Abstract:
A pipetting device ( 2 ) for removing fluid from a sample vessel ( 52 ) includes a pipetting needle ( 4 ) and an auxiliary cannula ( 18 ) for piercing a septum, designed to guide the pipetting needle axially through the auxiliary cannula. The pipetting device has a guide arm ( 6 ), on the lower end ( 10 ) of which is arranged an end plate ( 12 ) that is axially displaceable along the guide arm against a resilient resistance. A centering device ( 14 ) inserts into the end plate of the guide arm, and at least three centering fingers ( 26 ) with conical bevels ( 34 ) are constructed on the radial outside of the centering device, distributed around the circumference thereof, and forming a holding-down device for the sample vessel. The disclosed construction makes possible to reliably pierce the septum of a sample vessel and to easily pull the pipetting needle out of the septum again even with thin pipetting needles.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims foreign priority under 35 U.S.C. §119(a)-(d) to German Application No. 10 2015 221 024.3 filed on Oct. 28, 2015, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a pipetting device for sampling fluid from a sample vessel that is sealed on the top side with a septum, comprising a pipetting needle as well as an auxiliary cannula for piercing the septum, designed to guide the pipetting needle axially through the auxiliary cannula. 
         [0003]    Such a pipetting device became known, for instance, from the “Triathlon user manual” published by Spark Holland B. V., Pieter de Keyserstraat 8, NL-7825 VE Emmen, the Netherlands, from April 2000. 
       BACKGROUND 
       [0004]    The present invention relates generally to the area of the preparation of liquid samples in a sample vessel, particularly to the sampling fluid from a sample vessel or the filling of sample fluid into a sample vessel, wherein the sample vessel is sealed on the top side with a septum. The invention relates particularly to the preparation of samples to be analyzed using NMR spectroscopy. However, the applicability of the invention is not limited to this area. 
         [0005]    NMR spectroscopy is a widely used method of measurement with which chemical compounds can be analyzed. In NMR spectroscopy, a sample to be measured, situated in a test tube, is typically placed into a probehead and measured in the NMR spectrometer. 
         [0006]    A so-called XYZ liquid handler is typically used to prepare samples for NMR measurements. Such liquid handlers comprise a pipetting device that can be moved in three spatial directions (X, Y, Z). A plurality of samples to be studied are prepared in sample vessels, each of which is sealed with a septum. These septa must be pierced by a needle with a bevel in order to reach the sample. The XYZ liquid handler usually has a so-called septum piercing needle for this purpose. The septum piercing needle is typically used both for piercing the septum and for handling the sample fluid. 
         [0007]    A liquid handler in which freely hanging septum piercing needles are used became known from the company brochure “SamplePro Tube—Customer Information” published by Bruker BioSpin GmbH, Silberstreifen 4, D-76287 Rheinstetten and dated 27 April 2015. This liquid handler further comprises a holding-down device for sample vessels. 
         [0008]    This liquid handler enables simple monitoring of the sample position with the aid of the holding-down device through simultaneous measurement of a motor current during movement to a set Z-position (vertical position) for some but not all commonly used sample vessels. Particularly with densely arranged vessels with a small opening, it is not possible to check the sample position. 
         [0009]    Besides sample vessels, which can sometimes have a relatively large top-side opening (3 to 5 mm diameter), objects with very small inside diameters (such as NMR tubes, for example), among other things, must also be filled. There are NMR tubes that must be filled with prepared sample fluid which have a 1.0 mm or 1.7 mm outside diameter and correspondingly small inside diameter of only 0.7 mm or 0.8 mm or 1.4 mm. Such NMR tubes can only be approached with a thin needle with small outside dimensions. However, needles with outside diameters of less than 1 mm are too labile to pierce septa, which are up to 3 mm thick. 
         [0010]    Septum piercing needles can have a length of up to 200 mm with outside diameters between 0.5 mm and 1.5 mm. They are therefore relatively labile. It is then necessary to guide the needles in order to prevent buckling or bending of the needles. 
         [0011]    In the “Triathlon user manual” cited at the outset, a pipetting device for a liquid handler is described that has a piercing needle and a sample needle. The piercing needle is used to pierce a septum sealing a sample vessel. The analysis needle is arranged concentrically within the piercing needle and is used to handle sample fluids. 
         [0012]    Due to its construction and mechanism for moving the pipetting device, however, a liquid handler is prone to certain imprecisions in the spatial alignment of the pipetting device. Furthermore, the sample vessels used have (dimensional) tolerances. The positioning of the sample vessels in a holding device is also subject to certain (positional) imprecisions. This can result in deviations in the relative position between the pipetting device and the sample vessels, quite possibly on the order of +/−1 mm. Moreover, there are sample vessels with very narrow openings in NMR applications, such as so-called NMR SampleJet tubes, for example, which have an opening diameter of approximately 2 mm. The danger therefore exists of the needles used in the pipetting device of the liquid handler are not striking the center of the septum or even hitting an upper edge of the sample vessel. Such out-of-position of the needles must be avoided, since it would inevitably result in the destruction of the needles. 
