Positive-displacement pipette

Abstract: The invention relates to a positive-displacement pipette, in which there is a body and in its jet a cylinder part (3) equipped with a piston (4). In the body there is a sleeve-like gripping arm (9), with which the piston is gripped.

Description: This invention relates to the handling of liquid samples, and more especially to a positive-displacement pipette. Displacement pipettes which include detachable tips and pistons are already known. In the body of such a pipette there is a connecting arm movable in a bore and having at its lower end a gripping device formed by fingers. The fingers open out around the top of the piston rod when the end of the connecting arm is pushed out of the bore. When the connecting arm is pulled back into the bore, the fingers grip the piston rod, thus pulling the piston up. US-A-4,474,071 describes a pipette for dispensing liquids in which fingers of a sleeve are urged open radially to grip a flange provided on the piston of the pipette tip. US-A-4,672,857 describes a pipette for dispensing liquids in which depression of a shaft relative to the handle forces a rod, which serves as a gripping device, downwardly into engagement with the plunger of the pipette tip. Another pipette for dispensing liquids is furthermore described in US-A-4 967 604. In the pipette of the present invention there is a body, a cylinder part provided with a piston, and a sleeve-like gripping arm for gripping the piston. The advantage of the pipette according to the invention is above all its simple structure. Accordingly, the present invention provides a positive displacement pipette comprising: a body including a tip holder and a bore passing through the body and tip holder; a tip adapted for attachment to the tip holder and including a cylinder with a piston and a piston rod movable therein; and a gripping arm movable within the bore and adapted to grip the piston rod, wherein the gripping arm is a sleeve which is a frictional force fit around the rod of the piston when the tip is attached to the tip holder, and the sleeve accommodates a release rod by means of which the rod of the piston can be disengaged from the sleeve. A specific embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figures 1 to 3 show the pipette in cross section in its various functioning phases; Figure 4 shows an enlarged view of the piston mounting; Figure 5 shows a top plan view of the piston; and Figures 6 and 7 shows an enlarged view of another embodiment of cylinder and piston. The pipette shown in Figs. 1 to 3 includes a body which consists of a hand grip 1 and of a tip holder 2. The tip holder 2 narrows conically downwards. At its bottom end, attached by friction, there is a pipette tip 3. The tip 3 incorporates a lower cylinder section and a piston 4. At the lower end of the cylinder section there is a cylindrical bore accommodating the piston 4 which includes a seal 5 at its lower end. The top end of the tip 3 is in the form of a conical socket 6, the inner surface of which corresponds in shape to the outer surface of the lower end of the tip holder 2. Thus the pipette tip 3 can be attached to the tip holder 2 by friction. The piston 4 includes a rod which extends upwards beyond the lower edge of the tip holder 2 and a limiter 7, which is located at the bottom of the socket 6 when the piston is in its lower position so that the lower end of the piston is at the very bottom end of the cylinder section of the tip 3. Extending through the tip holder 2 there is a bore that is larger in its top end to form a shoulder 8. Extending through the bore from its bottom end to the hand grip, there is a sliding sleeve 9. The top end of the sleeve 9 is attached to a bottom limit sleeve 10, the outside diameter of which is bigger than the internal diameter of the bore in the tip holder 2. When the lower end of the tip holder 2 is pushed into the conical socket 6 of the tip 3, the sliding sleeve 9 is in its lower position with its bottom end level with the bottom end of the tip holder (Fig. 1). The sliding sleeve 9 therefore passes over and frictionally grips the top end of the rod of the piston 4. Frictional grip is ensured by holding elements (Fig. 4) formed by a circular constriction 11 in the sleeve 9 and by a corresponding groove 12 in the rod of the piston 4. Sliding of the sleeve 9 over the rod is made easier by means of elastic elements, which consist of axial grooves 13 located in the end of the rod (Figs. 4 and 5). Inside the sliding sleeve 9 there is a sliding release rod 14, the upper end of which is attached to a sliding element 15. When the rod of the piston is gripped by the sliding sleeve 9, the bottom end of the release rod 14 is located above the top end of the rod of the piston 4. When the tip 3 is to be removed, the release rod 14 is moved downwards whereby it pushes the rod of the piston 4 out from within the sliding sleeve 9. At the same time, the limiter 7 on the piston 4 is pressed against the bottom of the socket 6 of the tip 3 and pushes the tip 3 out of engagement with the tip holder 2 (Fig. 3). In the bottom end of the sliding element 15 there is a circular flange 16 supporting a holding sleeve 17 at the top end of which there is a limit ring 18 corresponding in size to the flange 16. The bottom end of the holding sleeve 17 extends beyond the bottom end of the sliding element 15 when the ring 18 of the holding sleeve is located against the flange 16. Around the sliding element 15 there is a coil spring 19, which urges the ring 18 on the holding sleeve 17 against the flange 16. At its top end, the coil spring 19 bears against a collar 21 attached to the top end of the sliding element 15. Over the sliding element 15 there is located a sliding bush 20, in the downward facing opening of which the top end of the sliding element 15 is located. Abutting the top surface of the sliding bush 20 there is an arm 22 to the top end of which there is attached a broad press button 23 extending above the hand grip 1. Around the press arm 22 there is mounted by threads a limit nut 24 sliding longitudinally in the hand grip 1. Twisting of the limit nut relative to the hand grip is blocked by guide bars. The top end of the hand grip 1 is closed by a calibration sleeve 25 located by threads 26. The press button 23 is so arranged that it slides in the calibration sleeve 25. There is a coil spring 27 around the sliding sleeve 9, the bottom end of the coil spring being located against the shoulder 8. The top of the spring 27 bears against the bottom limit sleeve 10 which is attached to the top end of the sliding sleeve 9. The spring 19 is more rigid than the spring 27. The spring 27 urges the bottom limit sleeve 10, the holding sleeve 17, the spring 19, the collar 21, the sliding element 15 and the sliding bush 20 to push the press arm 22 into its top position in which the limit nut 24 is positioned against the bottom surface of the calibration sleeve 25 (Fig. 2). Thus the upper limit of movement of the sliding sleeve 9 and hence the suction volume of the piston 4 attached to it are determined. The suction volume can be adjusted by turning the arm 22 by means of the button 23, whereby the longitudinal location of the nut 24 on the arm is changed. A volume indication is located in the hand grip 1 (not shown in Figs. 1 to 3), e.g. based on turning digital rings. The volume indication is calibrated by turning the calibration sleeve 25 so that the volume shown corresponds exactly to the desired suction volume. Pipetting is started by pushing the end of the tip holder 2 into the conical socket 6 and by pressing the button 23 against the force of the spring 27, until the bottom limit sleeve 10 meets the upper surface of the tip holder 2, whereby the sliding sleeve 9 grips the piston 4. When pressing the button 23, an increase in resistance is clearly felt as the force of the spring 19 resists further pressing movement. The button 23 is held in this position and the lower end of the tip 3 is placed into a vessel containing the liquid to be pipetted. The button 23 is then released, whereby the spring 27 returns the sliding sleeve 9 to its top position and the piston 4 attached into the sleeve sucks up liquid into the tip 3. The liquid is displaced from the tip 3 by again pressing the button 23 against the force of the spring 27, until the bottom limit sleeve 10 meets the upper surface of the tip holder 2. When the tip 3 is to be removed, the button 23 is pressed even lower against the force of the spring 19. The release rod 14 then pushes the piston 4 out of the sliding sleeve 9 and the limiter 7 presses the tip 3 away from the tip holder 2 (Fig. 3.). The tip 3 shown in Figs. 1 to 3 is adapted for e.g. volumes of 2-25 microliters. For larger volumes, a broader tip is needed in order to avoid piston movement becoming uncomfortably long. In Figs. 6 and 7, there are shown a tip 3 and a piston 4 adapted for larger volumes, e.g. 20-250 microliters. The bottom end 28 of the tip 3 is here conically tapered. The piston 4 has its bottom section 29 correspondingly shaped and its end 30 shaped to correspond to the opening in the bottom end of the tip. The seal 5 of the piston 4 is directly above the bottom section 29 which acts also as a limiter 7, which when pressed against the tapered end 28 of the tip 3 forces the socket 6 from the tip holder 2 of the pipette. Additionally, a guide ring 31 on the piston 4 enables the piston to stay upright in the tip 3.

