Abstract:
A handheld multichannel air displacement pipette is modular in construction and includes compliant joints and molded plastic components in place of various machined metal, glass, and ceramic components found in traditional multichannel pipettes. These improvements are made without sacrificing performance in channel-to-channel consistency, accuracy, and precision. The multichannel pipette is inexpensive to manufacture, lightweight, reliable, and easy to assemble and service.

Description:
FIELD OF THE INVENTION 
     The invention relates to liquid-handling pipettes, and more particularly to handheld multichannel air-displacement pipettes operable to measure and transfer multiple substantially identical small volumes of liquid simultaneously. 
     BACKGROUND OF THE INVENTION 
     Handheld multichannel air-displacement pipettes are well known, and have been commonplace in laboratory settings for decades. Such pipettes are particularly useful for fast, convenient transfer of liquid samples between microtubes and multi-well plates, for example. Generally, multichannel pipettes have multiple nozzles arranged in one or two evenly-spaced rows, and the nozzles are configured to receive disposable pipette tips similar or identical to tips used on single-channel handheld pipettes. See U.S. Pat. No. 4,779,467, which is hereby incorporated by reference as though set forth in full, for an example of a traditional multichannel pipette configuration. 
     Because multichannel pipettes are handled by laboratory personnel so frequently, and are often used for long uninterrupted periods of time, ergonomic considerations are paramount. However, in order to maintain accuracy and reliability, some tradeoffs have often been made. Multichannel pipettes are often uncomfortably large and heavy, with precision metal parts used to ensure accuracy, consistency, and durability. Not only are such metal parts heavy, they tend to be expensive to manufacture as a result of the labor-intensive processes necessary to assemble them. 
     Channel-to-channel consistency is an important quality for multichannel pipettes. Unintended variation may result in experimental anomalies or other poor results. Accordingly, as noted above, multichannel pipettes made with bulky, heavy parts have proven to be reliable scientific tools. However, in some cases, this weight can result in fatigue over long periods of usage. 
     Accordingly, a need exists for an adjustable multichannel pipette that avoids the limitations of the prior art. Such a pipette would include advantageous features, such as a compact design that is reliable, accurate, capable of precise fluid measurement, yet lightweight. These characteristics and features are achieved while retaining ease of assembly and service, and providing excellent channel-to-channel volume measurement consistency and performance. 
     SUMMARY OF THE INVENTION 
     A handheld multichannel air displacement pipette according to the invention replaces many of the bulky, heavy, metal components of traditional multichannel pipettes with smaller, lighter weight replacements. 
     The liquid end portion of a traditional multichannel pipette often includes a metal pushrod, multiple polished metal pistons, and other metal parts. These components are often made of stainless steel, a material that offers excellent resistance to corrosion and wear but is quite dense and heavy. A multichannel pipette according to the invention replaces the stainless steel pushrod and various other components with molded plastic counterparts without significantly sacrificing reliability, accuracy, or precision. 
     The pushbar in a multichannel pipette is a component that transfers the movement of a single plunger rod—often manually controlled by a pushbutton—into corresponding equal movement of multiple parallel pistons. The pushbar in a multichannel pipette according to the invention is guided and kept parallel to ensure channel-to-channel consistency via a bearing that tracks a single stainless steel guide shaft; the guide shaft is made hollow to further reduce weight. 
     In contrast to traditional multichannel pipettes, which often use individual springs for this purpose, head ends of the pistons are retained in the pushbar by way of small, light, stainless steel spring clips. These clips reduce or eliminate axial play between the pistons and the pushbar, and retain and help center the pistons automatically. 
     As with traditional handheld multichannel pipettes, a plurality of cylinders is mounted within the housing, each of which receives an air displacement piston mounted for axial movement therein in response to movement of the plunger rod, via the pushbar. Each of the cylinders is coupled to a nozzle with an open end extending from the bottom wall of the housing. As in traditional pipettes, the nozzles are used to mount and release disposable pipette tips. 
     In a multichannel pipette according to the invention, the nozzles and air displacement cylinders are also fabricated from molded plastic. A modular configuration allows for simple, tool-less removal and replacement of nozzles, cylinders, seals, and pistons as necessary once the housing has been opened. This modular construction facilitates simple manufacturing and greatly improved service. The molded parts also greatly reduce weight, by reducing the need for heavier precision-machined metal components in the cylinder, piston, pushrod, and pushbar assemblies. 
