EP0078724B1 - Positive-displacement precision pipette - Google Patents

Abstract

The pipette according to the invention has a body of the pipette at the lower end of which is fitted an end fitting of a calibrated capillary tube in which a piston is movable, and a gripping device at the upper end of the piston. It is characterized in that the piston has, in the region of its upper end intended to be gripped by the gripping device, a flange adapted to cooperate at its upper face with the free ends of the resilient fingers of the gripping device and by its lower face with an abutment surface arranged on the interior surface of the said end fitting of the capillary tube.

Description

The present invention relates to an improvement made to precision positive displacement pipettes intended for the sampling and distribution of small quantities of liquid.

In the prior art, such positive displacement precision pipettes are already known which are of the type comprising an essentially tubular pipette body at the lower end of which is arranged a calibrated capillary tube in which it moves in a sealed and against an upward elastic restoring force exerted by a main spring, a piston controlled in its downward movement by a control rod protruding from the upper end of said pipette body, as well as a clamping member of the upper end of the piston, which is of the type with elastic branches deviating radially by displacement against the force exerted by a second spring of resistance much greater than that of the main spring and which is integral in translation with said rod control. Such a positive displacement pipette is for example described in EP-A-0 014120 belonging to the applicant.

This general type of positive displacement pipette was then the subject of an improvement intended to allow simultaneous ejection of the capillary tip and of the piston, without the operator having to directly manipulate the capillary and / or the piston. It is clear that, when handling radioactive samples or any other contaminated liquid samples, it is crucial to be able to easily ensure such simultaneous ejection of the capillary tip and the piston which constitute the only organs of the incoming pipette in direct contact with the contaminated liquid withdrawn.

Such an improvement could be achieved using a precision displacement pipette of the aforementioned type having a calibrated capillary tube tip, able to cooperate by fitting with the free lower end of the pipette body, the orifice of this last end allowing the passage of at least part of the pinching member which, in the vicinity of the end of its downward stroke, comes into direct contact with a stop formed on the inner surface of the end piece to ensure the simultaneous ejection of said tip and of the piston which remains trapped in the capillary orifice. This improvement is described in detail in EP-A-0 032 469 filed in the name of the applicant and the lessons of which are incorporated into the present description.

In practice, such a positive displacement pipette has some drawbacks in terms of calibration. Indeed, perfect calibration of such a pipette can only be obtained with certainty if the upper end of the piston always occupies, from one operation to another, the same vertical position in the clamping member. To achieve such a result, it was planned to bring the free upper end of the piston into direct contact with the bottom of the central orifice formed between the elastic branches of the pinching member. It is clear that this correct positioning of the free upper end of the piston, on the one hand requires perfect machining of this end and thus prohibits in particular the presence of burrs often encountered during the manufacture of the pistons, and on the other hand is adversely affected by the presence of all kinds of soiling which, during the use of the pipette, can accumulate at the bottom of the pinching member. Such fouling of the bottom of the orifice formed between the branches of the clamping member will thus cause a downward shift of the piston in its capillary tube, which of course distorts the calibration of the pipette.

This type of pipette also had the disadvantage that, in order to raise the piston in its high limit position - reached when its upper free end comes into abutment at the bottom of the clamping member - it was necessary to exert pressure on the end free bottom of the piston protruding from the orifice of the capillary tube. This operation of correct positioning of the piston was usually carried out by the operator by pressing on the protruding part of the piston either with his finger or with the bench. This operation was therefore intolerable when the pipette was used to collect sterilized samples, since during the operation of correct positioning of the piston in its capillary tube, the operator risked contaminating the free end of the piston. and possibly the end of the capillary tube.

The present invention specifically aims to achieve a positive displacement pipette which overcomes the aforementioned drawbacks.

According to the present invention, the positive displacement precision pipette is characterized in that its piston has, in the vicinity of its upper end intended to be gripped by said gripping member, a collar capable of exerting abutment cooperation by its upper face with the free end of the elastic branches of the pinching member and by its lower face with an abutment surface formed on the inner surface of said capillary tube tip.

