Abstract:
A serial dilution is effected by sucking a determined increment of liquid to be diluted and air into an elastic tube and discharging both the air and the liquid through a nozzle into a diluting liquid by collapsing the tube at predetermined pressing points in a predetermined cycle.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to a method for accurately metering a precise increment of liquid and to apparatus for carrying out the same. It is particularly directed to a method and apparatus in which a precise increment of liquid is admitted into a collapsible chamber together with a quantity of air and the mixture of air and liquid discharged from the chamber through a nozzle by collapsing the chamber. Still more particularly the invention is directed to an improved method and apparatus for effecting serial dilution in which the increment of liquid to be diluted and air are drawn into an elastic tubing and both the air and the liquid are simultaneously discharged from the tubing through a nozzle into a measured amount of dilution liquid by collapsing the elastic tube, and in which the process is repeated as needed to effect the desired number of serial dilutions. 
     2. Prior Art 
     Serial dilution has heretofore been effected by drawing the liquid to be diluted into a syringe to a predetermined volume and thereafter squirting it into a measured volume of dilution liquid and repeating this operation step by step until the desired number of serial dilutions is effected. This method and apparatus has the disadvantage that even if the plunger stroke of the syringe is limited by means of end stops in order to permit some automization of the function, the inevitable leakage at the plunger packing could bring about a systematic volume error which is unacceptable in the handling of small liquid volumes, for example, of a magnitude of 25 to 50 microliters. Furthermore, any attempt to cushion the drawing in and expulsion strokes in automated apparatus is likely to introduce errors in the amount of liquid measured out, especially when very small liquid volumes are involved. 
     OBJECTS 
     It is an object of the invention to provide improved method and apparatus for measuring out a determined quantity of liquid. It is an object of the invention to effect an improved serial dilution. It is another object of the invention to provide a method and apparatus which permits faster and more accurate serial dilutions. It is another object of the invention to provide a method and apparatus avoiding the drawbacks inherent in the known techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described more particularly below with reference to the accompanying drawings in which: 
     FIG. 1 is a longitudinal section of the device having a plurality of pressing members. 
     FIG. 2 is a series of curves showing the programming of the pressing members shown in FIG. 1. 
     FIG. 3 is an alternative series of curves showing an alternative programming for the pressing members as shown in FIG. 1. 
     FIGS. 4 and 5 are fragmentary side elevations showing modified forms of the pressing heads of the pressing members. 
    
    
     SUMMARY OF THE INVENTION 
     The above objects and others which will become apparent as the description proceeds are accomplished in the present invention for method and apparatus by admitting air and a precise increment of liquid into a collapsible chamber having at least two serially connected collapsible positions which can be expanded to admit fluid and collapsed to discharge the same, said air being admitted by expanding one of said collapsible portions and said increment of liquid being admitted by expanding the other of said collapsible portions, and discharging the liquid and air through a nozzle by collapsing said collapsible portions whereby the air acts to cushion the collapse of said collapsible portions and to promote discharge of the liquid through said nozzle. More particularly, the invention is directed to an improved method and apparatus for effecting serial dilutions in which the liquid to be diluted is drawn into an elastic tubing together with air by expanding collapsed portions of the same and the volumes of air and liquid expelled by collapsing the tubing, the expanding and collapsing being effected at a plurality of press positions and programmed to effect the intake of air and then liquid and the expulsion of the two together. The air and the liquid preferably are expelled into the diluting liquid in order to effect a thorough mixing of the liquid to be diluted with the dilution liquid. 