     A multichannel pipette according to the invention further includes a compliant interface between the pushrod and the pushbar. This compliant interface allows for some radial (i.e. angular) play in the pushrod resulting from minor manufacturing variations or attachment inaccuracies between the pipette body and plunger rod, and the connection of those components to the pushrod within the multichannel pipette liquid end. Axial play, however, is minimized. As a result of the compliant interface, the pushbar is able to remain parallel and centered on the guide shaft to ensure channel-to-channel consistency, accuracy, and precision, while allowing small radial angular offsets of the pushrod. 
     Accordingly, as set forth above and described in further detail below, a handheld multichannel air displacement pipette according to the invention is modular in construction and includes molded plastic components as replacements for various machined metal, glass, and ceramic components in traditional multichannel pipettes. These improvements are made without sacrificing performance in channel-to-channel consistency, accuracy, and precision. The resulting multichannel pipette is lightweight yet robust and reliable, easier to assemble and service than traditional multichannel pipettes, and may be considerably less expensive to build. 
     Because of the light weight and low forces required to operate a multichannel pipette according to the invention, it offers improved ergonomics and less operator fatigue than heavier traditional models. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features, and advantages of the invention will become apparent from the detailed description below and the accompanying drawings, in which: 
         FIG. 1  is an external view of an exemplary complete handheld multichannel pipette, including a body and a liquid end according to the invention; 
         FIG. 2  is a view of an exemplary liquid end portion of the handheld multichannel pipette of  FIG. 1  with a portion of its housing and the tip ejector removed, allowing internal components to be seen; 
         FIG. 3  is a cutaway view of the liquid end portion of  FIG. 2 ; 
         FIG. 4  is an exploded view of a first embodiment of a pushrod, pushbar, piston, and cylinder assembly of an embodiment of a liquid end portion of  FIG. 2 ; 
         FIG. 5  is an exploded view of a second embodiment of a piston and cylinder assembly of an embodiment of a liquid end portion of a pipette according to the invention; 
         FIGS. 6 a  through 6 d    illustrate the operation of a compliant bayonet joint for connecting a pushrod to a pushbar in a liquid end of an exemplary pipette according to the invention; 
         FIG. 7  illustrates an exemplary spring clip used to retain pistons within a pushbar according to  FIG. 4 ; and 
         FIG. 8  illustrates in schematic form how the spring clip of  FIG. 7  holds pistons within a pushbar in an embodiment of a pipette according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described below, with reference to detailed illustrative embodiments. It will be apparent that a multichannel air displacement pipette according to the invention may be embodied in a wide variety of forms. Consequently, the specific structural and functional details disclosed herein are representative and do not limit the scope of the invention. 
     Referring initially to  FIG. 1 , an overview illustration of a handheld multichannel pipette  110  according to the invention is presented. 
     The pipette  110  includes a hand-holdable body  112  which contains a volume adjustment mechanism and a plunger button  114  that is movable axially toward the body. In the illustrated pipette  110 , as the plunger button is operated by applying pressure thereto, this movement is transferred through a plunger rod  116  (largely internal to the body) to a liquid end assembly  120 . In the disclosed embodiment, the liquid end assembly  120  includes a relatively low-profile housing  122 . 
     The housing  122  includes a rear portion  128  and a front portion  129 , the front portion  129  being detachable from the rear portion  128  to expose the internal components of the liquid end assembly  120  for manufacture or service, as will be illustrated and described in further detail below. 
     The operation of one form of exemplary manual pipette is explained in U.S. Pat. No. 5,700,959 to Homberg, entitled “Manual Pipette with Magnet Assist,” which is hereby incorporated by reference as though set forth in full herein. The plunger button  114  is spring-biased toward its uppermost position defined by a volume setting of the pipette. As explained in the Homberg patent, the user depresses the plunger button  114  toward a soft stop defining a home position, and past the home position to a blowout position while dispensing fluids. 
     Although  FIG. 1  illustrates a manual pipette, it will be recognized that electronically controlled motor driven pipettes may also be used. In such cases, the plunger rod  116  is driven axially in response to control from an embedded processor. 
