Abstract:
A system and method for determining when a defective or non-defective pipette tip has been acquired by a robotic device performing a sample transfer, prior to the insertion of the defective pipette tip into the fluid sample, thereby preventing waste of the sample or unacceptable handling of the sample. Furthermore, the system and method can effectively eject pipette tips, and in some circumstances, determine whether the ejection of the pipette tip was successful.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a system and method for verifying the integrity of the condition and operation of a pipetter device for manipulating fluid samples in test tubes. More particularly, the present invention relates to a system and method for an automated pipetter device that makes use of pressure transducers to detect the presence and integrity of filtered pipette tips on the nozzle of the device, and to sense liquid levels in test tubes from which the pipetter device draws fluid samples.  
         BACKGROUND OF THE INVENTION  
         [0002]    A variety of molecular biology methodologies, such as nucleic acid sequencing, direct detection of particular nucleic acids sequences by nucleic acid hybridization, and nucleic acid sequence amplification techniques, require that the nucleic acids (DNA or RNA) be separated from the remaining cellular and non-cellular sample components. This process generally includes the steps of collecting a sample containing the cells of interest in a sample tube. The sample is then treated with heat or heat plus reagent, which causes the cells to rupture and release the nucleic acids (DNA or RNA) into the solution in the tube. Alternatively, the sample tube is placed in a centrifuge and spun down to separate the cells from other sample components. The resulting pellet is then re-suspended with an appropriate buffer and lysed as described above. The lysed solution containing free nucleic acids is removed from the sample tube by a pipette or any suitable instrument. The solution is then transferred to other tubes or microtiter wells containing reagents necessary for the desired downstream application. One such application, the amplification and detection of specific nucleic acid sequences, requires the addition of priming sequences, fluorescein probes, enzymes, and other reagents. The nucleic acids are then detected in an apparatus such as the BDProbeTec® ET system, manufactured by Becton, Dickinson and Company and described in U.S. Pat. No. 6,043,880 to Andrews et al., the entire contents of which is incorporated herein by reference.  
           [0003]    In order to properly control a pipetter device to draw fluid from a sample container such as a test tube, it is necessary to know the level of the sample fluid in the tube so the pipette can be lowered to the appropriate depth. It is also necessary to detect whether the pipette tip has been properly connected to the pipetter device. Prior methods to detect the level of a fluid in a container include the use of electrical conductivity detection. This method requires the use of electrically conductive pipette tips connected to a sensitive amplifier which detects small changes in the electrical capacitance of the pipette tip when it comes in contact with an ionic fluid. Pipette tip detection in this known system is achieved by touching the end of the conductive pipette tip to a grounded conductor. Drawbacks of this approach include the higher cost of conductive pipette tips, and that the method only works effectively with ionic fluids. In other words, if the fluid is non-conductive, it will not provide a suitable electrical path to complete the circuit between the conductors in the pipette tip.  
           [0004]    A system and method for the measurement of the level of fluid in a pipette tube has been described in U.S. Pat. No. 4,780,833, issued to Atake, the contents of which are herein incorporated by reference. Atake&#39;s system and method involves applying suction to the liquid to be measured, maintaining liquid in a micro-pipette tube or tubes, and providing the tubes with a storage portion having a large inner diameter and a slender tubular portion with a smaller diameter. A pressure gauge is included for measuring potential head in the tube or tubes. Knowing the measured hydraulic head in the pipette tube and the specific gravity of the liquid, the amount of fluid contained in the pipette tube can be ascertained.  
           [0005]    Devices used in molecular biology methodologies can incorporate the pipette device mentioned above, with robotics, to provide precisely controlled movements to safely and carefully move sample biological fluids from one container to another. Typically, these robotic devices are capable of coupling to one or more of the aforementioned pipette tips, and employ an air pump or other suitable pressurization device to draw the sample biological fluid into the pipette tips. However, these robotic systems presently have no suitable mechanism to determine whether any of the pipette tips are defective or have been properly acquired by the robot.  
