Abstract:
A method for generating a vortex-like mixing action within a liquid held in a container by causing the container to move back and forth in a constant sinusoidal pattern at high speeds. Momentum forces acting upon the liquid solution cause it to generate an internal mixing motion of the liquid solution without the aid of extraneous mixing members.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for uniformly mixing sample liquids, reagents or other solutions. In particular, the present invention provides a method for rapidly and uniformly mixing a liquid in a container by generating a sinusoidal mixing action of the container.  
         BACKGROUND OF THE INVENTION  
         [0002]    Automated microbiology and clinical chemistry analyzers identify the presence of microorganisms and analytes in body fluids such as urine, blood serum, plasma, cerebrospinal fluid, sputum and the like. Automated microbiology and clinical chemistry analyzers improve productivity and enable the clinical laboratory to meet the workload resulting from high-test volume. Automated systems provide faster and more accurate results as well as valuable information to clinicians with regard to the types of antibiotics or medicines that can effectively treat patients diagnosed with infections or diseases. In a fully automated analyzer, many different processes are required to identify microorganisms or analytes and an effective type of antibiotic or medicine. Throughout these processes, patient sample liquids and samples in combination with various liquid reagents and antibiotics, are frequently required to be mixed to a high degree of uniformity producing a demand for high speed, low cost mixers that occupy a minimal amount of space.  
           [0003]    Analyzers like those described above perform a variety of analytical processes upon microbiological liquid samples and in most of these, it is critical that a patient&#39;s biological sample, particularly when in a liquid state, be uniformly mixed with analytical reagents or diluents or other liquids or even rehydrated compositions and presented to an analytical module in a uniformly mixed state. In a biochemical analyzer, other liquids like broth may need to be uniformly stirred before being used. Various methods have been implemented to provide a uniform sample solution mixture, including agitation, mixing, ball milling, etc. One popular approach involves using a pipette to alternately aspirate and release a portion of liquid solution within a liquid container. Magnetic mixing, in which a vortex mixing action is introduced into a solution of liquid sample and liquid or non-dissolving reagents, herein called a sample liquid solution, has also been particularly useful in clinical and laboratory devices. Typical of such mixing is disclosed in U.S. Pat. No. 6,382,827 wherein a liquid solution in a liquid container is mixed by causing a freely disposed, spherical mixing member to rapidly oscillate within the solution in a generally circular pattern within the container. The spherical mixing member is caused to rapidly move within the solution by revolving a magnetic field at high speed in a generally circular pattern in proximity to the liquid container. Magnetic forces acting upon the magnetic mixing member cause it to generate a mixing motion within the liquid solution.  
           [0004]    Ultrasonic mixing techniques like described in U.S. Pat. No. 4,720,374 employ ultrasonic energy applied from the exterior of the package and coupled into a reaction compartment so that a solid tablet of material within the compartment is dissolved or so that liquids contained therein are uniformly mixed. The container may include an array of sonication-improving projections mounted therein and spaced from each other to provide recirculating channels which communicate with both the tablet-receiving recess and the remainder of the volume of the container such that, in use, the projections act to confine a tableted material within a relatively high ultrasonic energy zone and simultaneously permit a flow of hydrating liquid from the high energy zone through the channels thereby to rapidly effect the dissolution of the tableted material.  
           [0005]    U.S. Pat. No. 6,382,827 mixes a liquid solution contained in a liquid container by causing a freely disposed, spherical mixing member to rapidly oscillate within the solution in a generally circular pattern within the container. The spherical mixing member is caused to rapidly move within the solution by revolving a magnetic field at high speed in a generally circular pattern in proximity to the liquid container. Magnetic forces acting upon the magnetic mixing member cause it to generate a mixing motion within the liquid solution.  
