DEVICE AND METHOD FOR DETERMINING THE QUALITY OF PULSED DISPENSING OF LIQUID ACCORDING TO THE AIR DISPLACEMENT PRINCIPLE

Abstract

A pipette device comprises a pipette channel filled with compressible working gas, a pipette piston movable along the pipette path, a piston drive, which drives the pipette piston, a control device, a data memory connected to the control device for signal transmission, a pressure sensor which detects the pressure of the working gas and which is connected to the control device, a position sensor which detects a position of the pipette piston and which is connected to the control device. The control device is designed to determine a quality of a dispensing sequence on the basis of a target residual quantity value, which represents the target residual quantity of dosing liquid remaining in the pipette channel, of a working gas pressure and of an end position of the pipette piston, in each case after the end of the dispensing sequence, and to output the determined quality.

Claims

1. A pipetting device, comprising: a pipetting channel extending along a channel path and filled at least in part with a compressible working gas, a pipetting piston accommodated movably in the pipetting channel along the channel path, a piston drive, configured for impelling the pipetting piston to move along the channel path in order to modify the pressure of the working gas in the pipetting channel by means of piston motion, a control device, configured for actuating the piston drive, a data memory linked with the control device so as to allow signal transmission, a pressure sensor that acquires the pressure of the working gas and that is linked with the control device so as to allow signal transmission, a piston position sensor that acquires a position of the pipetting piston and that is linked with the control device so as to allow signal transmission, where the control device is configured for impelling the pipetting piston to a pulsed dispensing motion for single-volume dispensing of a single volume of not more than 1 μl, where through the dispensing motion for single-volume dispensing, which dispensing motion for each single-volume dispensing comprises two motion sections in opposite directions along the channel path, an overpressure pulse is generated in the working gas for a duration not exceeding 50 ms, wherein the control device is configured to determine for a specified dosing liquid a quality of a dispensing sequence, which comprises at least one pulsed single-volume dispensing run, on the basis of a target residual quantity value, which represents a target residual quantity of dosing liquid remaining in the pipetting channel at the end of the dispensing sequence, a working gas pressure after the end of the dispensing sequence, and an end position of the pipetting piston after the end of the dispensing sequence and to output the determined quality discernibly.

2. The pipetting device according to claim 1, wherein the control device is configured to at least one of determine, in accordance with a starting quantity value which represents a starting quantity of dosing liquid in the pipetting channel before the beginning of the dispensing sequence, and a target dispensed quantity value which represents the target total quantity of dosing liquid to be dispensed during the dispensing sequence, a target residual quantity value which represents a target residual quantity of dosing liquid remaining in the pipetting channel at the end of the dispensing sequence; and determine, on the basis of a residual quantity working gas pressure-data correlation recorded in the data memory for the specified dosing liquid, a working gas pressure associated to the represented target residual quantity, and actuate the piston drive to adjust the determined working gas pressure in the pipetting channel, and after adjustment of the determined working gas pressure in the pipetting channel, to acquire the piston position of the pipetting piston, and compare the acquired piston position with a target piston position associated to at least one of the represented target residual quantity and the determined working gas pressure, and depending on the quality of the comparison result representing the dispensing sequence, to produce an output representing the determined quality.

3. The pipetting device according to claim 1, wherein the starting quantity value is a starting position of the pipetting piston at the start of the dispensing sequence.

4. The pipetting device according to claim 1, wherein the control device is configured to determine the target dispensed quantity value on the basis of the number of single-volume dispensing runs of the dispensing sequence and of the target single volumes to be dispensed associated to the single-volume dispensing runs.

5. The pipetting device according to claim 2, wherein the control device is configured to determine the target piston position on the basis of the starting quantity value and of the target dispensing quantity value.

6. The pipetting device according to claim 2, wherein the control device is configured to determine the target piston position from a data relation recorded for the dosing liquid in the data memory, in which for the dosing liquid a target piston position is associated to each of a number of represented target residual quantities.

