Abstract:
A bottom part of a positive-displacement pipetting system), including a body slideably housing a device for gripping a piston belonging to a capillary-piston assembly the capillary of which is adapted to be fitted onto an end-fitting of the body. The system also comprises means for ejecting the capillary-piston assembly arranged externally with respect to the body. The system is constructed so that a relative rotation between the ejection means and the gripping device causes it to pass from an open configuration enabling the piston to be released to a closed configuration enabling said piston to be retained, and/or the reverse.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to the field of positive-displacement pipetting. 
         [0002]    Such systems are intended to cooperate with consumables of the capillary-piston type, in which the piston is intended to be directly in contact with the sample to be collected, before being ejected or reused. Positive-displacement systems therefore have a different design from that of more conventional air-displacement systems, in which the piston is an integral part of the system. 
         [0003]    The invention more specifically relates to the ejection function of the capillary-piston assembly. It applies to all different types of sampling systems, namely, in particular, pipettes, manual or powered, as well as automated systems. 
       PRIOR ART 
       [0004]    Positive-displacement pipettes are normally used for sampling viscous, volatile or contaminating liquids. Their association with consumables of the “capillary-piston” type makes it possible to avoid contamination of the pipette. The same is true of automated systems operating according to the same principle. 
         [0005]    Such a positive-displacement pipette is known from the French patent application FR 2 446 672. 
         [0006]    The positive-displacement pipettes known from the prior art are provided with a control rod, the bottom end of which controls the displacement of a gripping device of the top end of a piston, belonging to a capillary-piston assembly intended to cooperate with the pipette. This gripping device is also called a “clamp”. 
         [0007]    The pipette is designed so as to be capable of exerting two successive downward strokes with the control rod, via a control button arranged at its top end. The first stroke of the control rod corresponds to the stroke for dispensing the collected sample. It is performed by opposing the return force of a first spring, preferably a compression force. The second stroke of the control rod corresponds to the presentation and opening of the gripping clamp of the piston. It is performed by opposing the return force of a second spring, preferably a compression force, arranged in the same direction as the first spring and having a stiffness that is clearly greater. 
         [0008]    More specifically, this second stroke results causes the jaws/jaw assembly of the clamp to be extracted from a sheath surrounding them. Once the jaws have been released from their sheath, they may easily allow penetration of the top end of the piston during installation of the consumable on the pipette, simultaneously allowing the fitting of the capillary on the end-fitting of the pipette. 
         [0009]    The first phase of raising the control rod, under the effect of the return force of the second spring, causes the jaws to be retracted into the sheath of the clamp, with the top end of the piston held by the jaws in the clamped position. The second phase of raising the control rod, under the effect of the return force of the first spring of lesser stiffness, causes the displacement of said rod as well as of the clamp holding the piston until it reaches the top position, with respect to the pipette body. 
         [0010]    In order to perform a sample collection, the operator must again perform the first stroke of the control rod with the piston attached, until the first spring is totally compressed, bringing the piston to its bottom collection point. A continuation of the stroke at this stage, i.e. an accidental initiation of the second stroke against the second spring, would cause an excessive downward displacement of the piston, and would lead to an error in the quantity of sample collected. An excessive over-stroke of the piston could even lead to the accidental ejection of the piston and the capillary, due to the release of the jaws of the clamp by the sheath surrounding them, and the bearing of said jaws on the capillary. Such an ejection is not only problematic in terms of productivity, but also causes non-negligible risks of liquid spray when the capillary-piston assembly falls into a liquid container. 
         [0011]    When the piston has reached its bottom point at the end of the first stroke, the consumable is immersed in the liquid to be collected. To perform the collection, the operator then progressively relaxes the pressure exerted by his or her thumb on the control button, enabling the control rod and the piston to rise under the effect of the return force of the first spring. During this rise, the liquid in contact with the bottom end of the piston is inserted into the capillary. 
         [0012]    For the dispensing, the operator again performs the first stroke of the control rod by pressing his or her thumb on the control button, placing the capillary in the container intended to receive the liquid. Again, in the event of an over-stroke of the piston, it risks being accidentally ejected with the capillary, thus again generating a risk of accidental spray of liquid. 