           [0006]    U.S. Pat. No. 5,824,276 cleans contact lens by applying a solution flow in an oscillatory fashion, so that the lens moves up and down within a container but does not contact the container for an extended time period. The method includes suspending the article in a solution within a container such that the article does not experience substantial or extended contact with the container interior. A predetermined flow of solution is passed into the container, thereby providing an upward force which, in conjunction with the buoyancy force, overcomes the downward gravitational force on the article, when the article is more dense than the solution. Alternatively, if the article has a lower density than the treatment solution, the flow is generated at the top of the container, to produce a substantially steady state effect.  
           [0007]    Accordingly, from a study of the different approaches taken in the prior art to the problems encountered with mixing of small volume solutions taken with the challenges of minimizing the physical size of a magnetic mixer, there is a need for an improved approach to the design of a simplified, space-efficient liquid sample and or sample-reagent mixer. In particular, there is a need for a mixer which enables rapid and uniform mixing of liquid solutions contained in one or more wells in a multi-well tray or of liquid solutions contained in tubes held in a sample tube rack without mechanisms to move the tray or tubes to a separate location for mixing. There is a further need for a method for liquid mixing that is of such high speed that multiple mixing processes may be achieved without adversely affecting the time required for liquid solution analysis. There is a even further need for a method for mixing device having a mixing motion that is unidirectional so as to positively affect the cost reduction of liquid solution analysis.  
         SUMMARY OF THE INVENTION  
         [0008]    Many of these disadvantages to the prior art are overcome by using the methods of this invention. This invention provides a method for generating a vortex-like mixing action within a liquid held in a container by causing the container to move back and forth in a constant sinusoidal pattern at high speeds. Momentum forces acting upon the liquid solution cause it to generate an internal mixing motion of the liquid solution without the aid of extraneous mixing members. The stroke and frequency may be optimized depending upon the geometrical sizes of the container. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will be more fully understood from the following detailed description thereof taken in connection with the accompanying drawings which form a part of this application and in which:  
         [0010]    [0010]FIG. 1 is a schematic plan view of a single conventional automated clinical analyzer like those known in the art;  
         [0011]    [0011]FIG. 2 is an enlarged partial schematic plan view of the automated analyzer of FIG. 1;  
         [0012]    [0012]FIGS. 3A and 3B are perspective views of a sample rack transport system useful in the analyzer of FIG. 1;  
         [0013]    [0013]FIG. 4 is a perspective view of a multi-well aliquot vessel useful in practicing the present invention;  
         [0014]    [0014]FIG. 5 is schematic plan view of a pair of automated clinical analyzers like those of FIG. 1, useful in practicing the present invention.  
         [0015]    [0015]FIG. 6 is a perspective view of an automated aliquot vessel array storage and handling unit integrated with a sampling track in which the present invention may be practiced to advantage;  
         [0016]    [0016]FIG. 6A is an enlarged perspective view of a portion of the automated aliquot vessel array storage and handling unit of FIG. 6 illustrating one embodiment of the present invention;  
         [0017]    [0017]FIG. 7 is a plan view of an aliquot vessel array useful in practicing the present invention;  
         [0018]    [0018]FIG. 7A is a sectional view of a portion of the aliquot vessel array of FIG. 7; and, FIG. 8 is an perspective view of a portion of an alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1, taken with FIG. 2, shows schematically the elements of a single convention automatic chemical analyzer  10  convenient for practicing the present invention and comprising a reaction carousel  12  supporting a outer cuvette circle  14  of cuvette ports  20  and an inner cuvette circle  16  of cuvette ports  22 , the outer cuvette circle  14  and inner cuvette circle  16  being separated by a open groove  18 . Cuvette ports  20  and  22  are adapted to receive a plurality of reaction cuvettes  24  typically formed as small, flat walled, U-shaped containers with an open central reaction portion closed at the bottom and with an opening at the top of cuvettes  24  to allow the addition of reagent and sample liquids. Reaction carousel  12  is rotatable using stepwise movements in a constant direction at a constant velocity, the stepwise movements being separated by a constant dwell time during which dwell time, carousel  12  is maintained stationary and an assay operation device  32  located proximate carousel  12  may operate on an assay mixture contained within a cuvette  24 .  