7. The pipetting device according to claim 2, wherein the control device is configured to actuate into motion a moveable component that can be actuated into motion by the control device as a discernible indication of the determined quality.

8. The pipetting device according to claim 2, wherein the pipetting device exhibits at least one of, at least one of an output device for visual and, acoustic and haptic information output and the control device is configured to actuate the output device to output visual and/or acoustic and/or haptic Information as a discernible indication.

9. The pipetting device according to claim 2, wherein the control device is configured, when the comparison of the acquired piston position with the target piston position yields that the acquired piston position is quantitatively nearer by more than a specified first tolerance interval to a pipetting aperture through which dosing liquid is dispensed from the pipetting channel than the target piston position, to produce a discernible output of information which represents the dispensing of a larger dosing liquid quantity than the target total quantity, and/or when the comparison of the acquired piston position with the target piston position yields that the acquired piston position is quantitatively further away by more than a specified second tolerance interval from the pipetting aperture than the target piston position, to produce a discernible output of information which represents the dispensing of a smaller dosing liquid quantity than the target total quantity, and/or when the comparison of the acquired piston position with the target piston position yields that the acquired piston position conforms within a specified tolerance band to the target piston position, to produce a discernible output of information which represents the dispensing of a dosing liquid quantity that conforms sufficiently to the target total quantity.

10. The pipetting device according to claim 1, wherein the pipetting device is configured for the aspiration of dosing liquid in the pipetting channel.

11. The pipetting device according to claim 1, wherein the dispensing sequence comprises more than ten single-volume dispensing runs.

12. The pipetting device according to claim 2, wherein in the data memory at least one residual quantity-working gas pressure data correlation is recorded for each of a large number of dosing liquids.

13. The pipetting device according to claim 6, wherein in the data memory at least one residual quantity-piston position data relation is recorded for each of a large number of dosing liquids, in which a target piston position is associated to each of a number of represented target residual quantities.

14. The pipetting device according to claim 1, wherein at least one characteristic substance value such as density, viscosity, and the like is recorded in the data memory for a large number of dosing liquids.

15. The pipetting device according to claim 1, wherein the pipetting device exhibits a temperature sensor which acquires the temperature of at least one of the working gas and of a dosing liquid quantity accommodated at least one of in the pipetting channel and of a section of the pipetting channel and transmits it to the control device

16. A method for determining the dosing quality of a dispensing sequence of a pipetting device working according to the air displacement principle, where the dispensing sequence comprises at least one pulsed single-volume dispensing run of a dosing liquid with a target single volume to be dispensed of less than 1 μl, which through generation of an overpressure pulse of less than 50 ms pulse duration in a compressible working gas present between a pipetting piston and a dosing liquid supply in the pipetting channel of the pipetting device is delivered from the dosing liquid supply, where the method comprises the following steps: determining a target residual quantity value, which represents a residual quantity of dosing liquid remaining in the pipetting channel at the end of the dispensing sequence, determining a working gas pressure associated to the represented target residual quantity which is required in order to maintain the target residual quantity in the pipetting channel in a predetermined state, actuating a piston drive for adjusting the determined working gas pressure in the pipetting channel, after adjustment of the determined working gas pressure in the pipetting channel: acquiring the piston position of the pipetting piston, comparing the acquired piston position with a target piston position associated to at least one of the represented target residual quantity and/or to the determined working gas pressure, and producing an output representing the determined dosing quality as a function of the comparison result representing the dosing quality of the dispensing sequence .

