Abstract:
A plug unit and system for connecting capillary tubes, especially for high-performance liquid chromatography, with a plug capillary tube projecting through a hole of a plug housing, which is detachably connectable to a bushing unit. The plug capillary tube front end projects into a capillary tube receptacle in the bushing unit with its end face essentially aligned opposite a front end of a bushing capillary tube or a bushing capillary tube opening of the bushing unit, the end face of which is butted against. The plug housing applies a force, with its end face facing the plug capillary tube end, directly or indirectly on an annular sealing element surrounding the plug capillary tube in the region of the front end of the plug capillary tube such that the front end of the plug capillary tube is sealed through deformation of the sealing element against the capillary tube receptacle opening.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation under 35 U.S.C. §120 and claims the priority benefit of co-pending U.S. application Ser. No. No. 13/132,578, filed Jul. 7, 2011, entitled “PLUG UNIT AND CONNECTION SYSTEM FOR CONNECTING CAPILLARY TUBES, ESPECIALLY FOR HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY,” which is the United States National Stage Application, under 35 U.S.C. §371, of International Application PCT/DE2009/001688, filed Nov. 27, 2009, which claims the priority benefit to German Patent Application No. 10 2008 059 897.6, filed Dec. 2, 2008, which applications are hereby incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field of the Invention 
         [0003]    The invention relates to a plug unit for connecting capillary tubes, especially for high-performance liquid chromatography. In addition, the invention relates to a connection system consisting of a bushing unit and such a plug unit. 
         [0004]    2. Description of Related Art 
         [0005]    In chromatography systems, liquids or gases are fed through suitable connection lines between the components of the relevant system. These connection lines, which can be made, for example, from stainless steel, have suitable connection systems, also called fittings, available at their ends to enable creating a tight connection with the connectors of the components. 
         [0006]    Such a connection system has already been described in 1975 in U.S. Pat. No. 3,880,452. A capillary tube forming the connection line between two components is here pushed into the capillary tube receptacle opening of a bushing unit or connection unit and fixed in the bushing by means of a threaded fastener that has a central hole for guiding the capillary tube. For sealing, one or more sealing elements that surround the capillary tube in their front end region are pressed into the capillary tube receptacle opening, which runs conically inward, by means of the threaded fastener when the capillary tube and bushing unit are connected. 
         [0007]    However, it is disadvantageous that the sealing position is not realized in the plane of the end surface perpendicular to the longitudinal axis of the capillary tube, but is instead offset rearward a certain distance from the end face in the axial direction. In this way, a dead volume is produced that has a disadvantageous effect, especially in high-performance liquid chromatography. In order to be able to guarantee the tightness of such connections for the extremely high pressures used in high-performance liquid chromatography, sealing elements are often used like those described, for example, in U.S. Pat. No. 4,619,473 as prior art ( FIG. 2 ). That publication involves annular sealing elements that are, for the most part, likewise made from stainless steel and that have, in longitudinal section, a conical profile of the outer diameter. Such a sealing element interacts with a conical receptacle opening in the bushing unit, wherein the conical receptacle opening has a larger angle than the sealing element relative to the longitudinal axis of the capillary tube. Here, when the sealing element is pressed into the receptacle opening, an extremely high, radially inward pressure is exerted by means of a threaded fastener onto the front region of the sealing element, so that the sealing position is realized. Through this pressure, however, a deformation of the sealing element and the capillary tube is usually generated, wherein the sealing element is pressed with its annular front edge into the outer periphery of the capillary tube. Such a deformation is undesired, especially because the sealing element is connected to the capillary tube here with a positive interference fit and non-positive-interference-fit connection and the sealing element can no longer be slipped onto the capillary tube easily in the axial direction. If the sealing connection is released and should such a plug element be screwed into another bushing unit, for example because a component of the chromatography system must be replaced, then a tight connection can indeed be produced, but due to tolerances or manufacture-dependent differences in the depth of the receptacle opening, it can no longer be guaranteed that the capillary tube will again apply a force with its end face onto the end face of the line to which it should connect. If the receptacle opening of the bushing unit of the exchanged component is longer in the axial direction than the previously used component, then an undesired dead volume is created. If the receptacle opening for the capillary tube in the exchanged component is shorter in the axial direction than the previously used component, then the capillary tube is actually deformed, possibly damaged, by the pressure of the threaded fastener, and a tight connection is no longer possible under some circumstances. This is because the sealing element mounted on the capillary tube with a positive interference fit and non-positive-interference-fit connection cannot move in the axial direction. 
