Abstract:
A drip shield includes cover members that define a protective canopy over sample receptacles to prevent unwanted material from being deposited into the receptacles. The cover members cooperate to define at least one access hole through the drip shield to permit access to a sample receptacle by a pipette tip through the access hole. One of the cover members is moveable with respect to another cover member between a closed stated defining the access hole and an open state permitting a pipette tip extending through the access hole to be laterally conveyed relative to the drip shield and out of the access hole. In a preferred embodiment, a system control feature automatically determines if a pipette tip might have been left in a sample receptacle and extending through the access hole of the drip shield and thereby cause the sample receptacle and pipette tip to be conveyed laterally relative to the drip shield while the one cover member moves from the closed to the open state to permit the pipette tip to be conveyed out of the access hole.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/772,575, filed Feb. 13, 2006, the disclosure of which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a drip shield for protecting against cross-contamination between sample tubes and is especially suited for use with an automated sampling system and sample tubes having penetrable caps. The drip shield permits a sample tube to be conveyed laterally beneath the drip shield even when a disposable pipette tip becomes dislodged from an automated sampling device and extends up from the sample tube through an access hole in the drip shield. 
     BACKGROUND OF THE INVENTION 
     Procedures for determining the presence or absence of specific organisms or viruses in a test sample commonly rely upon nucleic acid-based probe testing. To increase the sensitivity of these tests, an amplification step is often included to increase the number of potential nucleic acid target sequences present in the test sample. There are many amplification procedures in common use today, including the polymerase chain reaction (PCR), Q-beta replicase, self-sustained sequence replication (3SR), transcription-mediated amplification (TMA), nucleic acid sequence-based amplification (NASBA), ligase chain reaction (LCR), strand displacement amplification (SDA) and loop-mediated isothermal amplification (LAMP), each of which is well known in the art. See, e.g., Mullis, “Process for Amplifying Nucleic Acid Sequences,” U.S. Pat. No. 4,683,202; Erlich et al., “Kits for Amplifying and Detecting Nucleic Acid Sequences,” U.S. Pat. No. 6,197,563; Walker et al.,  Nucleic Acids Res.,  20:1691-1696 (1992); Fahy et al., “Self-sustained Sequence Replication (3SR): An Isothermal Transcription-Based Amplification System Alternative to PCR,”  PCR Methods and Applications,  1:25-33 (1991); Kacian et al., “Nucleic Acid Sequence Amplification Methods,” U.S. Pat. No. 5,399,491; Davey et al., “Nucleic Acid Amplification Process,” U.S. Pat. No. 5,554,517; Birkenmeyer et al., “Amplification of Target Nucleic Acids Using Gap Filling Ligase Chain Reaction,” U.S. Pat. No. 5,427,930; Marshall et al., “Amplification of RNA Sequences Using the Ligase Chain Reaction,” U.S. Pat. No. 5,686,272; Walker, “Strand Displacement Amplification,” U.S. Pat. No. 5,712,124; Notomi et al., “Process for Synthesizing Nucleic Acid,” U.S. Pat. No. 6,410,278; Dattagupta et al., “Isothermal Strand Displacement Amplification,” U.S. Pat. No. 6,214,587; and H ELEN  H. L EE ET AL.,  N UCLEIC  A CID  A MPLIFICATION  T ECHNOLOGIES : A PPLICATION  T O  D ISEASE  D IAGNOSIS  (1997). 
     A concern with amplification is the possibility of cross-contamination, since transferring even a minute amount of target-containing sample to a target-negative sample could lead to the production of billions of target sequences in the “negative” sample. As a consequence, a test may indicate a positive result for a sample actually lacking nucleic acid from an organism or virus of interest. The source of a contaminating sample transfer may be an aerosol or bubbles released from a sample tube when a cap component of the sample tube is removed or penetrated by a practitioner or instrument. To minimize such sources of contamination, collection devices which include penetrable caps having filtering means have been introduced for use with automated analyzers. Such collection devices include the APTIMA® Urine Specimen Collection Kit for Male and Female Urine Specimens (Gen-Probe Incorporated, San Diego, Calif.; Cat. No. 1040), which is an embodiment of the collection devices disclosed by Kacian et al., “Penetrable Cap,” U.S. Pat. No. 6,893,612. 
     The components of a penetrable cap generally exert a retention force against a fluid transfer device (e.g., pipette tip) as the fluid transfer device is being withdrawn from an associated sample tube. See, e.g., Ammann et al., “Automated Process for Isolating and Amplifying a Target Nucleic Acid Sequence,” U.S. Pat. No. 6,335,166 (an instrument for performing amplification assays on test samples which includes a robotic pipettor using disposable pipette tips for obtaining test sample from a sample tube is disclosed). The retention force may be attributable to, for example, the sealing material of the cap and/or filtering means included within the cap exerting a frictional force on the fluid transfer device. The retention force may also be caused by a swab used for specimen collection (e.g., a cervical, urethral or urinary tract specimen) which is angled in the sample tube or where multiple swabs have been inadvertently inserted into the same sample tube. Swabs used for specimen collection may be provided with a mid-section score line for snapping off the upper portion of the swab. (Such swabs are described by Pestes et al., “Cell Collection Swab,” U.S. Pat. No. 5,623,942, and one such commercial product is the APTIMA® Unisex Swab Specimen Collection Kit for Endocervical and Male Urethral Swab Specimens, available from Gen-Probe as Cat. No. 1041.) If properly broken, the remainder of the swab should fit along the inner wall of the sample tube below the cap. But, if the snap occurs above the score line, then when the swab is fitted into the sample tube, and the cap is screwed onto the sample tube, the swab may bow in such a way that it interferes with the path of a pipette tip inserted through the cap. 