           [0006]    Therefore, there exists a need for an improved system and method for determining the level of a fluid sample in a container. Also, there exists a need for a system and method for determining when a defective pipette tip has been acquired by a robotic device which is used in the fluid sample transfer process.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the invention to provide a system and method that effectively determines when a pipette tip has come into contact with a fluid sample in a container, to thus determine the level of fluid sample in the container, without the use of a specialized equipment, or restricted to applications in which only specific types of fluid samples can be used.  
           [0008]    It is therefore an additional object of the invention to use existing pipette technology to determine the condition of a pipette tip has been acquired by a robotic device performing a sample transfer, so that prior to insertion pipette tip into the fluid sample, it can be discarded if it is defective, thereby preventing waste of the sample or unacceptable handling of the sample.  
           [0009]    These and other objects of the invention are substantially achieved by providing a method for determination of a pipette tip&#39;s condition, comprising the steps of measuring pressure in a nozzle, acquiring a pipette tip with the nozzle, determining whether said pressure in the nozzle changes upon acquisition of the pipette tip, and ascertaining the condition of the acquired pipette tip based on the change in air pressure.  
           [0010]    Still another object of the invention is substantially achieved by providing another method for determination of a pipette tip&#39;s condition, comprising, measuring pressure in a nozzle, acquiring a pipette tip with the nozzle, determining a maximum air pressure in the nozzle upon acquisition of the pipette tip and ascertaining the acquired pipette tip&#39;s condition based on the rate of change in air pressure after the maximum air pressure was reached.  
           [0011]    A further object of the invention is substantially achieved by providing a method for discarding a non-defective pipette tip, comprising controlling an ejection assembly to engage said pipette tip from said nozzle, creating an air flow in said nozzle, determining whether said air flow causes a change in pressure in said nozzle and if said determining determines that substantially no pressure change has occurred ascertaining that the non-defective pipette tip has not been discarded.  
           [0012]    A system for determination of a pipette tip&#39;s condition, is provided comprising an air pump in communication with a nozzle, and a pressure transducer, adapted to measure a change in air pressure in the nozzle as the pipette tip is acquired by the nozzle.  
           [0013]    An additional system is provided according to the present invention for discarding a non-defective pipette tip, comprising an air pump with a nozzle, a pressure transducer, adapted to measure a change in air pressure in the nozzle as the pipette tip is acquired by the nozzle, and an ejection assembly adapted to eject a non-defective pipette tip.  
           [0014]    Another method according to the present invention is provided for detecting a level of liquid in a container using a pipette tip, comprising moving the pipette tip toward the liquid in the container without aspirating through said pipette tip while detecting for a change in air pressure in said pipette tip, and ascertaining that the pipette tip has entered the fluid holding container when said change in air pressure is detected.  
           [0015]    Lastly, another system according to the present invention is provided for detecting a level of fluid in a container using a pipette tip, comprising an air pump in communication with a nozzle, and a pressure transducer, adapted to measure a change in air pressure in the nozzle as the pipette tip is inserted onto the fluid holding container.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The novel features and advantages of the invention will be best understood by reference to the detailed description of the specific embodiments which follows, when read in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 illustrates a typical implementation of a robotic pipetting system for manipulating fluid samples which employs a system and method according to an embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a conceptual block diagram illustrating a cross sectional view of a pipetter device and pipette tip employed in the system shown in FIG. 1;  
         [0019]    [0019]FIG. 3 illustrates a frontal view of an industrial application of the pipetter device;  
         [0020]    [0020]FIG. 4 illustrates a right side view of the pipetter device;  
         [0021]    [0021]FIG. 5 illustrates a bottom perspective view of the pipetter device;  
         [0022]    [0022]FIG. 6 illustrates a front perspective view of the pipetter device;  
         [0023]    [0023]FIG. 7 illustrates a conceptual block diagram of a controller board assembly used with the system shown in FIG. 1;  
         [0024]    [0024]FIG. 8 illustrates a graph depicting an example of air pressure versus time during pipette tip acquisition, for a non-defective pipette tip;  
         [0025]    [0025]FIG. 9 illustrates a graph depicting an example of air pressure versus time during pipette tip acquisition, and its subsequent ejection, for a defective pipette tip;  
         [0026]    [0026]FIG. 10 illustrates a graph depicting an example of air pressure versus time during ejection of a non-defective pipette tip;  
         [0027]    [0027]FIG. 11 illustrates a graph depicting an example of air pressure versus time during insertion of a pipette tip into a fluid sample;  
         [0028]    [0028]FIG. 12 illustrates a flow diagram of an example of a first method according to an embodiment of the invention; and  
         [0029]    [0029]FIG. 13 illustrates a flow diagram of an example of a second method according to another embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    The various features of the invention will now be described with reference to the figures, in which like parts are identified with the same reference characters.  