• - la figure 1 illustre la partie basse d'une pipette à déplacement positif conforme à la présente invention;
• - la figure 2 représente une vue à plus grande échelle de la zone de la pipette comportant le perfectionnement selon l'invention, et
• - la figure 3 représente l'ensemble constitué par un embout à tube capillaire dans lequel se trouve introduit un piston, tel qu'ils peuvent être stockés, l'un dans l'autre, en vue de leur mise en place simultanée sur le corps de la pipette.

Other characteristics and advantages of the pipette in accordance with the present invention will appear on reading the detailed description given below, in particular with reference to the accompanying drawings:

• - Figure 1 illustrates the lower part of a positive displacement pipette according to the present invention;
• FIG. 2 represents a view on a larger scale of the area of the pipette comprising the improvement according to the invention, and
• - Figure 3 shows the set consti killed by a capillary tube tip into which is inserted a piston, as they can be stored, one inside the other, for their simultaneous placement on the body of the pipette.

The positive displacement pipette according to the invention comprises an essentially tubular body 10 which, in a particular embodiment, may consist of two separate parts which are secured to each other by screwing. The upper part or housing (not shown) allows for example to incorporate a mechanism for adjusting and displaying the volume to be withdrawn and distributed, and the tubular lower part 12 is used mainly for guiding the nipping member 14 of the piston 16. The particular adjustment and display mechanism is not part of the present invention and will therefore not be described here in detail. Such a mechanism is for example described in detail in EP-A-0 032 098 filed in the name of the applicant and the lessons of which are integrated into the present description.

Conventionally, at the lower end of the part 12 of the pipette body 10 is arranged a calibrated capillary tube 18 in which moves, in a sealed manner and against an upward elastic restoring force exerted by a main spring 20, a piston 16 controlled in its movement by a control rod 22 projecting from the upper end of the case of the pipette body 10.

According to the present invention, the pipette has a tip 24 which ends in a calibrated capillary tube 18 and which is able to cooperate by fitting with the free lower end of the pipette body 10. Advantageously, such a tip 24 will be made of a thermoplastic material chosen to resist the samples with which it comes into contact. It will for example be made of polypropylene or any other semi-rigid thermoplastic material.

In such a type of pipette, it is essential that the piston 16 has at least one sealing zone 26 coming into intimate contact with the inner surface of the capillary tube 18. Such a piston 16 can for example advantageously be produced in the form of a 'a stainless steel wire having a coating, for example a coating of polyethylene or teflon, at the sealing zone 26. As shown in Figure 1, such a coating can for example affect a shape generally cylindrical or else may be equipped at its periphery with at least one annular sealing lip coming into intimate contact with the internal surface of the capillary tube 18.

Such a type of pipette also comprises a clamping member 14 of the upper end of the piston 16, which is of the type with elastic branches 28 deviating radially by displacement against the force exerted by a second spring (not shown) resistance much higher than that of the main spring 20. It is clear that such a clamping member 14 must be secured in translation with the control rod 22 of the pipette.

The detail of embodiment of this clamping member 14 is not part of the present invention, and is therefore not illustrated in the accompanying drawings. Referring to EP-A-0 032 469, it can be seen that this pinching member can for example be in the form of a self-contained cartridge comprising a clamp proper ending at its lower end by elastic branches 28 and of which the other end is screwed into a clamp holder. This clamp support is able to slide, against the force exerted by the second spring and according to a partial stroke in the upper part of an orifice formed in the clamp holder 30. The latter clamp holder 30 slides itself inside the lower part 12 of the pipette body 10 against the force exerted by the main spring 20. It is important that the end of the elastic branches 28 has an outwardly flaring surface suitable for to be applied tightly against the conical lower opening 32 of the clamp holder 30, and this precisely under the action of the force exerted by the second spring.

