52 ,  54  are concentric and spaced apart a distance to receive between them the outer annular surface  20  and the portion of the plunger sidewall on which that annular surface  20  is formed. Advantageously, the cap&#39;s sidewalls  52 ,  54  conform to the shape of the tubular plunger&#39;s sidewall on which the annular surface  20  is formed. Thus, the inner and outer sidewalls  52 ,  54  are concentric, annular walls spaced apart a predetermined distance by bottom  56  on the top side of the cap  50  with a larger bottom  57  of the lower side of the cap  50 . The bottom  56  on the upward facing side of the annular cap  50  forms an annular surface extending between the facing portions of the cap&#39;s sidewalls  52 ,  54 , and has a radial width extending from the outside of the inner wall  54  to the inside of the outer wall  52 . The bottom  57  on the downward facing side of the annular cap  50  forms an annular surface that also extends between the sidewalls  54 ,  54 , but it has a radial width extending from the inside of the inner wall  54  to the outside of the outer wall  52 . 
     The inner sidewall  54  has a curved outer surface  62  ( FIG. 3B ) with a curvature selected to match the curvature of sidewall  36  of the plunger recess  32  so the sidewalls  54  and  62  nest together. This mating curved shape is believed to not only hold the cap  50  onto the end of the plunger  18  but also helps ensure the inner wall  54  pushes the filter  70  against the inner annular wall  34  to form a fluid tight seal. Thus, in use, the outer surface  62  of the inner annular sidewall  54  abuts the sidewall  36  of plunger recess  32  in the end of the plunger  18 , with the sidewalls  54 ,  36  forming a fluid tight seal. 
     The inner sidewall  54  has an inner surface  64  ( FIG. 3B ) shaped to fit over protrusion  14  ( FIG. 1C ) with a snug fit and optionally with a slight interference fit. Preferably, but optionally, the inner surface  54  is a segment of a conical surface (frustoconical), with the surface tapered to intersect longitudinal axis  27  above the ends of the sidewalls  54 ,  58 . Thus, the cap&#39;s inner sidewall  54  forms a shaped cavity  66  centered about longitudinal axis  27 . Advantageously, the cap&#39;s inner sidewall  54  is inclined outward toward the bottom end of the annular cap  50 , and preferably has that outward inclination along the entire length on the inside of the inner sidewall  54 . Advantageously, the top  68  ( FIG. 3B ) of the cap&#39;s inner sidewall  54  forms a circular opening that is the same as the circular opening on the inside of inner annular wall  34  of the plunger recess  32  ( FIG. 4C ), to form an hourglass shape in cross-section, with the filter  70  at the narrow point of the hourglass. The upper end of the inner sidewall  54  forms an annular surface  68  which abuts the filter  70  during use and pushes the filter against the inner annular surface  34  of the plunger  18 . 
     Referring to  FIGS. 2-4  and  11  and especially to  FIGS. 2C ,  3 B and  4 A- 4 C the formation of plunger assembly  69  is described in which the cap  50  is placed on the plunger  18  to hold a filter  70  and optionally a frit or support  72  in place. The outer wall  52  has an inner surface with a diameter that is slightly smaller than the largest diameter of the plunger flange  42  of the plunger  18  so that an interference fit is created when the upper end of the annular cap  50  is forced over the lower end of the plunger  18 . The cap&#39;s outer sidewall  52  is flexible enough so that it bulges slightly outward at the location of the plunger flange  42 , with the upper portion of sidewall  52  fitting into groove  40  in the lower end of the plunger. The plunger flange  42  advantageously forms a fluid tight seal with the outer wall  52  of the annular cap  50 . During use, the plunger flange  42  is preferably configured so that it has a taper that makes it easier to fit the plunger flange  42  into the cap  50 , but hinders removal of the cap from the flange after the parts are engaged. 
