Patent Publication Number: US-9415399-B2

Title: Device for mixing and separation of magnetic particles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 11/079,695, filed Mar. 15, 2005, now abandoned, which is a division of application Ser. No. 10/290,514, filed Nov. 8, 2002, now U.S. Pat. No. 6,884,357, which is a continuation-in-part of application Ser. No. 09/771,665, filed on Jan. 30, 2001, now U.S. Pat. No. 6,500,343, which is continuation-in-part of application Ser. No. 09/476,258, filed on Jan. 3, 2000, now U.S. Pat. No. 6,228,268, and a continuation-in-part of application Ser. No. 09/476,260, filed on Jan. 3, 2000, now U.S. Pat. No. 6,231,760, which is a division of application Ser. No. 08/902,164, filed Jul. 29, 1997, now U.S. Pat. No. 6,033,574. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for mixing and separation of magnetic particles for the purpose of isolating substances from a nonmagnetic liquid test medium. 
     2. Description of Related Art 
     Magnetic separation of biomolecules and cells based on magnetic particles and employing biospecific affinity reactions is advantageous in terms of selectivity, simplicity, and speed. The technique has proved to be quite useful in analytical and preparative biotechnology and is now being increasingly used for bioassays and isolation of target substances such as cells, proteins, nucleic acid sequences and the like. 
     As used herein, the term “receptor” refers to any substance or group of substances having biospecific binding affinity for a given ligand, to the substantial exclusion of other substances. Among the receptors susceptible to biospecific binding affinity reactions are antibodies (both monoclonal and polyclonol), antibody fragments, enzymes, nucleic acids, lectins and the like. The term “ligand” refers to substances such as antigens, haptens, and various cell associated structures having at least one characteristic determinant or epitope, which substances are capable of being biospecifically recognized by and bound to a receptor. The term “target substance” refers to either member of a biospecific binding affinity pair, i.e., a pair of substances or a substance and a structure exhibiting a mutual affinity of interaction, and includes such things as biological cells or cell components, biospecific ligands, and receptors. 
     Affinity separation refers to known process techniques where a target substance mixed with other substances in a liquid medium is bound to the surface of a solid phase by a biospecific affinity binding reaction. Substances, which lack the specific molecule or structure of the target substance, are not bound to the solid phase and can be removed to effect the separation of the bound substance or vice versa. Small particles, particularly polymeric spherical particles as solid phase, have proved to be quite useful, as they can be conveniently coated with biomolecules, provide a very high surface area, and give reasonable reaction kinetics. Separations of the particles containing bound target substance (bound material) from the liquid medium (free material) may be accomplished by filtration or gravitational effects, e.g., settling, or by centrifugation. 
     Separation of bound/free fractions is greatly simplified by employing magnetizable particles, which allows the particle bound substance to be separated by applying a magnetic field. Small magnetizable particles are well known in the art, as is their use in the separations involving immunological and other biospecific affinity reactions. Small magnetizable particles generally fall into two broad categories. The first category includes particles that are permanently magnetized, and the second comprises particles that become magnetic only when subjected to a magnetic field. The latter are referred to as paramagnetic or superparamagnetic particles and are usually preferred over the permanently magnetized particles. 
     For many applications, the surface of paramagnetic particles is coated with a suitable ligand or receptor, such as antibodies, lectins, oligo nucleotides, or other bioreactive molecules, which can selectively bind a target substance in a mixture with other substances. Examples of small magnetic particles or beads are disclosed in U.S. Pat. Nos. 4,230,685, 4,554,088, and 4,628,037. The use of paramagnetic particles is taught in publications, “Application of Magnetic Beads in Bioassays,” by B., Haukanes, and C. Kvam, Bio/Technology, 11:60-63 (1993); “Removal of Neuroblastoma Cells from Bone Marrow with Monoclonal Antibodies Conjugated to Magnetic Microspheres” by J. G. Treleaven et al. Lancet, Jan. 14, 1984, pages 70-73; “Depletion of T Lymphocytes from Human Bone Marrow,” by F. Vartdal et. al. Transplantation, 43: 366-71 (1987); “Magnetic Monosized Polymer Particles for Fast and Specific Fractionation of Human Mononuclear Cells,” by T. Lea et. al., Scandinavian Journal of Immunology, 22: 207-16 (1985); and “Advances in Biomagnetic Separations,” (1994), M. Uhlen et. al. eds. Eaton Publishing Co., Natick, Mass. 
