Patent Publication Number: US-2021178296-A1

Title: Filter device, column, and liquid chromatography device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-227009 filed on Dec. 17, 2019, the disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a filter device, a column, and a liquid chromatography device. 
     Background Art 
     For example, Japanese Patent Application Laid-Open (JP-A) No. H02-262054 describes a high-speed liquid chromatography filter in which a coupling member is fitted over a column body so as to house a column filter in a space in the coupling member. The column filter is configured including an outer filter layer with a large pore size, and an inner filter layer with a small pore size. 
     Filter devices that include plural filters with different pore sizes to each other are employed in order to prevent filter clogging over a long period of time. However, even in cases in which plural filters with different pore sizes are employed, filter clogging may still occur in a short period of time, requiring filter replacement. 
     SUMMARY 
     An aspect of the present disclosure is a filter device that includes: a first filter; a first retaining member that has a hollow shape, the first filter being fitted inside the first retaining member; a second filter that has a smaller pore size than the first filter; a second retaining member that is disposed downstream of the first filter and that has a hollow shape, the second filter being fitted inside the second retaining member; a spacer that has a hollow shape, that is disposed between the first retaining member and the second retaining member, and that maintains a non-contact state between the first filter and the second filter, and that includes a first contact portion contacting the first filter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram illustrating a liquid chromatography device applied with a column provided with a filter device of a first exemplary embodiment; 
         FIG. 2  is a side view illustrating a column provided with a filter device of the first exemplary embodiment; 
         FIG. 3  is an exploded side view illustrating a column provided with a filter device of the first exemplary embodiment; 
         FIG. 4  is a cross-section illustrating an upstream filter structure serving as an example of a filter device of the first exemplary embodiment; 
         FIG. 5  is an enlarged diagram illustrating part of an upstream filter structure serving as an example of a filter device of the first exemplary embodiment; 
         FIG. 6  is a front view illustrating a spacer of a filter device of the first exemplary embodiment; 
         FIG. 7  is a front view illustrating part of a first filter of a filter structure of the first exemplary embodiment; 
         FIG. 8  is a front view illustrating part of a second filter of a filter structure of the first exemplary embodiment; 
         FIG. 9  is a front view illustrating part of a first filter and a second filter of a filter structure in an overlapping state; 
         FIG. 10  is a front view illustrating a first modified example of a spacer of a filter device of the first exemplary embodiment; 
         FIG. 11  is a front view illustrating a second modified example of a spacer of a filter device of the first exemplary embodiment; and 
         FIG. 12  is a front view illustrating a third modified example of a spacer of a filter device of the first exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
     Explanation follows regarding a filter device, a column, and a liquid chromatography device of a first exemplary embodiment, with reference to the drawings. In this explanation, as an example the liquid chromatography device provided with the filter device is employed to measure the concentration of glycated hemoglobin (HbA1c) in whole blood. However, the measurement subject is not limited thereto, and moreover the filter device may be applied to devices other than the liquid chromatography device. 
     As illustrated in  FIG. 1 , a blood collection tube  16  is set in a device body  14  of a liquid chromatography device  12 . The liquid chromatography device  12  is capable of automatically measuring the concentration of glycated hemoglobin (HbA1c) in whole blood. 
     The device body  14  of the liquid chromatography device  12  includes plural (five in the example illustrated in  FIG. 1 ) eluent bottles  18 A,  18 B,  18 C,  18 D,  18 E. The eluent bottles  18 A to  18 E hold eluents A to E to be supplied to an analysis column  30 , described later. Each of the eluents has a different composition, component ratio, acidity level, osmotic pressure, or the like, according to the purpose. 
     The device body  14  further includes a sample preparation unit  20 , an analysis unit  24 , and a light measuring unit  26 . 
     The blood collection tube  16  is retained by the device body  14 . The blood collection tube  16  is moved to a position from which a sample can be taken using a nozzle  22  of the sample preparation unit  20 . 
     The sample preparation unit  20  includes the nozzle  22 , and a dilution tank  28 . The sample preparation unit  20  takes a sample of the blood in the blood collection tube  16  and introduces this blood to the dilution tank  28 . The blood diluted in the dilution tank  28  is then introduced to the analysis column  30 . 
     The nozzle  22  is capable of sucking in and expelling liquid, and may be applied to various liquids, in particular a blood sample from the blood collection tube  16 . The nozzle  22  can use suction to take a sample of the liquid, and can then expel the liquid. 