         [0008]    In the case of such a fitting, however, a small dead volume also can be rarely avoided if the end faces of the capillary tube and the line to be connected are directly opposite each other or contact each other, because the sealing position is not located in the region of the end face of the capillary tube or the line to be connected. 
         [0009]    In order to be able to compensate for such tolerances or in order to be able to use connection systems of different manufacturers with one and the same plug unit, a self-adjusting plug for high-performance liquid chromatography is described in U.S. Pat. No. 6,494,500, in which the capillary tube is biased axially toward the capillary tube receptacle openings of the bushing unit by means of a spring provided in the threaded fastener. For the seal, however, a replaceable ferrule is used that again has a conical construction in its front region, and to form the seal interacts with an inner wall of the ferrule receptacle opening that has a more pronounced conical construction. Here there is again the risk that the ferrule is “crimped tight” onto the capillary tube, especially when the sealing element is made from a metal, for example, stainless steel. 
         [0010]    In addition, in this plug unit it is disadvantageous that a support element for the helical spring must be mounted on the capillary tube, wherein this support element makes the production of such a plug unit more complicated. 
         [0011]    For avoiding a dead volume, a unit is known from U.S. Pat. No. 4,083,702 for connecting capillary tubes for gas chromatography in which the capillary tubes are similarly connected butted to each other with their end faces. The capillary tubes are fixed by means of ring elements that have a wedge shape in the longitudinal section and that interact with corresponding, conical recesses in the connector housing. In this case, however, the seal is simpler relative to a connection that is compatible with high-performance liquid chromatography, since the pressures used for gas chromatography are significantly lower, for example, up to 6 bar. 
         [0012]    In addition to the already mentioned disadvantages, the known connection systems feature the disadvantage that the risk arises that the sealing element will remain in place in the relevant receptacle opening of the bushing unit when the plug unit is removed and the capillary tube is pulled out from the bushing unit. 
         [0013]    In addition, the trend in high-performance liquid chromatography is toward using thinner capillary tubes, since these can be more easily bent to better adapt them to the existing installation conditions. Due to the production process, thinner capillary tubes also feature smaller tolerances in the inner diameter and smaller eccentricity of the inner diameter. To allow using capillary tubes with smaller outer diameters in connection with conventional bushing units or connection units, the tubes are provided at the ends with a sleeve for adapting the outer diameter to the diameter of the thicker, conventional capillary tubes. This represents an additional expense, however, and increases the dead volume of a connection. 
       SUMMARY OF THE INVENTION 
       [0014]    The invention provides a plug unit for connecting capillary tubes, especially for high-performance liquid chromatography, which can be used in a simple way for capillary tubes with different outer diameters in connection with bushing units with conventional diameters of receptacle openings. In addition, the present invention prevents the problem that, when the plug unit is removed, the sealing element detaches from the capillary tube and remains in the bushing unit. In addition, the invention provides a connection system with such a plug unit. 
         [0015]    In accordance with the present invention, in a simple way, contact of the end face of the capillary tube against the end face of a capillary tube to be connected or against the end face of an opening of a fluid channel in a bushing unit can be guaranteed in that the capillary tube has on its front end, a force-receiving region projecting in the radial direction past the inner diameter of the annular sealing element. In this manner, the present invention ensures that, for the connection of the plug housing with the bushing unit, the plug housing applies a force with its end face on the sealing element and the sealing element consequently applies a force on the force-receiving region of the plug capillary tube with an axial contact force. In addition, the sealing element has a construction that can be deformed sufficiently elastically or plastically to simultaneously guarantee sealing of the plug capillary tube relative to the bushing unit or the capillary tube receptacle opening. The sealing is performed in the region of the radially outward extending force-receiving region of the plug capillary tube, so that a dead volume is avoided. 
         [0016]    The force-receiving region advantageously has its greatest diameter in the plane of the end face of the plug capillary tube or in its direct vicinity, in order to prevent material of the sealing element from being pressed into an annular gap between the end face of the plug capillary tube and the end face of the bushing capillary tube or the floor of the receptacle opening in the bushing unit in which the bushing capillary tube opening is provided. 
         [0017]    The thicker section provided at the front end of the plug capillary tube in the form of the force-receiving region guarantees that when the plug capillary tube is pulled out from the capillary tube receptacle opening when a connection system is disassembled, the sealing element is also removed. A later removal of the seal from the receptacle opening, possibly with a special extraction tool, is avoided. 
