LIQUID INTERFACE ESTIMATION FOR LIQUID ASPIRATION

Abstract

A method for aspirating a first liquid medium from a sample container with a laboratory automation device, wherein the first liquid medium is above a second liquid medium, which has a higher density and/or viscosity as the first liquid medium comprises: detecting a surface position of a surface of the first liquid medium; calculating an estimated interface position of an interface between the first liquid medium and the second liquid medium from the surface position and calculating a safety position between the surface position and the estimated interface position by adding a safety offset to the estimated interface position; and lowering a pipette of the laboratory automation device into the sample container and aspirating the first liquid medium from the sample container with the pipette by generating an underpressure in the pipette until a pipette tip of the pipette reaches the safety position.

Claims

1. A method for aspirating a first liquid medium from a sample container with a laboratory automation device, wherein the first liquid medium is above a second liquid medium, which has a higher density and/or viscosity as the first liquid medium, the method comprising: detecting a surface position of a surface of the first liquid medium; calculating an estimated interface position of an interface between the first liquid medium and the second liquid medium from the surface position and calculating a safety position (z.sub.S) between the surface position and the estimated interface position by adding a safety offset to the estimated interface position; lowering a pipette of the laboratory automation device into the sample container and aspirating the first liquid medium from the sample container with the pipette by generating an underpressure in the pipette until a pipette tip of the pipette reaches the safety position.

2. The method of claim 1, wherein the estimated interface position is calculated by applying a mathematical formula to the surface position; wherein the estimated interface position is linearly dependent on the surface position.

3. The method of claim 1, wherein the safety offset comprises a statistical offset depending on the surface position; wherein the safety offset comprises a constant offset.

4. The method of claim 1, wherein the surface position of the surface of the first liquid medium is detected capacitively and/or by measuring change of pressure via the pipette tip, which is lowered towards the surface.

5. The method of claim 1, further comprising: during lowering, when the safety position has been reached, measuring a pressure in the pipette and detecting a measured interface position of the interface, when a slope of the pressure changes.

6. The method of claim 5, further comprising: providing a plausibility message for the interface detection, when a difference of the estimated interface position and the measured interface position is higher than a threshold.

7. The method of claim 1, further comprising: after detection of the measured interface position, stopping the pipette tip at the detected position and generating overpressure to dispense an amount of the second aspirated liquid medium and/or an amount of the first aspirated liquid medium from the pipette.

8. The method of claim 1, further comprising: before the safety position is reached, aspirating the first liquid medium from the sample container in several passes, wherein during each pass, an aspiration volume is aspirated from the sample container and dispensed into a further sample container.

9. The method of claim 8, further comprising: before the safety position is reached, measuring a pressure in the pipette during the lowering of the pipette and detecting a measured interface position of the interface, when a slope of the pressure changes.

10. The method of claim 9, further comprising: providing a plausibility message for the interface detection, when the measured interface position is detected.

11. The method of claim 8, wherein during each pass, the pipette is lowered into the sample container until the pipette tip has passed a lowering distance, which is chosen, such that the pipette tip passes the aspiration volume in the sample container; wherein a first aspiration rate during the lowering of the pipette for the lowering distance is lower than a second aspiration rate, after the lowering distance has been passed.

12. The method of claim 8, wherein the aspiration volume and/or a lowering distance of the pipette tip for each pass are chosen in dependence of the safety position.

13. A computer program for aspirating a first liquid medium of two liquid media of different density and/or viscosity from a sample container, which computer program, when being executed by a processor, is adapted to carry out the steps of the method of claim 1.

14. A computer-readable medium, in which a computer program according to claim 13 is stored.

15. A laboratory automation device, comprising: a pipetting arm for carrying a pipette; a pressure device for changing a pressure in a volume connected to the pipette for aspirating and dispensing a liquid medium in the pipette; a pressure sensor for pressure measurements in the volume connected to the pipette; a control device for controlling the pressure device and the pipetting arm and for receiving a pressure signal from the pressure sensor; wherein the control device is adapted for performing the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Below, embodiments of the present invention are described in more detail with reference to the attached drawings.

[0052] FIG. 1 schematically shows a laboratory automation device according to an embodiment of the invention.