     The analysis unit  24  includes the analysis column  30 , a manifold  32 , a liquid feeding pump  34 , and an injection valve  36 . 
     As illustrated in  FIG. 2  and  FIG. 3 , the analysis column  30  includes a substantially circular cylinder shaped column body  38 , and an upstream filter structure  40  and a downstream filter structure  42 , respectively provided at the two axial direction (length direction, arrow L 1  direction) ends of the column body  38 . The upstream filter structure  40  is an example of a “filter device”. A sample flows through the inside of the analysis column  30  in the arrow F 1  direction. “Upstream” and “downstream” refer to upstream and whole blood downstream in this flow direction. 
     A filler is retained inside the column body  38  in order to cause selective adsorption of hemoglobin in the sample. For example, a methacrylic acid/methacrylic acid ester copolymer may be employed as the filler. 
     The analysis unit  24  controls adsorption and desorption of biogenic components by the filler in the analysis column  30 , and supplies various biogenic components separated in the analysis column  30  to the light measuring unit  26 . As an example, the analysis unit  24  is set to a temperature in the region of 40° C. 
     The manifold  32  is connected to the eluent bottles  18 A,  18 B,  18 C,  18 D,  18 E via respective tubes  80 A to  80 E, and is connected to the injection valve  36  by a tube  84  with the liquid feeding pump  34  interposed between the manifold  32  and the injection valve  36 . The manifold  32  switches internal valves so as to selectively supply eluent to the analysis column  30  from a specific eluent bottle out of the plural eluent bottles  18 A to  18 E. 
     The liquid feeding pump  34  is provided partway along the tube  84 , and imparts a motive force in order to move the eluent to the injection valve  36 . 
     The injection valve  36  includes plural entry ports and exit ports (not illustrated in the drawings), and is capable of taking an introduction sample of a fixed quantity, and of introducing such an introduction sample into the analysis column  30 . 
     An injection loop  64  is connected to the injection valve  36 . The injection loop  64  is capable of retaining a fixed quantity (for example several μL) of liquid. Switching the injection valve  36  as appropriate enables selection of either a state in which the injection loop  64  is in communication with the dilution tank  28  and the introduction sample is supplied from the dilution tank  28  to the injection loop  64 , or a state in which the injection loop  64  is in communication with the analysis column  30  via a tube  85  and the introduction sample is introduced to the analysis column  30  from the injection loop  64 . For example, a six-way valve may be employed as the injection valve  36 . 
     The light measuring unit  26  is connected to a waste liquid tank  88  via, a tube  87 , Liquid discharged from the analysis column  30  is discarded in the waste liquid tank  88 . The light measuring unit  26  optically detects the hemoglobin contained in the eluent that has passed through the analysis column  30 . 
     The upstream filter structure  40  of the analysis column  30  includes a first retaining member  44  and a second retaining member  46 , as illustrated in  FIG. 3  and  FIG. 4 . The first retaining member  44  is a hollow ring-shaped member, and a first filter  48  is retained in a hollow portion at the radial direction inside of the first retaining member  44 . The second retaining member  46  is a hollow ring-shaped member that has substantially the same external diameter, internal diameter, and thickness (liquid feed direction length) as the first retaining member  44 . A second filter  50  is retained in a hollow portion at the radial direction inside of the second retaining member  46 . The first filter  48  is disposed upstream of the second filter  50 , Namely, the first filter  48  is disposed closer to the inlet of the analysis column  30 . As an example, the first filter  48  and the second filter  50  are configured by membrane filters or sintered filters configured to trap particles on their filter surfaces. Note that the first retaining member  44  and the second retaining member  46  do not necessarily have the same thickness as each other. 
     As described above, in the present exemplary embodiment, the first retaining member  44  and the second retaining member  46  have substantially the same external diameter R 1 , and substantially the same internal diameter R 2  and thickness. The first retaining member  44  and the second retaining member  46  are housed snugly in internal spaces of caps  74 ,  76 , described later. Moreover, in the present exemplary embodiment, the first retaining member  44  and the second retaining member  46  are both made from resin. 