         [0018]    In order to be able to use (thinner) capillary tubes with different thicknesses together with existing bushing units that were originally provided for thicker capillary tubes, only one suitable sealing element need be used with a suitable inner and outer diameter, as well as a plug housing with a matching axial hole for receiving the capillary tube. The additional use of adapter sleeves or the like is not required. 
         [0019]    In addition, a plug unit constructed in this way according to the invention could also be used for conventional, known bushing units with different, manufacturer-dependent capillary tube receptacle opening hole depths. In such embodiments, only the plug housing must be provided with connection means that allow the secure connection in any position within a correspondingly large axial tolerance range. This can also be realized in a simple way by a thread, in particular, a fine thread. Obviously, the thread must be selected to match the bushing unit. For bushing units of known plug units, a UFN10-32 thread is usually used, however only thread that satisfies this condition or is suitable for this purpose. 
         [0020]    For the use of a fine thread, that is, a thread with a very low pitch, it is also ensured that a high contact pressure could be exerted on the sealing element, in order to guarantee the required tightness even for the extremely high pressures used in high-performance liquid chromatography. The invention uses the fact that the system pressure active surface area, and thus also the force needed for sealing, is especially small when sealing at the base of the hole of the bushing unit. 
         [0021]    According to one embodiment of the invention, the force-receiving region of the plug capillary tube can be constructed rotationally symmetric to the longitudinal axis and can advantageously have an outer diameter increasing continuously toward the end face of the plug capillary tube. For example, a trumpet-shaped or conical structure of a force-receiving region could be produced through simple compression of the front region of a capillary tube. A corresponding tool, however, should have a pin that engages in the channel opening of the capillary tube during the compression process, in order to maintain the inner diameter at the original value. 
         [0022]    According to another embodiment, the force-receiving region of the plug capillary tube could have arms extending radially outward. 
         [0023]    The sealing element advantageously has a hole for receiving the plug capillary tube that corresponds, in the assembled state, essentially to the outer diameter of the plug capillary tube. Advantageously, the sealing element could have an elastic construction, so that it surrounds the outer periphery of the plug capillary tube with a biasing tension directed radially inward in the assembled state. 
         [0024]    To simplify applying it to the plug capillary tube, the sealing element could be constructed as a slotted ring element that has elasticity sufficient to allow it to be either pushed from the front onto the plug capillary tube or placed from the side onto the plug capillary tube. 
         [0025]    The outer diameter of the sealing element advantageously is formed to be larger than the outer diameter of the force-receiving region of the plug capillary tube. In this manner, it can be guaranteed that a sealing effect is also realized in a region radially outside of the end face of the plug capillary tube, for example, by pressing the sealing element onto the floor of a corresponding receptacle opening in the bushing unit in a region radially outside of the end face of the plug capillary tube. 
         [0026]    In such embodiments, the sealing element could be constructed so that it surrounds, with a positive interference fit, the force-receiving region of the plug capillary tube with its front region in the non-pressurized or no-tension state. The front end face of the sealing element, however, should not project past the end face of the plug capillary tube in the axial direction, in order to guarantee butted pressing of the end face of the plug capillary tube onto an opposite end face, and thus to avoid dead volume. 
         [0027]    A pressure piece that has an axial hole in which the plug capillary tube is received could also be provided in the axial direction between the end face of the plug housing or a corresponding part of the plug housing and the sealing element. In such embodiments, the hole has an inner diameter that is only slightly greater than the outer diameter of the capillary tube, at least in a front region of the pressure piece. In this way it is avoided that, in the case of a high pressure being exerted by the pressure piece onto the sealing element, material of the sealing element enters into the annular gap between the outer diameter of the plug capillary and the inner diameter of the axial hole of the pressure piece. The use of a pressure piece offers the advantage that there is no friction between the sealing element and the pressure piece if a threaded connection is used between the plug housing and the bushing unit, and consequently damage to or negative effects on the sealing element during the assembly process are avoided. In addition, the use of a pressure piece allows the use of a uniform plug housing for different capillary tube diameters, wherein in this case only the pressure piece, which is simple to produce, must be adapted to the outer diameter of the capillary tube. 
         [0028]    The invention will be explained in greater detail below with reference to the embodiments shown in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  shows a diagram of the components of a first embodiment of a connection system according to the invention, in a longitudinal section. 
           [0030]      FIG. 2  shows a diagram of the connection system according to  FIG. 1 , in the assembled state. 