     If the retention force is too great, attempts to remove the fluid transfer device from the associated sample tube could result in the sample tube being withdrawn from a sample carrier holding the sample tube. In a more extreme case, the retention force of the cap and the sample tube holding force of the sample carrier are each great enough that the sample carrier is lifted vertically as the fluid transfer device is being withdrawn from the sample tube. 
     Conventional sample carriers commonly rely upon springs to immobilize sample tubes, biasing the sample tubes against one or more opposing surfaces of the sample carriers. And more recently, a sample carrier has been described which further includes a top wall portion having a plurality of openings which are configured and arranged so that penetrable caps affixed to the vessel components of sample tubes are positioned snugly within the openings when the sample tubes are held by the sample carrier, thereby centering the sample tubes by restricting lateral movement of the corresponding caps within the openings. See Sevigny et al., “Sample Carrier and Drip Shield for Use Therewith,” U.S. Pat. Application Publication No. US 2003-0215365 A1. Furthermore, the sample carriers described include a mechanism, such as a sample tube blocking member, for ensuring that sample tubes remain in the sample carriers during automated sampling procedures when the retention force generated by a cap onto a portion of the fluid transfer device (e.g., pipette tips) is greater than the holding force of the sample carrier on an associated sample tube component. 
     Furthermore, a drip shield has been described for use in an automated sampling system to protect the contents of sample tubes held by sample carriers from fluid contamination, especially hanging droplets which may be dislodged from a robotic pipetting device during an automated sampling procedure. See Sevigny et al., “Sample Carrier and Drip Shield for Use Therewith,” U.S. Patent Application Publication No. US 2003-0215365 A1. By “automated sampling system” is meant a system for holding a sample tube in a generally upright orientation and conveying the sample tube by automated means (e.g., a transport mechanism) to a location within an apparatus where the contents of the sample tube may be accessed by an automated substance transfer mechanism, such as a robotic pipetting device, in order to effect a transfer of at least a portion of the contents to another location within the apparatus. The drip shield includes a cover member having one or more access holes, where each access hole is configured and arranged to provide non-interfering, vertical passage of an aligned pipette tip therethrough. The access holes are sized to permit access to the contents of only one sample tube at a time, where the sample tubes being accessed are present in a sample carrier positioned beneath the cover member. The diameter of each access hole is preferably the same as or smaller than the smallest diameter of any sample tube cap associated with a sample tube held by the sample carrier to minimize opportunities for contaminating the sample carrier and its contents. 
     A potential problem associated with the above-described sample carrier and drip shield configurations occurs when a disposable pipette tip is dislodged from a pipette tip mounting shaft of a robotic pipetting device while the pipette tip is inserted through an access hole in the drip shield and into the sample tube. The pipette tip can, for example, become unseated or dislodged when the frictional retention force created by the sample tube cap or a specimen collection swab (as described above) on the pipette tip, as the pipette tip is being withdrawn from the cap, exceeds the force required to dislodge the pipette tip from the pipette tip mounting shaft. When a pipette tip becomes dislodged and extends upward through an access hole of the drip shield, the sample carrier is prevented from advancing beneath the drip shield. To correct this problem, an operator must terminate operation of the apparatus, reach into the apparatus and remove the dislodged pipette tip or push the pipette tip far enough into the sample tube to clear the drip shield. This corrective procedure can be awkward and inconvenient—if not altogether impossible—if the sample transfer location is in a difficult to access location within the apparatus. Ideally, sample carriers could be conveyed away from the drip shield on a lateral transport path to a location where the operator could more easily reach and remove the dislodged pipette tip. 
     Accordingly, a need exists for a drip shield design that will allow the sample carrier to be conveyed laterally away from the drip shield when a dislodged pipette tip extends through an access hole of the drip shield, while still maintaining protection of the sample tubes being conveyed beneath the drip shield. 
     SUMMARY 
     Aspects of the invention are embodied in an apparatus for preventing unwanted materials from being deposited into receptacles carried by an automated conveyor. In one embodiment, the apparatus includes a first cover member and a second cover member configured to be moveable with respect to the first cover member between a closed state and an open state. In the closed state, the second cover member is operatively positioned with respect to the first cover member so that the first and second cover members cooperate to define one or more access holes for receiving pipette tips therethrough to access receptacles positioned beneath the apparatus. In the open state, the second cover member is operatively positioned with respect to the first cover member so that at least one of the access holes is laterally opened, thereby permitting a pipette tip extending through the access hole to be laterally conveyed relative to the apparatus and away from the location of the access hole. Thus, when a pipette tip disengages from a substance transfer mechanism, remaining within a receptacle and extending up through the access hole, an associated transport mechanism is able to move the receptacle to a location within the instrument at which a technician can access and remove the pipette tip. 