         [0013]    Furthermore, the friction of the septum retains a needle that is inserted into the septum. When the needle is pulled out, the septum, or a sample vessel comprising the septum, must therefore be actively pushed off. This can be accomplished by anchoring the sample vessel firmly on a work plate, although that requires an individualized and often elaborate mechanism for each position and vessel shape. 
       SUMMARY 
       [0014]    It is an object of the invention to provide a structurally simple pipetting device that makes it possible even when using very thin pipetting needles to reliably pierce the septum of a sample vessel and to easily pull the pipetting needle out of the septum again. 
         [0015]    This object is achieved by the present invention in a manner that is just as surprisingly simple as it is effective, namely in that the pipetting device comprises a guide arm on the lower end of which is arranged an end plate that is axially displaceable along the guide arm against a resilient resistance, in that a centering device is provided that can be inserted into the end plate of the guide arm, and in that at least three centering fingers with conical bevels are constructed on the radial outside of the centering device, distributed around the circumference thereof, forming a holding-down device for the sample vessel and able to engage around the top portion of the sample vessel from the outside during operation. 
         [0016]    Through use of the centering device, the pipetting device can be aligned precisely relative to the sample vessel. In this way, it can be ensured that the septum is always pierced in its center. The septum can be made, for example, of silicone, rubber, PTFE, or of various combinations of materials, such as silicone with a PTFE coating, for example. 
         [0017]    The auxiliary cannula is used to pierce the septum. For this purpose, it can be embodied so as to have an outside diameter that is large compared to the pipetting needle as well as a greater wall thickness. The auxiliary cannula also serves to guide and support the pipetting needle. The pipetting needle can be passed axially through the auxiliary cannula. An inside diameter of the auxiliary cannula therefore corresponds at least to an outside diameter of the pipetting needle. 
         [0018]    The centering of the pipetting device on the sample vessel is necessary particularly if an opening of the sample vessel that is sealed by the septum has a small diameter. In such cases, even small positioning errors result in an upper edge of the sample vessel being struck upon attempting to pierce the septum. The centering of the pipetting device on other sample vessels with a larger septum opening, such as 2 ml auto-sampler vials, for example, is also advantageous. A septum is substantially easier to pierce in the center than outside of the center. The stress on the auxiliary cannula is therefore less. 
         [0019]    The centering device can be inserted into the end plate of the guide arm. The end plate is arranged on the underside of the guide arm. The end plate is displaceable along the guide arm against a resilient resistance. The guide arm can be designed with a linear guide for the end plate for this purpose. The resilient resistance is preferably provided by a spring, particularly a cylindrical coil spring. Alternatively, a leaf spring, a gas spring, a rubber spring, or other mechanisms with elastically deformable elements can also be provided. The resilient resistance counteracts a movement of the end plate toward a top-side suspension of the guide arm. A home position into which the resilient resistance tries to push the end plate can be defined by a stop on the guide arm. 
         [0020]    The centering device has at least three centering fingers with conical bevels that are distributed along the radial outside circumference of the centering device. The lock-in range of the centering device is enlarged by the bevels of the centering fingers. Sample vessels that are initially arranged significantly to the side of a center axis of the centering device are caught by the centering fingers and pressed into the center of the centering device upon lowering of the centering device. In this way, it can be achieved that objects (i.e., sample vessels in particular) with outside diameters of 6 to 14 mm, for example, can be reliably captured and centered. Depending on their outside diameter, the centered sample vessels can abut with a top-side edge against the bevels of the centering device, or be received within a substantially cylindrical or cylindrical segment-shaped centering recess. 
         [0021]    During the operation of the pipetting device, the centering device is pressed by the resilient resistance against the sample vessel upon the lowering of the top-side suspension of the guide arm. In this way, it can be achieved that the sample vessel slides into the center of the centering device and that the sample vessel is held there securely. The centering fingers then engage around the sample vessel on the top side thereof from the outside. 
         [0022]    The auxiliary cannula is typically fastened to the guide arm above the displaceable end plate. The auxiliary cannula can be screwed into a projection of the guide arm for this purpose. A longitudinal axis of the auxiliary cannula is usually oriented parallel to a direction of longitudinal extension of the guide arm. 
         [0023]    Preferably, the auxiliary cannula is not displaceable in relation to the top-side suspension of the guide arm, particularly not in the axial direction—that is, along the guide arm. A lower, free end of the auxiliary cannula is designed to pierce the septum. If the end plate with the centering device is in a lower position, the free end of the auxiliary cannula is preferably arranged within the centering device. If the centering device is pushed upward against the resilient resistance, the free end of the auxiliary cannula then emerges from the centering device. The free end of the auxiliary cannula can then pierce the septum of the sample vessel. The centering function is maintained throughout the entire procedure, since the pipetting needle is displaced relative to the centering device, whereas the centering device is always pressed by the resilient resistance onto the sample vessel. 