           [0031]      FIG. 3  shows a diagram of another embodiment of a connection system according to the invention with a plug capillary tube consisting of two jacket layers, in the assembled state. 
           [0032]      FIG. 4  shows an oblique diagram of a slotted plug unit of another embodiment of a connection system that is arranged, together with a pressure piece and a sealing element, on a plug capillary tube for assembly with a bushing unit. 
           [0033]      FIG. 5  shows an embodiment of a connection system in which the plug unit is expanded by a housing with a two-part construction in which a filter is provided. 
           [0034]      FIG. 6  shows a side view schematic diagram of a slotted annular element embodiment of sealing element  40  according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The connection system  1  shown in the disassembled state in  FIG. 1  comprises a bushing unit  5 , which is shown only schematically with respect to its outer contours that are not relevant to the invention, and which can be arranged, for example, on a component of a system for high-performance liquid chromatography, such as, for example, on a separating column. In addition, the connection system  1  comprises a plug unit  3 . For its part, the plug unit  3  comprises, a plug capillary tube  10  that can be made, for example, from stainless steel, a plug housing  20  depicted in the form of a screw, a pressure piece  30  that, just like the plug housing  20 , can be made from stainless steel or another metal, and a sealing element  40  that can be made, for example, from a material, such as, a suitable plastic, that can deform sufficiently elastically and/or plastically. 
         [0036]    The screw-shaped plug housing  20  has a thickened head part that is provided with knurling  21  on the outer periphery and that is adjacent to the threaded part provided with a thread  22 . The plug housing  20  has a central hole with a step-like profile in the embodiment shown, wherein the inner diameter of the hole in the front region of the threaded part is only slightly larger than the outer diameter of the plug capillary tube  10  held in the central hole. The plug capillary tube  10  passes with its front end through the central hole of the plug housing  20 , wherein the pressure piece  30  is arranged, radially around the plug capillary tube  10 , in front of the plug housing  20  toward the end of the plug capillary tube  10 . In its rear region, the outer diameter of the rotationally symmetric pressure piece  30  corresponds to the inner diameter of a receptacle opening  53  of the bushing unit  5  in its rear region facing the plug unit  3 . The outer diameter of pressure piece  30  must be somewhat smaller than the inner diameter of the receptacle opening  53 , because otherwise the pressure piece may become jammed in the thread  52 . The outer diameter of pressure piece  30  should be as large as possible, however, so that the pressure piece is oriented in the axial direction on the plug housing as much as possible, and the hole of the capillary tube  10  and the opening of a bushing capillary tube  57  are aligned as well as possible, in order not to interfere with the flow. The outer diameter of the front region of the pressure piece  30  corresponds to the inner diameter of the receptacle opening  53  in its inward directed, cylindrical end region  56  that is closed off by the bottom wall  51  into which opens the bushing capillary tube opening  55  of the bushing capillary tube  57 . This outer diameter likewise must be somewhat smaller than the inner diameter of the cylindrical end region  56  of the receptacle opening  53 , so that the assembly and disassembly are not hindered. 
         [0037]    The sealing element  40 , which has an annular construction, is arranged on the capillary tube in front of the pressure piece  30  toward the end of this plug capillary tube  10 . 
         [0038]    The end of the plug capillary tube  10  has a constantly increasing outer diameter by means of which a force-receiving region  11  is formed. 
         [0039]    The maximum outer diameter of the force-receiving region  11  is less than the inner diameter of the receptacle opening  53  in its inner, cylindrical end region  56  that is delimited by the bottom wall  51 . The outer diameter of the sealing element  40  essentially corresponds to the inner diameter of this cylindrical, inner region of the receptacle opening  53 . 
         [0040]    In its front region facing the force-receiving region  11 , the inner diameter of the sealing element  40  is formed essentially complementary to the contours of the outer diameter of the force-receiving region  11 , so that the sealing element  40  can be pushed onto the force-receiving region  11  without a large pressure or without a large axial force. The contour of the inner diameter in the front region of the sealing element  40  is, however, advantageously constructed so that the pressureless sliding onto the force-receiving region  11  can take place only up to an axial position in which the end face of the sealing element  40  is still set back by a certain amount relative to the end face of the force-receiving region  11  or plug capillary tube  10 . 
         [0041]    For assembly of the plug unit  3 , the plug housing  20 , pressure piece  30 , and sealing element  40  can be slid from the front onto the plug capillary tube  10 , which in this case still has no force-receiving region  11  enlarged in the radial direction. The force-receiving region  11  can then be produced with a suitable tool, for example, by compression or the like. 