     As illustrated in  FIG. 5 , the first filter  48  is formed with numerous percolation holes  52 , each having a specific pore size D. The second filter  50  is formed with numerous percolation holes  54 , each having a pore size E that is smaller than that of the percolation holes  52  in the first filter  48 . Note that although the pore size D and the pore size E are respectively illustrated as uniform diameters in  FIG. 5 , in reality variation within a specific range may be present in the pore size D and the pore size E of the first filter  48  and the second filter  50 . However, the pore size E of the percolation holes  54  in the second filter  50  is smaller than the particle size of the filler in the column body  38 . Liquid introduced to the analysis column  30  by the pump passes through the first filter  48 , and then passes through the second filter  50 . As will be described later, in the present exemplary embodiment, a filtration subject flows from the first filter  48  that has the larger pore size to the second filter  50  that has the smaller pore size. Namely, the second filter  50  that has the smaller pore size is a filter disposed downstream of the first filter  48  that has the larger pore size. 
     In the present exemplary embodiment, the first filter  48  is configured by a porous resin (sintered resin product), and more specifically, is made of polyethylene. The resin first filter  48  is thus fitted inside and retained by friction in the hollow portion at the radial direction inside of the first retaining member  44  that is also made of resin. The outer side of the first filter  48  is surrounded by the first retaining member  44 . The thickness of the first filter  48  is the same as the thickness of the first retaining member  44 . 
     In the present exemplary embodiment, the second filter  50  is also configured by a porous resin (sintered resin product), and more specifically, is made of polyether ether ketone. The resin second filter  50  is thus fitted inside and retained by friction in the hollow portion at the radial direction inside of the second retaining member  46  that is also made of resin. The thickness of the second filter  50  is the same as the thickness of the second retaining member  46 . 
     A spacer  56  is disposed between the first retaining member  44  and the second retaining member  46 . The spacer  56  is a hollow ring-shaped member that has a specific thickness T (described in detail later). The spacer  56  contacts both the first retaining member  44  and the second retaining member  46 . In the present exemplary embodiment, the spacer  56  is made of resin. 
     An external diameter R 3  of the spacer  56  is substantially the same as the external diameter R 1  of the first retaining member  44  and the external diameter R 1  of the second retaining member  46 . However, an internal diameter R 4  of the spacer  56  is smaller than the internal diameter R 2  of the first retaining member  44 . A radial direction inside portion of the spacer  56 , more specifically a portion of the spacer  56  that is positioned further toward the radial direction inside than the internal diameter R 2  of the first retaining member  44 , is positioned further toward the inside than an inner ring edge (a radial direction inside edge of the ring shaped portion) of the first retaining member  44 , and is in contact with a downstream end face of the first filter  48 . 
     As is also illustrated in  FIG. 6 , in the first exemplary embodiment, the spacer  56  includes a first contact portion  58  that contacts the first filter  48  around the entire inner circumference of the spacer  56 . In other words, the internal diameter R 4  of the spacer  56  is smaller than the internal diameter R 2  of the first retaining member  44 , and thus the first contact portion  58  is a ring shaped portion including the inner ring edge of the spacer  56  (radial direction inside edge of the ring shaped portion) and serves as an example of a contact tab. In the present exemplary embodiment, the first contact portion  58  also contacts an upstream end face of the second filter  50 . 
     As illustrated in  FIG. 3 , the downstream filter structure  42  includes a ring shaped downstream retaining member  60 , and a downstream filter  62  is retained clamped between the column body  38  and the downstream retaining member  60 . Numerous percolation holes formed in the downstream filter  62  with a specific pore size have a pore size smaller than the particle size of the filler, such that the downstream filter  62  prevents the filler inside the column body  38  from being discharged to the exterior. 
     A third filter  66  is disposed further upstream than the upstream filter structure  40 . The third filter  66  is a depth filter in which a fibrous material is interwoven so as to form a filter structure, and traps foreign material in the filtration subject not only at the filter surface but also in tortuous internal passages. This discourages clogging even if the filtration precision has dropped. The pore size (mesh size) of the third filter  66  is larger than the pore size of the first filter  48 , and the thickness (liquid feed direction length) of the third filter  66  is thinner than that of the first filter  48  and the second filter  50 . As an example, the first filter  48  and the second filter  50  each have a thickness of from 0.5 mm to 4 mm, and the third filter 66 has a thickness of from 0.05 mm to 0.3 mm. Since the third filter  66  is thinner than the first filter  48 , spreading out that occurs as the eluent and sample flows through a space inside the third filter  66  can be kept to a minimum. 