Claims: A positive displacement pipette comprising: a body including a tip holder (2) and a bore passing through the body and tip holder; a tip (3) adapted for attachment to the tip holder (2) and including a cylinder with a piston (4) and a piston rod movable therein; and a gripping arm (9) movable within the bore and adapted to grip the piston rod, wherein the gripping arm is a sleeve (9) which is a frictional force fit around the rod of the piston (4) when the tip (3) is attached to the tip holder (2), and the sleeve (9) accommodates a release rod (14) by means of which the rod of the piston (4) can be disengaged from the sleeve (9). A pipette according to claim 1, including a bottom limit stop (10) attached to the sleeve (9) and which together with a surface in the body determines the bottom position of the sleeve in the body. A pipette according to claim 1 or claim 2, including a top limit stop (24) attached to the sleeve (9) and which together with a second surface in the body determines the top position of the sleeve in the body. A pipette according to claim 3, wherein the position of the second surface is adjustable in the direction of movement of the sleeve (9). A pipette according to claim 3 or claim 4, including a spring (27) associated with the sleeve (9) to urge the sleeve towards its top position. A pipette according to claim 5, including a sliding element (15) and a spring (19) associated with the release rod (14) and adapted to urge the releasing rod into a top position relative to the sleeve (9), the spring (19) being more rigid than the spring (27) associated with the sleeve. A pipette according to any one of claims 3 to 6, wherein the top limit stop is a nut (24) threaded on an arm (22) forming an extension of the sleeve (9), the nut being non-rotatable relative to the body. A pipette according to any one of claims 1 to 7, including a limit stop (7) attached to the piston (4) and a counter surface forming part of the tip to permit the tip (3) to be released from the tip holder (2) by pushing the piston (4). A pipette according to any one of claims 1 to 8, including holding elements (11,12) associated with the piston (4) or the sleeve (9) to enable the piston to be engaged by the sleeve, and elastic elements (13) associated with the piston or the sleeve to facilitate engagement of the piston (4) and the sleeve (9).

Inventors: HEINONEN MAUNO, HEINONEN, MAUNO
Applicants: LABSYSTEMS OY
Application year: 1992
Main classification: B01L3
IPC codes: B01L3