[0053] FIG. 2 shows a sample container and illustrates a method for determining an estimated interface position according to an embodiment of the invention.

[0054] FIG. 3 shows a flow diagram for a method for aspirating a liquid medium according to an embodiment of the invention.

[0055] FIG. 4 shows a diagram with a pressure over time generated during a method according to an embodiment of the invention.

[0056] The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

Detailed Description of Exemplary Embodiments

[0057] FIG. 1 schematically shows a laboratory automation device 10, which comprises an automatically movable pipetting arm 12 to which a pipette 14 is attached. As shown in FIG. 1, the pipette tip 16 of the pipette 14 may be lowered into a sample container 18 via the movable pipetting arm 12. For example, the container 18 may be a test tube with a centrifuged blood sample.

[0058] The pipetting arm 12 may move the pipette 14 and the pipette tip 16 in three dimensions, may lower the pipette tip 16 into the sample container 18 and may retract the pipette tip 16 therefrom.

[0059] In the sample container 18, liquid media 20a, 20b are contained, which have a different density and/or viscosity and which separate themselves in a vertical direction, for example under gravity or centrifugal forces. The two liquid media 20a, 20b are separated by an interface 22. The interface 22 may be seen as a layer or plane between the liquid media 20a, 20b. It has to be noted that there is surface or air/liquid interface 24, which is present between the liquid media 20a and the air of the environment.

[0060] For example, the first liquid media 20a may be blood plasma and the second liquid media 20b may contain blood cells, such as red and white blood cells. As shown in FIG. 1, the red and white blood cells also may be separated from each other and the white blood cells may form a small layer 26 between the plasma and the red blood cells. This layer 26 may be seen as a part of the interface 22.

[0061] As a further example, the two liquid media 20a, 20b may be two unmixable liquids, which are used for liquid-liquid extraction.

[0062] The laboratory automation system 10 furthermore comprises a pump 28, which is connected via a hose 30 with the pipette 14. With the pump 28, a pressure may be applied to the hose 30 and to the pipette 14, or more general to a volume 30 connected with the pipette 14, which causes the pipette 14 to aspirate or dispense a liquid medium 20a, 20b or any other fluid. For example, the pump 28 comprises a plunger 29, which is moved for generating underpressure and overpressure in the hose 30 and the pipette 14. Any other kind of pumps, e.g. peristaltic pumps or in general any source of vacuum and pressure may be used. The source of vacuum and pressure may be controlled via valves.

[0063] A pressure sensor 32, which may be attached to the hose 30 and/or the pipette 14, is adapted for measuring a pressure in the hose 30 and/or the pipette 14.

[0064] A control device 34 of the laboratory automation device 10, which may be a part of the laboratory automation device 10 or connected thereto, may control the pipetting arm 12, the pump 28 and may receive a pressure signal from the pressure sensor 32.

[0065] In general, liquid may be aspirated from the sample container 18 with the pipette 14 by generating an underpressure in the pipette 14, wherein the first liquid medium 20a of the two liquid media is aspirated andafter the interface 22 and the pipette tip 16 pass each otherthe second liquid medium 20b of the two liquid media is aspirated.

[0066] It may be that a pressure in the pipette 14 is measured with the pressure sensor 32, while the pipette tip 16 and the interface 22 move with respect to each other and the position of the interface 22 is detected, when a slope of the pressure changes.

[0067] FIG. 1 also shows a further sample container 36 into which the first liquid medium 20a will be dispensed as described below.

[0068] FIG. 2 shows the sample container 18 and illustrates a method for determining an estimated interface position z.sub.I of the interface 22 between the first liquid medium 20a and the second liquid medium 20b. The method may be performed by the laboratory automation device controlled by the control device 34.

[0069] At first, a surface position z.sub.0 of the surface 24 of the first liquid medium 20a is detected. This may be done by lowering the pipette 14 into the sample container and detecting the surface position z.sub.0 capacitively via the pipette tip 16 or by measuring a change of pressure by a pressure sensor via the pipetting tip, which is lowered towards the surface 24. The surface position z.sub.0 may be provided as a z-coordinate in vertical direction.