     According to a preferred form of the invention, collapsible tubing, which is elastic but substantially non-stretchable in its circumferential direction, is arranged to be collapsed and expanded (unloaded) at a plurality of pressing positions in a programmed sequence designed to first suck in air, then suck in liquid to be diluted, and then to eject both the liquid and the air simultaneously. The programming can be such that, when the liquid and air are simultaneously ejected into the dilution liquid, the thus diluted liquid can be repeatedly sucked in and ejected as necessary or desirable to effect thorough mixing of the two liquids. It is desirable to provide a tube that is open at both ends, one end open to the atmosphere and the other end adapted to be placed into communication with the liquid to be diluted or the dilution liquid as needed. By providing three pressing positions arranged to collapse and expand the tubing in a repetitive programmed sequence, several different types of operation can be effected. In a preferred form, three pressing positions are provided, a liquid intake position, the position nearest or closet to the liquid intake, an air inlet position, the position closet or nearest to the air intake end of the tube, and an intermediate position. Thus, with all the pressing positions in collapsed position, the programming can proceed in the following sequence: 
     1. Expand the air inlet and intermediate positions to draw in air, 
     2. Collapse the air inlet position, 
     3. Expand the liquid inlet position thereby sucking in liquid, and 
     4. Collapse the intermediate and liquid inlet positions (the liquid intake end is of course at this stage transferred to the dilution liquid in order that the volumes of air and liquid be discharged into the dilution liquid). 
     In apparatus according to the invention it is of course possible to arrange a plurality of tubes, preferably parallel to each other, provided only that the pressing members are sufficiently wide that they cover all of the tubes. In such manner it is possible to parallelly carry out several dilution series, whereby the dilution part volumes suitably are arranged in parallel rows on a matrix such that the dilution method can be entirely automated. 
     The movement of the pressing members can be achieved in several ways. As will be described below, the pressing members may be arranged on axially displaceable and guided rods which are actuated by rotating cams. One can, however, also arrange the pressing members at hydraulic or pneumatic piston cylinder units, the movements of which can be controlled electronically by circuits already well known to those skilled in the art. 
     Further, it is possible to constitute the pressing members of rotating cam discs, which during rotation directly engage the tube, the rotational axis of these cam plates or discs advantageously being arranged parallel to the tube. 
     The tube used in the apparatus according to the invention need only be elastic at said pressing positions and the tube may there consist of some type of rubber or elastomer which has the desired elastical properties and resistance to the liquids to be dispensed. In order to suck in liquid and air, it should have a good form recovery at least at the pressing position utilized for insucking of liquid. 
     The apparatus and the method according to the invention is primarily suitable for metering small amounts of liquid, for example 25-50 microliters. 
     Furthermore it is easy to vary the size of the increment by changing tube dimensions. Also, the use of parallel tubes is a simple way of varying the quantity, if the various tubes have different dimensions. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more specifically to the drawings, there is illustrated a preferred form of the invention. The metering apparatus according to FIG. 1 has a frame comprising an upper member 1, a bottom member 2, and side members 3 and 4. A guide block 5 is arranged between the side members 3 and 4. The pressing members 6, 7 and 8 are fitted at push rods 9, 10 and 11, respectively, which are guided in apertures 12, 13 and 14, respectively, in the block 5. The apertures and rods have such shape that rotation of the rods is avoided. The cam shaft 15 is rotatably mounted in the side members 3 and 4 above the rods 9-11 in the plane through the rods, and is provided with cam discs 16, 17 and 18, which cooperate with the rods 9-11, respectively. 
     A tube 19, one end of which communicates with the atmosphere, is pulled through openings 21 and 22 in the side members 3 and 4, respectively, and rests on the bottom member 2 under the pressing members. Springs 26, which rest against the block 5, urge the pressing members 6-8 upwardly so that the rods 9-11 at all times contact the cam discs 16-18. The other end of the tube 19 is provided with a nozzle 23, the point of which is narrow and elongated, the outflow channel having a small section area. In this manner the liquid squirted out attains a high velocity, whereby the formation of drops at the orifice of the nozzle is reduced. This is of significance when the device according to the invention is used for pumping increments in the range of 10-200 microliters, inasmuch as about 50 microliters correspond to one drop of water. 
     The cam shaft 15 is driven by means of a motor 25 via a coupling 27. This coupling is of the type which at actuation permits the motor 25 to rotate the shaft 15 one revolution or an angle of rotation corresponding to one squirting out and one sucking in cycle. In this manner the operator is given sufficient time to transfer the nozzle to the next volume of liquid to be diluted. 
     The vertical movement provided to the rods and their pressing members during a working cycle by the cam discs is illustrated in FIG. 2, wherein T refers to a pressing member position corresponding to an unloaded or expanded tube, and wherein 0° refers to a pressing member position corresponding to the tube being collapsed. The shape of the cam discs and their orientation on the shaft can easily be deduced from what is shown in FIG. 2. 