         [0024]    The pressing of the centering device against the sample vessel by the resilient resistance also has the effect that the sample vessel is not lifted when the auxiliary cannula is pulled out of the septum. The centering fingers that engage around and hold the top side of the sample vessel thus form a holding-down device for the sample vessel. In their capacity as seals, septa per se have a high level of friction, so the auxiliary cannula must be pulled out of them forcefully. Preferably, the resilient resistance is therefore set up and coordinated with the longitudinal position of the free end of the auxiliary cannula relative to the guide arm such that sufficient contact pressure of the centering device against the sample vessel still exists when the auxiliary cannula has already been pulled out of the septum to the greatest possible extent. The centering device on the resiliently supported end plate thus forms a general (universally usable) stripper that can be used in various types of sample vessel. Separate, elaborate devices for anchoring the sample vessels on a platform so that they cannot be lifted in the vertical direction are therefore not necessary when using the pipetting device according to the invention. 
         [0025]    With the present invention, the following advantages are also particularly achieved: 
         [0026]    Pipetting needles having different dimensions can be used, for example with a length between 100 mm and 250 mm, an outside diameter between 0.5 mm and 2.5 mm and/or an inside diameter between 0.25 mm and 1.6 mm. Depending on the application, it can be necessary to use pipetting needles having different outside diameters (0.5 mm to 2.5 mm) on the same system. The auxiliary cannula acts as a needle guide. Auxiliary cannulas can be given different inside diameters, each of which is only slightly larger than the outside diameter of the pipetting needle being used, whereby the pipetting needle is guided in a precise and reliable manner in the auxiliary cannula. 
         [0027]    The use of pipetting needles with small outside diameters is made possible. Particularly, the pipetting needles need not have such a stable design that they are able to pierce the septum. The auxiliary cannula can be short and relatively strong (large diameter, large wall thickness) in order to enable even thick septa to be pierced. The auxiliary cannula creates an opening through which the pipetting needle can be inserted into and removed from the sample vessel effortlessly and without stressing the pipetting needle. 
         [0028]    The centering device ensures central penetration of the septum, particularly even in the case of critical sample vessels with a small opening. Due to the conical bevels of the centering fingers in an area of contact with the sample vessel, the centering device has an overall conical shape, thereby creating a catch range that covers up to +/−3 mm depending on the radial positional deviations between sample vessel and centering device. That is, even a sample vessel that is initially arranged off-center relative to the centering device is caught by the centering device and aligned concentrically to the centering device when the centering device is pressed down on the same. The septum can then be pierced precisely in the center. 
         [0029]    The resilient supporting of the centering device on the guide arm ensures that the sample vessel is held down when the auxiliary cannula is pulled out. 
         [0030]    An embodiment of the monitoring cell according to the invention is especially preferred in which the centering device can be screwed into the end plate. This enables the centering device to be fastened easily and securely in the end plate. In particular, the centering device can also be replaced quickly. It can be necessary to change the centering device due to wear, or in order to adapt to the specific conditions of the application. 
         [0031]    An alternative embodiment to this configures the centering device to be engaged, clamped or inserted into the end plate with a bayonet socket. This enables the centering device to be changed very easily. In particular, such fastening mechanisms can also be operated safely and reliably by laboratory staff. 
         [0032]    An embodiment that is especially preferred is one in which an anti-twist device is provided that is embodied such that it enables a defined alignment of the centering device relative to the end plate, with the anti-rotation device having particularly at least two, preferably four, top-side projections of the centering device that engage laterally around the end plate. The anti-twist device ensures that the alignment of the centering device can no longer change during use. The precise alignment of the centering fingers is important in order to prevent conflicts with neighboring sample vessels. In particular, in the case of tightly arranged sample vessels, it is possible to utilize the clearances between the sample vessels to move the centering fingers past the sample vessels only if the centering device is oriented with precision. Typically, the sample vessels are arranged next to one another so as to be tightly packed in a quadratic pattern. Even a slight rotation of the centering device would negate the function, since the centering finger would collide with neighboring sample vessels. The projections arranged on the top side and a width of the end plate are coordinated with one another such that the centering aid adjusts itself automatically upon insertion and is secured in the correct alignment against rotation. 
         [0033]    Another advantageous embodiment is one in which a provision is made that the centering device has on its bottom side a preferably annular lift-off aid arranged coaxially to the auxiliary cannula. With the lift-off aid, possible droplet formation on the auxiliary cannula can be prevented from causing sample vessels to become stuck on the centering device as a result of adhesion and removed from their original position. To wit, liquid droplets cannot be completely prevented from adhering to both the pipetting needle, the auxiliary cannula and the centering device after pipetting or after a cleaning and washing procedure. Without the lift-off aid on the bottom side of the centering device, the danger exists that the sample vessel will “stick” as a result of adhesive forces of the liquid between the septum and/or sample vessel and the centering aid and be removed from its position, or that the pipetting device will be damaged upon further spatial movement of the pipetting device. Through the lift-off aid, the contact surface of the centering device is reduced so much that this effect is prevented. 