[0070] From the surface position z.sub.0, an estimated interface position z.sub.I of the interface 22 between the first liquid medium 20a and the second liquid medium 20b is calculated and therefrom a safety position z.sub.S is calculated, which is between the surface position z.sub.0 and the estimated interface position z.sub.I.

[0071] The estimated interface position z.sub.I is calculated by applying a mathematical formula to the surface position z.sub.0, such as

z.sub.I=p(z.sub.0z.sub.b)+z.sub.b

which applies to cylindrical sections of the sample container 18. z.sub.b is a base value, such as the position of the bottom of the sample container 18, and p may be a percentage value, such as 55% in the case of centrifuged blood sample. The estimated interface position z.sub.S may be linearly dependent on the surface position z.sub.0 in case of a cylindric container or any container with a flat bottom and walls perpendicular to the bottom. However, in case of a sample container 18 with a more complicated shape, also more complicated formulas may be used.

[0072] As a further example, for a round bottom tube, i.e. a container with circular cross-section with radius r and a hemispherical bottom cap, also with radius r, the interface position may be calculated by

z.sub.I=p[(z.sub.0z.sub.b)r/3]+r/3+z.sub.b

wherein z.sub.b is the coordinate of the bottom of the round bottom tube.

[0073] The parameters of the mathematical formula, such as z.sub.b and p, may be provided in the controller 34 and may be fixed with respect to the assay procedure.

[0074] The safety position z.sub.S is calculated by adding one or more safety offsets d.sub.1, d.sub.2 to the estimated interface position z.sub.I.

z.sub.S=z.sub.I+d.sub.1+d.sub.2

[0075] The statistical offset d.sub.1 may depend on the surface position z.sub.0, such as

d.sub.1=z.sub.I*s

where s is a constant value, which may depend on a variance of the interface position within the context of the application.

[0076] There also may be a constant offset d.sub.2, which for example is chosen, such that an interface detection by pressure measurements works properly, when starting from the safety position z.sub.S.

[0077] After the estimation process, the aspiration process starts. The pipette 14 of the laboratory automation device 10 is lowered into the sample container 18 and the first liquid medium 20a. The pipette tip 16 may be moved along a vertical direction, however, more complicated paths 38 (see FIG. 1) are possible, which may have components in a horizontal direction and/or inclined sections. The first liquid medium 20a is aspirated from the sample container 18 with the pipette 14 by generating an underpressure in the pipette until the pipette tip 16 of the pipette 14 reaches the safety position z.sub.S. During this first aspiration process, an interface detection may take place or not.

[0078] FIG. 3 shows a flow diagram for a method for aspirating the first liquid medium 20a and for transporting the first liquid medium 20a into the further sample container 36. The method may be performed by the laboratory automation device 10 controlled by the control device 34.

[0079] In step S10, the pipette 14 of the laboratory automation device 10 is lowered into the sample container 18 and the surface position z.sub.0 of the surface 24 of the first liquid medium 20a is detected, such as described with respect to FIG. 2. In particular, the pipette tip 16 is moved into the container 18, until the liquid level 24 being the boundary between the first liquid medium 20a and air is reached. The liquid level 24 may be detected with a capacitive method, for example as described above.

[0080] After that, the estimated interface position z.sub.I is calculated such as described above.

[0081] In step S12, the first liquid medium 20a is aspirated from the sample container 18 with the pipette 14 by generating an underpressure in the pipette until a pipette tip 16 of the pipette 14 reaches the safety position z.sub.S. When the pipette tip 16 is immersed in the first liquid 20a, liquid is aspirated from the sample container 18 with the pipette 14 by generating an underpressure in the pipette 14 with the pump 28.

[0082] This may be done in several passes, such as described above and below. After each pass, the liquid medium 20a in the pipette 14 may be dispensed into the further sample container 36.

[0083] In step S14, when the safety position z.sub.S has been reached, a pressure 40 (see FIG. 4) in the pipette 14 is measured during lowering the pipette further into the sample container 18. A measured interface position z.sub.M of the interface 22 is detected, when a slope 44 of the pressure 40 changes. This will be described in more detail with respect to FIG. 4.