         [0013]    Finally, once the operation of dispensing the liquid is completed, the operator may perform the second stroke of the control rod in order to cause the desired ejection of the capillary-piston consumable. Nevertheless, this ejection operation is performed using a single thumb of the operator, countering the return force of the second spring, which is necessarily substantial in order to oppose the return force of the first spring, and thus be capable of ensuring its function of delivering a sensory signal to the operator at the end of the first stroke. The high stiffness of the second spring thus makes the pipette perfectible in terms of ergonomics, especially because this disadvantage also occurs during the above-described operation of opening the clamp, before insertion of the piston. 
         [0014]    At least some of the disadvantages cited above are found in an identical or analogous manner in the current automatic positive-displacement pipetting systems. 
         [0015]    In addition, another disadvantage of the ejection system provided on the positive-displacement pipettes currently known is that it is based on a design far removed from that of the ejection system for conventional air-displacement pipettes. As operators are generally more accustomed to using such air-displacement pipettes, there is a need to harmonize the design of the ejection system of positive-displacement pipettes. 
       OBJECT OF THE INVENTION 
       [0016]    The invention is therefore intended to at least partially overcome the disadvantages mentioned above, involving the embodiments of the prior art. 
         [0017]    To this end, the invention relates to a bottom part of a positive-displacement pipetting system, including a body slideably housing a gripping device of the top end of a piston belonging to a capillary-piston assembly, the capillary of which is intended to be fitted onto an end-fitting of the body. According to the invention, the bottom part also comprises means for ejecting the capillary-piston assembly arranged outside said body and slideably mounted with respect to the body according to a longitudinal axis of the pipetting system, so as to be capable of cooperating with said capillary for the ejection thereof. In addition, the bottom part is designed so that a relative rotation between the ejection means and the gripping device, according to the longitudinal axis of the pipetting system, causes the gripping device to pass from an open configuration enabling the top end of the piston to be released, to a closed configuration enabling said end to be held, and/or to pass from the closed configuration to the open configuration. 
         [0018]    The invention is notable in that it is based on a design making it possible to dissociate the elements ensuring the control function of the pipette from those ensuring the ejection of the capillary-piston consumable. More specifically, according to the invention, the ejection of the consumable is performed by dedicated ejection means surrounding the body of the bottom part, in the same was as similar ejection means commonly seen on air-displacement pipettes. Consequently, unlike the embodiments of the prior art in which the ejection means integrate the control rod, the risks of accidental ejection of the consumable by said same control rod are advantageously reduced to none. 
         [0019]    Thus, during handling of the pipette, the operator may actuate the control rod without being concerned with risks of such a loss, thereby generally making it possible to generally improve ergonomics, reproducibility of collections, and productivity. This is also the case when the invention is applied to an automated positive-displacement sampling system. 
         [0020]    In addition, the force to be delivered in order to ensure the ejection of the consumable may be much lower than that previously required in order to counter the return force of the spring having a high stiffness, since it is no longer necessary to provide a high differential of spring stiffnesses in order to provide a sensory signal for the operator. Moreover, the low bearing force required in order to detach the capillary from the end-fitting of the pipette has no effect on the risk of accidental loss of the capillary-piston consumable, as explained above. This force is also low due to the possibility of bringing the gripping device into an open configuration during ejection of the piston-capillary assembly. Therefore, during the ejection, which occurs like a conventional ejection on an air-displacement pipette, there is no need to counter the friction force between the top end of the piston and the clamp. This results in better ergonomics for the operator. 
         [0021]    The relative rotation between the ejection means and the gripping device may be performed in different ways. A solution consisting in manually rotating the ejection means may be envisaged, but an automation of this movement is preferred. This automation is preferably performed by mechanical movement converting means, as will be described in examples below. For this, mechanical connections of the ramp/pin type are preferably implanted on the bottom part of the pipetting system according to the invention. 