     As illustrated, the liquid end assembly  120  includes eight protruding nozzles  130  arranged in a row. As described above, pipettes having six, eight or twelve nozzles in a single row, or sixteen or twenty-four nozzles in two rows of eight of twelve are currently available in traditional form. An embodiment of the invention employing eight nozzles in a single row will be discussed in detail herein, but the disclosed invention is applicable to multichannel pipettes of various configurations. 
     In operation, replaceable pipette tips may be and generally are attached to each of the nozzles  130 , for example as described in U.S. Pat. No. 6,168,761 to Kelly et al., which is hereby incorporated by reference as though set forth in full herein. When the pipette  110  is operated as described above, by manipulating the plunger button  114  to aspirate and dispense fluid through the pipette tips, air is displaced from the cylinders and out of the nozzles, driving liquid into or out of the attached pipette tips as desired by the user. 
     The liquid end assembly  120  includes a tip ejector  124  actuated by depressing an ejector button  118  (on the body  112  of the pipette), which transfers force to the tip ejector  124  through a mechanism internal to the pipette body  112  to an ejector sleeve  126  at a proximate end of the liquid end assembly. The ejector sleeve  126  transfers its axial movement to the tip ejector  124  via a projecting upper end  134  of the ejector  124  adjacent to the sleeve  126 . When the ejector button  118  is depressed, the tip ejector  124  is caused to move axially, pushing off any tips mounted then on the nozzles  130 . When the ejector button  118  is released, the ejector  124  is spring-biased back to its rest position adjacent to the housing  122 , allowing new tips to be mounted. 
     In the disclosed embodiment, the housing  122  and external portions of the pipette body  112  are made from a suitable rigid polymer such as a polybutylene terephthalate (PBT) and polycarbonate blend. 
     Referring now to  FIG. 2 , the liquid end assembly  120  of  FIG. 1  is illustrated with the front portion  129  ( FIG. 1 ) of the housing  122  removed to expose the internal components of the liquid end assembly  120 . The exemplary liquid end assembly shown in  FIG. 2  and described herein is designed to allow the front portion  129  to be easily removed to service the internal components, which are primarily anchored to the rear portion  128  of the housing  122 . Accordingly, the liquid end assembly  120  is still operational and may be observed and inspected with the front portion  129  removed. 
     To attach the front portion  129  and the rear portion  128  together, the rear portion  128  includes a plurality of L-shaped features  212  and  214  arranged to interlock with counterpart features on the front portion  129 . To attach the front portion  129  to the rear portion  128 , the front portion  129  ( FIG. 1 ) is placed over the rear portion  128  and slid into place, causing the L-shaped features  212  and  214  to interlock with the corresponding features on the front portion  129 ; screws  216  are then fastened to prevent the portions  128  and  129  from sliding apart. The screws  216  and L-shaped features  212  and  214  cooperatively act to keep the housing  122  closed around its entire periphery. To remove the front portion  129  from the rear portion  128 , the screws  216  are removed and the portions are slid apart. This is considered a particularly advantageous assembly method, resulting in easy assembly and disassembly and light weight. Other configurations and means of attachment between the front portion  129  and the rear portion  128 , and among the various components of the liquid end assembly  120  are, of course, possible and are in keeping with the scope of the present invention. 
     In the disclosed embodiment, a coupling nut  210  attaches the liquid end assembly  120  to the body  112  of the pipette  110  ( FIG. 1 ). The coupling nut  210  easily screws and unscrews from a threaded distal portion of the pipette body  112 . 
     A pushrod  218  protrudes from a proximal end of the liquid end assembly  120 . The pushrod  218  includes a cupped receptacle  220  at its proximal end, adapted for coupling with a rounded adjoining distal end portion extending from the plunger rod  116  in the pipette body  112 . A bias spring  312  ( FIG. 3 ) urges the pushrod  218  upward and toward the pipette body  112 , which keeps the pushrod  218  and the plunger rod  116  closely coupled. This joint may be disassembled simply by loosening the coupling nut  210  and pulling the plunger rod and pushrod apart. 
     The joint between the pipette body  112  and the liquid end assembly  120  is free to rotate, even when the coupling nut  210  attaches the liquid end assembly  120  to the body of the pipette  112 . Accordingly, a user of the pipette is free to position the liquid end assembly  120  in any desired radial orientation for convenient operation. 