17. A computer program product on a data medium, comprising a sequence of operating instructions executable through an electronic computing unit, which executed on an electronic computing unit, which is linked with a pipetting device according to claim 1, so as to allow signal transmission, effects the execution of the the following steps: determining a target residual quantity value, which represents a residual quantity of dosing liquid remaining in the pipetting channel at the end of the dispensing sequence, determining a working gas pressure associated to the represented target residual quantity which is required in order to maintain the target residual quantity in the pipetting channel in a predetermined state, actuating a piston drive for adjusting the determined working gas pressure in the pipetting channel, after adjustment of the determined working gas pressure in the pipetting channel: acquiring the piston position of the pipetting piston, comparing the acquired piston position with a target piston position associated to at least one of the represented target residual quantity and the determined working gas pressure, and producing an output representing the determined dosing quality as a function of the comparison result representing the dosing quality of the dispensing sequence.

18. The pipetting device according to claim 1, wherein the dispensing sequence comprises more than 20 single-volume dispensing runs.

19. The pipetting device according to claim 1, wherein the dispensing sequence comprises more than 30 single-volume dispensing runs.

Description

[0078] The current application is explained in further detail below by reference to the enclosed drawings. The figures show:

[0079] FIG. 1 A pipetting device according to the invention, in which a pulsed dispensing method according to the invention is executed, immediately after the quasi-synchronous aspiration of a predetermined quantity of dosing liquid,

[0080] FIG. 2a The pipetting device of FIG. 1 after generating a first underpressure in the working gas referred to the holding pressure of FIG. 1, for creating a gas volume between the pipetting aperture and the aspirated dosing liquid,

[0081] FIG. 2b The pipetting device of FIG. 2a after increase of the pressure of the working gas between pipetting piston and aspirated dosing liquid, for displacing the meniscus located nearer to the pipetting aperture towards the pipetting aperture,

[0082] FIG. 2c The pipetting device of FIG. 2b after generating a second underpressure in the working gas referred to the holding pressure of FIG. 1, for creating a gas volume between the pipetting aperture and the aspirated dosing liquid and a meniscus located nearer to the pipetting aperture with a defined shape,

[0083] FIG. 3a The pipetting device of FIG. 2c during the abrupt generation of an overpressure pulse with the pipetting piston at the lower dead point of its whip-like motion for pulsed dispensing,

[0084] FIG. 3b The pipetting device of FIG. 3a after completion of the whip-like piston motion for dispensing a single dosed volume of 0.50 μl,

[0085] FIG. 3c The pipetting device of FIG. 3b with working gas pressure adjusted in the working gas through appropriate piston motion in accordance with the expected target residual quantity of dosing liquid for pre-conditioning of the dosing liquid supply in the pipetting channel for subsequent asynchronous single-volume dispensing,

[0086] FIG. 4 A rough schematic flowchart of a method according to the invention for determining the quality of the dispensing sequence of FIGS. 3a and 3b.

[0087] In FIGS. 1 to 3c, a pipetting device according to the invention is generally denoted by 10. This comprises a pipetting channel 11 with a cylinder 12, which extends along a channel path K configured as a straight channel axis. In this pipetting channel 11, a pipetting piston or in brief “piston” 14 is accommodated movably along the channel path K.

[0088] The piston 14 comprises two end caps 16 (for clarity, only the lower one is labeled with a reference number in FIGS. 1 to 3c), between which a number of permanent magnets 18 (in the current example, three permanent magnets 18) are accommodated. In order to achieve along the channel path K a sharply separated magnetic field, the permanent magnets 18 are polarized along the channel axis K and arranged pairwise with same-named poles pointing towards each other. This arrangement results in a magnetic field emanating from piston 14, which as far as possible is uniform around the channel axis K, i.e. essentially rotation-symmetrical with regard to the channel axis K, and which along the channel axis K exhibits a high gradient of the magnetic field strength, such that opposite-named polarization zones interchange alternatingly with sharp separation along the channel path K. Thereby, for example by means of a Hall sensor arrangement as a position sensor 17, a high positional resolution can be achieved in the position acquisition of the piston 14 along the channel axis K and very efficient coupling of an external magnetic field to the piston 14 can be achieved.