     In one embodiment, the first cover member includes one or more slots extending from an end portion thereof, and the second cover member includes a cover portion which, when in the closed state, covers all but a portion of the one or more slots, the uncovered portion of the slots composing the access holes. The cover portion of the second cover member may have an edge, for example an undulating edge, that is engaged by a pipette tip extending through one of the access holes as the pipette tip is conveyed laterally, thereby causing the second cover member to move relative to the first cover member from the closed state to the open state. 
     Other aspects of the invention are embodied in a system for transferring substance to or from each of a plurality of receptacles. The system includes a substance transfer mechanism, a transport mechanism, and a drip shield. The substance transfer mechanism is used in conjunction with a pipette tip and is adapted to insert a pipette tip removably engaged by the substance transfer mechanism into a receptacle to transfer substance to or from the receptacle. The transport mechanism is adapted to move a plurality of receptacles in generally upright orientations into an operative location with respect to the substance transfer mechanism, where the substance transfer mechanism can access each receptacle to insert a pipette tip into the receptacle. The drip shield is disposed over a portion of the transport mechanism at the operative location for preventing unwanted materials from being deposited into receptacles carried by the transport mechanism. In one embodiment, the drip shield includes a first cover member and a second cover member configured to be moveable with respect to the first cover member between a closed state and an open state. In the closed state, the second cover member is operatively positioned with respect to the first cover member so that the first and second cover members cooperate to define one or more access holes for receiving a pipette tip therethrough to access a receptacle positioned beneath the drip shield. And in the open state, the second cover member is operatively positioned with respect to the first cover member so that at least one of the access holes is laterally opened, thereby permitting a pipette tip extending through the access hole to be laterally conveyed by the transport mechanism relative to the drip shield and away from the location of the access hole. 
     Still further aspects of the invention are embodied in a method for transferring substance to or from a receptacle and for preventing unwanted materials from being deposited into the receptacle comprising. The method includes providing, at a substance transfer location, an apparatus, such as described above, for preventing unwanted materials from being deposited into receptacles carried by an automated conveyor. The method further includes transporting a receptacle to the substance transfer location; accessing the receptacle through one of the access holes of the apparatus with a pipette tip engaged by a substance transfer mechanism; and transferring substance into or out of the pipette tip accessing the receptacle through the access hole. After the transferring step, if the pipette tip is no longer engaged by the substance transfer mechanism, the receptacle is conveyed laterally with respect to the apparatus. If the pipette tip no longer engaged by the substance transfer mechanism is extending through the access hole, the second cover member is caused to move from the closed state to the open state to permit the pipette tip extending through the access hole to be laterally conveyed by the transport mechanism relative to the apparatus and away from the location of the access hole. 
     Other objects, features, and characteristics of the present invention, including the methods of operation and the function and interrelation of the elements of structure, will become more apparent upon consideration of the following description and the appended claims, with reference to the accompanying drawings, all of which form a part of this disclosure, wherein like reference numerals designate corresponding parts in the various figures. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a drip shield according to a first embodiment of the invention positioned above a sample transport carousel holding a sample carrier. 
         FIG. 2  is a cross-section of a portion of the drip shield, transport carousel, and sample carrier taken in the direction  2 - 2  in  FIG. 1 . 
         FIG. 3  is a top perspective view of a cover plate of a drip shield according to the first embodiment of the invention. 
         FIG. 4  is a bottom perspective view of the cover plate shown in  FIG. 3 . 
         FIG. 5  is a top perspective view of a shutter of a drip shield according to the first embodiment of the invention. 
         FIG. 6  is a bottom perspective view of the shutter shown in  FIG. 5 . 
         FIG. 7  is a partial top plan view showing a drip shield arranged above a sample transport carousel with a pipette tip extending through one of the access holes provided in the drip shield. 
         FIG. 8  is a partial top plan view of the drip shield arranged above the sample transport carousel with the shutter of the drip shield partially deflected with respect to the cover plate by engagement of the laterally moving pipette tip with an edge of the shutter. 
         FIG. 9  is a partial top plan view of the drip shield arranged above the sample carousel with the shutter of the drip shield further deflected relative to the cover plate so as to permit the pipette tip extending out of a sample tube to be moved laterally away from the drip shield. 
         FIG. 10  is a partial top plan view of the drip shield arranged above the sample carousel with the shutter of the drip shield now moving back toward its undeflected position as the pipette tip continues to move laterally away from the drip shield. 
         FIG. 11  is a partial top plan view of the drip shield arranged above the sample carousel with the pipette tip extending out of a sample tube now moved completely away from the drip shield and the shutter of the drip shield moved back to its undeflected position relative to the cover plate. 
         FIG. 12  is a top perspective view of a second embodiment of the drip shield of the invention. 
         FIG. 13  is a top plan view of a cover plate of the second drip shield embodiment. 
         FIG. 14  is a top perspective view of a shutter of the second drip shield embodiment. 
         FIG. 15  is a bottom perspective view of the shutter of  FIG. 14 . 
         FIG. 16  is a top perspective view of the second drip shield embodiment with the shutter shown moved to a deflected position relative to the cover plate. 
         FIG. 17  is a top perspective view of a third embodiment of the drip shield of the invention. 
         FIG. 18  is a top plan view of a cover plate of the third drip shield embodiment. 
         FIG. 19  is a top plan view of a shutter of the third drip shield embodiment. 
         FIG. 20  is a flow chart showing a procedure to be followed when a pipette tip has been left in a sample tube. 