     The illustrated liquid end assembly  120  includes eight nozzles  130 , each situated at the distal end of a corresponding cylinder  230 . The cylinders  230  are affixed to a bottom wall  232  of the rear portion  128  of the housing  122 ; each cylinder  230  is fabricated from a molded polymer material and is formed with a radial protruding flange  234  adapted to fit into a corresponding cylinder slot  236  defined by the housing  122 . When the flange  234  is fully inserted into its cylinder slot  236  and the front portion  129  of the housing  122  is affixed to the rear portion  128 , the cylinders  230  are effectively anchored in place. In the disclosed embodiment, the flange  234  is keyed in such a way that it fits into the corresponding cylinder slot  236  in only one orientation, to ensure that all cylinders are appropriately and correctly aligned. 
     As in traditional multichannel pipettes, each cylinder  230  receives an axially movable piston  238 , and each piston  238  is anchored at its proximal end to an axially movable pushbar  240 . In turn, the pushbar  240  is attached to the pushrod  218 . Accordingly, then, when the pushrod  218  is moved axially during operation of the pipette  110  via depression of the plunger button  114 , that axial movement of the pushrod  218  is transferred to the pushbar  240  and the pistons  238 , each of which moves axially a distance equal to the movement of the plunger button  114 . Each of the pistons  238  forms an air-tight seal with its corresponding cylinder  230 , and accordingly, the movement of the pistons  238  into and out of the cylinders  230  displaces a substantially equal amount of air in each cylinder and the attached pipette tips, allowing liquid to be aspirated into or dispensed from the tips. 
     In the disclosed embodiment, the cylinders  230  and at least portions of the pistons  238  are molded from a suitable rigid chemical-resistant polymer, such as polyetherimide (PEI). This material and construction have shown to provide satisfactory sealing performance, light weight, and pipetting accuracy. 
     As seen in  FIGS. 2 and 3 , the pushbar  240  is mounted within the liquid end assembly  120  on a guide shaft  242  extending axially through the liquid end assembly  120  and mounted to the rear portion  128  of the housing  122 . As will be illustrated below in connection with  FIG. 4 , the pushbar  240  employs a brass bearing to track the guide shaft and maintain a strict perpendicular relationship between the pushbar  240  and the pushrod  218 , thereby assuring consistent performance across the channels of the disclosed multichannel pipette. Advantageously, the pushbar  240  is also fabricated from PEI, and the guide shaft  242  is a hollow stainless steel rod. The stainless steel guide shaft  242  is dimensioned to ensure that it remains straight and rigid within the liquid end assembly  120 , but thin enough to ensure the liquid end assembly does not include excess weight. 
     The rear portion  128  of the housing  122  further includes linear projections  244 . These linear projections  244  are positioned to guide the rear of the pushbar  240 , preventing it from rotating about either the guide shaft  242  or the pushrod  218  without imparting substantial friction to the axial movement of the pushrod  218  and pushbar  240 . 
     Each piston  238  includes a proximal flared head  250  sized to fit into a corresponding piston head slot  252  formed in the pushbar  240 . As illustrated, the pushbar  240  includes eight slots, one for each of the eight pistons  238  in the eight-channel pipette shown. The piston head slots  252  are sized to accommodate the flared heads  250  of the pistons  238 , with some extra space made available above the flared heads  250  for retention spring clips (discussed below in connection with  FIGS. 4 and 5 ). 
     With the illustrated configuration, a multichannel pipette liquid end assembly  120  according to the invention is easily assembled and disassembled. As noted above, each of the cylinders  230  fits into a corresponding cylinder slot  236  defined by the rear portion  128  of the housing  122 . Similarly, as the flared piston heads  250  fit into corresponding piston head slots  252  in the pushbar  240 , each individual cylinder  230  and corresponding piston  238  may be added to or removed from the liquid end assembly  120  by simply sliding those parts away from the rear portion  128  of the housing  122 . This facilitates simple manufacturing and service. If only one nozzle  130  or cylinder  230  has been damaged or is operating incorrectly, that specific component may be replaced without disturbing any of the other channels of the pipette  110 . 
       FIG. 3  illustrates many of the same components shown in  FIG. 2 , but in cutaway form with the operation of the cylinders  230  more clearly visible. 