[0089] The end caps 16 are formed preferentially of a low-friction, graphite-comprising material, as is known for example from commercially available caps of Airpot Corporation in Norwalk, Conn., (USA). In order to be able to exploit the low friction provided by this material as fully as possible, the pipetting channel 11 comprises preferentially a glass cylinder 12, such that during movement of the piston 14 along the channel axis K the graphite-comprising material slides with extremely low friction along a glass surface.

[0090] The piston 14 thus forms a rotor of a linear motor 20, whose stator is formed by the coils 22 surrounding the pipetting channel 11 (here only four coils are shown as an example).

[0091] It is pointed out expressly that FIGS. 1 to 3c show merely a rough schematic longitudinal cross-section of a pipetting device 10 according to the invention, which should by no means be understood as being to scale. Furthermore, plural components are depicted by an arbitrary number of components, such as for example three permanent magnets 18 and four coils 22. In actual fact, both the number of permanent magnets 18 and the number of coils 22 can be greater or indeed smaller than the number shown.

[0092] The linear motor 20, more precisely its coils 22, are actuated via a control device 24 that is linked with the coils 22 so as to allow signal transmission. A signal can also be the transmission of electric current for sending current through the coils and thus for generating a magnetic field through these.

[0093] At the dosing-side end 12a of the cylinder 12, a pipetting tip 26 is installed detachably in a manner which in principle is known. The connection of the pipetting tip 26 with the dosing-side longitudinal end 12a of the cylinder 12 is likewise shown only in rough schematic form.

[0094] The pipetting tip 26 defines a pipetting space 28 in it interior, which at the coupling-distal longitudinal end 26a is accessible solely through a pipetting aperture 30. The pipetting tip 26 extends the pipetting channel 11 during its coupling to the cylinder 12 up to the pipetting aperture 30 and is part of the pipetting channel 11.

[0095] In the example shown in FIG. 1 of the pipetting device 10 immediately after completion of a conventional, quasi-synchronous aspiration process by the pipetting device 10, a quantity and/or a supply 32 of dosing liquid 33 is accommodated in the pipetting space 28 and thus in the pipetting device 10.

[0096] Between the piston 14 and the dosing liquid supply 32 there is located permanently working gas 34, which serves as a force mediator between the piston 14 and the dosing liquid supply 32. Preferentially there is located between the piston 14 and the dosing liquid supply 32 only the working gas 34, possibly modified negligibly in its chemical composition due to taking up volatile constituents from the dosing liquid 33.

[0097] Even with a completely emptied pipetting tip 26, the working gas 34 is arranged between the piston 14 and a dosing liquid 33, since the pipetting tip 26 is immersed in an appropriate dosing liquid reservoir for the aspiration of dosing liquid 33, such that in this state a meniscus of the dosing liquid 33 is present at least at the pipetting aperture 30. Thus, in every state of the pipetting device 10 that is relevant for a pipetting process, working gas 34 is located permanently and completely between the piston 14 and a dosing liquid 33 and separates them from each other.

[0098] More precisely, the working gas 34 is located between a dosing-side end face 14a of the piston 14, which in this example is formed by an end face of the end cap 16 pointing in the axial direction—relative to the channel path K—towards the pipetting aperture 30 and a meniscus 32a located further away from the pipetting aperture of the dosing liquid supply 32 accommodated in the pipetting space 28 as a liquid column.

[0099] For the sake of clarity, only FIG. 1 shows a temperature sensor 19 connected with the control device 24 so as to allow signal transmission, which acquires the temperature of the working gas 34 and transmits it to the control device 24. This makes possible temperature compensation of all the temperature-dependent data recorded in the data memory 25 and of all the acquisition results of further sensors.

[0100] Proceeding from the state shown in FIG. 1, preparation for a pulsed dispensing process of the pipetting device 10 according to the invention and the pulsed dispensing process itself are described below:

[0101] With reference to FIGS. 2a to 2c, preparation of the pipetting device 10 and preconditioning of the dosing liquid supply 32 are described, with which the accuracy of the pulsed dispensing process shown in FIGS. 3a and 3b can be improved considerably. This means essentially that smaller minimum dispensed doses with higher repetition accuracy can be delivered than without the appropriate preparation.