It can be seen in FIG. 1 that the assembly of this cartridge which constitutes the pinching member of the piston 16 is elastically urged upwards under the action of the force exerted by the main spring 20. For this purpose, the holder clamp 30 has a shoulder 34 against which the main spring rests. By its lower end, the main spring 20 comes into fixed support on a shoulder 36 formed on the internal surface of the lower part 12 of the pipette body 10 , in the vicinity of its free lower end. The clamp holder 30 can advantageously come into upper abutment against an adjustment screw (not shown), the variable vertical position of which makes it possible to adjust the useful stroke for taking and dispensing the liquid sample.

The present invention is however not limited to such a type of adjustable pipette, but can of course also extend to fixed pipettes, i. e. pipettes whose adjusting screw would be replaced by an upstroke limit stop which remains fixed in position relative to the pipette body 10.

The stop for limiting down the useful stroke for dispensing the pipette is made at a shoulder 38 formed in the vicinity of the lower part of the clamp holder 30. This shoulder 38 is able to cooperate with the shoulder corresponding 36 formed in the vicinity of the lower end of the pipette body 10. Such a lower limit position of the clamp-holder member 14 is illustrated in FIG. 2. It will be noted that this position is obtained at the end of the compression complete with main spring 20.

The opening 40 of the free lower end of the pipette body 10 is arranged so as to allow at least a portion of the clamping member 14 to be passed which, after compression of the second spring and at the end of the downward stroke, ensures the simultaneous ejection of said end piece 24 and of the piston 16 which remains trapped in the capillary tube 18.

To obtain such a simultaneous ejection of the piston 16 and of the tip 24 of the capillary tube 18, while eliminating the aforementioned drawbacks of the prior art, the piston 16 has a flange 42 located near its upper end intended to be gripped by the elastic branches 28 of the clamping member 14. This flange 42 has an upper face capable of cooperating with the free end of the elastic branches 28 of the clamping member 14, as well as a lower face capable of cooperating with a abutment surface 44 formed on the inner wall of the end piece 24 with capillary tube 18, preferably in the vicinity of the junction between the upper flared part of the end piece and the capillary tube 18 proper.

As shown in particular in Figure 2, the flange 42 has a transverse dimension substantially equal to the outer diameter of the elastic branches 28, in the separated state, of the clamping member 14. This flange 42 has on its underside at least one peripheral annular zone defining an abutment surface which is substantially perpendicular to the axis of the piston 16. This particular arrangement makes it possible to ensure good abutment cooperation between the underside of the flange 42 and the abutment surface 44 formed on the inner wall of the end piece 24. At its upper face, the flange 42 is shaped so as to be perfectly applied against the profile of the opening zone of the pinching member 14. Since in practice the elastic branches 28 of the pinching member have, in the vicinity of their free end, a bevelled internal face intended to facilitate the introduction of the upper end of the piston 16, the upper face of the flange 42 also has a central part affecting a slightly conical profile.

Finally, the upper free end of the piston 16 ends, beyond said flange 42, by a bulged portion 46 which is essentially cylindrical. This bulged part 46 can also advantageously have a cone shape of very slight slope inverted with respect to the conical central profile 50. This bulged part 46 only intended to facilitate the gripping of the piston 16 between the elastic branches 28 of the pinching member 14 has an axial dimension, extending from the free upper end of the piston 16 to the upper face of the flange 42, which remains significantly less than the depth of the central orifice 48 formed between the elastic branches 28 of the member pinching 14.

The lower face of the flange 42 comprises, in its central part, centering means, cooperating for example with the stop 44 formed on the inner wall of the end piece 24 with capillary tube 18. These means which make it possible to ensure centering of the upper part of the piston, advantageously take the form of a conical central profile 50 formed on the underside of the flange 42, the largest diameter of this conical central profile 50 being slightly less than the inside diameter of the capillary tube 18 It is clear that in practice, the collar 42, the bulged part 46 formed on its upper face and the centering means 50 provided on its lower face are advantageously made in one and the same piece of molded plastic.

When it is desired to use the positive displacement pipette according to the invention, it is of course necessary to equip the lower part of the housing with the aid of a nozzle 24 with capillary tube 18, in which a piston 16 moves. do there are two possibilities.