     The illustrated embodiment represents an eight-channel liquid end, where each channel has a 50 μl displacement capacity. In this embodiment, it will be noted that each cylinder  230  includes a cylinder body  360 , a skirted seal  362 , and a seal retainer  364 . The seal retainers  364  are fabricated from molded polyoxymethylene (POM), while the skirted seals  362  are EPDM (ethylene propylene diene monomer) rubber, which has been found to provide advantageous sealing properties in a liquid end according to the invention, while having useful chemical resistance, wear resistance, and long life. The seals remain stationary near the proximal ends of the cylinders  230 ; each skirted seal  362  forms a substantially air-tight seal against an outer surface portion of its corresponding piston  238 . 
     It is anticipated that different pipette capacities may call for different seal designs. For example, a 10 μl or 20 μl multichannel liquid end assembly may include compressed nitrile rubber o-rings, held between the cylinder body and a seal retainer, for sealing against the axially moving piston. Such o-rings would also constitute stationary seals. Or larger-volume liquid end assemblies (such as 100 μl, 200 μl, or larger) may include moving seals, in which an annular EPDM rubber lip seal is affixed near the proximal end of each of the axially moving pistons, and the lip seal moves axially with the piston, sealing against a smooth inner surface of the cylinder. Such an alternative embodiment is illustrated in  FIG. 5  and discussed below. These alternative seal designs are also well adapted to displace air within pistons in multichannel pipettes. Other piston and cylinder seal configurations are well known and may be employed within the scope of the present invention. 
     Advantageously, a suitable perfluoropolyether (PFPE) or other grease may be used to lubricate the sealing surfaces between each piston  238  and the corresponding cylinder seal  362  described above, at the interface between the seals and the piston (in the illustrated embodiment) or the seals and the cylinder walls (in the moving-seal embodiment described above). PFPE grease is substantially inert, insoluble, chemical resistant, and heat resistant, and tends to avoid migration; these characteristics are considered particularly useful in lubricating pipette seals. 
     Several inventive features of the disclosed multichannel liquid end assembly  120  ( FIG. 1 ) are clearly illustrated in  FIG. 4 , which illustrates the pushrod  218 , the pushbar  240 , a spring clip  450 , and a single piston-cylinder assembly including a piston  412 , a seal retainer  414 , a skirted seal  416 , and a cylinder  418 . 
     As noted above, the pushbar  240  ( FIG. 2 ) is coupled to a tubular brass bearing  420  ( FIG. 4 ). The liquid end assembly  120  ( FIG. 1 ) is assembled so that an annular inner surface of the bearing tracks the stainless steel guide shaft  242  ( FIG. 2 ), which has a smooth, polished outer surface. The interface between the smooth guide shaft  242  and the brass bearing provides a low-friction guide upon which the pushbar  240  is free to move axially within the housing  122 , with minimal radial play. In the disclosed embodiment, the brass bearing  420  is press-fit into a sleeve defined by the pushbar  240 . 
     The bearing  420  fits closely upon the guide shaft  242 , and accordingly, the pushbar  240  is prevented from skewing and tends to remain perpendicular to the guide shaft  242 . It will be noted that pushbar skew is a significant contributor to channel-to-channel volume inconsistencies in multichannel pipettes. Although central channels, located close to the pushrod  218 , may have close to the desired performance, outboard channels (near either end of the row of nozzles  130 ) may have either a shorter-than-intended or longer-than-intended stroke as a result of pushbar skew or misalignment. The guide shaft  242  and bearing  420  arrangement set forth herein has been found to counteract such accuracy-defeating skew. 
     The pushrod  218  is coupled to the pushbar  240  through a compliant bayonet joint  430 . A distal end  432  of the pushrod  218  includes a T-shaped flange  434 . The T-shaped flange  434  fits into a corresponding bayonet slot  436  defined by the pushbar  240 , and during manufacture of the liquid end assembly  120 , a resilient o-ring  438  (nitrile rubber in the disclosed embodiment) is placed over the T-shaped flange  434  of the pushrod, the T-shaped flange  434  is inserted into the bayonet slot  436 , and the pushrod  218  is rotated approximately ninety degrees to lock the bayonet joint  430  together. The T-shaped flange  434 , bayonet slot  436 , and o-ring  438  cooperatively result in a compliant joint that is substantially free of axial play yet able to accommodate some radial angular movement of the pushrod, such as that caused by axial displacement of the cupped receptacle  220  ( FIG. 2 ) when the liquid end assembly  120  is mounted to a pipette body  112 . This ability to tolerate some minor misalignment between the pipette body  112  and the liquid end assembly  120 , without any substantial effect on the accuracy of liquid measurement using a pipette  110  according to the invention, is an advantageous feature of a pipette according to the invention. 