     The magnetic separation process typically involves mixing the sample with paramagnetic particles in a liquid medium to bind the target substance by affinity reaction, and then separating the bound particle/target complex from the sample medium by applying a magnetic field. All magnetic particles except those particles that are colloidal, settle in time. The liquid medium, therefore, must be agitated to some degree to keep the particles suspended for a sufficient period of time to allow the bioaffinity binding reaction to occur. Examples of known agitation methods include shaking, swirling, rocking, rotation, or similar manipulations of a partially filled container. In some cases the affinity bond between the target substance and the paramagnetic particles is relatively weak so as to be disrupted by strong turbulence in the liquid medium. In other cases biological target substances such as cells, cellular fractions, and enzyme complexes are extremely fragile and will likewise be disrupted or denatured by excess turbulence. 
     Excess turbulence is just one of several significant drawbacks and deficiencies of apparatus and methods used in the prior art for biomagnetic separations. The specific configuration of a magnetic separation apparatus used for separating particle-bound target complex from the liquid medium will depend on the nature and size of magnetic particles. Paramagnetic particles in the size range of 0.1 to 300 μm are readily removed by means of commercially available magnetic separation devices. Examples of such magnetic separation devices are the Dynal MPC series of separators manufactured by Dynal, Inc., Lake Success, N.Y.; and BioMag Separator series devices manufactured by PerSeptive Diagnostics, Cambridge, Mass.; and a magnetic separator rack described in U.S. Pat. No. 4,895,650. These devices employ permanent magnets located externally to a container holding a test medium and provide only for separation. Mixing of the paramagnetic particles in the test medium for affinity binding reaction must be done separately. For example, Dynal MPC series of separators requires a separate mixing apparatus, a Dynal Sample Mixer, for agitating the test media. The process must be actively monitored through various stages of mixing, washing, and separation, and requires significant intervention from the operator. Accordingly, the efficiency of these devices is necessarily limited by the skill and effectiveness of the operator. 
     U.S. Pat. No. 4,910,148 describes a device and method for separating cancer cells from healthy cells. Immunoreactive paramagnetic particles and bone marrow cells are mixed by agitating the liquid medium on a rocking platform. Once the particles have bound to the cancer cells, they are separated from the liquid medium by magnets located externally on the platform. Although such mixing minimizes the liquid turbulence, it does not provide an efficient degree of contact between the particles and the target substance. Moreover, the utility of this device is limited to the separation of cells from relatively large sample volumes. 
     U.S. Pat. No. 5,238,812 describes a complicated device for rapid mixing to enhance bioaffinity binding reactions employing a U-tube-like structure as mixer. The U-tube is rapidly rocked or rotated for 5 to 15 seconds to mix the magnetic particles in the test medium, and then a magnet is brought in close proximity to the bottom of the U-tube to separate the magnetic particles. As stated in the &#39;812 patent, its utility is limited to treating very small volumes (&lt;1000 μl) of test medium. 
     U.S. Pat. No. 5,336,760 describes a mixing and magnetic separation device comprising a chamber attached to a platform with one or more magnets located close to the container and an intricate mechanism of gears and motor to rotate the platform. Immuno-reactive paramagnetic particles are mixed in the test medium by first placing a stainless steel “keeper” between the chamber and the magnet to shield it from the magnetic field. Then the platform is rotated between vertical and horizontal positions. The particles in the test medium are mixed by end-over-end movement of the chamber. Following the mixing, the “keeper” is removed so that the magnetic particles are captured by the exposed magnetic field. Besides requiring a complicated mechanism, agitation of the liquid medium by end-over-end rotation does not mix relatively buoyant particles efficiently, and the liquid turbulence will tend to shear off or damage the target substance. 