         [0022]    Preferably, the gripping device has a jaw that is mobile between a first position bringing the device into the closed configuration and a second position bringing the device into an open configuration, the jaw comprising one of the two elements among the first pin passing through said body and a first ramp cooperating with the first pin, the other of the two elements being provided on said ejection means. In addition, the passage of the jaw from the first position to the second position, and the reverse, is performed by a relative displacement of the first pin along said first ramp. 
         [0023]    Preferably, said gripping device is equipped with resilient return means returning the mobile jaw to its first position or to its second position. 
         [0024]    Preferably, the gripping device comprises one of the two elements among a second pin passing through said body and a second ramp cooperating with the second pin, the other of the two elements being provided on said ejection means. In addition, the bottom part of the pipetting system is designed so that, during a downward displacement of the gripping device, the latter passes from the open configuration to the closed configuration by a relative rotation between the ejection means and the gripping device caused by the relative displacement of the second pin along said second ramp. It is preferably this relative rotation that causes said jaw to pass from its first position to its second position. 
         [0025]    Preferably, the second ramp is designed so that, after passing into the closed configuration of the gripping device, this closed configuration is maintained as the gripping device rises. A pipetting sequence can then be implemented using conventional pipette control means. 
         [0026]    Preferably, said body comprises one of the two elements among a third pin and a third ramp cooperating with the third pin, the other of the two elements being provided on said ejection means. In addition, the bottom part of the pipetting system is designed so that, during a downward displacement of the ejection means, said gripping device is brought from the closed configuration to the open configuration by rotation of the ejection means with respect to said body, caused by the relative displacement of the third pin along said third ramp. In addition, this makes it possible to obtain a helical displacement of the ejection means by applying a simple downward translation movement on them. The thrust of the capillary and the release of the piston are therefore performed simultaneously, for perfectly optimized ergonomics. 
         [0027]    Preferably, the third ramp is designed so that, after passing into the open configuration of the gripping device, this open configuration is maintained as the ejection means rise. Consequently, the gripping device advantageously remains in the open configuration, ready to cooperate with a new capillary-piston assembly. 
         [0028]    Preferably, the first, second and third ramps are provided on the interior surface of the ejection means. Alternatively, they may be placed, at least one, on the body and/or the gripping device. 
         [0029]    Preferably, said ejection means include a ring surrounding the body and rotatably bearing an ejection sheath also surrounding the body. It is thus said sheath that is intended to be capable of pivoting with respect to the gripping device, in order to bring the latter into the open or closed configuration. In this regard, it is indicated that the aforementioned relative rotation is preferably performed by rotation of the ejection means about the longitudinal axis of the pipetting system, keeping, according to said same axis, the gripping device locked in rotation. A reverse technical solution may, however, be envisaged, without going beyond the scope of the invention. Similarly, a mixed solution in which the ejection means and the gripping device turn in opposite directions is also possible. 
         [0030]    The invention also relates to a positive-displacement pipetting system including a bottom part as described above. 
         [0031]    Preferably, it is a pipette, manual or powered, comprising a top part forming a handle equipped with an ejection button connected to said ejection means. In this case, said ejection button is preferably distinct from a control button for displacement of the piston. 
         [0032]    Alternatively, the pipetting system may be an automated system. 
         [0033]    The invention also relates to a pipetting method using a positive-displacement pipetting system as described above, including the following successive steps: 
         [0034]    (a) fitting the capillary of the capillary-piston assembly onto an end-fitting of the pipetting system, and inserting the top end of the piston of the assembly into the gripping device during the displacement thereof into a bottom position, during which insertion a relative rotation is performed between said ejection means and the gripping device in order to bring the latter from the open configuration to the closed configuration, so as to retain the top end of the piston; 
         [0035]    (b) collection and dispensing of a sample by actuating a control rod connected to the gripping device; 
         [0036]    (c) ejection of the capillary-piston assembly by a downward displacement of said ejection means thrusting the capillary, during which ejection a relative rotation is performed between said ejection means and the gripping device in order to bring the latter from the closed configuration to the open configuration, releasing the top end of the piston. 