     
    
    
     DETAILED DESCRIPTION 
     While the present invention may be embodied in a variety of forms, the following description and accompanying drawings are merely intended to disclose some of those forms as specific examples of the present invention. Accordingly, the present invention is not intended to be limited to the forms or embodiments so described and illustrated. Instead, the full scope of the present invention is set forth in the appended claims. 
     As shown in  FIG. 1 , a sample carrier  10  is provided to carry a plurality of receptacles, such as sample tubes  300 , and may be adapted for use with a sample carrier conveying means, such as a sample carousel for rotating a plurality of sample carriers within an automated sampling system. One such sample carousel  400  is disclosed by Ammann et al. in U.S. Pat. No. 6,335,166 and is illustrated in  FIG. 1 . This particular sample carousel  400  is formed of milled, unhardened aluminum and includes an annular trough  401  about the periphery of a ring  402  and a plurality of raised, radially extending dividers  403 . The dividers  403  divide the trough  401  into nine arcuate sample carrier receiving wells  404  which can be configured to accommodate the sample carriers  10 . The individual sample carrier receiving wells  404  are dimensioned to maintain the sample carriers  10  in an upright position as sample tubes  300  held by the sample carriers  10  are indexed under a substance transfer mechanism, such as a robotic pipettor (not shown), for retrieving sample material for analysis. An example of a robotic pipettor for use with the present invention is the Robotic Sample Processor, Model No. RSP 9000, available from Cavro Scientific Instruments, Inc. of Sunnyvale, Calif. 
     Those of ordinary skill in the art will appreciate that, as an alternative to the carousel  400  shown in  FIG. 1 , a sample carrier conveying means may comprise a linear transport conveyor or a transport conveyor of some other configuration. 
     The sample carriers  10  can be used in combination with a device for protecting sample tubes  300  during sampling to limit opportunities for cross-contamination. Such a device is provided by a novel drip shield  200  depicted in  FIGS. 1-19  for preventing unwanted materials from being deposited into the sample tubes  300 . (Reference herein to “drip shield  200 ” is a general reference to any of the illustrated drip shields  200 A-C.) Drip shield  200 A shown in  FIG. 1  is an assembly which includes a first cover member, for example in the form of a cover plate  201 A, and a second cover member, for example in the form of a shutter  240 A, which are dimensioned and cooperate to form a canopy over a sample carrier  10  positioned thereunder. (Reference herein to “cover plate  201 ” and to “shutter  240 ” are general references to any of the illustrated cover plates  201 A-C and shutters  240 A-C, respectively.) Thus, in the illustrated embodiment, the drip shield  200  has an arcuate shape corresponding to the preferred arcuate shape of the sample carrier  10 , as shown in  FIG. 1 . That is, the general shape and configuration of the drip shield can be selected to conform to the shape and configuration of the sample carrier and associated conveying means. For example, a drip shield implemented in combination with a linear conveying means would have a linear shape. 
     Two access holes, identified in  FIG. 1  as a first, or inner, access hole  202 A and a second, or outer, access hole  203 A, extend through the drip shield  200  and provide access to sample tubes  300  centered below the access holes. (Reference herein to “access hole  202 ” or “access hole  203 ” is a general reference to any of the illustrated access holes  202 A-C and  203 A-C.) The access holes  202 ,  203  are dimensioned to permit non-interfering passage therethrough by pipette tips carried by a robotic pipettor, but are small enough so that a top surface  204  of the drip shield  200  can function to catch hanging droplets which are dislodged from the pipette tips during sample transfer procedures. Therefore, the diameters of the first and second access holes  202 ,  203 , respectively, are preferably about the same as or less than the smallest diameter of any cap  310  of a sample tube  300  to be carried by a sample carrier  10 , as shown in  FIG. 2 . 
     Cap  310  may be a penetrable closure device having one or more frangible seals and, optionally, a filter means (e.g., as described in Kacian, et al., U.S. Pat. No. 6,893,612) which are pierceable by the pipette tip. Other closure devices that can be pierced by a pipette tip include those disclosed by Anderson et al., “Penetrable Cap,” U.S. Pat. No. 6,716,396. The sample tubes  300  may also contain a specimen retrieval device configured to enable a practitioner to collect an amount of specimen material and deposit the material, along with the specimen retrieval device, into a sample tube  300 . An exemplary specimen retrieval device is the cell collection swab described in Pestes, et al., U.S. Pat. No. 5,623,942. 
     The access holes  202 ,  203  are arranged on the drip shield  200  so that the first access hole  202  is positioned above a first or inner row of longitudinally or arcuately aligned sample tubes  300  and the second access hole  203  is aligned above a second or outer row of longitudinally or arcuately aligned sample tubes  300 . As the sample carrier  10  is indexed clockwise or counterclockwise under the drip shield  200  by the sample carousel  400 , the next sample tube  300  in each row of tubes can be presented under one of the access holes  202 ,  203  for access by a robotic pipettor. The access holes  202 ,  203  are preferably angularly offset on the drip shield  200  to further minimize opportunities for contamination resulting from released hanging droplets of sample material. In a preferred mode, the access holes  202 ,  203  are arranged on the drip shield  200 , as shown in  FIG. 1 , so that the third sample tube in the second or outer row of aligned tubes is positioned beneath the second access hole  203  as the first sample tube in the first or inner row of aligned tubes is positioned beneath the first access hole  202  when the sample carrier  10  is conveyed in a counterclockwise direction. 