         [0020]    Two temperature-controlled reagent storage areas  26  and  28  each store a plurality of reagent cartridges  30 , cartridges  30 , for example being a multi-compartmented reagent container like those described in U.S. Pat. No. 4,720,374, or co-pending application Ser. No. 09/949,132 assigned to the assignee of the present invention, and sold under the tradename FLEX(tm) cartridge by Dade Behring Inc, Deerfield, Ill., and containing reagents as necessary to perform a given assay. A selectively-opened lid (not shown) covers each of reagent storage areas  26  and  28  to allow access to cartridges  30 ; for simplicity, only three reagent cartridges  30  are schematically illustrated in FIG. 2 as disposed beneath a cut out portion of reagent storage area  26  however similar reagent cartridges  30  are disposed within reagent storage area  28 . Shuttle means (not shown) move individual cartridges  30  to access ports for conventional reagent aspiration and dispense probes  27  and  29 . As shown, storage area  28  may be conveniently located external to the circumference of outer cuvette circle  14  and reagent storage area  26  may be conveniently located internal to the circumference of inner cuvette circle  16 .  
         [0021]    A clinical analyzer  10  like those on which the present invention may be performed has a plurality of conventional assay operation devices  32  disposed proximate carousel  12  and at which are positioned individual computer controlled electromechanical devices, such as sensors, reagent add stations, mixing stations, and the like, as required to perform the myriad of actions required in well known clinical assays. Such devices and their operation are well known in the art and need not be described herein. See, for example, U.S. Pat. Nos. 5,876,668, 5,575,976 and 5,482,861 and the references cited therein.  
         [0022]    An indexing drive for the reaction carousel moves the reaction vessels in the constant direction a predetermined numbers of incremental steps. The length of the circumference of cuvette circles  14  and  16 , the separation distance between cuvette ports  20  and  22 , the number of cuvette ports  20  and  22 , and the number of increments per indexing are selected so that any given cuvette ports  20  and  22  returns to its original starting position after a fixed number of incremental steps. Thus, all cuvette ports  20  and  22  on the reaction carousel  12  return to their original location in a full operational cycle time which is determined by the fixed number of incremental steps multiplied by the sum of dwell time at each assay device and the time required for a stepwise movement.  
         [0023]    Incoming sample specimens to be tested are contained in sample tubes  40  mounted in sample tube racks  42  and transported into the arc of a conventional liquid sampling arm  44 , for example, by a bi-directional incoming, as indicated by open arrow  36 A, and outgoing, as indicated by open arrow  36 B, sample tube transport system  36 , described in co-pending application Ser. No. 09/992,917 assigned to the assignee of the present invention. A magnetic drive system  50  useful in analyzer  10  is seen in the perspective drawing FIG. 3A as having at least one bi-directional linear drive transport mechanism  52  comprising, for example, a first belt  51  endlessly circulating around a pair of first pulleys  53 , one of the first pulleys  53  being coupled to a first bi-directional motor  54 , the first belt  51  and first pulleys  53  being mounted beneath and in close proximity to the operating surface of analyzer  10  which defines input and output lanes. In an exemplary embodiment of the present invention only a single incoming sample tube transport system  50  is employed and only a single bi-directional linear drive transport mechanism  52  is required. It should be understood that any of several mechanisms are capable of providing the bi-directional linear drive transport mechanism  52  used within the present invention, for instance a bi-directional motor coupled to a linear drive screw, or a pneumatic operated plunger, both supporting the magnetic housings and having a magnet therein.  