         [0037]    Preferably, for step (a), said relative rotation between said ejection means and the gripping device is performed automatically as the gripping device descends, and, for step (c), said relative rotation between said ejection means and the gripping device is performed automatically as the ejection means descend. As mentioned above, if the desired automation can take any form considered to be appropriate by a person skilled in the art, it is preferably implemented by mechanical movement conversion means such as mechanical connections of the ramp/pin type or the like. 
         [0038]    Other advantages and features of the invention will become clear in the non-limiting detailed description below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    This description will be provided in view of the appended drawings, wherein: 
           [0040]      FIG. 1  shows a perspective view of a positive-displacement sampling pipette, according to a preferred embodiment of the present invention; 
           [0041]      FIG. 2  shows a cross-section view of the bottom part of the pipette shown in the previous FIG., with the pipette in a configuration free of the capillary-piston assembly, and with the control button in the top position; 
           [0042]      FIG. 3  shows a transverse cross-section view with ramp/pin connections specific to the present invention; 
           [0043]      FIG. 4  is a schematic side view of the ejection means, showing ramp/pin connections specific to the present invention; and 
           [0044]      FIGS. 5   a  to  8   d,  based on  FIGS. 2 to 4 , show the pipette in different configurations during a pipetting cycle. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0045]    In reference first to  FIG. 1 , a positive-displacement sampling pipette  1  is shown according to a preferred embodiment of the present invention. 
         [0046]    Throughout the following description, the terms “top” and “bottom” are to be considered with the pipette held vertically, in the pipetting position or close to this position. 
         [0047]    The pipette  1  has an exterior body, the top part of which forms a handle  2  for the operator, and the bottom part  4  of which is narrower, ending toward the bottom with an end-fitting  6  on which a capillary is intended to be fitted. The bottom part  4  is preferably screwed onto the handle-forming body  2 , by a threaded ring  5 , so as to facilitate the assembly/disassembly. 
         [0048]    The pipette integrates a control rod  8 , slideably housed inside the exterior body of the pipette. The rod  8  is arranged according to the longitudinal axis  10  of the pipette. Its top end projects upward from the handle-forming body  2 , and has a control button  12  intended to be actuated by the thumb of an operator holding the body  2  with one hand. The rod  8  is slideably housed through a screw for adjusting the volume to be sampled (not shown). As partially shown in  FIG. 2 , the rod  8  may include a stroke reduction box  11 , for facilitating the rod displacement operation performed by the operator. 
         [0049]    In a known manner, the rotation of the control rod  8  by its button  12  enables the adjustment screw to be displaced with respect to the exterior body of the pipette according to axis  10 , and thus causes the volume of the sample to be collected to be modified. 
         [0050]    The bottom end of the control rod  8  bears axially against a gripping device in the form of a clamp  20 , slideably mounted in a bore hole  18  of the hollow body  19  of the bottom part integrating the end-fitting  6  at its bottom end. 
         [0051]    As is shown in  FIG. 2 , the clamp  20  has a body  24  in contact with the bottom end  16  of the control rod, extended downward by one or more stationary jaws, also referred to as a jaw assembly. As an indicative example, a stationary jaw  25  is provided, made in a single piece with the clamp body  24 , said stationary jaw extending, for example, over an angular sector on the order of 270° or more. In addition, the clamp  20  comprises a mobile jaw  26  pivotably mounted on the body  24 , according to an axis  27  orthogonal to the axis  10 . As will be described in detail below, by pivoting according to said axis  27 , the mobile jaw  26  may adopt a first position bringing the clamp into the closed configuration, as well as a second position shown in  FIG. 2  bringing the clamp into the open configuration. 
         [0052]    Resilient return means  32  enable the mobile jaw  26  to be returned to its first position ensuring a gripping of the clamp  20 . To do this, the resilient return means urge the jaws/jaw assembly  25 ,  26  radially inwardly, preferably by surrounding said same jaw assembly. They may thus be a spring  32  with a general annular shape, the diameter of which may be increased when it is stressed radially outwardly. In the example shown in  FIG. 2 , the spring  32  takes the form of a spiral spring clamping the exterior surface of the jaws  25 ,  26 . 