     In a liquid end assembly  120  according to the invention, the compliant bayonet joint  430  may be kept in its affixed and locked orientation (ninety degrees rotated after insertion) by one or more protruding features on an upper surface  440  of the pushbar  240  under the bayonet slot  436 ; compression of the resilient o-ring  438  urges the T-shaped flange  434  against that upper surface  440 , and accordingly, even a small protrusion extending from the upper surface  440  will assist in avoiding undesired pushrod rotation. 
     It will be noted that other forms of compliant joints may be substituted for the bayonet joint  430  in a pipette  110  according to the invention. In the disclosed embodiment, the bayonet joint  430  is captive. However, in alternative embodiments, non-captive joints (similar to the cupped joint at the proximal end of the pushrod  218 ) or ball-and-socket joints may afford similar advantages, provided any non-captive joint is sufficiently spring-biased together to avoid undesired slack. 
     As described above with reference to  FIG. 3 , the cylinder assemblies employed in a liquid end assembly  120  according to the invention include several components: the seal retainer  414 , the skirted seal  416 , and the cylinder  418 . The skirted seal  416  is inserted into a suitably sized recess  460  at a proximal end of the cylinder  418 , and the seal retainer  414  snaps into place over the seal  416 , securing the seal  416  firmly in position within the cylinder  418 . This assembly process can be accomplished without tools, as the disclosed seal retainer  414  includes at least one resilient tab  462  adapted to snap into a mating receptacle  464  defined by the cylinder  418 . Disassembly is also simple, as a tool may be employed to simply depress the tab  462  within the receptacle  464 , allowing the seal retainer  414  to be withdrawn from the cylinder  418 . It will be noted that the tab  462  and receptacle  464  are positioned proximal to the seal  416  within the cylinder  418 , and accordingly, the airtightness of the seal between the piston  412  and the cylinder  418  is not affected thereby. 
     As the piston  412  moves axially within the cylinder  418  the skirted seal  416  seals against an outer surface of the piston  412 , thereby displacing air within the cylinder  418 . In an embodiment of the invention, particularly low-volume pipettes (e.g. less than 100 μl according to the invention, the sealing portion of the piston  412  may be made from polished stainless steel. As noted above, stainless steel is considerably more dense and heavy than plastic, but it provides excellent surface smoothness and hardness, and in low-volume pipettes this material would not add appreciably to the overall weight of the device. 
     A spring clip  450  is interposed between the piston  412  and the pushbar  240 , positioned against an upper abutment surface  454  of the pushbar  240 . The spring clip  450  serves to secure and center the piston  412  in a desired position within the pushbar  240 . The spring clip will be discussed in further detail below, with reference to  FIGS. 7 and 8 . 
     As noted above, a different embodiment of piston-cylinder assembly may be employed in multichannel pipettes according to the invention, particularly larger-volume embodiments (e.g., 100 μl or larger). Such an alternative configuration is illustrated in  FIG. 5 . 
     The 200 μl piston-cylinder assembly of  FIG. 5  includes a piston  512 , preferably fabricated from a molded polymer material (such as the PEI employed in various other portions of a pipette according to the invention). 
     During operation of the pipette, the piston  512  moves into and out of a corresponding cylinder  580 . An annular seal ring  582  comprising an EPDM rubber lip seal is affixed near the proximal end of each of the axially moving pistons, and the lip seal moves axially with the piston  512 , sealing against a smooth inner surface  586  of the cylinder. In the disclosed embodiment, the seal ring  582  is kept in a preferred position on the piston  512  by a notch  584  defined by the piston. When the pipette is assembled, the seal ring  582  is stretched over the piston and released in the desired position; the notch  584  keeps the seal ring  582  in place during operation via an interference fit. In an embodiment of the invention, the interface between the seal ring  582  and the inner surface  586  of the cylinder  580  is lubricated with a suitable perfluoropolyether (PFPE) grease. 
     The bayonet joint  430  ( FIG. 4 ) between the pushrod  218  and pushbar  240  ( FIGS. 2-4 ) is illustrated in greater detail in  FIGS. 6 a  through 6 d   , which is schematic and illustrative in nature. 