         [0024]    Alternately, first belt  51  may driven by motor  54  only in an incoming direction, for example along the direction of arrow  36 A. In a similar manner, magnetic drive system  50  may additionally comprise a second belt  57  endlessly circulating around a pair of second pulleys  58 , one of the second pulleys  58  (only one such second pulley  58  is visible) being coupled to a second bi-directional motor  59 , the second belt  57  and second pulleys  58  being mounted beneath and in close proximity to the output lane portion of the operating surface of analyzer  10 , indicated by open arrow  36 B. Second belt  57  is driven by second motor  59  in an outgoing direction  36 B opposite to the incoming direction  36 A. Motors  54  and  59  are typically stepper motors independently controlled by computer  15  and have drive gears coupled to pulleys  53  and  58  which are preferably formed as pulley gears interlaced with gear teeth formed on belts  51  and  57 . The magnetic drive system  50  is described here in terms of a pulley-and-belt drive mechanism, however, any of a number of bi-directional linear drive mechanisms may be employed to achieve the purpose of linearly moving a sample tube rack  42  in either of two opposing directions. FIG. 3B illustrates a plurality of sample tube racks  42  magnetically coupled to each drive belt  51  and  57  by means of a plurality of upright posts  59  generally equally spaced apart by a predetermined distance, and, as seen in FIG. 3B, the plurality of upright posts  60  are attached to belts  51  and  57  at that same predetermined distance. Posts  60  are adapted by any of various mechanical techniques, such as screws, snaps, welds, etc., to secure the plurality of magnetic sample tube racks  42  to belt  51  and  57 .  
         [0025]    A fluid sampling arm  44  supports a conventional liquid sampling probe  46  and is mounted to a rotatable shaft  48  so that movement of sampling arm  44  describes an arc intersecting the sample tube transport system  36  and an aliquot strip transport system  62  described later and adapted to transport multi-well aliquot vessels  64 , like that seen in FIG. 4, to a conventional sample/reagent aspiration and dispense arm  66  located proximate reaction carousel  12 . Sampling arm  44  is operable to aspirate liquid sample from sample tubes  40  and to dispense a sample aliquot portion into one or more of a plurality of aliquot wells  62 W in aliquot vessels  62 , depending on the quantity of sample required to perform the requisite assays and to provide for a sample aliquot to be retained by analyzer  10  within an environmental chamber  38 . Sample/reagent aspiration and dispense arm  66  is controlled by computer  15  and is adapted to aspirate a controlled amount of sample from wells  52 W via a conventional liquid probe  66 P and to dispense an appropriate amount of aspirated sample into one or more cuvettes  24  for assay testing for one or more analytes. After sample has been dispensed into reaction cuvettes  24  in cuvette ports  20  and  22 , conventional transfer means move aliquot strips  52  as required between aliquot strip transport system  50  and an environmental chamber  38  as described in co-pending application Ser. No. 09/827,045 also assigned to the assignee of the present invention, or, optionally, to a waste disposal area, not shown. Patient liquid specimens contained in open sample tubes  40  are identified by reading bar coded indicia placed thereon using a conventional bar code reader to determine, among other items, a patient&#39;s identity, the tests to be performed, if a sample aliquot is desired to be retained inside environmental chamber  38  and if so, for what period of time. It is also common practice to place bar coded indicia on sample tube racks  42  and employ a large number of conventional bar code readers installed throughout analyzer  10  in order to ascertain, control and track the location of both sample tubes  40  and sample tube racks  42 . Such reader devices and the techniques for tracking are well known in the art and are not seen in FIG. 1 nor need be discussed further.  