         [0053]    In addition, each jaw  25 ,  26  has a chamfered end so as to facilitate the insertion of the piston, as will be explained below. 
         [0054]    In addition, the clamp body  24  has a first shoulder  38  facing downward, opposite and at a distance from a shoulder  40  formed on the bottom part  4 , near the end-fitting  6 . A return spring  42  is housed in contact between said two shoulders  38 ,  40 , so as to form a return spring in the top position of said clamp  20  and the control rod located in contact, in its extension. The return force developed by said compression spring  42  effectively causes the control rod  8  to adopt its top position with respect to the exterior body, a conventional top stop (not shown) being provided for this purpose on the same exterior body. 
         [0055]    The clamp  20  comprises a second shoulder  44 , also facing downward opposite and at a distance from the shoulder  40 . The second shoulder  44  is located further down and radially internally with respect to the first shoulder  38 . It is thus surrounded by the spring  42 . As will be described below, it is intended to form a bottom stop for the clamp  20  and the control rod  8  in contact on said same clamp. In the top position shown in  FIG. 2 , the clamp  20  is located outside of the end-fitting  6 , at a distance from it, toward the top. 
         [0056]    One of the particular features of the present invention lies in the presence of ejection means  46  of a capillary-piston assembly, mounted externally around the hollow body  19 , and sliding relative to the latter in the direction of the axis  10 . 
         [0057]    These means  46  comprise an intermediate ring  47  surrounding the hollow body  19  of the bottom part, said ring rotatably bearing, toward the bottom, an ejection sheath  49  also surrounding said body  19 . The sheath has a bottom end  51  arranged near the end-fitting  6  and intended to bear on the fitted capillary in order to eject it. 
         [0058]    The intermediate ring  47  is borne by an upper arm  52 , which extends upward, optionally passing through an orifice  54  of the handle  2 , to a top end attached to an ejection button  48 . Said button  48  is arranged in the top part of the pipette, near the main control button  12 , but distinct from the latter. It is also intended to be simply actuated by the thumb of an operator holding the body  2  with one hand. 
         [0059]    In reference to both  FIGS. 2 and 4 , different elements specific to the present invention are shown. First, the mobile jaw  26  extends upward, having a first radially outwardly-facing pin  54  passing through a slot  56  formed in the hollow body  19 . The pin  54  bears on a first ramp  58  provided on the interior surface of the sheath  49 . The ramp  58  is a circumferential ramp, and its pin-receiving surface  54  has, in reference to axis  10 , a progressive radial spacing. In fact, as is best seen in  FIG. 3 , the ramp  58  has, in particular, an area  58   a  close to the axis  10 , an area  58   b  at a distance from said same axis, and an area  58   c  of transition between areas  58   a  and  58   b.  These areas  58   a,    58   c,    58   b  follow one another in the circumferential direction. 
         [0060]    When the pin  54  is in the second position shown in  FIGS. 2 and 3 , it contacts the close area  58   a  of the ramp  58 , causing the bottom end of the jaw  26  to be located farther from the stationary jaw  25 , and therefore the clamp to be placed in the open configuration in which the piston of a capillary-piston assembly can easily be inserted. The adoption of this second position of the mobile jaw  26  is tolerated by the deformation of the spring  32  surrounding the jaw assembly. 
         [0061]    The mechanical connection formed by the pin  54  and the ramp  58  is such that, when the pin is displaced relative to the ramp toward the distant area  58   b,  it causes a pivoting of the mobile jaw  26  according to axis  27 , under the action of the return force of the spring  32 . The mobile jaw  26  is thus displaced from its second position to its first position by a relative displacement of the pin  54  along the ramp  58 , and the reverse. In the first position, which will be described below, the jaws  25 ,  26  close to one another place the clamp  20  in the closed configuration ensuring the holding of the piston in the clamp by friction. 
         [0062]    In addition, another mechanical connection is provided, including a second pin  60  projecting radially outwardly from the clamp body  24 , and passing through a slot  64  formed in the hollow body  19 . A second ramp  62  is associated with the pin  60 . The pin  60  bears on said second ramp  62 , also provided on the interior surface of the sheath  49 . 