         [0031]    [0031]FIGS. 1 and 2 illustrate a typical implementation of a robotic pipetting system pipetter device and pipette tip, for manipulating fluid samples which employs a system and method according to an embodiment of the invention. Pipetter device  200 , attached to the end of a robotic arm  102 , can acquire disposable pipette tips  202  from a holder onto the pipetter device nozzle  204 .  
         [0032]    The disposable pipette tips  202  are used to transfer biological (fluid) samples  218  from one container  216  in a diagnostic process to another. Each fluid sample  218  transfer requires a new pipette tip  202  to prevent cross contamination between fluid samples  218 . Additionally, each pipette tip  202  contains a filter  206  that prevents the fluid sample  218  from contaminating the nozzle  204  of the pipetter device  200 . As shown in FIG. 2, the pipetter device  200  employs a pressurization apparatus such as air pump  210 , with piston  210 A. The interior portion of air pump  210  is an air pump chamber  214  and is in communication with pressure transducer  208 , which measures the air pressure within the cavity formed within air pump  210  nozzle  204  of pipetter device  200  and pipette tip  202 . Shown also in FIG. 2 are originating position  212  and overdrive position  224 , which conveys the extent of travel of piston  210  within air pump  210 . These features will be discussed in detail below.  
         [0033]    FIGS.  3 - 6  illustrate various views of an industrial application of the pipetter device  200  and pipette tip  202  shown in FIGS. 1 and 2. FIG. 3 illustrates a frontal view. In FIG. 3, motor  302  is shown connected to lead screw  304 . Lead screw  304  is, in turn, also connected to piston drive bar  306 . Piston drive bar  306  is connected to actuating bars  310 A and  310 B, and both actuating bars  310 A,  310 B are connected to ejection bar  312 . Springs  310 A (left side) and  310 B (right side) act upon body part  314  to resist downward motion of piston drive bar  306 , and actuating bars  310 A,  310 B and ejection bar  312 . However, springs  308 A,  308 B are chiefly intended to assist in returning the aforementioned components to their resting position. The combination of motor  302 , lead screw  304 , piston drive bar  306 , springs  308 A,  308 B, actuating bars  310 A, B and ejection bar  312 , comprise the tip ejection assembly.  
         [0034]    The tip ejection assembly is designed to facilitate easy insertion of pipette tips  202  into nozzles  204 , yet provide a reliable means and manner for proper ejection of used and/or defective pipette tips  202 . Ejection bar  312  performs the physical ejection of pipette tips  202 . Ejection bar  312  has a plurality of holes; each hole allowing nozzle  204  to pass through it, so that it might be received into a pipette tip  202 . However, pipette tip  202  cannot pass through ejection bar  312 , because at the very bottom of pipette tip  202 , there is a flange  203  having a dimension larger than the body of pipette tip  202  and larger than the diameter of the holes in ejection bar  312 . Additionally, there are pipette tip adapters  316 , with upper adapter flange  318 A and lower adapter flange  318 B. Upper adapter flange  318 A and lower adapter flange  318 B mate with pipette tip  202 , providing a two-point seal that inn turn provides an air-tight interface between pipetter device  200  and pipette tip  202 .  