     As the plunger flange  42  seals against the outer wall  52  of the annular cap  50 , the outer surface of the inner wall  54  of the annular cap seals against the inward facing surface  36 , or sidewall  36 , of the plunger recess  32  in the end of the tubular plunger  18 . The flange or rib  58  on the outer wall  52  of the cap  50  fits into the groove  40  on the plunger. The flange  58  may abut the upper side of the groove  40  in order to limit relative motion of the outer sidewall  52 . The flange  58  within the groove  40  also provides resistance to removing the cap  50  from the plunger. 
     Likewise, the inner wall  54  both seals against the plunger  18  and hinders removal of the cap from the plunger. In the depicted embodiment the sidewall  36  is convex toward the axis  27  and the outer surface of sidewall  54  is curved (concave) to mate with convex sidewall  36 . The cap&#39;s inner sidewall  54  engages the plunger sidewall  36  to form a fluid tight seal and help hold the parts together. Other mating shapes could be used on the surface  36  and sidewall  54 . 
     Preferably, the bottom  56  of the annular cap  50  is sized to accept the annular bottom  20  of the plunger  18 , with the optional end sealing member  38  abutting the bottom  56  ( FIG. 4   c ). Thus, the annular cap  50  may be forced over the plunger flange  42  on the bottom end of the plunger  18 , with the plunger flange  42  forming a snap lock to hold the cap in place, and the cap forming fluid tight seal with the bottom end of the plunger. Further, the annular cap  50  has an outer wall  52  that seals against the outer surface of the plunger  18  at the bottom end of the plunger and advantageously hinders removal of the cap from the plunger. The annular cap  50  further has an inner wall that seals against the inner surface of the plunger  18  (sidewall  36 ) at the bottom end of the plunger and advantageously also hinders removal of the cap from the plunger. The annular cap  50  defines a fluid passage  66  which the cap  50  places in fluid communication with the opening in the lower end of tapered fluid holding cavity  30 . 
     Advantageously, the cap&#39;s inner wall  54  has a height h corresponding to that of protrusion  14  ( FIG. 1C ) which height also preferably places the upper, annular end  68  ( FIG. 3B ) of that sidewall  54  immediately adjacent or abutting the top wall  34  of the plunger recess  32  and filter  70  but such that the bottom of the tapered fluid holding cavity  30  ( FIG. 4B ,  4 C) is aligned along axis  27  with the top of shaped cavity  66 . Referring to  FIGS. 4B-4C , the cap&#39;s inner sidewall  54  preferably leaves a slight gap between its upper end  68  and the top wall  34  of plunger recess  32 . A filter  70  and preferably but optionally a frit or support  72  are placed in that slight gap. The filter  70  is preferably below the frit or support  72  so that the frit or support  72  can support the filter against upward fluid pressure from shaped cavity  66  into fluid holding cavity  30 . The outer periphery of filter  70  and frit or support  72  are clamped between the top wall  34  of the plunger recess  32  ( FIG. 2C ) and the top end  68  of the inner sidewall  54 . There may be any number of filters  70  or frits/supports  72 , in various arrangements as partially illustrated in  FIG. 11 . The number and arrangement of filters  70  and frits or supports  72  will vary with the specific application and design, as will the shape of the filters and frits or supports. In particular, one or more filters  70  may be on the upstream or downstream side of the frit or support  27 , and the filters may be separate from or attached to the frit or support  72 . Further, because the sidewall  36  of recess  34  is curved, the filter  70  and frit or support  72  may have a diameter large enough to be placed adjacent the larger diameter portion of the sidewall  36  ( FIG. 2C ) that is immediately adjacent the top wall  34  ( FIG. 2C ), with the curvature of the sidewall temporarily holding the filter and/or frit or support in place. Moreover, the diameter of the seal  70  and any frit or support  72  are preferably large enough to extend past sealing member  35  on the top wall  34  of cavity  32  in the end of the plunger  18 . The sealing member  35  helps provide a fluid tight seal between the top wall  34  and end  68  of the cap&#39;s inner wall  54  and any intervening filter or frit/support. 