     As illustrated in  FIG. 4 , the third filter  66  is attached to the first retaining member  44 , and contacts the first filter  48  on the upstream side of the first filter  48 . In other words, the first filter  48  configures a structure downstream of the third filter  66  that supports the third filter  66  by contacting the entire downstream face of the third filter  66 . Namely, a filtration face of the third filter  66  is in contact with the first filter  48 . In the present exemplary embodiment, the third filter  66  is a filter configured by a resin nonwoven fabric. Although it is difficult to fix the filter edge of a thin filter using a retaining member or the like without causing a reduction in the effective filtration surface area, in the present exemplary embodiment, the downstream face of the third filter  66  and the first filter  48  are in contact with one another, such that the third filter  66  is pressed against the first filter  48  by the flow of liquid, thus fixing the third filter  66 . Since the third filter  66  is a depth filter with large pores, the likelihood of blocked pores arising in the first filter  48  at a contact location between the third filter  66  and the first filter  48  is low, and clogging therefore does not occur. 
     The caps  74 ,  76  are respectively mounted to the upstream side and downstream side of the column body  38  by being screwed into place. The upstream filter structure  40  is retained gripped between the column body  38  and the cap  74 , and the downstream filter structure  42  is retained gripped between the column body  38  and the cap  76 . The downstream face of the second filter  50  is in contact with the filler retained in the column body  38 . Each of the caps  74 ,  76  is formed with a flow path  78  through which the sample flows. The column body  38  and the respective filters are integrated together in order to form the analysis column  30 . This enables effort required when replacing the column body  38  and the respective filters to be alleviated. 
     Explanation follows regarding operation of the present exemplary embodiment. 
     In the liquid chromatography device  12  illustrated in  FIG. 1 , the nozzle  22  is used to take a blood sample from the blood collection tube  16 , and this blood sample is supplied to the dilution tank  28 . A diluent solution is also supplied to the dilution tank  28  from a preparation liquid tank (not illustrated in the drawings) in order to prepare an introduction sample in the dilution tank  28 . 
     The introduction sample prepared in the dilution tank  28  is supplied into and retained in the injection loop  64 . The injection valve  36  is then switched so as to introduce the sample retained in the injection loop  64  into the analysis column  30 . When the sample is introduced into the analysis column  30 , components including sA1c, HbA0, and mutant Hb are adsorbed by the filler. The injection valve  36  is then switched as appropriate so as to sequentially supply the eluents A to E into the analysis column  30  according to a predetermined control sequence. 
     The eluent containing the various types of separated hemoglobin is then discharged from the analysis column  30 . The eluent is supplied via a tube  86  to a light measuring cell of the light measuring unit  26 , and is then guided via the tube  87  to the waste liquid tank  88 . 
     In the light measuring unit  26 , light from a light source is shone consecutively on the eluent. Transmitted light that has passed through the eluent is split using a beam splitter and picked up by light receiving elements. A chromatogram is computed and obtained by a control section of the light measuring unit  26  based on the light reception results of the light receiving elements. 
     The upstream filter structure  40  of the analysis column  30  of the present exemplary embodiment includes the third filter  66 , the first filter  48 , and the second filter  50  in sequence from the liquid feed direction upstream side. The filter pore size of these filters decreases on in sequence from the upstream side toward the downstream side. Accordingly, larger sized foreign material is removed in sequence from the upstream side. 
     The upstream side of the column body  38  is contacted by the second filter  50 , thus suppressing the filler in the column body  38  from escaping (leaking) upstream. 
     The spacer  56  is disposed between the first retaining member  44  that retains the first filter  48  and the second retaining member  46  that retains the second filter  50 . The spacer  56  creates a non-contact state between the first filter  48  and the second filter  50 , such that a space is created between the first filter  48  and the second filter  50 . 
     Note that  FIG. 7  schematically illustrates the percolation holes  52  of the first filter  48 . Similarly,  FIG. 8  schematically illustrates the percolation holes  54  of the second filter  50 . Note that although the shapes of the percolation holes  52 ,  54  are approximated to true circles in  FIG. 7 ,  FIG. 8 , and  FIG. 9 , described later, in reality elliptical holes or polygonal holes may also be present. 
     At a boundary location between the first filter  48  and the second filter  50 , the percolation holes  52  of the first filter  48  illustrated in  FIG. 7  and the percolation holes  54  of the second filter  50  illustrated in  FIG. 8  would overlap each other. As illustrated in  FIG. 9 , such overlapping of the percolation holes  52  and the percolation holes  54  would form fine holes  68  each having a smaller pore size than the pore size of the percolation holes  54 . Since foreign material would pass through the fine holes  68  less readily than through the percolation holes  54  due to the smaller opening area, clogging would be liable to occur at the boundary location between the first filter  48  and the second filter  50 . Such clogging would reduce the effective filtration surface area of the filters, which would for example lead to a drop in flow speed and an increase in the pressure of the sample being filtered. 