         [0034]    The lift-off aid can also particularly be formed by ring segments. The lift-off aid ensures that, after the auxiliary cannula is pulled out of the septum, the centering device detaches from the sample vessel and can be lifted off of the same. For this purpose, a contact surface between the septum and the centering device is reduced by the lift-off aid. The lift-off aid prevents large-surface contact of the centering device with the septum. Instead, one or more small areas of contact are created. Particularly, a very narrow (e.g., with a width of less than 1.0 mm or less than 0.6 mm), annular area of contact can be established. In this way, adhesive forces or the like between the centering device and the sample vessel and/or the septum can be reduced so much that the weight of the sample vessel is sufficient to overcome these forces. 
         [0035]    One preferred embodiment is characterized in that a ventilation channel is established for the sample vessel between the auxiliary cannula and the pipetting needle when the pipetting needle passes axially through the auxiliary cannula. This enables the precise removal of a predetermined volume of sample fluid from the sample vessel or the precise filling of a predetermined volume of sample fluid into the sample vessel. 
         [0036]    With the pumps usually used for pipetting, the conveyance (aspirate and dispense) of exact volumes is pressure-dependent. A pressure equalization between the surroundings and an interior space of the sample vessel is therefore necessary in order to precisely adhere to a desired quantity of liquid (volume of sample fluid to be aspirated or dispensed). After the septum is pierced, the sample vessel must therefore be ventilated in order to prevent under-pressure when the sample fluid is aspirated out of the sample vessel, or over-pressure during the dispensing of sample fluid into the sample vessel. To establish the ventilation channel, spacing is preferably set up between an outer wall of the pipetting needle and an inner wall of the auxiliary cannula. In particular, an outside diameter of the pipetting needle can be selected so as to be smaller than an inside diameter of the auxiliary cannula. 
         [0037]    The pipetting needle can then be arranged concentrically or off-center in the auxiliary cannula. Alternatively or in addition, the outside of the pipetting needles and/or the inside of the auxiliary cannula can also be provided with a continuous notch in the longitudinal direction, such as in the manner of an engraving. In any case, a ventilation channel is opened for the sample vessel that enables a pressure equalization between the surroundings and the interior space of the sample vessel. In this way, it can be achieved that the same pressure conditions are always prevalent in the sample vessel during pipetting. 
         [0038]    One advantageous embodiment makes a provision that the centering fingers of the centering device are beveled on the bottom side in the circumferential direction of the centering device. This enables conflicts with neighboring sample vessels to be minimized, i.e., the centering fingers can be prevented from colliding with directly adjacent sample vessels. 
         [0039]    One embodiment is preferred in which the centering device has indentations in the shape of a circular segment (circular section) between the centering fingers. In this way, it can be achieved that only a small amount of space needs to be furnished between neighboring sample vessels (e.g., only 3 mm or even only 2 mm) in order to enable the centering device to engage around them. The centering fingers project into free spaces between mutually adjacent sample vessels, whereas the indentations are arranged between the neighboring sample vessels. This minimizes the space required between the sample vessels. 
         [0040]    The sample vessel is typically arranged in a tightly packed manner with other sample vessels on a work platform. For example, 96 NMR tubes can be arranged next to one another very densely—that is, with only a small amount of spacing—in the well plate format. By virtue of the centering device with indentations, they can be approached with precision using the pipetting device. Another function of this centering device is making a determination of whether a selected sample vessel is located in an approached position or whether this position is unoccupied. To achieve this, the centering device must be able to travel lower than a centering position (at least lower than the vertical position of the centering device in which the centering device is seated so as to be centered on the sample vessel). To enable this lower position between the other vessel positions to be reached, the centering aid is also designed with indentations having the shape of a circular segment. 
         [0041]    In one especially preferred embodiment, a provision is made that the centering fingers are arranged so as to be distributed uniformly around the periphery of the centering device. The sample vessels are typically arranged uniformly in holders. This results in uniformly distributed free spaces into which the centering device can engage with uniformly distributed fingers. Furthermore, this ensures that the centering device is supported on the sample vessel equally in all horizontal directions. 
         [0042]    One embodiment that is very especially preferred is one in which the centering device has four centering fingers that are respectively arranged opposite one another in pairs. As a rule, the sample vessels are arranged uniformly in rows and lines in holders. This results in four free spaces on the sample vessel respectively arranged opposite one another in pairs. The centering device can engage into these free spaces if the centering fingers are respectively arranged opposite one another in pairs. 
         [0043]    An embodiment is also advantageous in which the centering device can also be inserted into the end plate of the guide arm without a tool and removed from the end plate. This enables the especially fast and easy replacement of the centering device. Particularly, this makes it possible even for technically untrained laboratory staff or support personnel to safely and flawlessly exchanging the centering device. It can be necessary to replace the centering device if sample vessels with different outside diameters and/or other pipetting needles and/or other auxiliary cannulas are to be used. 