     As shown in  FIG. 6 a    (and as previously described with reference to  FIG. 4 ), the pushrod  218  is inserted through a resilient o-ring  438  and into a bayonet slot  436  defined by the pushbar  240 . After insertion into the bayonet slot  436 , the pushrod is rotated ninety degrees to lock the T-shaped flange  434  into position within the pushbar  240 . As discussed above, and as illustrated in  FIG. 6 b   , the pushbar  240  includes projections  620  protruding from an upper surface  440  ( FIG. 4 ) of the pushbar  240 ; these projections tend to prevent the pushrod  218  and T-shaped flange  434  from rotating back into an unlocked orientation while the bayonet joint  430  is held together via compression of the o-ring  438 . 
     In  FIG. 6 a   , it will be observed that the o-ring  438  remains slightly compressed, and the T-shaped flange  430  is urged against the upper surface  440  of the pushbar  240 , while the bayonet joint  430  is fully assembled. Accordingly, then, as the pushrod  218  is moved axially during operation of a pipette according to the invention, there is substantially no axial play between the pushrod  218  and the pushbar  240 , and the pushbar accurately tracks the movement of the plunger rod of the pipette, and the pushbar remains perpendicular to the guide shaft  242 . 
     As noted above, however, the bayonet joint  430  is compliant, permitting some radial angular play in the pushrod  218  as shown in  FIG. 6 b   . Although one side of the o-ring  438  may be more compressed than the other, and the pushrod  218  and the pushbar  240  are no longer perpendicular, it will be observed that the compliant bayonet joint  430  permits the pushbar  240  to remain perpendicular to the guide shaft  242 , enabling excellent accuracy and channel-to-channel consistency in a multichannel pipette according to the invention. The angle of the pushrod  218  depicted in  FIG. 6 b    is exaggerated for effect, and in practice any deviation observed is likely to be considerably smaller, with minimal effect on the overall accuracy of liquid measurement. 
       FIGS. 6 c  and 6 d    illustrate the compliant bayonet joint  430  from a side of the liquid end assembly according to the invention, perpendicular to the view offered in  FIG. 3 . As shown in  FIG. 6 c   , the T-shaped flange  434  is wider than the bayonet slot  436  (once rotated into position), and the flange  434  is urged against the upper surface  440  of the pushbar  240 . And as shown in  FIG. 6 d   , radial displacement of the cupped receptacle  220  ( FIG. 2 ) will allow the pushrod  218  to angle somewhat from perpendicular to the pushbar  240 —and in doing so, the T-shaped flange  434  may rock slightly away from the upper surface  440  of the pushbar  240  (though the T-shaped flange  434  may be contoured to minimize this effect in practice). However, the pushbar  240  will remain perpendicular to the guide shaft  242  as desired, and the accuracy and performance of the pipette will tend to be maintained. 
     Clearly, then, the compliant bayonet joint  430  used in connection with the cupped receptacle  220  of the pushrod  218  is particularly advantageous, in that it allows a pipette according to the invention to be operated—even with some misalignment of parts—without applying any significant torque or moment to the pushbar. 
     The spring clip  450  is illustrated in greater detail in  FIG. 7 . As observed above, the spring clip  450  is preferably stamped and formed from stainless steel, but it should be noted that other suitable resilient materials may be used in this application, including but not limited to other metals or plastics. The material should be selected for adequate performance; stainless steel has been found to provide a good balance of durability, weight, cost, and resilience. 
     The illustrated spring clip is adapted to be positioned between the pushbar  240  and its pistons, such as the piston  412  ( FIG. 4 ). The spring clip  450  shown in  FIG. 7  accommodates four pistons, and accordingly two such spring clips are required for an eight-channel pipette, and three are required for a twelve-channel pipette. Other available configurations will be apparent. 
     The pushbar  240  illustrated in  FIG. 4  is provided with one or more apertures  452  or recesses to accommodate the attachment tabs  712  on the spring clip  450 . With the spring clip  450  inserted into place in the  240 , the attachment tabs  712  extend into corresponding apertures  452 , preventing the spring clip from undesired lateral movement under force from a piston  412  or other influence. The cooperative tabs  712  and apertures  452  keep the spring clip  450  securely in place. As noted above, the spring clip  450  includes a flat surface  710  that is positioned securely against the flat upper abutment surface  454  of the pushbar  240 , and the cooperative tabs  712  and apertures  452  keep it in that desired position. 