     U.S. Pat. No. 5,110,624, relates to a method of preparing magnetizable porous particles and describes a flow-through magnetically stabilized fluidized bed (MSFB) column to isolate proteins from cell lysate. The MSFB column is loosely packed with a bed of magnetizable particles and equipped with means of creating a stationary magnetic field that runs parallel to the flow of solution through the column. The particles are maintained in a magnetically stabilized fluidized bed by adjusting the rate of flow of the solution and the strength of the magnetic field. This is a complicated technique requiring precise adjustment of the flow rate and magnetic strength so that the combined effect of fluid velocity and magnetic attraction exactly counterbalances the effect of gravity on the particles. Moreover, the design of MSFB is not optimized for use with small test volumes, and cannot be made optimal for applications such as bioassays or cell separations. 
     International patent application WO 91/09308 published Jun. 27, 1991 discloses a separating and resuspending process and apparatus. This application teaches that rotation of a magnet around the container containing paramagnetic particles induces the particles to remain as a compact aggregate (in close proximity to the magnet source) and roll over one another. The application teaches that this method fails to produce resuspension of the particles. The application WO 91/09308, discloses that the magnetic particles must be subjected to sequential magnetic fields situated opposite each other in order to effect resuspension. The application describes a device comprising a chamber located between two electromagnets, which are energized and de-energized to aggregate the magnetic particles alternately at the two magnets. The application teaches that alternately energizing and de-energizing the two electromagnets at a sufficiently rapid rate keeps the particles suspended in the center of the chamber. This method limits movement of the particles to a relatively small distance, significantly reducing the collision frequency between particles and the target substance, necessary for affinity binding which is a major reason for mixing the paramagnetic particles in the liquid medium. Moreover, a significant fraction of the particles, particularly particle-cell complexes may escape the magnetic field by gravitational settling to the bottom of chamber and will be lost during aspiration of the liquid medium following the aggregation step. 
     Japanese patent No. JP58193687 entitled Agitation And Separation Of Microscopic Material is directed to separation of microorganisms by mixing magnetized ultra-fine magnetic wire with microorganisms containing magnetic particles. The mixing is accomplished by a rotary magnetic field, which also acts to separate the microorganisms after a mixing step. This patent is concerned with separation of microorganisms that contain internally ultra-fine magnetic particles. Such microorganisms are well known in the art, a particular example being magneto spirillium, a bacteria known to synthesize ultra fine magnetic particles. Such microorganisms would not and cannot be used as magnetic particles for mixing and separation of a target species as envisioned by the present invention. The Japanese patent&#39;s requirement for linearly-connected ultra-fine magnetic particles refers to a wire which is most likely used to create a high gradient magnetic field (HGMF) to collect or precipitate the magnetite-containing bacteria over the surface of these wires. Such a technique has no application to the process of affinity separation of a target substance from a liquid test medium as envisioned by the present invention since it relies on the magnetic properties of the micro-organisms (the target substance itself) to effect a reaction. 
     The applicable known procedures have shortcomings, including the requirement for separate mechanically complex mixing mechanisms, as well as various process constraints and inefficiencies. The present invention provides devices and methods for magnetic mixing and separation which are of relatively simple construction and operation, which can be adapted to process large or small volumes of test liquid, and which can process multiple test samples simultaneously. Additionally, the invention provides a single device for both mixing and separation and a method which maximizes the mixing efficiency of the paramagnetic particles in the liquid medium without causing detrimental liquid turbulence, using an angular acceleration of at least 0.84 radians/second/second (hereinafter referred as rads/s 2 ). 