         [0044]    The scope of the invention also encompasses a method for detecting the presence and, optionally, the position of a sample vessel using the pipetting device according to the invention described above. This method includes:
   a. Establishing of a vertical target position for the centering device;   b. Lowering the centering device on the guide arm while monitoring a current vertical position of the centering device until a resistance counteracts further lowering or until the centering device has reached a predefined lowest position;   c. Evaluating the vertical position of the centering device reached in step b),
       i. with a vertical position below the target position signifying that no sample vessel was found,   ii. with a vertical position that corresponds to this target position signifying that the centering device is centered on a sample vessel, and   iii. with a vertical position above this target position signifying that the centering device is located on a sample vessel but is not centered.   
       
 
         [0051]    This method makes it possible to identify automatically whether the sample vessel is present and whether the centering device is centered on the sample vessel. The automatic detection of whether the sample vessel is present and whether the centering is correct increases the reliability of the automation substantially. It is actively checked for this purpose whether a sample vessel is located at the intended position and preparation can be carried out with sample fluid from the sample vessel. 
         [0052]    In the prior art, a bar code reader is used for this purpose, or images are created and subjected to a relatively elaborate software-based evaluation. In the case of tightly packed, small sample vessels, however, this is only possible with great effort and expense. An additional gripper module is then necessary, for example, which transports the sample vessel to a barcode reader or which scans the position at which the sample vessel is expected using an additional sensor. This additional effort can be avoided according to the invention. 
         [0053]    The contacting of the centering device with the sample vessel can be determined, for example, by monitoring a current through a drive motor for moving the pipetting device vertically. Upon contacting a sample vessel, this current typically rises. The rise in the current can be detected using software. The current vertical position of the centering device can be determined directly using a distance measurement device or indirectly from operating states (e.g., the profile of current consumption over time) of the drive motor. 
         [0054]    In case “i.,” no sample vessel was found. Then a preparation request will be cancelled. The process continues to the next preparation request. In situation “ii.,” everything is ok, and a preparation is processed normally with the sample vessel. In situation “iii.,” no centering occurred. This is a critical state that can result in the destruction of the auxiliary cannula and/or pipetting needles or to the loss of the sample. In this case, the preparation is aborted. 
         [0055]    The invention further relates to a method for removing sample fluid from a sample vessel using a pipetting device according to the invention, wherein the above-described method according to the invention for detecting a sample vessel is first carried out, and wherein the removal process is aborted in cases i. and iii., and the following additional steps are carried out only in case ii.:
       d) Further lowering a top-side suspension of the guide arm against the resilient resistance, so that the auxiliary cannula pierces the septum of the sample vessel;   e) Axially guiding the pipetting needle through the auxiliary cannula into the sample vessel;   f) Aspirating sample fluid from the sample vessel into the pipetting needle;   g) Pulling-out the pipetting needle from the sample vessel;   h) Lifting the top-side suspension of the guide arm until the auxiliary cannula has been pulled out of the septum, the holding-down device pressing against the top side of the sample vessel; and   i) Further lifting the top-side suspension of the guide arm, whereby the centering device detaches from the sample vessel.       
 
         [0062]    The method according to the invention can also be used analogously for dispensing sample fluid into a sample vessel. 
         [0063]    This method makes it possible to perform sample preparation in the sample vessel in a reliable and safe manner. Particularly, separate anchoring of the sample vessel on a work platform can be omitted. Through the centering device, it is also ensured that the septum of the sample vessel is pierced in the center. The pipetting needle can thus be reliably protected from damage. 
         [0064]    After the auxiliary cannula has pierced into the sample vessel, the thinner pipetting needle is then pushed in, and sample fluid is removed or added. Through a ventilation channel between the auxiliary cannula and the pipetting needle, pressure equalization can occur simultaneously. After the pipetting needle is pulled out, the guide arm also moves upward along with the auxiliary cannula, with the resiliently supported end plate initially remaining with the centering device due to the spring pressure on the sample vessel. After the auxiliary cannula has been pulled completely out of the septum, the guide arm is lifted further, with the end plate being pressed with the centering device by the resilient resistance into a home position that can be defined by a stop on the guide arm. In the home position, a bias of the resilient resistance can remain. 
         [0065]    Through the bias, it can be ensured that the centering device is pressed against the sample vessel during the entire process of the auxiliary cannula being pulled out of the septum, with the contact pressure force exceeding the force required for the withdrawal of the auxiliary cannula at all times. The home position of the end plate on the guide arm is preferably reached only after the auxiliary cannula has been pulled completely out of the septum. After the end plate has returned with the centering device into the home position, the centering device also detaches again from the sample vessel. 
         [0066]    Additional advantages of the invention follow from the description and the drawing. Likewise, the features cited in the foregoing and in the following can each be utilized according to the invention individually or together in any combination. The embodiments that are shown and described must not be understood as an exhaustive enumeration, but rather as examples intended to illustrate the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0067]    The invention is illustrated in the drawing and is explained in further detail with reference to exemplary embodiments. 