     To address this, in the present exemplary embodiment, the spacer  56  is used to create the non-contact state between the first filter  48  and the second filter  50 . Since there is no boundary location between the first filter  48  and the second filter  50 , locations (the fine holes  68  illustrated in  FIG. 9 ) having a smaller opening area than the percolation holes  54  do not arise. Accordingly, the present exemplary embodiment is capable of suppressing clogging of the first filter  48  and the second filter  50 . Suppressing such clogging enables a drop in the filtration performance of the filter device to be suppressed, and moreover enables increases in the lifespans of the first filter  48  and the second filter  50 . 
     Moreover, in the upstream filter structure  40  of the present exemplary embodiment, the spacer  56  is provided with the first contact portion  58 . The first contact portion  58  contacts the downstream face of the first filter  48 . Accordingly, even if the first filter  48  is pushed toward the downstream side by the flow of the sample being filtered in the arrow F 1  direction (see  FIG. 4 ), the first filter  48  is suppressed from slipping or detaching from the first retaining member  44 , enabling the spacing between the first filter  48  and the second filter  50  to be maintained. For example, the first filter  48  can be suppressed from moving downstream and contacting the second filter  50 . Maintaining the spacing between the first filter  48  and the second filter  50  achieves such effects as suppressing a reduction in the filtration performance of the filter device, and increasing the lifespans of the first filter  48  and the second filter  50 . Moreover, a second contact portion (not illustrated in the drawings) is provided on the opposite face to the first contact portion  58 , and the second contact portion contacts the upstream face of the second filter  50 . Although the second contact portion may be omitted since the spacer  56  is upstream of the second filter  50 , such a second contact portion is capable of preventing the second filter  50  from slipping or detaching from the second retaining member  46 . 
     As illustrated in  FIG. 4 , the upstream filter structure  40  of the present exemplary embodiment includes the third filter  66  that is provided further upstream than the first filter  48 . The third filter  66  covers the first filter  48 , and the pore size (mesh size) of the third filter  66  is larger than the pore size D of the percolation holes  52  of the first filter  48 . This enables foreign material with a relatively large particle size to be removed from the sample being filtered first, by the third filter  66 . Foreign material with a smaller particle size can then removed by the first filter  48 , and foreign material with an even smaller particle size can then removed by the second filter  50 . 
     The third filter  66  is further upstream than the first filter  48 , and contacts the first filter  48 . The first filter  48  therefore supports the third filter  66 , enabling the third filter  66  to be suppressed from slipping toward the downstream side or deforming due to the flow of the sample being filtered in the arrow F 1  direction. For example, since the third filter  66  is configured from nonwoven fabric, the third filter  66  flexes easily. However, since the third filter  66  contacts and is supported by the filtration face of the first filter  48 , flexing of the third filter  66  is suppressed. 
     Note that the thickness T of the spacer  56  is not limited, as long as the thickness T is sufficient to place the first filter  48  and the second filter  50  in the non-contact state as described above. However, a lower limit for the thickness T of the spacer  56  is preferably set with respect to the pore size D (mm) of the percolation holes  52  of the first filter  48  so as to satisfy the relationship D×10≤T. So doing achieves a sufficient spacing between the first filter  48  and the second filter  50 . The resulting gap enables, for example, a space in which the filtration subject that has passed through the first filter  48  can flow to be secured. This configuration is preferable since any obstruction to the flow of eluent can be suppressed even if foreign material contained in the filtration subject is trapped at the surface of the second filter  50 . 
     Moreover, an upper limit of the thickness T of the spacer  56  is preferably set so as to satisfy the relationship T≤0.2 mm. With increasing flow path length to the column body  38 , the sample spreads out more before reaching the column body  38 , resulting in decreased separation precision. However, setting the thickness T of the spacer  56  within the above range enables good separation precision to be maintained. Moreover, in the present exemplary embodiment, since the upstream filter structure  40  is employed to remove foreign material from the sample flowing into the column body  38 . an increase in pressure in the column body  38  can be prevented, enabling a decrease in separation precision to be suppressed. 