     In the position 0°, which is the position illustrated in FIG. 1, the tube is collapsed in all positions. Then the pressing member 6, which merely functions as a shut-off valve, and the pressing member 7 are lifted so that air can be sucked into the tube (90°). Then, at 180°, the pressing member 6 is lowered such that the air feed is shut off. At 270° the pressing member 8, which because of its large area is also a displacement member, is lifted so that liquid can be sucked into the nozzle 23. The nozzle is then transferred to next dilution volume, into which the volume just sucked up is discharged by driving the pressing members 7 and 8 downwards to collapse the tube 19. 
     FIG. 3 shows alternative pressing member movements for the device according to the invention. The work function is equivalent to the case when one works with a tube, one end of which is closed to the atmosphere, the air cushion being sucked in through the nozzle 23, so that only two pressing members are necessary to carry out the work cycle. However, with a device that works according to FIG. 3, more dipping movements are necessary than with the device which operates according to FIG. 2. By sucking in the air cushion through the nozzle channel one achieves, however, the advantage that any micro drops that may have gathered at the point of the nozzle will then be sucked into the nozzle, whereby one avoids the possibility that micro drops are gathered into one drop which eventually will fall off and disturb the dilution. 
     The area of the pressing members 7 and 8, which is exposed to the tube 19, determines the displacement effect of the pressing member. Thus in FIG. 1, pressing member 6 gives relatively little displacement as compared with pressing member 8, whereas that of pressing member 7 is intermediate. In the sequence of FIG. 2, the area depressed by pressing member 7 is roughly proportional to the air sucked in and retained when positions 6 and 8 are collapsed, whereas that of pressing member 8 is roughly proportional to the amount of liquid sucked in. Thus, by varying these respective areas, the relative amounts of air and liquid can be varied. Generally speaking the ratio of air to liquid (vol/vol) is approximately 1 or less as shown in FIG. 1 by the fact that the area depressed by the pressing member 7 is approximately equal to but less than that depressed by the pressing member 8. It nevertheless can be varied widely on either side, but generally speaking, it is desirable to keep the ratio between 5:1 and 1:5 (vol/vol). 
     In order to make it possible to adjust the pressing members, especially pressing member 8, for different desired displacement volumes, the pressing member may be U-shaped in accordance with FIG. 4 and may be provided with and adjustment screw 30, which permits setting of different distances between the legs 31 of the pressing member. Thus, by squeezing the legs 31 closer together, the area subtended by the pressing member is made smaller and the displacement volume correspondingly less. 
     If one wishes to calibrate the displacement volume or to compensate said volume for shape changes of the tube at the pressing position, one can as shown in FIG. 5 arrange an adjustment flange 32 at the pressing member 8, this flange being adjustable in relation to the member 8. A displacement of the flange 32 up or down gives a very small adjustment of the displacement volume. 
     If one wishes to use other drive means, for example, hydraulic or pneumatic cylinders, for the pressing members, one can equally well use the pattern shown in FIG. 2 or 3 to control the drive means. 
     Even though the movement pattern of FIG. 2 is preferred, it should be appreciated that these patterns can be varied in many ways without departing from the concept of the invention. One can for example, if desired, by simple modifications of the control curves according to FIG. 2 or 3, prolong the sucking-in step and shorten the feeding-out step during a pumping cycle, instead of having them approximately equal in duration. 
     The apparatus illustrated can be modified in many ways within the scope of the invention. Thus, the device shown in FIG. 1 may be turned upside down, the springs 26 then being unnecessary. Further, the pressing members may be changeable so that one readily can alter displacement volumes. Further, the movement pattern shown in FIGS. 2 and 3 can be varied in many ways, the essential thing being that the movement of the pressing members is programmed to effect admission of air, then admission of a precise increment of liquid, and then concurrent or simultaneous discharge of the air and the liquid. Also, two or more tubes may be arranged in parallel with each pressing member actuating all of the tubes. 
     The dimension of the nozzle channel can be adapted to the properties of the fluid which is metered, the minimum section area being determined by the allowable pressure in the tube whereas the maximum section area is determined by the risk that the fluid will flow out by gravity. 
     It is to be understood that the invention is not to be limited to the exact details of operation or structure shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art.