     In the illustrated (four-channel) version of the spring clip  450 , a plurality of slots  714  are defined by the spring clip  450 , which straddle walls  456  of the pushbar  240  between adjacent channels thereof. The spring clip  450  provides a resilient finger  716  for each of the channels it supports, and the resilient finger  716  is provided with a protruding bump  718  cooperative with an indentation  468  on the proximal flared head  470  of the piston  412  ( FIG. 4 ). With the piston  412  inserted partway into the cylinder  418 , the entire piston-and-cylinder assembly may be installed into the liquid end assembly  120  by pushing the flange  472  of the cylinder  418  into a corresponding slot  236  ( FIG. 2 ) in the rear portion  128  of the housing  122 , and also pushing the flared head  470  of the piston  412  into the pushbar  240 , with the indentation  468  defined by the flared head  470  engaging the protruding bump  718  on the spring clip  450 . Accordingly, installation and removal of cylinders and pistons from a multichannel pipette according to the invention may be accomplished without tools, and features on the cylinder  418  and housing  122  (the flanges  472  and cylinder slots  236 , respectively) and features on the piston  412  and spring clip  450  (the indentation  468  and protruding bump  718 ) serve to maintain the cylinder assembly in proper alignment. 
     The piston  512  illustrated in  FIG. 5  also includes an indentation  568  defined in a proximal flared head  570 , and accordingly, the piston-cylinder assembly of  FIG. 5  assembles with its pushbar and corresponding spring clip in a similar manner, though dimensions may vary to accommodate whether it is employed in a smaller-volume or larger-volume pipette according to the invention. 
       FIG. 8  illustrates the role of the spring clip  450  ( FIGS. 4, 7 ) in retaining pistons  412  within the pushbar  240 . 
     As described above with reference to  FIGS. 4 and 7 , the spring clip  450  inserts into the pushbar  240 , with a flat surface  710  ( FIG. 7 ) of the spring clip  450  held against the flat upper abutment surface  454  of the pushbar  240 . With the spring clip  450  so positioned, each of the fingers  716  ( FIG. 7 ) of the spring clip  450  projects into a corresponding piston head slot  252  defined by the pushbar  240 . When a piston  412  is inserted into its corresponding piston head slot  252 , the finger  716  and its protruding bump  718  urge the proximal flared head  470  of the piston  412  against a lower surface  812  of the piston head slot  252 . The protruding bump  718  projects into the indentation  468  ( FIG. 4 ) of the flared head  470 , preventing the piston  412  from sliding out of the piston head slot  252 . The piston/cylinder assembly of  FIG. 5  is configured similarly, with the protruding bump  718  of the spring clip  450  projecting into the indentation  568  of the flared head  570  of the piston  512  employed in that embodiment. 
     As noted above with reference to  FIGS. 4 and 7 , the spring clip  450  defines a plurality of slots  714  between adjacent fingers  716 . These slots  714  fit around walls  456  between adjacent piston head slots  252 , permitting a single spring clip  450  to be employed for multiple channels in a multichannel pipette according to the invention. The spring clip  450  illustrated herein includes four fingers  716  to retain four pistons  412 ; other configurations are possible. 
     In operation, the spring clip  450  (and its fingers  716  and protruding bumps  718 ) continues to resiliently urge the flared heads  470  of the pistons  412  against the lower surface  812  of the piston head slot  252  with little or no axial play, ensuring accurate performance. Although the spring clip  450  is designed to deform and allow the pistons  412  to move away from the lower surface  812  during assembly and service, during operation the flared heads  470  of the pistons  412  ordinarily remain depressed against the lower surface  812 . However, it should be noted that some temporary flex may be tolerated during operation without loss of accuracy, as long as the pistons remain in the proper position at the beginning and end of each pipetting stroke. 
     It should be observed that while the foregoing detailed description of various embodiments of the present invention is set forth in some detail, the invention is not limited to those details and a pipette made according to the invention can differ from the disclosed embodiments in numerous ways. In particular, it will be appreciated that embodiments of the present invention may be employed in many different liquid-handling applications. It should be noted that functional distinctions are made above for purposes of explanation and clarity; structural distinctions in a system or method according to the invention may not be drawn along the same boundaries. Hence, the appropriate scope hereof is deemed to be in accordance with the claims as set forth below.