     The shutter  240  is disposed over a portion of the cover plate  201 , and at least a portion of the shutter  240  is movable relative to the cover plate  201 . As shown in  FIG. 4 , the cover plate  201 A may include a series of three longitudinally or arcuately extending runners  208  which are spaced apart from each other and depend from a bottom surface  209  of the cover plate  201 A so as to define a first channel  220  and a second channel  221 . (The reference number “ 208 ” refers generally to all three runners shown in the figures, whereas the reference number “ 208 A” refers to the center runner and the reference number “ 208 B” refers to the two edge runners.) Channels  220  and  221  provide clearance under the drip shield  200  to accommodate taller sample tubes (possibly taller sample tubes without caps). The runners  208 A,  208 B preferably include tapered ends  211 ,  212 , respectively, as shown in  FIG. 4 . The tapered ends  211 ,  212  of the runners  208 A,  208 B are provided to facilitate proper seating of sample carriers  10  which have not been fully inserted into sample carousel receiving wells  404  prior to rotation, whether the sample carousel  400  is being rotated clockwise or counterclockwise. 
     It should be understood that the runners  208  and channels  220 ,  221  are optional and are not required for the effective operation of the invention. 
     The sample carrier  10  shown in  FIGS. 1 and 2  includes tabs  223 ,  224  (see  FIG. 2 ) extending laterally from a lower end of the carrier  10 . The sample carrier shown is described in Knight et al., “Sample Tube Holder,” U.S. Pat. Application Publication No. US 2006-0266719 A1. Other exemplary sample carriers are described in: Dale et al., “Sample Carrier and Drip Shield for Use Therewith,” U.S. Patent Application Publication No. US 2003-0017084 A1; Sevigny et al., “Sample Carrier Having Sample Tube Blocking Means and Drip Shield for Use Therewith,” U.S. Patent Application Publication No. US 2003-0215365 A1; and Aviles et al., “Sample Carrier Having Releasable Locking Mechanism,” U.S. Pat. No. 7,132,082. When the sample carrier  10  is in a sample transfer location (i.e., under the drip shield  200 A) tabs  223  and  224  engage blocking elements which prevent the sample carrier  10  from being lifted out of the receiving well  404  of the sample carousel  400 . More specifically, tab  223  extends into a gap  225  defined between the top of the sample carousel  400  and the bottom of a block element  226  mounted in a fixed position with respect to a stationary surface  216 . Similarly, tab  224  extends into a gap  227  defined between the top of the sample carousel  400  and the bottom of block element  228  mounted in a fixed position with respect to the stationary surface  216 . Thus, tabs  223  and  224  prevent the carrier from being lifted out of the receiving well  404 . 
     Preferably, the distance between the bottom of the drip shield  200  and the top of a sample tube cap  310  is about 0.36 inches (9.14 mm). 
     The drip shield  200  can be maintained in fixed relationship over sample carriers  10  being indexed on the sample carousel  400  therebelow by means of mounting posts  215  fixed to the stationary surface  216  of the automated sampling system, as illustrated in  FIGS. 1 and 2  and more fully described by Ammann et al. in U.S. Pat. No. 6,335,166. The drip shield  200  can be secured to these mounting posts  215  using screws, bolts or like mechanical fasteners. Preferred are screws  217  mated with threaded holes (not shown) in the mounting posts  215  and inserted through three counter-bored through-holes  218  located on the periphery of the cover plate  201 A, as shown in  FIG. 3 . The holes  218  may be countersunk in the top surface  204  of the cover plate  201 A. This is especially necessary with respect to the screw  217  that is adjacent to the shutter  240 A so that the screw head does not interfere with movement of the shutter  240 A relative to the cover plate  201 A. Cover plate  201 A also includes three bored openings  219  aligned with the through-holes  218  (see  FIG. 4 ) which receive the top ends of the mounting posts  215 . 
     Components of the drip shield  200  of the present invention are preferably made of a substantially non-conductive plastic, such as acrylonitrile-butadiene-styrene (ABS), which can be obtained from GE Plastics of Pittsfield, Mass. as Cycolac® MG47. The materials used should be selected to resist corrosion by chemicals and reagents that the sample carrier  10  and drip shield  200  may be exposed to during use. ABS is readily available, durable and easily machined. 
     Referring to  FIGS. 3 and 4 , the cover plate  201 A includes an outer slot  230  and an inner slot  232  that is somewhat shorter in length than the outer slot  230 . Slots  230  and  232  are preferably arcuate in shape having a curvature generally corresponding to the curvature of the arrangement of sample tubes  300  in a sample carrier  10 , which is also the curvature of the path of movement of a sample tube  300  on the sample carousel  400 . Outer slot  230  has a rounded closed end  231 , and inner slot  232  has a rounded closed end  233 , both ends  231 ,  233  being located in a portion of the cover plate  201 A that is interior to the outer perimeter of the cover plate. The slots  230 ,  232  extend from their respective ends  231 ,  233  to open ends at a peripheral edge of the cover plate  201 A. 
     A shutter bracket  234 A projects outwardly from an edge of the cover plate  201 A and is provided for attaching the shutter  240 A thereto. The shutter bracket  234 A includes mounting holes  235  and perimeter ledge  236  which extends beyond a side wall  238  of the shutter bracket  234 A. 