         [0035]    To eject pipette tips  202 , motor  302  turns lead screw  304 , which in turn forces piston drive bar  306  down. As piston drive bar  306  moves down, it forces actuating bars  310 A,  310 B down. This movement causes ejection bar  312  to move down, until ejection bar  312  encounter flanges  203  of pipette tips  202 . Flange  203  and ejection bar  312  come in contact and as ejection bar  312  continues its downward movement, it ejects pipette tips  202  from its mated connection with nozzle  204 . Then, motor  302  reverses and all the components of the tip ejection assembly move in the opposite direction. Springs  308 A,  308 B, which were compressed by the downward motion now decompress and assist in forcing the entire ejection assembly to its resting position. FIGS.  4 - 6  show different views of pipetter device  200  and pipette tip  202 . FIG. 4 is a right side view; FIG. 5 is a bottom-perspective view; and FIG. 6 is a front-perspective view.  
         [0036]    [0036]FIG. 7 illustrates a conceptual block diagram of a controller board assembly used with the system shown in FIG. 1. It is well known in the art that a robotic arm  102  may be controlled by a controller board  726  that is part of controller assembly  700 . Controller board  726  may contain processor  716  and memory  718  that stores executable software (system software)  722  that controls operation of robotic arm  102 , and pipetter device  200 .  
         [0037]    In general, controller assembly  700  will be designed to be able to control numerous robotic arms  102 . The number of robotic arms  102  able to be controlled by a single controller board is dependent upon several factors, including, but not limited to, the processing capability of processor  716  on the controller board, data acquisition rates, amount of memory, difficulty of tasks the robotic arms must perform, and how much data must be acquired about environmental conditions or the manufacturing process itself.  
         [0038]    As further shown in FIG. 7, a typical controller assembly includes controller board  726 , data and control cables  704 A-C and  706  that can be coupled to display  724 , motor  702  (that can control movement of piston  210 A), pressure transducer  208  and robotic arm  102 . Data and control cables  704 A-C might also be one continuous cable in some particular applications. As discussed above, controller board  726  includes memory  718 , which contains system software  722 , and can be connected by internal bus  724  to processor  716 . Processor  716  can be connected to network card  720 , by a second internal bus  726 , which can transfer collected data to and from network computer  730 . Processor  716  can communicate with analog-to-digital converter (ADC)  714  and input/output devices (I/O)  708 A by internal bus  724 . I/O  708 B is a different type of interface. Because it receives analog signals, these often require special cabling and coupling techniques to prevent the coupling of noise onto the signal. I/O  708 B are often separated from purely digital signals for these reasons. The received analog signal from I/O  708 B is first processed by AMP/filter  714 , which may contain an amplifier, filter, or even a level shifter, depending on the nature of the analog signal and ADC  712 .  
         [0039]    Controller assembly  700 , used in conjunction with an embodiment of the invention, is shown having a single ADC  712  and amplifier circuit  714 . In general, the amplifier  714  might also include a filter, which might be necessary depending on the nature of the analog signal received by controller board  726 . Controller board  726  communicates with robotic arm  102  via control/data bus  704 B. Control bus  704 A transmits control data from processor  716  to robotic arm  102 , and receives data from robotic arm  102 , which is reported to processor  716 . In this manner, motion control data is given to robotic arm  102 , and motion data that reports the movement of robotic arm  102  is fed back to processor  716 , providing a means for checking the movement and positioning of robotic arm  102 . Such data can include relative and absolute position in three axes (x, y and z), and relative and absolute velocity, acceleration and even angular velocities and acceleration measurements in the three axes.  