         [0042]    As far as this is possible, the force-receiving region  11  of the plug capillary tube  10  could also be produced in advance, and in this case, the remaining components of the plug unit  3  could be slid onto the capillary tube from the other end of the plug capillary tube  10 . 
         [0043]    The sealing element  40  could also be constructed as an annular element slotted in the axial direction, such as the sealing element  40  shown in  FIG. 6 , having slot  41 , so that the sealing element  40  can then be pushed forward past the enlarged force-receiving region  11  onto the plug capillary tube  10 , or can be placed on the plug capillary tube  10  from the side. For this purpose, the material of the sealing element  40  must obviously have sufficient elasticity. 
         [0044]    After sliding the components  20 ,  30  and  40  onto the plug capillary tube  10  and the production of the force-receiving region  11 , the plug unit  3  could then be inserted into the bushing unit  5 . 
         [0045]    The insertion takes place in a simple way in that the plug housing  20  is slid forward without force onto the capillary tube  10  toward the force-receiving region  11 , until the pressure piece  30  and the sealing element  40  are moved into their non-pressurized, front position. Subsequently or simultaneously, the plug unit  3  is introduced into the receptacle opening  53  of the bushing unit  5  and the thread  22  of the threaded part of the plug housing  20  is screwed into the inner thread  52  of the receptacle opening  53 . During the screwing-in process, the front end face of the plug housing  20  exerts an axial force via the pressure piece  30  onto the sealing element  40  that exerts, for its part, axial force via the force-receiving region  11  onto the plug capillary tube  10 . In this way the plug capillary tube  10  is pressed with its end face butted against the bottom wall  51  of the receptacle opening  53 , wherein the centering of the plug capillary tube  10  in the inner, cylindrical region of the receptacle opening  53  is guaranteed by means of the sealing element  40  and the front region of the pressure piece  30 . The axial force exerted via the pressure piece  30  onto the sealing element  40  is increased through further turning of the plug housing  20  into the bushing housing  50  of the bushing unit  5 . During its elastic and/or plastic deformation, the sealing element  40  transmits corresponding pressure forces onto the force-receiving region  11  and the bottom wall  51  in the bushing housing  50  as well as onto the cylinder wall of the cylindrical inner part of the receptacle opening  53 . A corresponding good sealing effect is produced in this way, wherein very high pressure forces can be exerted onto the sealing element through merely minimal forces for turning the plug housing  20  into the bushing housing  50 . 
         [0046]    From the diagram in  FIGS. 1 and 2 , it is easily seen that, for different outer diameters of plug capillary tubes  10 , pressure pieces  30  or sealing elements  40  must be used that are simply adapted according to their inner diameter, while the plug housing  20  can remain the same, assuming that the central hole in the front region of the threaded part  22  is large enough to receive the relevant plug capillary tube  10 . 
         [0047]    Different depths of receptacle openings  53 , which could vary depending on tolerances or manufacturers, could also be compensated, provided that the threaded part  22  of the plug housing  20  has a sufficient length in the axial direction. 
         [0048]    The construction of a plug unit  3  shown in the drawing also guarantees, for different depths of the receptacle opening  53  of the bushing unit  5 , that the end face of the plug capillary tube  10  is always positioned with a defined force against the bottom wall  51  of the receptacle opening  53 . 
         [0049]    For disassembly of the connection system  1 , the plug housing  20  need only to be unscrewed from the bushing housing  50  and the plug capillary tube  10  pulled out from the receptacle opening  53  together with the pressure piece  30  and the sealing element  40 . The diameter increase in the region of the force-receiving region  11  of the plug capillary tube  10  reliably guarantees that the sealing element  40  does not remain in the receptacle opening  53 , especially in the inner, cylindrical region, when the plug capillary tube  10  is pulled out, which would then require it being removed in a laborious way. If the plug unit  3  is then to be installed again, possibly in another (similar) bushing unit  5 , then the sealing element  40  can also be exchanged. 
         [0050]    If it is not possible, as described above, to push a new sealing element past the force-receiving region  11  onto the capillary tube (or to push on the plug unit  3  from the other end of the plug capillary tube  10 ), the front-most end of the plug capillary tube  10  with the force-receiving region  11  can optionally be cut so that a new force-receiving region  11  can be produced after new components that are required for the plug unit  3 , especially a new sealing element  40 , are slid on. 