         [0026]    Analyzer  10  is controlled by computer  15  based on software written in a machine language, like that used on the Dimension® clinical chemistry analyzer sold by Dade Behring Inc, of Deerfield, Ill., and widely used by those skilled in the art of computer-based electromechanical control programming. At least two reagent aspiration and dispense arms  27  and  29  comprising a pair of conventional liquid reagent aspiration and dispense probes,  27 P and  29 P, respectively, are independently mounted and translatable between reagent storage areas  26  and  28 , respectively. Probes  27 P and  29 P are shown in FIG. 1 in two operating positions, with one probe,  29 P, adapted to remove reagent from a reagent container in storage area  28  and to dispense aspirated reagent into cuvettes  22  and  24  located in cuvette circles  14  and  16  and with the other probe,  27 P, adapted to remove reagent from a reagent container in storage area  26  and to dispense aspirated reagent into cuvettes  22  and  24  located in cuvette circles  14  and  16 . Probes  27 P and  29 P typically comprise an ultrasonic mechanism used for hydrating, aspirating, dispensing and mixing reagents. The hydrating, aspirating, dispensing and mixing mechanisms have features well known in the art and need not be described further.  
         [0027]    Cuvette load and unload stations  61  and  63  are positioned proximate outer cuvette carousel  14  and are conventionally adapted to load cuvettes  24  into cuvette ports  20  and  22  seen in FIG. 2 formed in outer cuvette carousel  14  and inner carousel  16  using for example a translatable robotic clamp  64 . Conventional sample processing devices  32  (FIG. 2), are positioned at selected circumferential locations about the reaction carousel  12  in order to access reaction cuvettes  24 . Processing devices  32  are adapted to provide, among other processing steps, for mixing together of the sample liquid and the reagent liquid contained in cuvettes  24 , for washing the sample liquid and the reagent liquid contained in cuvettes  24 , and for magnetic separation of tagged magnetic particles from free tags or reagent liquid contained in cuvettes  24 .  
         [0028]    Various assay analyzing stations  67  may be located proximate outer reaction carousel  12  and are adapted to measure light absorbence in or emission from cuvettes  24  at various wavelengths, from which the presence of analyte in the sample liquid may be determined using well-known analytical techniques. Stations  67  typically comprise conventional photometric, fluorometric or luminescent measuring devices adapted to perform an interrogating measurement at any convenient time interval during which reaction carousel  12  is stationary.  
         [0029]    Drive means are provided for independently rotating outer reaction carousel  12  about an axis, the drive means typically comprising gear teeth disposed on the carousel  12  and interlacing with pinion gears mounted on the shaft of a motor. The drive means may be of conventional design and are not illustrated.  
         [0030]    [0030]FIG. 5 illustrates a method for operating a pair of automatic clinical analyzers like analyzer  10  seen in FIG. 1, identified as analyzers  11  and  13 , linked together by a bi-directional sample rack shuttle  69  that optimizes sample assay throughput irregardless of the mix of different assays required to be performed for different samples to be tested as described in co-pending application Ser. No. 60/376,256 also assigned to the assignee of the present invention. In this instance, the individual computers  15  of analyzers  10  may be cooperatively controlled by a stand-alone computer  17  so programmed using well known techniques, or a single one of the computers  15  may alternately be programmed so as to control both analyzers  10  and  11 .  
         [0031]    Analyzers  11  and  13  are essentially identical to one another except that the menu of assays capable of being performed thereon is selectively different as explained later. For convenience in describing operation of the pair of automatic clinical analyzers  10 , the leftmost analyzer is identified an analyzer  11  and the rightmost is identified as analyzer  13 . In this arrangement, the bi-directional incoming and outgoing sample tube transport system  36  of the analyzer  10  of FIG. 1 is converted into a one-way incoming sample tube transport system  68 , indicated by open arrow  36 A, adapted to receive all sample tube racks  42  having all sample tubes  40  to be analyzer by either analyzer  11  or  13 . Any sample tube rack  42  may then be transferred from incoming sample tube transport system  68  by a conventional tube rack transfer mechanism  70  operable between analyzer  10  and bi-directional sample rack shuttle  69  and shuttled from sample rack shuttle  66  via another conventional tube rack transfer mechanism  72  onto analyzer  13  as directed by computer  17 . In this arrangement, the sample tube transport system  36  of analyzer  11  may be converted into a one-way outgoing transport system  74 , indicated by open arrow  36 B, adapted to dispose of all sample tube racks  42  having sample tubes  40  with samples finally analyzed by either analyzer  11  or  13 . Operation and features of a transport mechanism like sample rack shuttle  69  are well known in the art, for example as discussed in U.S. Pat. Nos. 6,117,392 and 6,117,683 and 6,141,602, and are thus not provided here. Generally, conveyor belts, hooks, magnetic devices, or the like may be employed in the design of shuttle  69 , tube rack transfer mechanisms  70  and  72  and transport systems  68  and  74 .  