     Referring to  FIGS. 5 and 6 , the shutter  240 A includes a cover portion  241  with a flexible arm  250  and a mounting bracket  251 A. The cover portion includes a sloped end  242 , a inner edge  249  and an undulating edge  243 . Undulating edge  243  is generally defined by a first rounded fillet  244 , a first straight portion  245 , a curved transition  246 , a second rounded fillet  247 , and a second straight portion  248  which terminates at inner edge  249 . 
     The flexible arm  250  is in the form of a curved flexible strap arranged transversely to the plane of the cover portion  241  and extends from a peripheral edge of the cover portion  241 . 
     The mounting bracket  251 A includes a flat portion  252  with mounting holes  253  formed therein and a raised edge  254  extending along one side of the flat portion  252  until it merges with the edge of the flexible arm  250 . 
     The shutter  240 A is installed onto the cover plate  201 A by placing the cover portion  241  of the shutter  240 A on the top surface  204  of the cover plate  201 A and engaging the flat portion  252  of the mounting bracket  251 A of the shutter  240 A with the bottom surface  239  of the shutter bracket  234 A of the cover plate  201 A. Suitable fasteners, such as screws, rivets, or bolts, extend through the mounting holes  253  of the mounting bracket  251 A into the mounting holes  235 , which may be threaded, of the shutter bracket  234 A. The raised edge  254  of the mounting bracket  251 A preferably has a thickness corresponding to the width of the perimeter ledge  236  of the shutter bracket  234 A so that the raised edge  254  will seat below the perimeter ledge  236 , and the edge of the flexible arm  250  will seat below a ledge  239  extending along a portion of the cover plate  201 A to present a clean, flush appearance. 
     The cover plate  201  and the shutter  240  of are preferably machined from ABS due to the intricacy of the various features of the respective parts. Edges of the cover plate  201  and the shutter  240  are preferably rounded or chamfered so as to remove sharp edges. This is especially important in areas where the shutter and cover plate are to move relative to each other, so as to prevent one part from getting caught on the edge of the other. Persons of ordinary skill in the art will recognize that it may be possible, especially with embodiments including fewer intricate features, to form the cover plate and/or the shutter by injection molding. Furthermore, while the embodiments of the drip shield  200  shown in the drawings comprise assemblies of two or more pieces, it is contemplated that drip shields that are functionally equivalent to those shown in the drawings may be formed as a single, integrated structure. 
     As best shown in  FIGS. 7-11 , portions of the cover plate  201 A and the shutter  240 A cooperate to define the inner access hole  202 A and outer access hole  203 A. More specifically, shutter  240 A covers all but a portion of the outer slot  230  so that the first fillet  244  of the shutter  240 A and the rounded end  231  of the outer slot  230  form the perimeter of the outer access hole  203 A (see  FIG. 11 ). Similarly, shutter  240 A covers all but a portion of the inner slot  232  so that the second fillet  247  of the shutter  240 A and the rounded end  233  of the inner slot  232  cooperate to form the perimeter of the inner access hole  202 A. 
       FIGS. 7-11  show a sequence which demonstrates how a drip shield embodying aspects of the invention permits a pipette tip extending out of a sample tube and through one of the access holes  202 A or  203 A can be conveyed laterally relative to the drip shield. As shown in  FIG. 7 , a pipette tip  350  is disposed within and extends through the outer access hole  203 A of the drip shield  200 A. The drip shield  200 A shown in  FIG. 7  is in a closed state, as the shutter  240 A is in a position with respect to the cover plate  201 A to define the access holes  202 A and  203 A. To convey the pipette tip laterally relative to the drip shield  200 A, the transport carousel  400  is moved in a counterclockwise direction, as illustrated by arrow A in  FIGS. 8-11 . 
     As the transport carousel  400  continues to move in direction A, the pipette tip  350  engages the undulating edge  243  of the shutter  240 A. Initially, the pipette tip  350  slides along the first straight portion  245 , preferably having a slight forward inclination relative to the counterclockwise movement of the tip  350  so that the tip  350  does not get caught behind any back angled features of the edge of the shutter. The tip  350  then slides along the curved transition  246  of the shutter  240 A as the carousel  400  continues to rotate. The lateral, counterclockwise movement of the pipette tip  350  and the engagement of the pipette tip  350  along the first straight portion  245  and curved transition  246  causes the shutter  240 A to move laterally in the direction indicated by arrow B as the flexible arm  250  begins to flex outwardly, as shown in  FIG. 8 . 
     As shown in  FIG. 9 , continued counterclockwise movement of the sample carousel  400  in the direction A causes the pipette tip  350  to continue to slide along the undulating edge  243  of the shutter  240 A past the second straight portion  248 , which preferably has a slight forward inclination relative to the direction of movement of the pipette  350 , and then along the inner edge  249 . The continued movement of the pipette tip  350  along the undulating edge  243  of the shutter  240 A during the lateral, counterclockwise movement of the pipette tip  350  causes the shutter  240 A to further deflect laterally in the direction B, thereby permitting the pipette tip  350  to move laterally relative to the drip shield  200 A within the outer slot  230  of the cover plate  201 A. The drip shield  200 A shown in  FIG. 9  is in an open state, as the shutter  240 A has now been moved with respect to the cover plate  201 A so as to open the access holes  202 A and  203 A and permit the pipette tip  350  to be conveyed laterally with respect to the drip shield  200 A. 