           [0068]      FIG. 1A  shows a schematic representation of a pipetting device according to the invention with an end plate in a home position; 
           [0069]      FIG. 1B  shows the pipetting device of  FIG. 1  with the end plate in a pipetting position; 
           [0070]      FIG. 2A  shows a schematic representation of a centering device for a pipetting device according to the invention in an oblique view from below; 
           [0071]      FIG. 2B  shows an oblique view from above of the centering device of  FIG. 2A ; 
           [0072]      FIG. 3A  shows a schematic representation of sample vessels and a centering device for a pipetting device according to the invention in case i. during a method according to the invention; 
           [0073]      FIG. 3B  shows the arrangement of  FIG. 3A  in a view along sectional line A-A; 
           [0074]      FIG. 3C  shows a schematic representation of sample vessels and a centering device for a pipetting device according to the invention in case ii. during a method according to the invention; 
           [0075]      FIG. 3D  shows the arrangement of  FIG. 3C  in a view along sectional line A-A; 
           [0076]      FIG. 3E  shows a schematic representation of sample vessels and a centering device for a pipetting device according to the invention in case iii. during a method according to the invention; 
           [0077]      FIG. 3F  shows the arrangement of  FIG. 3E  in a view along sectional line A-A; 
           [0078]      FIG. 4A  shows a schematic view of a pipetting device according to the invention above a sample vessel before a pipetting procedure; 
           [0079]      FIG. 4B  shows the arrangement of  FIG. 4A  with centering device placed on the sample vessel; 
           [0080]      FIG. 4C  shows the arrangement of  FIG. 4A  with auxiliary cannula inserted into the septum of the sample vessel; 
           [0081]      FIG. 4D  shows the arrangement of  FIG. 4A  with pipetting needle introduced into the sample vessel; 
           [0082]      FIG. 4E  shows the arrangement of  FIG. 4A  with pipetting needle pulled out of the sample vessel; 
           [0083]      FIG. 4F  shows the arrangement of  FIG. 4A  with pipetting device lifted from the sample vessel after conclusion of the pipetting procedure; 
           [0084]      FIG. 5A  shows a first embodiment of a very thin sample vessel for a pipetting device according to the invention; 
           [0085]      FIG. 5B  shows a second embodiment of a very thin sample vessel for a pipetting device according to the invention; 
           [0086]      FIG. 6A  shows a third embodiment of a sample vessel for use with a pipetting device according to the invention; 
           [0087]      FIG. 6B  shows a fourth embodiment of a sample vessel for use with a pipetting device according to the invention; 
           [0088]      FIG. 7  shows a pipetting device with a freely hanging septum-piercing needle from the prior art. 
       
    
    
     DETAILED DESCRIPTION 
       [0089]      FIG. 1A  of the drawing shows a schematic view of a preferred embodiment of a pipetting device  2  according to the invention. The pipetting device  2  comprises a pipetting needle  4  and a guide arm  6 . The guide arm  6  has a top-side suspension  8 . An end plate  12  is arranged at a lower end  10  of the guide arm  6 . A centering device  14  is screwed into the end plate  12 . A projection  16  is constructed on the guide arm  6  above the end plate  12 . An auxiliary cannula  18  is screwed into the projection  16 . 
         [0090]    The end plate  12  with the centering device  14  can be displaced along the guide arm  6  against a resilient resistance, which is formed here by a spring  20 . In order to enable the displaceability, the guide arm  6  has a linear guide  22  with a stop  24 . In  FIG. 1A , the end plate is in a home position into which it is pushed by the spring  20 . Here, the stop  24  defines the home position relative to the upper end  8  of the guide arm  6 . In the depicted home position, a free end of the auxiliary cannula  18  is surrounded by the centering device  14 . 
         [0091]    Here, the centering device  14  has four centering fingers  26  mutually opposing one another in pairs on its bottom side. The centering fingers  26  are distributed here uniformly over the periphery of the centering device  14 ; that is, all of the centering fingers  26  are spaced at an equal distance from one another. Here, an anti-twist device  28  is constructed on the top side of the centering device  14 . Here, the anti-twist device  28  comprises four projections  30 . Two of the projections  30  engage laterally around the end plate  12 . In this way, it can be ensured that the centering device  14  does not twist with respect to the end plate  12  during the operation of the pipetting device  2 . The projections  30  are coordinated here with the end plate  12  such that the centering device  14  can be screwed into the end plate  12  and unscrewed from the end plate  12  without a tool, i.e., with bare hands. 
         [0092]      FIG. 1B  shows the pipetting device  2  of  FIG. 1A , although here the end plate  12  is displaced upward with the centering device  14  along the guide arm  6  toward the upper suspension  8 . The spring  20  has been compressed as a result. The free end  32  of the auxiliary cannula  18  now projects out of the bottom side of the centering device  14 . 
         [0093]      FIG. 2A  shows a schematic oblique view from below of a centering device  14  for a pipetting device according to the invention (not shown). Here, the centering device  14  has four uniformly distributed centering fingers  26  mutually opposing one another in pairs. Each centering finger  26  has a conical bevel  34  pointing radially inward. The conical bevels  34  form together a holding-down device for a sample vessel (not shown). A cylindrical-segment-shaped centering recess  38  adjoins the conical bevels  34  of the centering fingers  26  toward the top. The centering fingers  26  are also beveled here on the bottom side in the circumferential direction of the centering device  14  by angled surfaces  40 . As a result of the conical bevels  34  and the angled surfaces  40 , the centering fingers  26  thus have an overall faceted shape at the bottom end and taper to a point. 