     The filter  70  is preferably, but optionally made of Teflon, nylon, glass fiber or other filter materials such as PVDF (polyvinyldifloride) or PES (polyethersulphone), etc. The support or frit  72  is preferably made of the same plastic as the plunger  18  or cap  50 , but porous or otherwise allowing fluid passage. Frit  72  may be made of Teflon or glass mesh. Both the filter  70  and fit or support  72  may be very thin when the volume to be passed there through is measured in micro liters. The spacing between the inner sidewall  54  of the cap  50  and top wall  34  of the recess in the plunger  18  are selected to hold these thin parts (filter and/or frit/support) in place without breaking the support  72  and with enough force so fluid may be forced through the filter  70  during use. The inner wall  54  and plunger  18  thus clamp the filter  70  and/or frit or support  70  in position. If the filter is strong enough by itself to safely withstand fluid forced through it during use, the frit/support  72  may be omitted. If not, then a fit or support  72  of suitable thickness and strength is provided. Because the plunger  18  and/or a thicker plunger sidewall may be made of stiffer material than the annular cap  50 , or because the shape of the lower end of the plunger  18  makes it much stiffer along the axis  27  at the location of the cap  50 , it is believed possible to force the cap  50  and its inner wall  54  sufficiently tight against the plunger  18  so as to clamp the filter  70  between the inner wall  54  and top wall  34  that a support or frit  72  may not be needed for all applications. Thus, the plunger assembly  69  advantageously has both a filter  70  and frit or support  72  interposed between the annular cap  50  and the lower end of the plunger  18 , but may omit the fit or support  72 . 
     As reflected in  FIG. 4C , when the annular cap  50  is snap-fit onto the bottom end of the tubular plunger  18  to form the plunger assembly  69 , the facing portions of curved sidewall  36  and inner sidewall  54  on the sidewall  36  and inner wall are preferably not in an interference fit. Further, the bottom end of the plunger  18  adjacent the bottom end of tapered fluid holding cavity  30  is thicker and thus stiffer than the upper portion of the plunger, thus providing a stiffened plunger end. The tapered fluid holding cavity  30  in the bottom end of the plunger  18  has an opening that is aligned with the opening of shaped cavity  66  in the annular cap  50 . The diameter of the tapered fluid holding cavity  30  preferably narrows along the longitudinal axis  27  as you move toward the shaped cavity  66  and the cavity  66  narrows toward the fluid holding cavity  30  along the longitudinal axis as you move toward the fluid holding cavity  30 , to form a generally hourglass shape, or to form a venturi shape. As shown in  FIGS. 4A and 4B , the resulting plunger assembly  69  has a lower portion of a generally uniform outer diameter except for an outward extending protrusion adjacent the location of flange  58  at the end of cap  50  and/or plunger flange  42 . 
     Referring to  FIGS. 1C ,  4 B,  4 C, and  5 A,  5 B and  5 C, the plunger assembly of  FIG. 4  is placed into the open end  80  of vial  10 . That open end  80  advantageously has an inwardly and downwardly tapered opening to make it easier to align and insert the bottom end of plunger assembly  69  into the vial  10 . Advantageously, but optionally, a rib  82  extends around the top end of the vial to provide extra strength around the opening to the vial. The inner diameter of the sidewall  16  of the vial  10  has a diameter corresponding to the diameter of the bottom end of the annular cap  50  or slightly larger. But the inner diameter of the vial&#39;s sidewall  16  is smaller than the diameter of the plunger assembly at the location of the cap flange  58 , and preferably at the diameter of the plunger flange  42  so as to create an interference fit at both locations when the plunger assembly  69 . An interference fit of a few thousandths of an inch (about 0.1 mm or less) at the location of rib  58  and plunger flange  42  within the vial  10  is believed suitable when the volume of fluid being filtered is measured in micro liters, but the amount of interference fit will vary with the materials selected. 