     The foregoing explanation describes the first contact portion  58 , formed in a ring shape at a radial direction inside portion of the spacer  56  by setting the internal diameter R 4  of the spacer  56  smaller than the internal diameter R 2  of the first retaining member  44 , as an example of the first contact portion provided to the spacer  56 . However, since it is sufficient that the first contact portion  58  contact the first filter  48 , the first contact portion  58  is not limited to the shape and placement described above. For example, the first contact portion  58  may be configured by protruding tabs as illustrated in  FIG. 10  to  FIG. 12 . 
     In a first modified example illustrated in  FIG. 10 , protruding tabs  70  are formed protruding from the inner circumference of the spacer  56  toward the radial direction inside at two opposing locations. In the first modified example, the internal diameter R 4  of the spacer  56  is equal to the internal diameter R 2  of the first retaining member  44 . Accordingly, the first contact portion can be formed with a simple structure by the protruding tabs  70  protruding from the inner circumference of the spacer  56  in this manner. 
     In a second modified example illustrated in  FIG. 11 , the protruding tabs  70  of the first modified example illustrated in  FIG. 10  protrude from the inner circumference of the spacer  56  toward the radial direction inside at four locations. In the second modified example illustrated in  FIG. 11 , since the number of the protruding tabs  70  is greater, the effect of suppressing slipping and detachment of the first filter  48  is greater than that of the first modified example illustrated in  FIG. 10 . On the other hand, in the first modified example illustrated in  FIG. 10 , since the number of the protruding tabs  70  is fewer than in the second modified example illustrated in  FIG. 11 , a smaller overall area of the percolation holes of the first filter  48  is blocked by the protruding tabs  70 , thus enabling a greater flow path area to be secured in practice. Even the second modified example illustrated in  FIG. 11  in which four of the protruding tabs  70  are formed is capable of securing a greater flow path area than the first contact portion  58  having the shape illustrated in  FIG. 6 . 
     In a third modified example illustrated in  FIG. 12 , a bridging tab  72  is provided spanning across the diameter of the inner circumference of the spacer  56 . In other words, the bridging tab  72  has a shape achieved by extending the two protruding tabs  70  of the first modified example illustrated in  FIG. 10  in their protruding directions until they join up. The flow speed of the filtration subject flowing through the radial direction inside of the first retaining member  44  is faster the closer it is to a radial direction central portion thereof. The bridging tab  72  of the third modified example illustrated in  FIG. 12  also contacts the first filter  48  at this central portion, and is thus highly effective in suppressing slipping and detachment of the first filter  48 . 
     By contrast, in the first contact portion  58  in the example illustrated in  FIG. 6 , the first contact portion  58  contacts the first filter  48  around its entire circumference, and is thus capable of suppressing slipping and detachment of the first filter  48  around its entire circumference. Since the first contact portion  58  can be formed simply by reducing the internal diameter R 4  of the spacer  56 , there is no need to form the protruding tabs  70  or the bridging tab  72  and the shape remains simple. Note that the modified examples of the first contact portion may similarly be adopted for the second contact portion. The first contact portion and the second contact portion do not have to have the same shape as each other. For example, the first contact portion  58  may have the shape described in the exemplary embodiment illustrated in  FIG. 6 , whereas the second contact portion may have the shape of the first modified example illustrated in  FIG. 10 . 
     The materials employed for the first retaining member  44 . the second retaining member  46 , the first filter  48 , and the second filter  50  are not limited to the resins described above, and for example some or all of these components may be configured from metal. When fitting the resin first filter  48  into the resin first retaining member  44 , the press fitting may incur some slight deformation (tightening) in order to achieve retention. In such cases, the first retaining member  44  and the first filter  48  responds to the deformation of the other thereof, enabling a seal to be formed at a tight fitting portion between the first retaining member  44  and the first filter  48 . Due to being made of resin, the second retaining member  46  and the second filter  50  have a similar relationship, such that the second retaining member  46  and the second filter  50  responds to the deformation of the other thereof, enabling a seal to be formed at a tight fitting portion between the two. 
     In the exemplary embodiment described above, the first retaining member  44  and the second retaining member  46  are both hollow ring-shaped members. Instead of a circular outer circumference and a circular inner circumference, these hollow members may each have an elliptical or polygonal outer circumference and inner circumference as viewed along the arrow F 1  direction. Alternatively, these hollow members may each have a combination of a circular and a polygonal shape, such as a polygonal shaped outer side and a circular inner side. Namely, as long as they have a structure with a closed curved shape as viewed along the arrow F 1  direction, the first retaining member  44  and the second retaining member  46  are capable of respectively retaining the first filter  48  and the second filter  50  at the inside thereof.