         [0094]    Here, the centering device  14  has a lift-off aid  42  on the bottom side between the centering fingers  26  within the centering recess  38 .The lift-off aid  42  is designed here in the shape of a cylindrical ring. The lift-off aid  42  also defines an upper stop surface  44  for a sample vessel within the centering recess  38 . 
         [0095]      FIG. 2B  shows an oblique top view of the centering device  14  of  FIG. 2A . The centering device  14  is designed here with circular-segment-shaped indentations  46  between neighboring centering fingers  26 . A threaded portion  48  for engaging in a corresponding thread of an end plate (not shown) is constructed on the top side of the centering device  14 . Here, the centering device  14  has a projection  30  above each of the centering fingers  26 . The projections  30  form together an anti-twist device  28  for the centering device  14 . In the circumferential direction of the centering device  14 , each of the projections  30  has two chamfers  50 . The chamfers  50  facilitate the locking (snapping-in) of the anti-twist device  28  on an end plate. 
         [0096]      FIG. 3A  shows a schematic view of a centering device  14  of a pipetting device according to the invention (not shown) and eight sample vessels  52  during a method according to the invention. 
         [0097]      FIG. 3B  shows the arrangement of  FIG. 3A  in a side view at sectional plane A-A. Here, the centering device  14  was lowered in the middle between two sample vessels  52 . Four centering fingers  26  of the centering device  14  engage in free spaces between the sample vessels  52 . Since the middle space for a sample vessel is empty, the centering device  14  was able to be lowered so far that a current vertical position  54  of the centering device  14  lies below a vertical target position  56  for the centering device  14  (case i.). The vertical target position  56  corresponds here to a height of the sample vessels  52 . The centering device  14  can be lowered here even farther to a predefined lowest position  58 . 
         [0098]      FIG. 3C  shows a schematic view of a centering device  14  of a pipetting device according to the invention (not shown) and nine sample vessels  52  during a method according to the invention. 
         [0099]      FIG. 3D  shows the arrangement of  FIG. 3C  in a side view at sectional plane A-A. Here, a sample vessel  52  is also located in the middle space below the centering device  14  (in  FIG. 3C , this sample vessel  52  is covered by the centering device  14 ). The centering device  14  was therefore able to be lowered just so far that the current vertical position  54  of the centering device  14  corresponds to the vertical target position  56  (case ii.). The sample vessel  52  below the centering device  14  is engaged around by centering fingers  26  of the centering device  14 . 
         [0100]      FIG. 3E  shows a schematic view of a centering device  14  of a pipetting device according to the invention (not shown) and nine sample vessels  52  during a method according to the invention. 
         [0101]      FIG. 3F  shows the arrangement of  FIG. 3E  in a side view at sectional plane A-A. Here, the centering device  14  is shifted so far in relation to the middle sample vessel  52  that its centering fingers  26  can no longer engage in free spaces  60  between the sample vessels  52 . Instead, the centering fingers  26  rest on the top side of the sample vessels  52 . Accordingly, the centering device  14  has remained in a vertical position  54  above the target position  56  (case iii.). 
         [0102]      FIG. 4A  shows a schematic cutaway side view of a pipetting device  2  according to the invention above a sample vessel  52  before a pipetting procedure. An end plate  12  with a centering device  14  is pressed downward by a spring  20  into a home position, so that a free end  32  of an auxiliary cannula  18  is surrounded by the centering device  14 . A center axis  62  of the centering device  14  corresponds to a longitudinal axis of the auxiliary cannula  18 . Here, the sample vessel  52  is arranged below the centering device  14  such that a vessel axis  64  is offset laterally in relation to the center axis  62 . The center axis  62  of the auxiliary cannula  18  points here to a top side  66  of a lid  68  of the sample vessel  52 . The lid  68  seals the sample vessel  52  with a septum  70  that is arranged below a central opening  72  in the lid. A sample fluid  74  is located in the sample vessel  52 . 
         [0103]      FIG. 4B  shows the pipetting device  2  and the sample vessel  52  of  FIG. 4A , but now with the centering device  14  positioned on the sample vessel  52 . The centering device  14  was first moved closer to the sample vessel  52  by lowering an upper suspension  8  of a guide arm  6  of the pipetting device  2  until centering fingers  26  of the centering device  14  were able to engage around the sample vessel  52  from the outside. Upon further lowering, the lid  68  of the sample vessel  52  came into contact with the conical bevels  34  of the centering fingers  26 . The sample vessel  52  was aligned by the centering device  14  such that the vessel axis  64  now coincides with the center axis  62  of the centering device  14  and the identical longitudinal axis of the auxiliary cannula  18 . 