     As shown in  FIG. 10 , continued counterclockwise rotation of the transport carousel  400  in the direction A has now moved the pipette tip  350  beyond the outer slot  230  of the cover plate  201 A. The shutter  240 A, urged by the elasticity of the flexible arm  250 , now begins to move in the direction indicated by arrow C back toward its undeflected position (i.e., toward the closed state). As the shutter  240 A moves back toward its undeflected position, the pipette tip  350  slides along the edge  242  of the shutter  240 A, which is preferably sloped as shown so that the pipette tip  350  remains in contact with the shutter for a longer period, allowing for a more gentle return of the shutter  240 A to its undeflected position. 
     The estimated force required to deflect the shutter and permit the pipette tip to be moved laterally away from the drip shield is 1-2 pounds force. Ideally, the amount of force required to deflect the shutter is as low as possible, while still permitting smooth, consistent shutter return. As will be appreciated by persons of ordinary skill in the art, the deflection force of the shutter  240 A of the embodiment shown in  FIGS. 1-11  will depend on the material stiffness, the length of the arm  250 , and the cross-section of the arm  250 . 
     Finally, in  FIG. 11  the tip  350  has moved completely away from the drip shield  200 A, and the shutter  240 A has now moved back to its undeflected position relative to the cover plate  201 A. The sample carrier  10  can now be moved to a location where an operator can access and remove the dislodged pipette tip  350 . 
     A second embodiment of a drip shield according to the invention is indicated by reference number  200 B in  FIGS. 12-16 . This drip shield  200 B, shown in  FIG. 12 , is an assembly which includes a cover plate  201 B and a shutter  240 B which are dimensioned and cooperate to form a canopy over a sample carrier positioned thereunder. 
     As shown in  FIG. 13 , cover plate  201 B, like cover plate  201 A, includes slot  230  with rounded closed end  231  and slot  232  with rounded closed end  233 . Cover plate  201 B further includes a shutter bracket  234 B that is provided for attaching the shutter  240 B thereto and projects outwardly from an edge of the cover plate  201 B. A first spring element  260  is mounted on the shutter bracket  234 B. Spring element  260  may comprise a torsional spring element, such as Stock No. TO-5085LS, available from Century Spring Corp. of Los Angeles, Calif. 
     Referring to  FIGS. 14 and 15 , the shutter  240 B includes a cover portion  241  and a mounting bracket  251 B. The cover portion  241  includes a sloped end  242 , a inner edge  249  and an undulating edge  243 . Undulating edge  243  is generally defined by a first rounded fillet  244 , a first straight portion  245 , a curved transition  246 , a second rounded fillet  247 , and a second straight portion  248  which terminates at inner edge  249 . 
     The mounting bracket  251 B includes a spring element  262 . The shutter  240 B is installed onto the cover plate  201 B so that the shutter  240 B can pivot with respect to the cover plate  201 B. The spring elements  260 ,  262  cooperate to bias the shutter  240 B in the undeflected position (see  FIG. 12 ) with respect to the cover plate  201 B. 
     As best shown in  FIG. 12 , portions of the cover plate  201 B and the shutter  240 B cooperate to define the inner access hole  202 B and outer access hole  203 B. More specifically, the first fillet  244  of the shutter  240 B and the rounded end  231  of the outer slot  230  form the perimeter of the outer access hole  203 B. Similarly, the second fillet  247  of the shutter  240 B and the rounded end  233  of the inner slot  232  cooperate to form the perimeter of the inner access hole  202 B. 
     The drip shield  200 B works in much the same way as the drip shield  200 A described above. When a pipette tip (such as pipette tip  350  shown in FIGS.  2  and  7 - 11 ) is dislodged from a pipette tip mounting shaft of a robotic pipettor and extends through one of the access holes  202 B or  203 B of the drip shield  200 B, the pipette tip can be conveyed laterally by means of a sample transport mechanism (e.g. sample carousel  400 ). The pipette tip engages the undulating edge  243  of the shutter  240 B, thereby pivotally deflecting the shutter  240 B against the force of the spring elements  260 ,  262  (see  FIG. 16 ). Deflection of the shutter  240 B permits the pipette tip  350  to be conveyed laterally out of slot  230  or  232  and away from the drip shield  200 B. After the pipette tip  350  has been moved laterally away from the drip shield  200 B, the spring elements  260 ,  262  bias the shutter  240 B back to its undeflected position, as shown in  FIG. 12 . 
     A third embodiment of a drip shield according to the invention is indicated by reference number  200 C in  FIG. 17 . The drip shield  200 C shown in  FIG. 17  is an assembly which includes a cover plate  201 C and a shutter  240 C which are dimensioned and cooperate to form a canopy over a sample carrier positioned thereunder. 
     As shown in  FIG. 18 , cover plate  201 C, like cover plates  201 A and  201 B, includes slot  230  with rounded closed end  231  and slot  232  with rounded closed end  233 . Cover plate  201 C further includes mounting holes  266  for attaching the shutter  240 C to the cover plate  201 C as described below. 
     Referring to  FIG. 19 , the shutter  240 C includes two cover portions  271 ,  275 , each with a flexible arm  272 ,  276 , respectively, and a mounting bracket  251 C connecting the two cover portions. Cover portion  271  includes a sloped end  274 , a inner edge  280  and an actuating edge  273 . Similarly, cover portion  275  includes a sloped end  278 , a inner edge  281  and an actuating edge  277 . 