         [0063]    The ramp  62  has a complex shape that will be described in detail in reference to  FIG. 4 . First, it has a general U-shape, with a first flank  62   a  positioned parallel to the axis  10 , and formed in the thickness of the hollow body  19 . In the bottom part, the first flank  62   a  is followed by an actuation area  62   b,  which is both axially and circumferentially inclined. It is an area similar to a helical ramp portion. Optionally, at the outlet of this actuation area  62   b,  the ramp  62  includes a holding area  62   c  extending axially downward, and having a width corresponding substantially to that of the pin  60 . Then, the second axial flank  62   d,  parallel to the first, is provided. 
         [0064]    When the control button is in the top position, like the clamp  20  in the open configuration as shown in  FIGS. 2 to 4 , the pin  60  bears on a top part of the first flank  62   a.    
         [0065]    In addition, another mechanical connection is provided, including a third pin  68  projecting radially outwardly from the hollow body  19 . A third ramp  70  is associated with the pin  68 . The pin  68  bears on said third ramp  70  also provided on the interior surface of the sheath  49 . 
         [0066]    The ramp  70  has a complex shape that will be described in detail in reference to  FIG. 4 . First, it has a general U-shape, with a first flank  70   a  positioned parallel to the axis  10 , and formed in the thickness of the hollow body  19 . In the bottom part, the first flank  70   a  is connected to a base  70   b  positioned transversally in a plane orthogonal to the axis  10 . Opposite the first flank  70   a  is a second flank having a bottom actuation part  70   c,  which is both axially and circumferentially inclined. Its circumferential inclination is produced in a direction opposite that of the inclination of the actuation area  62   b  of the second ramp. In this case it is also an area similar to a helical ramp portion. The second flank then extends with a dead zone  70   d  symmetrical with the actuation area  70   c,  relative to a transverse plane of the pipette. The dead zone enables the insertion of the pin  68  during assembly of the pipette to be facilitated. 
         [0067]    In reference now to  FIGS. 5   a  to  8   d,  the operation of the pipette  1  will be described. 
         [0068]    First, in reference to  FIGS. 5   a  to  5   d,  the operator holding the pipette by the handle  2  engages the end-fitting  6  in a capillary  80  of a capillary-piston assembly  84 , preferably arranged in a box, also called a “rack”. By exerting a vertical downward pressure on the pipette, the capillary  80  is fitted onto the end-fitting  6 , like the fitting of a capillary or a conventional cone on the end-fitting of a conventional air-displacement pipette. 
         [0069]    Then, the operator presses on the control button  12  so as to bring the control rod  8  and the clamp  20  toward the bottom position. This displacement causes a movement of the second pin  60  along the first flank  62   a  of the second ramp  62  to the bottom of said first flank. During this first descent phase of the clamp, the angular position of the sheath  49  with respect to the body  19  and the clamp  20  is maintained. The mobile jaw  26  does not therefore undergo any displacement along its pivot axis  27 , and its second position is retrained by the bearing of the pin  54  on the close area  58   a  of the first ramp  58 . The relative position of the third pin  68  on the ramp  70  remains unchanged, namely, the pin  68  bears against the base  70   b  and the first flank  70   a.  During this first descent phase of the clamp allowed by the compression of the spring  42 , the top end  86  of the piston  82  has begun its insertion between the jaws  25 ,  26 , which are spaced apart. 
         [0070]    When the displacement of the rod  8  and the clamp  20  is continued downward, still against the spring  42 , the second pin  60  bears on the actuation area  62   b  of the second ramp  62 , as shown in  FIGS. 6   a  to  6   d.  The bearing caused by the relative displacement of the pin  60  along the ramp area  62   b  pushes the sheath  49  so as to rotate in the direction of the arrow  77  of  FIG. 6   b.  Also, during this second descent phase of the clamp  20  in which the end of the insertion of the top end  86  of the piston  82  in the clamp  20  is performed, a rotation of the sheath  49  according to axis  10  is also automatically created, which causes the clamp to be brought into the closed configuration. In fact, during said rotation of the sheath  49 , the clamp  20  remains fixed in rotation according to axis  10 , but the pin  54  is displaced along the ramp  58  so as to reach the distant area  58   b,  while pivoting according to its axis  27  under the effect of the spring  32 . Still during this automatic rotation of the sheath, which remains axially fixed, the third pin  68  is displaced along the base  70   b  of the ramp until it arrives in contact with the bottom of the actuation area  70   c.    