         [0104]      FIG. 4C  shows the pipetting device  2  of  FIG. 4A, 4B , but with the upper suspension  8  of the guide arm  6  having been lowered even farther compared to the state in  FIG. 4B . The free end  32  of the auxiliary cannula  18  has thus pierced the septum  70  of the sample vessel  52 . The centering device  14  is pressed by the compressed spring  20  onto the sample vessel  52 . The centering device  14  has the effect that the auxiliary cannula  18  has pierced the septum  70  precisely in the center. 
         [0105]      FIG. 4D  shows the pipetting device  2  of  FIGS. 4A-4C , but unlike in  FIG. 4C , a pipetting needle  4  has now been inserted into the sample vessel  52 . The pipetting needle  4  was guided axially through the auxiliary cannula  18  for this purpose. A ventilation channel  76  is constructed between the pipetting needle  4  and the auxiliary cannula  18  by an outer surface of the pipetting needle  4  being spaced apart from an inner surface of the auxiliary cannula  18 . To achieve this, an outside diameter of the pipetting needle  4  is about 10% less than an inside diameter of the auxiliary cannula  18 . The pipetting needle  4  was advanced so far that it has dipped into the sample fluid  74 . Sample fluid  74  was then aspirated into the pipetting needle  4 . 
         [0106]      FIG. 4E  shows the pipetting device  2  of  FIGS. 4A-4D , but unlike in  FIG. 4D , the pipetting needle  4  with suctioned sample fluid has been pulled upward out of the sample vessel  52 . 
         [0107]      FIG. 4F  shows the pipetting device  2  of  FIGS. 4A-4E , but the auxiliary cannula  18  has now been pulled out of the septum  70  and the centering device  14  has been lifted from the sample vessel  52 . Upon lifting of the upper suspension  8  of the guide arm  6 , the auxiliary cannula  18  was first pulled out of the septum  70 . Meanwhile, the centering device  14  was also pressed by the spring  20  against the sample vessel  52 . After the centering device  14  had reached its home position relative to the upper suspension  8 , the centering device  14  was also removed upward from the sample vessel  52  as the upper suspension  8  continued to be lifted. The pipetting procedure is thus concluded. 
         [0108]      FIG. 5A  shows a first embodiment of a very thin sample vessel  52  for a pipetting device according to the invention (not shown). The sample vessel  52  is embodied here as a first NMR tube (NMR sample vessel) with an outside diameter of greater than 2.5 mm and a length of greater than 150 mm. The sample vessel  52  comprises a substantially hollow-cylindrical body  78  and a lid  68 . A central opening  72  is constructed in the lid  68  and is sealed by a septum  70 . An outside diameter  80  of the lid  68  is about 8 mm here. An inside diameter  82  of the body  78  corresponds to an opening diameter of the opening  72  and is about 2.5 mm here. 
         [0109]      FIG. 5B  shows a second embodiment of a very thin sample vessel  52  for a pipetting device according to the invention (not shown). The sample vessel  52  is embodied here as a second NMR tube (NMR sample vessel) with an outside diameter of less than 2.5 mm and a length of less than 150 mm. The second embodiment of the sample vessel  52  of  FIG. 5B  corresponds in its basic structure to the first embodiment of  FIG. 5A . However, an outside diameter  80  of the lid  68  is only about 6 mm here. An inside diameter  82  of a substantially hollow-cylindrical body  78  corresponds here to an opening diameter of a central opening  72  in the lid  68  and is about 1.0 mm here. 
         [0110]      FIG. 6A  shows a third embodiment of a sample vessel  52  for a pipetting device according to the invention (not shown). Here, the sample vessel  52  is embodied as an auto-sampler vial. The sample vessel  52  comprises a vial body  84  and a lid  68 . A central opening  72  is constructed in the lid  68  and is sealed by a septum  70 . An outside diameter  80  of the lid  68  is about 11 mm here. An opening diameter  86  of the central opening  72  in the lid  68  is about 3 mm here. 
         [0111]      FIG. 6B  shows a fourth embodiment of a sample vessel  52  for a pipetting device according to the invention (not shown). Here, the sample vessel  52  is embodied as a cryo-vial. The sample vessel  52  comprises a vial body  84  and a lid  68 . The vial body  84  is conical (tapering toward the bottom). A central opening  72  is constructed in the lid  68  and is sealed by a septum  70 . An outside diameter  80  of the lid  68  is about 14 mm here. An opening diameter  86  of the central opening  72  in the lid  68  is about 4 mm here. 
         [0112]      FIG. 7  shows a pipetting device  102  with a freely hanging septum-piercing needle  104  from the prior art. In this pipetting device  102  from the prior art, the septum piercing needle  104  is used both to pierce a septum of a sample vessel (not shown) and for aspirating sample fluid from the sample vessel. Besides the septum piercing needle  104 , the pipetting device  102  comprises a guide arm  106 . A holding-down device  108  is constructed on the bottom side of the guide arm  106 . The septum piercing needle  104  projects through a recess  110  that is constructed on the holding-down device  108 , without being guided or supported in the recess  110 . A longitudinal engraving  112  on the outside of the septum piercing needle  104  serves to ventilate the sample vessel when the septum piercing needle  104  has pierced the septum of the sample vessel.