     SUMMARY OF THE INVENTION 
     According to the present invention, the affinity separation of a target substance from a liquid test medium is carried out by mixing magnetic particles bearing surface immobilized ligands or receptors to promote specific affinity binding reaction between the magnetic particles and the target substance. The liquid test medium with the magnetic particles in a suitable container is removably mounted in the apparatus of the present invention. In one preferred embodiment, a single magnetic field gradient is created in the liquid test medium. This gradient induces the magnetic particles to move towards the inside wall of the container nearest to the magnetic source. Relative movement between the magnetic source and the aggregating magnetic particles is started using a preferred angular acceleration of about 1.05 to about 4.19 rads/s 2  to mix the magnetic particles in the test medium and is continued for a sufficient time to ensure optimum binding of the target substance by affinity reaction. In addition, concurrently with the relative movement, the magnetic source may be moved from one end of the container to the other thereby effectively scanning along the length of the container by the magnetic field gradient. When the relative movement between the magnet and the magnetic particles is stopped, the magnetic particles are immobilized as a relatively compact aggregate on the inside wall of the container nearest to the magnetic source. The test medium may then be removed while the magnetic particles are retained on the wall of the container and may be subjected to further processing, as desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein: 
         FIG. 1  shows a perspective view of a preferred embodiment of the invention, which includes a stationary magnet placed next to a mobile container partially filled with a liquid test medium containing magnetic particles. 
         FIG. 2  shows a perspective of an alternate preferred embodiment of the invention, which includes a mobile magnet placed next to a stationary container partially filled with a liquid test medium containing magnetic particles. 
         FIG. 3  shows a perspective of another preferred embodiment of the invention, which includes a row of mobile magnets placed next to corresponding stationary containers, which are rotationally displaced by a common mechanism. 
         FIG. 4  shows a perspective of another preferred embodiment of the invention, which includes a row of stationary magnets placed next to corresponding rotatable containers, which are rotated by a common mechanism. 
         FIGS. 5 a , 5 b , 5 c , 5 d , 5 e  and 5 f    schematically illustrate the steps of a method according to the invention for mixing and separation of a target substance employing magnetic particles using the preferred embodiment of  FIG. 2 . 
         FIG. 6  shows a perspective view of a magnetic field gradient cavity in a test liquid medium according to the invention caused by one permanent magnet placed close to the container. 
         FIG. 7  shows a perspective view of a magnetic field gradient cavity in a liquid test medium according to the invention caused by two magnets placed at the opposite sides of the container. 
         FIG. 8  shows a perspective view of multiple magnetic field gradient cavities in a liquid test medium according to the invention caused by a vertical array of six permanent magnets placed close to the container. 
         FIG. 9  shows a perspective view of multiple magnetic field gradient cavities in a liquid test medium according to the invention caused by two vertical arrays of permanent magnets placed at the opposite sides of the container. 
         FIG. 10 a    shows a perspective top view of another preferred embodiment of the invention, which includes two electromagnets placed at opposite sides of the container. 
         FIG. 10 b    shows a perspective top view of yet another preferred embodiment of the invention, which includes a ring of electromagnets surrounding the container. 
         FIGS. 11 a  and 11 b    schematically illustrate the magnetic field lines created in a container by two magnets placed on opposite sides of the container. 
         FIG. 12  shows a perspective view of yet another alternate preferred embodiment of the invention which includes a row of magnets mounted on a vertically mobile assembly moveable by a linear drive mechanism and which can be positioned by a sliding mechanism at a desired distance from the corresponding rotatable containers, which are rotated by a common mechanism. 
         FIG. 13  A shows an isometric view of yet another alternate preferred embodiment of the invention, which includes magnets mounted in two concentric circular arrangement on a static plate and the containers inserted in a circular pattern of holes in a rotor plate which by rotation alternately positions the opposite sides of the containers in front of the each circular array of magnets. 
         FIG. 13  B shows a view corresponding to  FIG. 13A  without the rotor plate and showing concentrically arranged magnets in a staggered pattern and the containers positioned in from of each magnet. 
         FIG. 13  C shows an exploded view corresponding to  FIG. 1A , and showing various components of the embodiment. 
         FIG. 14  A shows yet another alternate preferred embodiment of a invention which includes containers inserted in a linear pattern the holes of a static plate and a linear array of magnets mounted in a linear pattern on a moveable support plate which by horizontally moving back and forth alternately brings the magnets on the opposite sides of the containers. 
         FIG. 14  B shows the partially cut away view corresponding to  FIG. 14  A and showing the details of the linear sliding mechanism of a groove in the side plate and the moveable support plate. 
         FIG. 14  C shows a view corresponding to  FIG. 14  A without the container holding plate and showing linear arrays of staggered magnets and the corresponding containers. 