[0084] When a difference of the estimated interface position z.sub.I and the measured interface position z.sub.M is higher than a threshold, a plausibility message 39a for the interface detection is provided. The threshold may be a fixed threshold or may depend on the surface position z.sub.0. For example, the controller 34 may generate data that the sample container 18 should be examined by a technician and/or should be discarded. The plausibility message 39a may contain such data.

[0085] After interface detection, a post-detection procedure may be performed. In particular, after the detection of the interface 22, the pipette 14 may be withdrawn from the sample container 18 and/or an amount of liquid medium from the pipette 14 may be dispensed such that an aspirated amount of second liquid medium 20b, and if desired a fraction of the first liquid medium 20a, is discarded.

[0086] For example, after detection of the measured interface position z.sub.M, the pipette tip 16 may be stopped at the detected position and an overpressure may be generated to dispense an amount of the second aspirated liquid medium 20b and/or an amount of the first aspirated liquid medium 20a from the pipette 14.

[0087] In the end, the first liquid medium 20a in the pipette 14 may be dispensed into the further sample container 36.

[0088] It also may be that the pipette 14 moves back to the first liquid container 18 and aspirates the remaining amount of the first liquid medium 20a with interface detection between air and the first liquid medium 20a. This is possible since the position of the interface position between the first liquid medium 20a and the second liquid medium 20b is now known.

[0089] FIG. 4 shows a diagram with a pressure signal or pressure curve 40, which has been recorded during a movement of the pipette tip 16 through the interface 22. FIG. 4 is an example of a pressure signal 40, which was recorded during aspiration of blood plasma, when entering from the plasma into the erythrocytes. When the pipette tip 16 goes through the interface 22, the slope of the pressure 40 changes.

[0090] In particular, the diagram shows the time t on the horizontal axis and pressure p on the vertical axis. At time t=0, the underpressure is generated and in a first time interval 42, the first liquid medium 20a starts to enter the pipette tip 16. In this time interval 42, the pressure 40 varies, since the flow in the pipette tip 16 has not yet stabilized. After the stabilization phase in the time interval 43, the slope of the pressure 40 is constant. The volume rate of the pump 28 has been adjusted to the amount of first liquid medium 20a flowing into the pipette 14. At point 44, the pipette tip 16 enters the second liquid medium 20b, which is more viscous and/or more dense as the first liquid medium 20a and in the following time interval 46, the slope of the pressure 40 changes (it deceases). This slope change is detected by the controller 34, which then stops the pump 28 and the movement of the pipette tip 16. After the stop of the pump 28, the pressure starts to increase again.

[0091] Due to the first time interval 42, it may be beneficial to introduce a waiting time t.sub.w after starting the pump 28, before the interface detection 22 becomes active, to ignore the pressure change when the first liquid medium 20a starts to enter into the empty pipette tip 16. The fixed offset d.sub.2 may be chosen in dependence of the movement speed of the pipette 14, such that the waiting time t.sub.w times the movement speed is less than the fixed offset d.sub.2.

[0092] Furthermore, it may be beneficial to stop immediately the movement of the pipette tip 16 and the pump 28 upon interface detection to avoid aspiration of a large amount of the second liquid medium 20b and to minimize a contamination risk.

[0093] To further improve the interface detection, it may be beneficial, that the pipette tip 16 solely moves when the pump 28 has a constant speed and/or volume rate. If the pump 28 is in an acceleration or deceleration phase, then the interface detection may be more difficult and/or not so accurate. It may be that the pipette tip 16 is solely moved, when a pump 28 generating the underpressure operates at a constant volume rate. When the pump 28 comprises a plunger 29, the pipette 14 may be solely moved, when the plunger 29 is moved with constant speed.

[0094] Returning to FIG. 3, step S12 may be performed in several passes. Before the safety position z.sub.S is reached, the first liquid medium 20a may be aspirated from the sample container 18 in several passes, wherein during each pass, an aspiration volume is aspirated from the sample container 18 and dispensed into the further sample container 36.

[0095] The amount of first liquid medium 20a in the pipette 14 is estimated, which may be determined from the volume capacity of the pipetting tip 16 and the volume rate of the pump 28. When an aspiration volume of the pipette 14 has been filled into the pipette 14, the pipette may be withdrawn from the sample container 18 and the content of the pipette 14 may be dispensed into the second sample container 36.