         [0040]    Controller assembly  700  communicates in a similar fashion with motor  302 . Control/data bus  704 A transmits control data to motor  302 , which controls the movement of piston  210 A of air pump  210 . Pressure transducer  208  outputs an analog pressure transducer (APT) signal  732 , transmitted on analog signal line  706 , which is connected to I/O  708 B on controller board  726 . For use in biotech and pharmaceutical industries, pressure transducer  208  is capable of detecting pressure with a resolution of 0.5 psi. After being received on I/O  708 B, APT signal  732  is input to AMP/filter  714 , which then outputs conditioned APT signal  734  to ADC  712 . ADC  712  converts conditioned APT signal  734  to a digital word, which can be processed by processor  716 . In this manner, processor  716  ascertains the air pressure in pipetter device  200 , and the methods of the invention including determining the volume of liquid in pipette device  200 , determining whether or not pipette tip  202  has entered fluid sample  218 , and determining whether or not a defective pipette tip  202  has been acquired by the robotic arm, and if not defective, when it has been discarded.  
         [0041]    [0041]FIG. 8 illustrates a graph depicting an example of air pressure versus time during pipette tip acquisition, for a non-defective pipette tip. During pipette tip  202  acquisition, robotic arm  102  moves pipetter device  200  to a holder that contains one or more pipette tips  202  (time T 0  in FIG. 8). Robotic arm  102  then positions nozzle  204  of the pipetter device  200  over a pipette tip  202  and pushes the nozzle  204  into pipette tip receptacle  202 A (time T 1  in FIG. 8). As nozzle  204  is pushed into the pipette tip  202 , air is forced through the filter  206 . This occurs between T 1 and T   2  in FIG. 8. Referring back to FIG. 2, air would flow through nozzle  204 , filter  206  and out opening  220  of pipette tip  202 . Because filter  206  restricts airflow, a momentary increase in air pressure is produced. In describing the embodiments of the invention, the convention used is that any increase in air pressure recorded by pressure transducer  208  is shown as a positive value (above the x axis). This is the situation when air enters pipette tip  202 . If air is released, or a vacuum created, air pressure is shown decreasing or becoming a negative value.  
         [0042]    Pressure transducer  208  mounted between the nozzle  204  and air pump  210  detects this momentary increase in air pressure and allows system software  722  to identify that a non defective pipette tip  202  has been acquired, and that filter  206  is in pipette tip  202 .  
         [0043]    At time T 2 , the air pressure measured by transducer  208  has reached a maximum, and begins to decay from time T 2  to T 3 . During the period of time from T 2  to T 3 , filter  206  allows the air pressure to decrease to  0 . This occurs because filter  206  is porous. The periods T 1 to T   2 , and T 2  to T 3  are dependent upon the type of filter  206  (i.e. what materials and manufacturing method used), and how fast nozzle  204  is inserted into pipette tip  202  (for the T 1 to T   2  period). In some applications, it is necessary for the air pressure to return to 0. Note that for a defective pipette tip  202 , which was completely blocked, i.e., little or no porosity in filter  206 , the air pressure versus time diagram would look similar to that of FIG. 8. The chief difference would be that the time it would take for air to escape from pipette tip  202 , through filter  206  (if at all possible), would be much longer. This is shown in FIG. 8 as the dashed lines in FIG. 8. Note that the dashed line of FIG. 8 eventually does return to zero at time T 3′ . As such, it may be possible to differentiate between a non-defective pipette tip  202  and a defective pipette tip  202  due to a completely or partially blocked filter  206 , by way of examining the rate of decay of the air pressure versus time, after a maximum air pressure had been reached after insertion of pipette tip  202 . Although this may have to be done on a trial basis, such a method can ensure the detection of defective pipette tips  202  due to blocked filters  206 .  
         [0044]    If an increase in air pressure is not detected between T 1 and T   2 , system software  722  will instruct robotic arm  102  to reject pipette tip  202  and acquire a new pipette  202  tip from the next location. Ejection of a defective pipette tip  202  is discussed in detail with respect to FIG. 9.  
         [0045]    [0045]FIG. 9 illustrates a graph depicting an example of air pressure versus time during pipette tip acquisition, and its subsequent ejection, for a defective pipette tip. At time T 0  in FIG. 9, robotic arm  102  is moving to acquire pipette tip  202 . At time T 1 , pipette tip  202  is acquired, and the nozzle is inserted in the period of time defined between T 1 and T   2 . As previously discussed, if a non-defective pipette tip  202  was acquired, there would be a positive change in air pressure measured by pressure transducer  208 . However, in this instance, pipette tip  202  is defective, and system software  722  notes that no change in air pressure has occurred. Therefore, from time T 2  to T 3 , robotic arm  102  moves pipette device  200  to a position in which defective pipette tip  202  can be discarded.  