[0102] Immediately after aspiration of the predetermined quantity of dosing liquid supply 32 into the pipetting tip 26, the meniscus 32a located further away from the pipetting aperture of the dosing liquid supply 32 motionless in the pipetting space 28 and thus in the pipetting channel 11 exhibits a concave shape due primarily to gravity. Likewise, a meniscus 32b located nearer to the pipetting aperture exhibits a convex shape due primarily to gravity.

[0103] Proceeding from the state of the pipetting device 10 immediately after aspiration of the predetermined quantity of dosing liquid supply 32 into the pipetting tip 26 (s. FIG. 1), the control device 24 sends current into the coils 22 in such a way that the pipetting piston 14 is moved in the sense of generating a (first) underpressure in the working gas 34, i.e. in a direction G away from the pipetting aperture 30 against the dispensing direction P.

[0104] As a result, the dosing liquid supply 32 provided in the pipetting channel 11, more precisely inside the pipetting space 28 of the pipetting tip 26, is displaced along the channel axis K away from the pipetting aperture 30 into the pipetting channel 11, more precisely into the pipetting tip 26. The dosing liquid supply 32 provided in the pipetting channel 11 is confined in the direction towards the pipetting piston 14 by the meniscus 32a located further away from the pipetting aperture 30 and is confined in the direction towards the pipetting aperture 30 by the meniscus 32b located nearer to the pipetting aperture. Due to the displacement of the dosing liquid supply 32 away from the pipetting aperture 30, a gas volume 35 forms between the pipetting aperture 30 and the meniscus 32b located nearer to the pipetting aperture.

[0105] In the case, for example, of a taken-up quantity of dosing liquid supply 32 of 40 μl, the gas volume 35 immediately before triggering of the pulsed dispensing overpressure pulse equals preferentially 4 to 10 μl, especially preferentially 4 to 6 μl.

[0106] Due to the displacement away from the pipetting aperture 30 of the meniscus 32b located nearer to the pipetting aperture and therefore later releasing the dosed droplet, the meniscus 32b present at the pipetting aperture 30 after the aspiration with an undefined shape, in particular undefined convex curvature, obtains a more strongly defined shape. After creation of the gas volume 35 as per FIG. 2a, the shape of the meniscus 32b located nearer to the pipetting aperture is essentially flat.

[0107] To this end, a data memory 25 linked with the control device 24 so as to allow data transmission stores a previously experimentally determined dosing liquid quantity-working gas pressure data correlation, in which a working gas pressure is assigned to the quantity 32 of dosing liquid 33 accommodated or present in the pipetting channel 11, whose adjustment as pressure of the working gas 34 effects an essentially flat meniscus 32b located nearer to the pipetting aperture. The dosing liquid quantity-working gas pressure data correlation is the residual quantity-working gas pressure data correlation mentioned in the introduction to the description.

[0108] The pipetting device exhibits a pressure sensor 38 which acquires the pressure of the working gas 34 in the pipetting channel 11 and transmits it via a signal or data link to the control device 24. The control device 24, to which the just now aspirated starting quantity of dosing liquid 33 is known due to the aspiration operation controlled by it, reads out in the data memory 25 from the dosing liquid quantity-working gas pressure data correlation the working gas pressure assigned to the starting quantity as a target working gas pressure or computes it, by interpolation if necessary, and moves the piston 14 in accordance with the signal supplied by the pressure sensor 38 in such a way that the pressure of the working gas 34 equals the target working gas pressure.

[0109] The shape of the meniscus 32b located nearer to the pipetting aperture depends, for example, on the the surface tension of the dosing liquid 33, on its density, on its viscosity, and on the wettability of the walls of the pipetting tip 26 by the dosing liquid 33.