According to a first possibility, there are pistons 16 and nozzles 24 with capillary tubes stored in a separate state. In such a case, pressure is exerted on the push button fitted to the free upper end of the control rod 22 to achieve compression of the first and second springs. This operation leads to the downward movement of the movable assembly of the pipette, as well as the spacing of the elastic branches 28 of the clamping member 14. It is then possible to introduce the bulged upper part 46 of the piston 16 between the elastic branches spaced apart 28 from the pinching member 14.

This introduction and this positioning of the piston 16 takes place in a very approximate vertical position, the main thing being to ensure that the piston 16 is gripped by the pinching member 14. Then, the end piece 24 is force fitted with capillary tube 18, taking care that the piston 16 penetrates unhindered inside the capillary tube 18. To do this, it is advantageous not to use as abutment surface 44 a continuous circular recess, but rather a plurality of 'pins or ribs with a constant angular distribution. This particular arrangement of the abutment surface 44 in fact prevents it from coming into abutment with the tip of the piston 16 against a continuous surface in an offset. It will also be noted that penetration into the end piece 24 and the capillary tube 18 is facilitated by the presence of an inclined inner wall 52 of the end piece 24.

When the piston 16 and the capillary tube end piece 24 have thus been rapidly put in place, it is then essential to ensure that the piston 16 occupies the correct lower limit position in its capillary tube, that is to say say that you need to adjust the pipette zero or calibrate it. Thanks to the arrangement according to the present invention, such calibration can be carried out very easily by simple pressure exerted on the push button fitted to the upper end of the control rod 22 of the pipette. Such pressure on the com rod mande 22 fully compresses the first spring 20 and provides partial compression of the second spring until it reaches a position illustrated in FIG. 2. During this additional stroke, following partial compression of the second spring, the arms elastic 28 of the pinching member 14 move apart then descend until they press on the upper face of the flange 42 of the piston 16. By continuing the compression of the second spring, an additional race is created towards the bottom of the crew movable until the underside of the flange 42 abuts on the internal abutment surface 44 formed on the inner wall of the end piece 24. At this stage the operator perceives a feeling of hard point and interrupts his pressure exerted on the control rod 22. It is thus certain that the piston 16 occupies a correct position in the capillary tube 18, the calibration of the pipette thus being carried out very simply and with perfect precision e.

Note that in the position illustrated in Figure 2, the second spring is not yet fully compressed. Additional pressure exerted on the push button of the control rod 22 ensures complete compression of the second spring beyond the hard point corresponding to the position illustrated in FIG. 2. In such a case, the elastic branches 28 of the pinch member 14 always remains apart, which allows the release of the gripping of the piston 16, then during said complementary downward stroke these elastic branches 28 exert pressure on the flange 42, which is transmitted to the abutment surface 44 and therefore to the nozzle 24 which can thus be separated from the rest of the pipette. Such an ejection can be obtained without any risk of losing the nozzle in normal operation, for example by virtue of the particular embodiment described. It can be seen in FIG. 2 that the inner surface of the upper part of the end piece 24 has at least one annular zone 54 projecting or recessed capable of cooperating elastically with a corresponding annular zone 56 recessed or projecting formed on the surface lateral lateral view of the lower part 12 of the pipette body. These annular zones 54, 56 projecting or recessed have been arranged so as to allow a large guide range of the tip 24 on the end of the pipette, for example of the order of 5 to 10 mm and preferably on the order of 6 to 8 mm. On the other hand, these annular zones 54, 56 projecting or recessed are also arranged so as to ensure a short engagement and disengagement stroke of the end piece 24, of the order of approximately 2 mm.