     Preferably, as the cap&#39;s outer sidewall  54  passes over flange  42  ( FIG. 2A ,  4 A) the sidewall  54  bulges slightly outward a distance sufficient to form a primary seal between the annular cup  50  and the sidewall  16  of the vial  10 . This primary seal provides a sufficient interference fit so that fluid in the vial  10  does not pass this primary seal. A secondary seal is provided by the interference fit formed by the rib  58  pressing against the inside of the vial&#39;s sidewall  16 . Advantageously no fluid enters the area between the seals provided by plunger flange  42  and the cap&#39;s rib  58  on the outer sidewall  52 . Since the fluid in the vial  10  does not pass the fluid seal formed by the plunger flange  42  forcing the cap sidewall  52  outward to abut the inside of the vial&#39;s sidewall  16 , fluid flows into the plunger  18  as the plunger and vial bottom  12  move toward each other. The inner surface of vial sidewall  16  is preferably slightly tapered to allow the parts to be more easily molded and to increase the slight interference fit between the primary and secondary seals provided by plunger flange  42  and outwardly extending rib  58  on the annular cap  50 . A slight taper of a few thousandths of an inch or less (about 0.1 mm or less) over the height of the vial is believed suitable when the vial is about an inch (25 mm) high. 
     As the plunger assembly  69  moves toward the vial&#39;s bottom  12 , the shaped cavity  66  in the annular cap  50  on the end of the plunger assembly  69  is aligned with and fits over the protrusion  14  at the vial bottom  12 . The protrusion is preferably frusto-conical in shape but other configurations may be used as shown in  FIGS. 6-10  and discussed later. More importantly, the shape of the protrusion  14  is shaped to fit snugly into the shaped cavity  66 . As the plunger assembly  69  is advanced along axis  27  toward the vial&#39;s bottom vial  12 , the seal between the plunger assembly  69  and the vial sidewall  16  forces fluid toward the vial bottom  12 , through the shaped cavity  66  and the filter  70  and into the body of the plunger  18  represented by cavities  30 ,  28 . 
     Referring further to  FIG. 5B  and  FIG. 1C , as the bottom end of the plunger assembly  69  reaches and passes the top of the protrusion  14 , the remaining fluid in the vial  10  is forced between the inclined sides of the protrusion  14  and the mating sides of shaped cavity  66  to force the fluid through the filter  70  and into the fluid holding cavity  30  and/or  28 . Referring further to  FIG. 5C  and  FIG. 1C , when the bottom end of plunger assembly  69  abuts the bottom  12  of the vial  10 , almost all or all of the fluid along the vial&#39;s bottom  12  and protrusion  14  is forced through the filter  70  and the top of the protrusion  14  preferably abuts the filter  70 , and ideally may slightly push against the filter  70  but not break the filter and any frit or support  72 . The channels  15  in the vial&#39;s bottom  12  and sides of protrusion  14  form fluid passageways to allow fluid in the vial to pass through filter  70 . As the cap&#39;s outer sidewall  52  seals against the vial sidewall  16  fluid is forced toward vial bottom  12  and as the cap&#39;s bottom  57  is pushed against the vial&#39;s bottom  12  any trapped fluid is forced into the shallow channels  15  and forced through filter  70 . Indeed, if the plunger assembly  69  is forced against the vial bottom  12 , the bottom  57  of annular cap  50  may be forced slightly into the shallow channels  15  and provide sufficient pressure to move any trapped fluid through the channels  15  and through the filter  70  as the top of the protrusion  14  abuts the filter. Channels a few thousandths of an inch or less deep (about 0.1 mm or less) and about the same in width are believed suitable. Four to eight channels are believed suitable but the number and size of the channels  15  will vary with the particular design and size of the vial  10 . 