[0110] In accordance with FIG. 2b, the control device 24 can subsequently drive the coils 22 to move the pipetting piston 14 in terms of a pressure elevation in the working gas 34, i.e. displace the pipetting piston 14 in the dispensing direction P towards the pipetting aperture 30. Thereby, the dosing liquid 32 provided in the pipetting tip 26 is displaced back again in the direction towards the pipetting aperture 30, but not beyond it. The gas volume 35 between the pipetting aperture 30 and the meniscus 32b located nearer to the pipetting aperture becomes thereby smaller or even vanishes completely.

[0111] Further, the control device 24 can once again impel the coils 22 to move the pipetting piston 14 in terms of decreasing the pressure of the working gas 34, i.e. in the direction G away from the pipetting aperture 30, whereby once again a gas volume 35 is formed and/or enlarged between the pipetting aperture 30 and the meniscus 32b located nearer to the pipetting aperture of the dosing liquid 32. Again the control device 24 adjusts in the working gas 34 the previously determined target working gas pressure. Through the back-and-forth motion of the dosing liquid 32 in the pipetting tip 26, as shown in FIGS. 2a to 2c, for the same dosing liquid 33 the same shape, preferentially essentially flat, of meniscus 32b located nearer to the pipetting aperture is always formed at the end of the generation of the second underpressure as per FIG. 2c, which is advantageous for the subsequent pulsed dispensing process, as shown and described in FIGS. 3a and 3b. The advantage consists in the decrease of the minimum dispensable liquid quantity and in the achievable repeatability of same when aliquoting.

[0112] The central point of the inventive idea of the current application is a whip-like motion of the piston 14. This whip-like motion is manifested in several kinds of configurations.

[0113] Due to the provided preferred linear motor 20, the piston 14 can be moved with enormous motion dynamics along the channel axis K. For dispensing a small quantity of liquid, about 0.5 μl of the dosing liquid 33, the piston 14 is first moved rapidly in the sense of generating a pressure elevation in the working gas 34 (here: dispensing direction P) towards the pipetting aperture 30. The control device 24 actuates the coils 22 of the linear motor 20 in such a way that the piston 14 executes such a large stroke D that the dosing-side end face 14a of the piston 14 sweeps out along the stroke D a multiple, about 40 times, of the predetermined single-dosing volume 36 (see FIG. 3b). The piston 14 is then located in the position shown in FIG. 3a at the lower dead point of its motion in the dispensing direction P, whereupon the piston 14 is impelled to an opposite motion in the aspiration direction G, i.e. in the sense of decreasing the pressure of the working gas 34.

[0114] The motion of the piston 14 in the dispensing direction P lasts less than 10 ms. When the piston 14 reaches its lower dead point, no part of the dosing liquid supply 32 has yet detached itself from the pipetting tip 26. The meniscus 32b located nearer to the pipetting aperture is shown in a shape preparatory to droplet release. The shape of the meniscus 32b is chosen only for illustration purposes, in order to visualize that a release of a dosing liquid droplet 36 (s. FIG. 3b) is imminent. The meniscus 32a located further away from the pipetting aperture is shown concavely curved, in order to show the effect of the overpressure pulse on the dosing liquid supply 32.

[0115] The piston is moved in the dispensing direction at a maximum velocity of about 10,000 μl/s and to that end accelerated with an acceleration of up to 8×10.sup.6 μl/s.sup.2 and slowed down again. The maximum velocity, however, occurs only briefly. This means that the piston 14, in the aforementioned case in which its dosing-side end face 14a sweeps out in the course of the dispensing motion a volume of about 40 times the single-dosing volume 36, i.e. about 20 μl, requires about 6 to 8 ms for this dispensing motion.