[0096] At the beginning of every pass, the air-liquid interface, i.e. the level 24, may be detected, for example with a capacitive method. However, for the second and further passes, the safety position z.sub.S and/or the estimated interface position z.sub.I are not calculated any more.

[0097] During each pass, the pipette 14 is lowered into the sample container 18 until the pipette tip 16 has passed a lowering distance l.sub.1 or l.sub.2, which is chosen, such that the pipette tip 16 passes the aspiration volume in the sample container 18. The aspiration volume and/or the lowering distance l.sub.1 or l.sub.2 of the pipette tip 16 for each pass may be chosen in dependence of the safety position z.sub.S. The sum of the lowering distances l.sub.1 and l.sub.2 may be chosen, such that at the end of the last pass during step S12, the safety position z.sub.S is reached.

[0098] There may be one, two or more passes during step S12. The last lowering during step S14 along a lowering distance 13 which may go beyond the estimated interface position z.sub.I may be seen as a further pass. The lowering distance 13 may be from the safety position z.sub.S to the estimated interface position z.sub.I subtracted by an offset, which may have the same length as the difference between the safety position z.sub.S and the estimated interface position z.sub.I.

[0099] It also may be that during step S12, interface detection takes places, i.e. the detection process of the interface 22 between the first liquid medium 20a and the second liquid medium 20b. In this case, a detected interface may indicate a problem with the sample in the sample container 18.

[0100] In other words, not only after but also before the safety position z.sub.S is reached, also a pressure 40 in the pipette 14 may be measured during the lowering of the pipette 14 and a measured interface position z.sub.M of the interface 22 may be detected, when a slope 44 of the pressure 40 changes. This may be done analogously, as described with respect to step S14.

[0101] When the interface 22 and/or the measured interface position z.sub.M is detected during step S12, i.e. before the safety position z.sub.S is reached, then a plausibility message 39b for the interface detection may be provided. As in step S14, this also may contain data indicating that there is a problem with the sample and/or that the sample should be checked by a technician.

[0102] When interface detection is performed during step S12 or step S14, a first aspiration rate during the lowering of the pipette 14 for the lowering distance l.sub.1, l.sub.2 and/or l.sub.3 (or until the interface 22 is detected) is lower than a second aspiration rate, after the lowering distance has been passed. When the interface 22 is detected, the lowering is stopped.

[0103] In this case, the movement during the lowering may be done rather fast and/or with a speed, such that the aspirated liquid medium 20a, 20b is enough for interface detection. When the end of the lowering distance 11, 12 is reached, the movement may be stopped and first liquid medium may be aspirated until the pipette 14 is filled up to the aspiration volume.

[0104] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or controller or other unit, such as an FPGA, may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS AND NOMENCLATURE

[0105] 10 laboratory automation device [0106] 12 pipetting arm [0107] 14 pipette [0108] 16 pipette tip [0109] 18 sample container [0110] 20a first liquid medium [0111] 20b second liquid medium [0112] 22 interface [0113] 24 surface, liquid level (between first liquid medium and air) [0114] 26 layer [0115] 28 pressure device, pump [0116] 29 plunger [0117] 30 hose [0118] 32 pressure sensor [0119] 34 control device [0120] 36 further sample container [0121] z.sub.0 surface position [0122] z.sub.S safety position [0123] z.sub.I estimated interface position [0124] z.sub.M measured interface position [0125] z.sub.b position of the bottom of the sample container [0126] d.sub.1 statistical offset [0127] d.sub.2 fixed offset [0128] s constant depending on a variance of the interface position [0129] 38 path [0130] 39a plausibility message [0131] 39b plausibility message [0132] 40 pressure [0133] 42 stabilizing time interval [0134] 43 constant slope time interval [0135] 44 change of pressure slope [0136] 46 constant slope interval [0137] 48 stop of pump [0138] t.sub.w waiting time [0139] l.sub.1 first lowering distance [0140] l.sub.2 second lowering distance [0141] l.sub.3 third lowering distance