According to another possibility, it is possible to store, for example on a rack, the assembly constituted by the end piece 24 with capillary tube and the piston 16 already put in place as illustrated in FIG. 3. 'examination of this figure that the underside of the collar 42, and in particular the peripheral annular zone perpendicular to the axis of the piston 16, plays here an additional function of holding the piston 16 in the capillary tube 18, thus avoiding any risk for the piston to slide downwards and consequently to escape from the end piece 24 with capillary tube 18. It will be understood on examining this figure 3 the essential advantage of providing the inner surface of the collar 42 centering means, for example in the form of a cone 50, in order to ensure the correct positioning of the bulged upper part 46 of the piston 16 which is intended to penetrate into the orifice 48 formed between the elastic branches 28 of the pinch member 14. These means s centering indeed allow to obtain a certainty of coincidence, when one comes to engage the lower part of the body of the pipette in the nozzle 24. This operation is obviously carried out having taken care beforehand to ensure a pressure on the push button of the control rod 22 against the first and second springs, in order to separate the elastic branches 28 from the clamping member 14, intended to receive the bulged end 46 of the piston 16. In such a case, the calibration, as explained above, is carried out automatically, since the piston 16 already occupied its correct position in the capillary tube 18. It is however possible to repeat the previous operation in order to have the certainty of good calibration.

It will easily be understood that such an arrangement leaving a vacuum at the bottom of the clamping member, eliminates all the effects of any fouling of the bottom of the orifice 48 of the clamping member 14.

Of course, the present invention is not limited to the particular embodiments described, but it is perfectly possible, without departing from the scope of the present invention, to imagine a number of alternative embodiments.

Claims (9)

1. Precision pipette of the positive displacement type intended for taking and distributing small quantities of liquid, of the type comprising an essentially tubular pipette body at the lower end of which is fitted in a removable manner a tip with a calibrated capillary tube in which a piston is displaced in a sealed manner and in opposition to an elastic force drawing it back upwards which is exerted by a principal spring, the said piston being controlled in its displacement by a control rod extending beyond the upper end of the said pipette body, and a device for gripping the upper end of the piston which is for the type having elastic branches parting radially by displacement in opposition to the force exerted by a second spring with much greater resistance than that of the principal spring and which is integral in translation with the said control rod, the orifice at the lower free end of the pipette body permitting the passage of at least one part of the gripping device which, after compression of the second spring and close to the end of its downward course, ensures the simultaneous ejection of the said tip and of the piston which remains trapped in the capillary tube, characterised in that the piston (16) has in the vicinity of its upper end intended to be held by the said gripping device (14) a flange (42) capable of co-operating for stop purposes with its upper face on the free end of the elastic branches (28) of the gripping device (14) and with its lower face on a stop surface (44) arranged on the interior surface of the said tip (24) with a capillary tube (18).

2. Pipette as claimed in claim 1, characterised in that the said flange (42) has a transverse dimension substantially equal to the external diameter of the elastic branches (28) of the gripping device (14) in their parted state.

3. Pipette as claimed in one of claims 1 and 2, characterised in that the said flange (42) has a lower face of which at least an annular peripheral zone defines a stop surface substantiallly perpendicular to the axis of the piston (16).

4. Pipette as claimed in one of claims 1 to 3, characterised in that the said flange (42) has an upper face shaped in such a way as to be perfectly applied against the profile of the opening zone of the gripping device (14).

5. Pipette as claimed in claim 4, characterised in that the said flange (42) has an upper face the central part of which has a conical profile.

6. Pipette as claimed in one of claims 1 to 5, characterised in that the upper free end of the piston (16) ends beyond the said flange (42) in an essentially cylindrical enlarged part (46) intended to penetrate into the opening of the gripping device (14), and the axial dimension of this enlarged part (46) is less than the depth of the opening (48) of the gripping device (14).

7. Pipette as claimed in one of claims 1 to 6, characterised in that the lower face of the said flange (42) has in its central part means for centering the upper part of the piston (16).

8. Pipette as claimed in claim 7, characterised in that the said centering means take the form of a central conical profile (50) arranged on the lower face of the flange (42), the greatest diameter of this central conical profile (50) being slightly less than the internal diameter of the capillary type (18).

9. Pipette as claimed in one of claims 1 to 8, characterised in that the flange (42), the enlarged part (46) arranged on its upper face and the centering means (50) provided on its lower face are made in one single piece from moulded plastics material.

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