         [0046]    In rejecting pipette tip  202 , robotic arm  102  moves from pipette tip  202  acquisition location, to an area where used or defective pipette tips  202  can be discarded, usually a waste container. This occurs from time T 2  to time T 3 . Pipette tips  202  are ejected from pipetter device  200  by over-driving the air pump  210  piston  210 A to overdrive position  224  in air pump chamber  214 , which engages the tip ejector assembly, and ejects defective pipette tips  202  into a waste container. The process by which this occurs was described above in detail with respect to FIGS.  3 - 7 . Because pipette tip  202  is defective (i.e. no filter  206 ). There will be no change in air pressure, even though piston  210 A has moved to overdrive position  224 . All the air simply escapes through the unrestricted opening  220  of pipette tip  202 .  
         [0047]    As piston  210 A then moves to its originating position, which occurs at time T 4 , the air pressure will not change. This is because there is no restriction to the flow of air within pipetter device  200 . After rejecting defective pipette tip  202 , robotic arm  102  can move pipetter device  200  to its starting position, or to a position to acquire a new pipette tip  202 . While robotic arm is moving pipetter device  200 , piston  210 A is recovering from its overdrive operation.  
         [0048]    [0048]FIG. 10 illustrates a graph depicting an example of air pressure versus time during ejection of a non-defective pipette tip. In FIG. 10, it is assumed a non-defective tip has already been acquired, and may have been used, but that in any case, it is desirable to eject it, and to acquire a new pipette tip  202  for a new use.  
         [0049]    At time T 1 , in FIG. 10, motor  302  is beginning to move piston  210 A to overdrive position  224 . This action also caused lead screw  304  to engage the tip ejection assembly, which ultimately causes ejection bar  312  to force the non-defective pipette tip(s)  202  off nozzle(s)  204 . Because these are non-defective pipette tips  202 , filter  206  will restrict air being forced out of air pump chamber  214 , and air pressure will rise. Pressure transducer  208  measures this air pressure rise and this information is communicated to controller board  726 , and ultimately processor  716 .  
         [0050]    At time T 2 , the tip ejection assembly has moved to a position where ejection bar  312  should force pipette tip  202  away from nozzle  204 . Between time T 2  and T 3  there will be a sudden decrease in air pressure, and the measured air pressure should, for a proper ejection, drop to a reading of, or about, zero. In general the ejection period could be sudden, but it might also be gradual; however, in a proper ejection of a non-defective pipette tip  202  the decrease in air pressure from T 2  to T 3  will be very quick. Therefore, at some short time later T 4 , a subsequent air pressure reading should indicate at, or about, zero, indicating no significant air pressure measured by pressure transducer  208 .  
         [0051]    If, however, at time T 4 , there is still a significant air pressure reading, this might indicate the ejection of pipette tip  202  was not successfully accomplished. The measured air pressure would then be indicated by the dashed lines in FIG. 10. Processor  716  recognizes that the air pressure should have returned to zero by the time T 4 , or even T 5 , but it has not. Therefore, it will attempt the tip ejection process again. As in the case of a non-defective pipette tip  202  acquisition, discussed in reference to FIG. 8, air pressure will eventually begin to reduce because of the porous nature of filter  206 . This is shown in the drop of pressure at T 5 . From time T 5  to T 6  piston  210 A returns to its originating position  212 , and causes the air pressure to return to, or about, zero. At some time later T 7 , the ejection process will begin again. Measured air pressure will rise, and at time T 8  the ejection assembly will again have moved to the position where ejection should have occurred.  
         [0052]    Thus, by measuring the air pressure through pressure transducer  208 , processor  716  can quickly determine whether non-defective pipetting tip  202  was properly ejected, and if not, re-active the tip ejection procedure.  