     The cap  50  on the bottom end of plunger  18  thus forms an annular male protrusion formed by the outer surface of the cap&#39;s outer sidewalls  52  and the inner surface of the cap&#39;s inner sidewall  54 , with the shaped recess  66  at the center of the male protrusion. The annular male protrusion is effectively formed by the bottom  57  and sidewalls  52 ,  54  of the cap  50  that is fastened to the bottom end of the plunger  18 . A complimentary shaped annular, female recess is formed by the vial&#39;s sidewall  16 , bottom  12  and protrusion  14 . Likewise the male protrusion  14  in the vial is shaped to mate with the female recess of the shaped cavity  66 . The male protrusions and female recesses fit together to squeeze any fluid between them toward the center protrusion  14  and into the body of the tubular plunger and fluid holding cavity  30  and/or  28 . The mating annular shapes of the end of the plunger (provided by the shape of the bottom portion of the annular cap) on the one hand and the annular recess of the vial on the other hand, provide means for minimizing the dead volume at the bottom of the vial, with the shaped protrusion  14  and correspondingly shaped recess  66  directing the fluid through filter  70  and into the body of plunger  18  and with the channels  15  in the vial bottom  12  and sides of the protrusion  14  allowing any small trapped portions of the fluid to be forced through the channels and through filter  70  abutting protrusion  14 . While the annular space is circular in the depicted embodiment of  FIGS. 1-5 , the annular space need not be cylindrically based as the mating shape of the protrusion  14  and recess  66  may be non-circular, and the shape of the vial&#39;s sidewall  16  and plunger  18  need not be cylindrical but may be any tubular shape, including shapes with continuous curves such as circular or oval, or shapes with straight sides including triangular, quadrilateral or multilateral. Thus, as used herein the use of “annular” is not limited to shapes or volumes having concentric circles, but includes shapes other than circles. For example,  FIGS. 7-8  show a protrusion  14  with a cross-section shape of a triangle and quadrilateral, respectively, but in a cylindrical vial  10 , while  FIGS. 9-10  show protrusions with three and four sides, respectively, in vials  10  having the same number of sides. The space surrounding the protrusion  14  and enclosed by the vial sidewall  16  is still “annular” as used herein. As used herein, the space is annular when the vial sidewall  16  is non-circular in cross section as in  FIGS. 9-10 , and had, for example cross-sectional shapes that were continuous curves such as circles or ovals, or shapes having straight sides including three, four or more sides joined by corners that were preferably, but optionally rounded. As seen in  FIGS. 7-10 , the shaped cavity  66  in cap  50  conforms to the shape of the protrusion to form a snug fit. In multi-sided configurations of the protrusion  14  the corners are preferably rounded but may be sharp as shown, with the corners of the shaped cavity  66  preferably conforming to the corners of the protrusion to form a snug fit. 
     Further, the outwardly inclined surfaces of the vial&#39;s male protrusion  14  and of the cap&#39;s inner sidewall  54  cooperate with plunger  18  to provide means for deforming the outer sidewall  52  and bottom  57  of the cap  50  against the vial&#39;s sidewall  16  and bottom  12 , respectively, to reduce the fluid retained between the plunger assembly  69  and vial&#39;s sidewall  16  and bottom  12  as the plunger is moved against the bottom  12  of the vial. The primary seal between the vial&#39;s sidewall and the plunger flange  42  and bulging cap sidewall  52  is an interference fit which forces fluid in the vial  10  downward toward the vial bottom  12  as the plunger  18  moves the primary seal toward the vial&#39;s bottom  12  where the trapped fluid is directed by the mating surfaces of annular cap  50 , channels  15  and protrusion  14  to pass through the filter  70 . As the bottom  57  and inner sidewall  54  of the annular cap  50  approach a snugly fit against the vial&#39;s bottom  12  and protrusion  14 , any remaining fluid is forced through channels  15 , through filter  70  and into the plunger cavity or cavities  28 ,  20 . 