[0116] The dosing liquid supply 32 is too inert to follow this piston motion. Instead, a pressure-elevating pulse is transmitted from the piston 14 via the working gas 34 to the dosing liquid supply 32 in the pipetting tip 26. Proceeding from the picture shown in FIG. 3a, the piston 14 is now accelerated as immediately as possible back in the aspiration direction G, where in the current case the movement stroke A in the aspiration direction is smaller than the stroke D of the movement in the dispensing direction to the extent that during the course of the movement in the aspiration direction A the end-side piston surface 14a sweeps out a volume referred to as “aspiration volume” which is smaller by the single-dosing volume 36 than the volume swept out during the piston's motion in the dispensing direction P, referred to hereunder as “dispensing volume”.

[0117] Having said that, this does not have to be thus. The aspiration volume can also be exactly as large as the dispensing volume. An aspiration volume reduced by the single-dosing volume 36, however, has the advantage that the position of the meniscus located nearer to the pipetting aperture does not change after the pipetting, which is advantageous especially in the aliquoting operation.

[0118] In the end position shown in FIG. 3b of the pipetting device 10 after the end of the pulsed dispensing process, the dosing-side end face 14a is distant by a resulting stroke H from the starting position of FIG. 2c, where in the pictured example the piston surface of the piston 14 multiplied by the resulting stroke H equals the single-dosing volume 36. For improved recognizability, the depiction of the stroke H in the drawing is not to scale and exaggerated.

[0119] The motion in the aspiration direction too, proceeds at the quoted maximum velocity, such that this motion also requires about 6 to 8 ms. With additional dwell times at the lower dead point, which can arise through overcoming the static friction limit, and taking into account any motion overshoots of the piston 14 about its rest position that may occur, the entire piston motion up to reaching the end position, as shown in FIG. 3b, takes place in about 14 to 30 ms.

[0120] Only after the reversal of the piston's motion from the aspiration direction to the dispensing direction is a defined single-dosing volume 36 in the form of a droplet ejected away from the pipetting aperture 30. This droplet moves along the notionally extended channel path K to a dosing destination placed under the pipetting aperture 30, for instance a container or a well. After ejecting the dosing liquid droplet 36, the meniscus 32b located nearer to the pipetting aperture can still overshoot briefly.

[0121] The pipetting tip 26 can exhibit a nominal pipetting space volume that significantly exceeds the single-dosing volume, about 200-400 μl, preferentially 300 μl.

[0122] The motion of the piston 14 in the aspiration direction, in turn, proceeds so rapidly that a pressure-decreasing pulse is transmitted from the dosing-side end face 14a to the dosing liquid supply 32 in the pipetting space 28.

[0123] The pressure-elevating pulse of the piston's motion in the dispensing direction forms the steep rising flank of an overpressure pulse, whose steep falling flank forms the pressure-decreasing pulse of the piston's motion in the aspiration direction. The temporally shorter the individual piston motion, the steeper the flank of the pressure-modifying pulse assigned to it. Thus, the two pressure-modifying pulses acting in opposite senses can define a “hard” overpressure pulse with steep flanks.

[0124] The impinging of the thus formed “hard” overpressure pulse on the meniscus 32a located further away from the pipetting aperture of the dosing liquid supply 32 leads to the extremely precise repeatable dispensing result.

[0125] Surprisingly, the here presented dispensing process is independent of the size of the chosen pipetting tip 26. The same piston motion described above would lead to exactly the same result even with a considerably smaller pipetting tip of, for instance, a nominal pipetting space volume of 50 μl, provided that the same working gas and the same dosing liquid continue to be used with unchanged dispensing parameters.

[0126] Thus the current pipetting device according to the invention and the presented pulsed dispensing method according to the invention are outstandingly suitable for the aliquoting of liquids from even large supplies 32 of dosing liquid 33 accommodated in pipetting tips 26. Even over many aliquoting cycles, the dispensing behavior of the pipetting device 10 does not change under otherwise the same conditions. The dispensing behavior of the pipetting device 10 according to the invention is therefore also independent of the filling ratio of a pipetting tip 26 coupled to the cylinder 12, as long as it is sufficiently filled for pulsed dispensing.