         [0053]    [0053]FIG. 11 illustrates a graph depicting an example of air pressure versus time during insertion of a pipette tip into a fluid sample. During the transfer of fluid samples  218  there is a need to limit the depth pipette tip  202  is submerged into container  216  to prevent overflowing and to minimize fluid build-up on the outer surface of pipette tip  202 . This is accomplished by monitoring the pressure within pipette tip  202  as it is submerged into fluid sample  218  to ascertain when pipette tip  202  insertion has occurred.  
         [0054]    The presence of fluids  218  in a container  216  is determined by measurement of the signal generated by pressure transducer  208 . Even a short insertion, e.g. several millimeters, of pipette tip  202  into fluid sample  218 , will cause a pressure change, readily ascertainable by pressure transducer  208  and system software  722 .  
         [0055]    However, the insertion of pipette tip  202  into fluid  218  by several millimeters to achieve reliable results may not be, under some circumstances, advantageous. Sometimes there is very little fluid to be spared, or, the fluid needs to be transferred as rapidly as possible. Therefore, and alternative method for ascertaining when pipette tip  202  insertion has occurred is to move pipette tip  202  through the air-to-liquid interface  222  while pump  210  is aspirating. In this manner, an adequate signal is achieved when opening  220  of pipette tip  202  initially penetrates fluid  218 . This approach allows detection of lower volumes of fluid  218  in small containers  216 . Detection of volumes as small as a milliliter are possible because pipette tip  202  needs only penetrate the air-to-liquid interface  222  a very small amount.  
         [0056]    Referring to FIG. 11, prior to insertion of pipette tip  202  into fluid sample  218 , robotic arm  102  moves pipetter device  200  into position during the period of time from T 0  to T 1 . From time T 1 to T   2 , pipette tip  202  is moved into fluid sample  218 . As pipette tip  202  is submerged into the fluid, fluid sample  218  compresses the air inside of pipette tip  202 . This compression registers as pressure reading P 1 . After a predetermined pressure is reached, P 1 , system software  722  commands robotic arm  102  to stop moving pipette tip  202  further into container  216 . This occurs at time T 2 . Pipetter device  200  then aspirates fluid sample  218  into opening  220  of pipette tip  202 , which is submerged in fluid sample  218 . This occurs from time T 2  to T 3 , and the pressure changes from P 1  to P 2 . P 2  is negative because air pump  210  is creating a vacuum to draw fluid sample  218  into pipette tip  202 . As fluid is drawn into pipette tip  202 , robotic arm  102  moves pipette tip  202  downward into container  216  at a speed based on the rate of aspiration and the diameter of the container  216 .  
         [0057]    The volume of fluid aspirated into the pipette tip can be verified using pressure transducer  208 . For example, U.S. Pat. No. 4,780,833, the contents of which are incorporated herein by reference, describes a system and method for determining the volume of a liquid sample drawn into a similar pipetter device  200 , by measuring the head pressure above the fluid column with knowledge of the fluid&#39;s specific gravity.  
         [0058]    At time T 3  aspiration of pipette tip  202  is stopped. The measured air pressure settles from P 2  to P 3 . P 3  is the air pressure that corresponds directly to the volume of liquid in pipette tip  202 . P 2  is the air pressure equal to the volume of aspirated fluid plus the friction force of the aspirated fluid sample  218 A to pipette tip  202  (inner wall surface) interface, due to surface tension. As the fluid is drawn up, it resists movement through friction; that friction is caused by, or directly proportional to, the surface tension of the fluid. When aspiration ceases, so does movement of the fluid and the friction due to the fluid&#39;s surface tension. Thus, at time T 4 , the measured air pressure is equivalent to the weight of aspirated fluid sample  218 A, and through use of its specific gravity (which is known, a priori), the fluid&#39;s volume is likewise known.  