         [0032]    As seen in FIG. 6, an automated aliquot vessel array storage and handling unit  80  is disposed proximate aliquot strip transport system  62  and is adapted in a manner so that aliquot vessel arrays  64  may be automatically transferred from a vertically translatable array elevator  82  from any of three aliquot vessel array inventory shafts  83  within aliquot vessel array storage unit  80  onto one of several pairs of parallel aligned aliquot vessel array sampling tracks  84 . Aliquot vessel arrays  42  are mounted within aliquot vessel array storage unit  80  between pairs of sampling tracks  84  having flared open ends suitable for discharging and receiving an aliquot vessel array  42 . Two aliquot vessel arrays  42  are seen located between a pair of sampling tracks  84 . The lengthwise positioning of an aliquot vessel array  42  between sampling tracks  84  is provided by a rotary step motor  86  adapted to independently move aliquot vessel arrays  42  in either direction between a pair of sampling tracks  84 , the motors  86  being connected for example by a connecting rod  88  to each aliquot vessel array  42  (as seen in FIG. 6A). Each aliquot vessel array  64  has a protruding and downwardly projecting zero-backlash hitch  100  described hereinafter adapted to secure aliquot vessel array  64  to a pin portion  101  of connecting rod  88 .  
         [0033]    [0033]FIG. 7 is a plan view of the aliquot vessel array  64  useful in practicing a first embodiment of the present invention showing spatial relationships between parallel first and second side walls  90  and  91  extending lengthwise along the longer orthogonal edges  92  of a generally rectangular aliquot base plate  93 . An ordered array of open wells  42 W is formed between the pair of parallel side walls  90  and  91 , and separated therefrom by notched side flanges  94 . Front orthogonal edge  95  and rear orthogonal edge  96  of base plate  116  are further seen to be formed mutually parallel to one another with a zero-backlash hitch  100  formed in the central region  97  of the front shorter perimeter portion  98  between the array of open wells  64 W and front orthogonal edge  95 . An important feature of the aliquot vessel array  64  is the zero-backlash hitch  100  formed in the central region  97  of the front shorter perimeter portion  98  between the shorter orthogonal edge  95  and the array of open wells  64 W. Sectional line A-A in FIG. 7A is enlarged to show details of zero-backlash hitch  100  comprising an opening  101  in base plate  93  and a pair of semi-circular sleeves extending downwardly, a frontal sleeve  102  formed to slant backwards from the front of aliquot vessel array  64  towards a rear sleeve  104  formed generally perpendicularly to base plate  93 . The pair of semi-circular sleeves  102  and  104  are spaced apart a distance so that a downwardly descending finger-latch portion  89  of connecting rod  88  may be inserted between the frontal sleeve  102  and rear sleeve  104  in such a manner that the backwards slanting frontal sleeve  102  biases finger-latch portion  89  securely against rear sleeve  104 , thereby ensuring that aliquot vessel array  42  may be accurately positioned within track  84  and secured to motor  86 . The backwards slanting frontal sleeve  102  provides zero-backlash locations to aliquot vessel array  64  throughout a repeated number of movements in both directions between tracks  84 . Aliquot vessel array  64  is repeatedly moved to a single sampling location in track  84  whereat multiple aliquots of sample are aspirated from wells  64 W, wells  64 W being environmentally sealed with a conventional laminate covering (not shown) and punctured by an aspiration needle. It is important that aliquot vessel array  64  be accurately positioned within track  84  by zero-backlash hitch  140  so that only a single aspiration puncture is made in the laminate covering during multiple sample aspirations thereby minimizing sample evaporation losses during subsequent storage of the aliquot vessel array  102 .  