         [0051]    As the material for the sealing element  40 , for example, PEEK or a material with suitable chemical and physical properties can be used. 
         [0052]      FIG. 3  shows a diagram of another plug unit  1  in the assembled state similar to  FIG. 2 , wherein the plug capillary tube  10  has an outer jacket layer  12  that is made from a material that can deform slightly plastically, and that surrounds an inner jacket layer. The outer jacket layer can be made, for example, from plastic that can deform thermally, while the inner jacket layer is made from a material, for example, stainless steel, glass, fused silica, or another plastic, which is suitable for conducting the relevant liquid. These materials are obviously also suitable for a plug capillary tube  10  that has a wall made from only a single material (cf., e.g., the embodiment shown in  FIGS. 1 and 2 ). 
         [0053]    In the embodiment according to  FIG. 3 , the inner jacket layer could also project beyond the front part of the outer jacket layer  12  deformed into a trumpet-like shape wherein, in the installed state of the plug unit  1  this projecting part can reach into the part of the channel formed in the bushing housing for the liquid to be conducted adjacent to the force-receiving region  11  of the front region  56  of the receptacle opening  53 . Thus, for the production of the part of the plug capillary tube  10  enlarged like a trumpet, a front sub-region of the outer jacket layer  12  can be pushed back and deformed plastically. However, in this way there is the disadvantage that a dead volume not carrying a flow is produced between the front end of the inner jacket layer and the force-receiving region  11 , if the outer diameter of the inner jacket layer does not essentially correspond to the inner diameter of the channel in the bushing housing  50 . This means, however, the observance of correspondingly smaller production tolerances, which is associated with corresponding expense. However, it is possible in a simple way to shape the front face of the enlarged front region of the outer jacket layer  12  to be flush with the front end of the inner jacket layer, in that the projecting part of the inner layer is cut so that it is flush after the production of the force-receiving region. 
         [0054]      FIG. 4  shows a plug unit  5  in which the plug housing  20  has a radial slot  23  that allows the plug housing  20  to be pushed onto the plug capillary tube  10  from the side, or that allows the plug capillary tube  20  to be inserted via the slot  23  into the coaxial mounting position within the plug housing  20  provided for this purpose. This simplifies the assembly or disassembly as well as the exchange of the plug housing  20 . 
         [0055]      FIG. 5  shows an embodiment of a connection system  1  in which the plug unit  5  is expanded by a housing  60 ,  80  that has a two-part construction and in which a filter (or a fritted filter)  71  is provided. The first, pot-shaped housing part  60  holds the disk-shaped filter  71  whose radially outer edge region is held between the end face of the second housing part  80  and the floor of the first, pot-shaped housing part  60 . The outer edge region simultaneously takes on a sealing function, and for this purpose can be formed from a suitable material. The radially inner region of the filter  71  represents the actual filter region  70 , and can be constructed in a known way for this purpose. The second housing part  80  is constructed in its rear region as the bushing housing  50 , and features a corresponding receptacle opening  53 . 
         [0056]    The front region of the first housing part  60  engages in the rear recess in the plug housing  20 , wherein the plug housing  20  and the housing part  60  are preferably constructed so that they are connected to be axially rotatable relative to each other. This can be effected, for example, by a connection of the parts locking in the axial direction. In this case, the plug capillary tube  10  is advantageously connected rigidly and tightly to the housing  60 ,  80 , especially to the first housing part  60 , for example, through fusion, wherein the channel of the plug capillary tube  10  is connected fluidically with the receptacle opening  53  in the second housing part  80  via a through-flow region formed in the housing  60 ,  80  in which the filter  71  is arranged. The plug capillary tube  10  must then be cut accordingly with respect to length. The rotating connection of the plug housing  20  guarantees that, for the assembly of the entire plug unit  5  (that comprises, in this case, the housing  60 ,  80  and the fritted filter  71 ) the capillary tube does not also rotate when the plug housing  20  is screwed into the receptacle opening  53  of the bushing housing. 
         [0057]    The simple, tool-free installation of the plug unit  5  into the bushing unit  3  enables the filter to be exchanged or cleaned quickly. 
         [0058]    The structural and functional features described above only in connection with one actual embodiment could obviously also be combined with each other to form other possible embodiments. 
         [0059]    The above-described illustrative embodiments are meant to illustrate the principles of the invention, but not to limit the scope of the invention. Various other embodiments and modifications to these illustrative embodiments may be made by those skilled in the art without departing from the scope of the present invention.