         [0059]    From time T 4  to T 5 , robotic arm  102 , at the command of system software  322 , moves pipette device  200  to another location where another container,  216 A, might be located to dispense the aspirated fluid into. At time T 5 , piston  210 A begins pumping the aspirated fluid out, and at time T 6  the desired amount of fluid has been expelled. The resultant pressure, P 4  or P 4  might still be negative (i.e., in the case that only a small amount of aspirated fluid was pumped out, and there is still a negative pressure retaining the fluid) or positive (i.e., in the case that all or nearly all of the fluid pumped out, requiring greater “pumping” force).  
         [0060]    [0060]FIG. 12 illustrates a flow diagram of a first method according to an embodiment of the invention. The flow diagram illustrated in FIG. 12 shows the steps in a method for detecting defective pipette tips, as discussed above. The method begins with step  1202 , in which pressure transducer  208  measures a first air pressure, which is recorded by processor  716 . In step  1204 , robotic arm  102  moves pipetter device  200  such that nozzle  204  may be inserted over pipette tip receptacle  202 A of pipette tip  202 . In step  1206 , a second air pressure is measured and recorded, soon after the pipette tip  202  has been inserted over nozzle  204 . Processor  716  then compares the first air pressure to the second air pressure: If the second air pressure is greater than the first air pressure, then a non-defective pipette tip  202  has been acquired by robotic arm  102 , and it may be used for acquiring fluids (yes path  1210  from decision box  1208 ).  
         [0061]    If however, the first and second air pressure are substantially the same, i.e., there has been no change in air pressure in the acquisition of pipette tip  202  by robotic arm  102 , then processor  716  determines that a defective pipette tip  202  has been acquired, and can discard it, using the ejection process discussed in reference with FIG. 9 (no path  1212  from decision box  1208 ).  
         [0062]    [0062]FIG. 13 illustrates a flow diagram of a second method according to another embodiment of the invention. The flow diagram illustrated in FIG. 13 shows the steps in a method for determining whether a non-defective pipette tip has been ejected, as discussed above. The method according to FIG. 13 begins with step  1302 . In step  1302 , air pressure is measured continuously by pressure transducer  208 , and recorded by processor  716 . Then, in step  1304 , processor  716  decides to eject the non-defective pipette tip  202 , and causes robotic arm to engage the tip ejection assembly. Engaging the tip ejection assembly means that motor  302  begins to overdrive air pump  210 , and turn lead screw  304 , etc., as described with reference to FIGS.  3 - 6 . As the piston bar reaches its overdrive position  224 , processor  716  again monitors the measured air pressure: At this point, the tip ejection assembly should have forced pipette tip(s)  202  off nozzle(s)  204 . Therefore, in step  1308 , processor  716  compares the air pressure just before piston bar  210  reached overdrive position  224 , and the air pressure just after piston bar reached overdrive position  224 , to determine whether a substantial and sudden decrease in air pressure has occurred. This decrease in air pressure would be caused by air being suddenly released when pipette tip  202  was forcibly ejected from nozzle  204 , and the pressurized air in air pump chamber  214  and pipette tip receptacle  202 A was released into the atmosphere. If there was a sudden and substantial decrease in the measured air pressures, then pipette tip  202  was properly ejected (yes path  1310  from decision box  1308 ).  
         [0063]    If however, there was no sudden and substantial decrease in the air pressure between the time just before piston bar  210  reached overdrive position  224 , and the air pressure just after piston bar reached overdrive position  224 , the processor  716  determines that pipette tip  202  was not properly ejected (no path  1712  from decision box  1708 ). It will cause piston bar  210  to return to an intermediate position (i.e., between its originating position and overdrive position) and begin the process of ejecting pipette tip  202  again (i.e., it returns to step  1304 ). It may do this several times before pipette tip  202  is properly ejected.  
         [0064]    The embodiments described above are merely given as examples and it should be understood that the invention is not limited thereto. It is of course possible to embody the invention in specific forms other than those described without departing from the spirit and scope of the invention. Further modifications and improvements, which retain the basic underlying principles disclosed and claimed herein, are within the spirit and scope of this invention.