     While preferred, it is not believed necessary for both the protrusion  14  and inner sidewall  54  to both have lower portions inclined outward from axis  27 . But is preferred as it is believed that the outward inclination of the protrusion  14  mating sidewall  54  of the annular cap  50  help deform the cap&#39;s outer sidewall  52  outward against the vial&#39;s sidewall  16  and toward the vial&#39;s corner where the vial&#39;s sidewall  16  joins bottom  12  so as to squeeze any remaining fluid into and through the shallow channels  15  through the filter  17 . 
     The downward movement of the plunger  18  and cap  50  are limited by the cap bottom  57  abutting the vial bottom  12 . At about the time the bottom  57  of the cap on the plunger assembly  69  reaches the top of the protrusion  14 , or shortly before, the upper flange  26  on the plunger abuts the wall  16  and advantageously forms a further fluid tight seal. The upper flange  26  is located on the plunger  18  to engage the sidewall  16  when the cap&#39;s bottom  57  on the plunger assembly  69  is adjacent the top of protrusion  14 . Likewise, the diameter of flange  26  is selected relative to the diameter of the wall  16  to form a suitable seal. The engagement of upper flange with the vial sidewall  16  helps stabilize the plunger  18  as it is pushed into the vial, and helps align the bottom of cap  57  with the vial bottom  12 . An optional fill line  84  may be provided on the outer surface of the filter vial  10  to indicate a preferred fluid level for use with the vial. As desired, printed indicia  86 , such as “fill line” may also be provided adjacent the fill line  84  to more clearly identify the purpose of the line. 
     There is thus advantageously provided an annular cap  50  with inner and outer sidewalls  54 ,  52 , respectively. The inner sidewall is preferably outwardly inclined or curved at the bottom of the sidewall. The annular cap  50  is preferably molded of an elastomeric material, and preferably one that is incompressible. The annular cap  50  and inclined sides cooperate with protrusion  14  having an outwardly inclined wall at the bottom  12  of the vial  10 , so as to force the outer wall  52  of the annular cap  50  toward the vial&#39;s sidewall  16  in order to force fluid away from the sidewall and toward the bottom  12  of the vial where it is forced upward past the cap&#39;s inner sidewall  54 , through the through filter  17  and into cavities  30  and/or  28  in the tubular plunger  18 . 
     The plunger  18 , annular cap  50  and vial  10  are preferably molded of a suitable plastic, preferably one that does not contaminate samples placed in the vial  10 . The fluid samples are commonly chemical or biological samples, but any fluid to be separated may be used. The vial and annular cap are preferably made of polyolefin, preferably polypropylene, or other suitable polymer that does not react with the intended fluid to be separated using the vial  10 . The plunger  18  is advantageously made of a stiffer material, or thicker material than the cap  50  so it is sufficiently stiff to allow plunger  18  to be forced into the cap  50  and to further deform the cap  50  into and against the vial&#39;s bottom  12  and protrusion  14 . That stiffness may be achieved by using a different material, or by configuring the plunger  18  to be stiffer, as by a thicker sidewall at the location during use of the sealing surfaces formed by flange  40  and rib  58 . 