     The mounting bracket  251 C includes fastener holes  268 . Shutter  240 C is installed onto the cover plate  201 C by means of suitable mechanical fasteners (such as screws  270 ) inserted through fastener holes  268  into mating holes  266  (which may be threaded) formed in cover plate  201 C. 
     As best shown in  FIG. 17 , portions of the cover plate  201 C and the shutter  240 C cooperate to define the inner access hole  202 C and outer access hole  203 C. More specifically, the actuating edge  273  of cover portion  271  and the rounded end  231  of the outer slot  230  form the perimeter of the outer access hole  203 C. Similarly, the actuating edge  277  of the cover portion  275  and the rounded end  233  of the inner slot  232  cooperate to form the perimeter of the inner access hole  202 C. Note that cover portion  275  is shorter than cover portion  271 , just as inner slot  232  is shorter than outer slot  230 . 
     When a pipette tip (such as pipette tip  350  shown in  FIGS. 7-11 ) is dislodged from a pipette tip mounting shaft and extends through one of the access holes  202 C or  203 C of the drip shield  200 C, the pipette tip may bemoved laterally by means of a sample transport mechanism (e.g. sample carousel  400 ). The pipette tip engages the actuating edge  273  or  277  of the shutter  240 C, thereby deflecting the corresponding cover portion  271  or  275  against the resilience of the corresponding flexible arm  272  or  276 . With a cover portion  271  or  275  of the shutter  240 C deflected, the pipette tip can be moved laterally out of the slot  230  or  232  and away from the drip shield  200 C, the pipette tip sliding along edge  280  or  281  as it continues to move laterally relative to the drip shield  200 C. After the pipette tip has been moved laterally away from the drip shield  200 C, the flexible arm  272  or  276  biases the corresponding cover portion  271  or  275  back to its undeflected position, as shown in  FIG. 17 . The rounded end  274  or  278  of the cover portion  271  or  275  permits a gentle return of the cover portion  274  or  275  to its undeflected position as the pipette tip disengages from the cover portion. 
     Still further alternative drip shield configurations include flexible rubber or foam flaps (as opposed to a rigid shutter) disposed over slots formed in the cover plate. The flaps cover all but a portion of the slots so as to define access holes permitting access to sample tubes beneath the drip shield, but flex out of the way of a pipette tip being conveyed laterally through the slot. 
     An apparatus incorporating a drip shield according to the present invention may include sensors and automated control (e.g., a programmed microprocessor) to provide automated warnings and appropriate sample carrier movement when a pipette tip is dislodged from an automated sampler device and is left in the sample tube, extending through the drip shield. As described in, e.g., Ammann et al., U.S. Pat. No. 6,335,166, under normal sampling procedures, a disposable pipette tip is placed onto the end of a pipette tip mounting shaft of a robotic pipetting device prior to transferring sample from a particular sample tube  300 . When sample transfer from that sample tube is complete, the disposable pipette tip is discarded, typically into a waste container. In the apparatus shown in  FIG. 1 , the robotic pipetting device moves the transfer pipette tip into a pipette tip chute  500 , moving the disposable pipette tip through a slot  501  formed in the side of the chute  500 , where the disposable pipette tip is stripped off the robotic pipetting device and directed into a waste container (not shown) by the pipette tip chute  500 . As sensor is provided (e.g., an optical sensor, not shown) to confirm that a pipette tip is in fact on the pipette tip mounting shaft prior to the stripping procedure. 
     The procedure for determining if a dislodged pipette tip extends through the drip shield is illustrated in the flow chart of  FIG. 20 . Step  320  represents the automated sampling procedure whereby sample is transferred from a sample tube. The sample tube may be provided with a penetrable cap having one or more frangible seals and, optionally, a filter means (e.g., as described in Kacian, et al., U.S. Patent No. 6,893,612), and the automated sampling procedure  320  may include piercing the seal(s) and filter means of the penetrable cap with the pipette tip to access the contents of the sample tube. In step  322 , the pipetting device is moved to an operative position with respect to a tip sensor to determine if the disposable pipette tip is still secured on the pipette tip mounting shaft. If the pipette tip is present (step  324 ), the pipette tip is ejected and normal operation continues (step  328 ). If the pipette tip is not present (step  326 ), an error warning (e.g., an alarm and/or a visual warning, such as a light or a warning icon on a graphical user interface) is provided, and the sample transport (e.g., carousel  400 ) moves the sample tube away from the drip shield to a place where the sample tube can be accessed by the operator to determine if a pipette tip is stuck in the sample tube (step  330 ). If there is no pipette tip in the sample tube (step  334 ) the operator is instructed to terminate the run (step  338 ). If a pipette tip is stuck in a sample tube (step  332 ), the operator removes the pipette tip and restarts the apparatus to continue operation (step  336 ). 
     All disclosures referred to herein are hereby incorporated by reference in their entireties. No disclosure referred to herein is admitted to be prior art to the claimed invention. 
     While the present invention has been described and shown in considerable detail with disclosure to certain preferred embodiments, those skilled in the art will readily appreciate other embodiments of the present invention. Accordingly, the present invention is deemed to include all modifications and variations encompassed within the spirit and scope of the following appended claims.