         [0071]    In the end-of-stroke position shown in  FIGS. 6   a  to  6   d,  the closed clamp  20  holding the piston  82  is in bottom abutment on the lower body  19 , by contact between the shoulders  44 ,  40 . 
         [0072]    At the end of this step, the piston is located in the bottom stop position in the capillary. Thus, to simplify the sampling process, the control rod  8  is preferably held in the bottom position until the sample is collected, during which the control rod rises with the piston so as to create the aspiration of the liquid. 
         [0073]    During this rise of the piston and the clamp schematically shown in  FIGS. 7   a  to  7   d,  the closed configuration of said clamp  20  is of course maintained in order to hold the piston by friction. During the rise, the second pin  60  is displaced along the second flank  62   d  of the ramp  62 , while the relative position of the third pin  68  and the third ramp  70  remains unchanged. No relative rotation movement is therefore produced between the sheath  49  and the clamp  20  or the body  19 . 
         [0074]    It is then the dispensing of the sampled liquid that is performed, in a manner identical to that performed for the gripping of the piston, but with the mobile jaw  26  held in its first position of clamping the piston. In fact, the stroke is the same, bringing the control rod  8  into the bottom position until it comes into contact between the shoulders  44 ,  40 . 
         [0075]    Finally, the ejection of the entire consumable  84  is performed, using ejection means  46  actuated by the ejection button  48 . This ejection step is schematically shown in  FIGS. 8   a  to  8   d.    
         [0076]    The axial bearing on the ejection button  48  causes the ring  47  and the sheath  49  to slide downward, according to axis  10 . But during this descent of the ejection means, the bearing caused by the relative displacement of the pin  68  along the actuation area  70   c  of the third ramp  70  pushes the sheath  49  so as to rotate according to the direction of the arrow  79  in  FIG. 8   b,  i.e. in a direction opposite that shown in  FIG. 6   b  when the capillary-piston assembly is placed on the pipette. Also, during this descent phase of the ejection means, a rotation of the sheath  49  according to axis  10  is automatically created, which causes the clamp to be brought into the open configuration. Indeed, during this rotation of the sheath  49 , which in fact undergoes a helical displacement according to axis  10 , the clamp  20  and the body  19  remain fixed in rotation according to axis  10 , but the pin  54  is displaced along the ramp  58  so as to reach the close area  58   a  of the first ramp, while pivoting according to its axis  27  against the return force of the spring  32 . Also during this downward displacement of the sheath  49 , which bears with its bottom end  51  on the capillary  80  for ejection thereof, the second pin  60  is displaced circumferentially so as to again reach the first flank  62   a,  at a top part thereof. 
         [0077]    In other words, during the helical movement of the sheath  49 , a thrust of the capillary via the bottom end  51  of the sheath and an opening of the clamp  20  releasing the top end  86  of the piston  82  are created. The assembly  84  may thus be easily ejected, in an innovative manner specific to the present invention. 
         [0078]    Finally, it is indicated that, as the ejection means rise, the open configuration of the clamp  20  is of course retrained in order to keep the pipette ready for a future use. During this rise, the third pin  68  is displaced along the first flank  70   a  that it has already contacted at the end of the ejection operation, due to the low width between the top end of the actuation area  70   c  and the first flank  70   a.  Once the ejection means have returned to the top position, the pin  68  is again at the junction between the flank  70   a  and the base  70   b.  Moreover, during the rise of the ejection means  46 , the relative position of the second pin  60  and the second ramp  62  remains unchanged. No relative rotation movement between the sheath  49  and the clamp  20  or the body  19  is therefore produced. 
         [0079]    Of course, various modifications may be made by a person skilled in the art to the invention described above, solely as non-limiting examples.