         [0034]    An important and surprising discovery of the present invention is that a vortex-like mixing action may be generated within a liquid contained in wells  64 W of aliquot vessel array  64  by high speed linear movements in opposite directions of aliquot vessel array  42  between sampling tracks  84 . It has been found that such bi-directional movement along a linear path, like that defined by sampling tracks  84  is highly effective in thoroughly mixing liquid sample and reagents and/or re-suspending a liquid mixture of different liquid constituents. It has further been found that the optimum length of such bi-directional linear movements and the frequency at which the direction of movement is changed are dependent upon the diameter of well  42 W or other larger container like that shown in FIG. 8.  
         [0035]    In a first operational example of the present invention, a red liquid with specific gravity slightly greater than 1.0 is placed into the bottom of wells  64 W. The red liquid is then covered with a layer of water resulting in a stratified sample, red liquid on the bottom with clear water on the top. In this example, wells  64 W are approximately 2-3 mm in diameter and about 1 cm in depth. Stepper motor  86  is operated so as to provide a bi-directional constant linear stroke in a stroke of about 3-6 millimeters at a constant frequency in a range of frequencies varying between about 10 to 40 Hz. Consequently, aliquot vessel array  64  is moved in a constant sinusoidal pattern selected from the range of sinusoidal patterns comprising 3-6 millimeters and frequencies varying between about 10 to 40 Hz. Examination of the fluid in wells  42 W discloses that in the frequency ranges between about 10-15 Hz and between about 30-40 Hz, very little suspension or mixing of the red liquid occurs within the water sample. However, when the stepper motor  86  is operated so that aliquot vessel array  42  is moved with bi-directional linear stroke of 3-6 millimeters in a range of frequencies varying between about 20 to 30 Hz, a uniform suspension or mixing of the red liquid occurs within the water sample in a time period of about 1-3 seconds.  
         [0036]    In an alternate operational example of the present invention depicted in FIG. 8, liquid water colored with red food dye is placed into reagent containers  124  supported in a reagent tray  122 , the reagent tray  122  being suspended by a pin-and slotted lever  125  from a flexible belt  126 . In this example, reagent containers  124  are approximately 10-15 mm in diameter and about 2 cm in depth. Reagent containers  124  are allowed to remain stationary until the dye settles to the bottom of the reagent containers  124  and the water is clear. Flexible belt  126  is seen as being driven by a pair of pulleys  128 , one pulley  128  of which is mounted to stepped motor  130 . Stepper motor  130  is operated so as to provide a bi-directional constant linear oscillation of the reagent containers  124  supported in a reagent tray  122  in a stroke of about 8-12 millimeters at a constant frequency in a range of frequencies varying between about 10 to 40 Hz. Consequently, reagent containers  124  supported in a reagent tray  122  are moved in a constant sinusoidal pattern selected from the range of sinusoidal patterns comprising about 8-12 millimeters stroke and frequencies varying between about 5 to 40 Hz. Examination of the fluid in reagent containers  124  discloses that in the frequency ranges between 5-10 Hz and between about 15-40 Hz, very little re-suspension or mixing of the red dye occurs within the water sample. However, when the stepper motor  86  is operated so that reagent containers  124  are moved with bi-directional linear stroke of 3-6 millimeters in a range of frequencies varying between about 10 to 15 Hz, a uniform re-suspension or mixing of the red dye occurs within the water sample in a time period of about 1-3 seconds.  
         [0037]    It is to be understood that the embodiments of the invention disclosed herein are illustrative of the principles of the invention and that other modifications may be employed which are still within the scope of the invention. Accordingly, the present invention is not limited to those embodiments precisely shown and described in the specification but only by the following claims.