     There is also advantageously provided a method for filtering fluid samples using a filter vial. The annular cap  50  is fit onto the end of the tubular plunger  18 , with the cap&#39;s inner wall  54  placing the cavity defined by that inner wall in fluid communication with the internal fluid holding cavity  28 ,  30  of the tubular plunger  18 , and with the cap&#39;s inner wall also clamping the filter  70  against the top wall  34  of the plunger recess  32  in the end of the plunger  18 . Advantageously the step of placing the annular cap  50  on the end of the plunger  18  also includes snap-fitting the cap over a barb or plunger flange  42  on the plunger to releasably fasten the cap to the plunger. The cap  50  forms an annular shaped protrusion on the end of the plunger holding the filter covering the shaped cavity  66  formed by the inner sidewall  54  of the annular cap. Fluid is placed in vial  10 , preferably up to the level indicated by fill line  84 , before or after the cap is placed on the plunger. The bottom of the plunger and cap are then inserted into the opening  80  of the vial and pushed toward the vial bottom  12  (or vice versa), with the bulge in outer cap wall  52  formed by plunger flange  42  and/or the flange  58  on the cap  50  forming a fluid tight seal with the vial wall  16 . Further relative movement of the plunger assembly  69  toward the vial bottom  12  forces fluid through the shaped cavity  66  in the annular cap  50 , through filter  70  and into one or both internal cavities  28 ,  30  of the plunger. As the bottom end  57  of the plunger assembly  69  passes the protrusion  14 , the annular end of the plunger assembly enters the annular bottom of the vial  10 , with the mating shapes forcing fluid upward, past central protrusion  14 , through the shaped cavity  66  and filter  70  and into the cavities  28  and/or  30  in the body of the tubular plunger  18 . The process may include placing a cap on the upper end of the tubular plunger  18 , preferably before affixing the annular cap to the bottom end of the plunger, and preferably before forcing fluid into either fluid holding cavity  28 ,  30  inside the plunger  18 . 
     The above apparatus and process are believed to force much more of the fluid sample through the filter  70  than prior filter vials. When small fluid samples are used the improvement in the volume filtered is especially noticeable. The filter vial  10  is believed especially suitable for fluid samples of about 10 μl, but may be used with larger samples where maximum sample recovery is important. Thus, the fill line  84  is preferably located to contain about 10 micro liters between the fill line and the bottom  12 . The method and apparatus are even more useful when the fluid samples are smaller than 10 μl, such as samples of 1 through ten micro liters in one micro-liter increments. While the working volume of the vial  10  is preferably about 10 micro liters or smaller, the total volume of the vial may be about 30 micro liters or slightly larger. A vial  10  having an internal diameter of about one third inch (8 mm) in diameter and about one inch (25 mm) long with walls about 40 thousandths of an inch (1 mm) thick is believed suitable. While the plunger and vial are especially suited for small fluid samples, the design is applicable to larger parts. The cost of the fluid filtered by the vial, plunger, annular cap and filter can be significant, and even small volumes of fluid may be important and costly. Thus, this disclosure is not limited to fluid samples measured in a few micro liters but may include much larger samples of milliliter volumes and even larger. 
     The plunger  18  is preferably made of an equally strong or stronger and harder material than the annular cap  50 . The thickened end of the plunger  18  formed by the tapered internal fluid holding cavity  30  allows a stronger end. But the fluid holding cavity  30  need not be tapered and could be the same diameter as fluid holding cavity  28  in the upper portion of the plunger. The fluid holding cavity  30  could also have a stepped interior shape with one or more steps that reduce the diameter of the cavity  28  as it approaches end  20 . Regardless of the shape of the cavity  28 , it is preferred that the cavity result in thickened wall of the plunger  18  at the groove  40  and flange  58  which at least partially fits into that groove. A thickened plunger wall provides a strong support to urge the seals formed by flanges  58 ,  42  into sealing contact with the cap  50  and sidewall  16  of vial  10 . Preferably the plunger  16  is of the same material as the annular cap  50 , but the cap&#39;s outer sidewall  52  is made thin enough to flexibly extend over plunger flange  42  and into groove  40 . 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including various ways of shaping the flanges  45 ,  58  and protrusion  14  and shaped cavity  66 . For example, as shown in  FIGS. 7-8 , the protrusion  14  and shaped cavity  66  may have various mating configurations other than the preferred frusto-conical shape shown in  FIGS. 1 ,  5  and  6 , including, but not limited to oval shapes and multisided configurations.  FIGS. 7-8  show protrusions  14  and correspondingly shaped cavities  66  having three sides and four sides respectively. If flat sided protrusions  14  are used the corners of the protrusion and mating cavity  66  in the cap are preferably, but optionally rounded as the curved corners are believed to make the parts easier to make. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.