[0127] Due to inertia, the piston's motion may possibly not follow completely exactly the control signal motivating the motion. At points of large dynamic forces—namely at the reversal of the direction of motion from the dispensing direction to the aspiration direction, but also when the piston stops—the piston can tend to overshoot. In the event of doubt, therefore, what should be decisive are the control signals motivating the motion, which are the mapping of a target movement.

[0128] FIG. 3c shows the pipetting device during assessment of the quality of the previously described asynchronous dispensing sequence, which comprises only one asynchronous single-volume dispensing run but which can also comprise several single-volume dispensing runs, as they occur in aliquoting. FIG. 4 shows the sequence of the quality determination.

[0129] In Step S10, the control device 24 determines by difference formation, from the known starting quantity of dosing liquid 33 in the pipetting channel 11 and from the target total quantity to be dosed in the previous dosing sequence of FIGS. 3a and 3b, the target residual quantity of dosing liquid 33 remaining in the pipetting channel after the dispensing sequence in FIG. 3c.

[0130] In the following Step S12, the control device 24 reads out in the data memory 25 from the dosing liquid quantity-working gas pressure data correlation or residual quantity-working gas pressure data correlation recorded there, a working gas pressure assigned to the target residual quantity determined in Step S10.

[0131] In Step S14, the control device 24 actuates the piston 14, taking into account the signal of the pressure sensor 38, to move in such a way that in the working gas 34 the working gas pressure read out in Step S12 and assigned to the target residual quantity prevails. The dosing liquid supply 32 remaining in the pipetting channel 11 is then again, under the assumption that it conforms sufficiently accurately to the target residual quantity, in a dispensing-ready “preconfigured” state with appropriate distance of the meniscus 32b located nearer to the pipetting aperture from the pipetting aperture 30 and with an essentially flat shape of the meniscus 32b.

[0132] In Step S16, the control device 24 reads out in the data memory 25 from a target residual quantity-target piston position data relation recorded there, in which for the dosing liquid 33 a target piston position is assigned to each of a number of target residual quantities, the target piston position assigned to the target residual quantity determined in Step S10. The target piston position can be quoted as a piston position difference value, and determined by the control device 24 through difference formation based on the piston position P1 acquired by the position sensor 17 and stored (s. FIG. 2c) for the preconditioned starting quantity of dosing liquid supply 32 immediately before the dispensing sequence.

[0133] In Step S18, the position sensor 17 acquires the current piston position P2 (s. FIG. 3c) after adjustment of the assigned working gas pressure and transfers it to the control device 24.

[0134] In Step S20, the control device 24 compares the piston position P2 acquired in Step S18 with the target piston position determined in Step S16.

[0135] Proceeding from the comparison performed in Step S20, in Step S22 the control device 24 sends to the output unit 39 (shown only in FIG. 3c), when the piston position P2 is further away from the pipetting aperture 30 than the target piston position determined in Step S16 by more than a permissible tolerance value, a message that the actually dispensed dosing liquid quantity 36 was impermissibly smaller than the target dispensed quantity.

[0136] Proceeding from the comparison performed in Step S20, in Step S24 the control device 24 sends to the output unit 39 (shown only in FIG. 3c), when the piston position P2 corresponds to the target piston position determined in Step S16 within a predetermined tolerance band, a message that the actually dispensed dosing liquid quantity 36 conforms sufficiently to the target dispensed quantity.

[0137] Proceeding from the comparison performed in Step S20, in Step S26 the control device 24 sends to the output unit 39 (shown only in FIG. 3c), when the piston position P2 is nearer to the pipetting aperture 30 than the target piston position determined in Step S16 by more than a permissible tolerance value, a message that the actually dispensed dosing liquid quantity 36 was impermissibly larger than the target dispensed quantity.

[0138] All particulars relating to the embodiment example refer to operation of the pipetting device 10 in an atmosphere of air at 20° C. and a pressure of 1013 hPa.