         FIG. 14  D shows a partial top view corresponding to  FIG. 14  A showing the positions of magnets in the three rows of magnet arrays and the corresponding position of the containers. 
         FIG. 15  shows an isometric view of yet another preferred embodiment of the invention for the 96-well microplate format containers and linear arrays of magnets mounted on two independently moveable support structures, which by alternate movements in the vertical direction bring the magnets between the wells of the 96-well microplate. 
         FIG. 15  A shows an exploded view corresponding to  FIG. 15  and showing different parts of the device and their respective positions. 
         FIG. 15  B shows a side view corresponding to  FIG. 15  A illustrating the relative positions of magnets on the two support structures. 
         FIG. 15  C shows a top view corresponding to  FIG. 15  with the positions of the magnets between the wells of the 96-well microplate brought in by the vertical motion of one support structure. 
         FIG. 15  D shows a top view corresponding to  FIG. 15  with the positions of the magnets between the wells of the 96-well microplate brought in by the vertical motion of second support structure. 
         FIG. 16  A shows an isometric view of yet another preferred embodiment of the invention, which includes a container, mounted in a fixed holder and positioned between two concentric rotor discs fixed to a rotating shaft. 
         FIG. 16  B shows a side view, corresponding to  FIG. 16  A and shows the positions of the container and the two magnets mounted on the two rotors which upon rotation alternately positions the magnets on the opposite sides of the container. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide an apparatus and method for mixing and separating samples containing paramagnetic particles, which maximize the mixing efficiency of the particles without causing significant liquid medium turbulence. 
     The invention permits rapid, efficient, and clean separation of a target substance from test media and is particularly useful in the affinity magnetic separations of organic, biochemical, or cellular components of interest from, for example, assay reaction mixtures, cell cultures, body fluids and the like. The invention includes a novel mixing system wherein the magnetic particles are mixed within a relatively motionless test liquid by magnetic means disposed external to the container holding the test liquid. The invention also includes an apparatus and method wherein magnetic particles while mixing and confined in a magnetic zone are concurrently linearly displaced to scan large volumes of test medium for affinity separation with a small concentration of magnetic particles. The invention provides an apparatus in which both the processes of mixing and separation are carried out by a common magnetic means disposed in a single apparatus, thereby making it simpler and more practical to use. 
     The apparatus of the invention comprises at least one container for holding a test medium, external magnetic means to generate a magnetic field gradient within the test medium, and means for creating a magnetically induced movement of the magnetic particles within the test medium. The apparatus of the invention may also include a linear motion mechanism to move the magnetic means for scanning large volume of the liquid test medium. The container for performing the described mixing and separation is preferably of cylindrical configuration, made of a nonmagnetic material such as glass or plastic. Preferably, the container has at least one opening for receiving the test medium containing the magnetic particles. 
     The magnetic means may comprise one or more permanent or electromagnets disposed externally to the container for generating magnetic field gradients within the liquid test medium. In a preferred embodiment, the magnet is a permanent magnet of a rare earth alloy such as anisotropic sintered materials composed of neodymium-iron-boron or samarium-cobalt. The magnet is disposed external to the container so as to define a magnetic field gradient cavity in a desired cross-section of the test medium. The tam cavity is employed because the magnetic field gradient acts to confine or concentrate the magnetic particles much as if they were enclosed within a cavity. The magnetic field strength in the cavity is normally stronger at a part of the internal surface of the container closer to the magnet (locus of magnetic force) than it is elsewhere in the cavity and becomes negligible outside the cavity. As a result, magnetic particles in the test medium near this locus are subject to considerably greater magnetic force than those farther from it and tend to aggregate as a relatively compact mass on the inner surface of the lateral wall of the container closest to magnetic means. As the particles are all clustered in the vicinity of the magnetic means, they also tend to stick to each other by non-magnetic forces of compression and surface tension. The degree of compression in the aggregated particles depends on the field strength of magnetic means and is particularly relevant in the case of particles with diameters of a few microns, such as are usually employed in affinity separation. Such compacted particles can remain aggregated even after the removal of the magnetic field and usually require vigorous shaking of the test medium to re-disperse. A carefully balanced magnetic field strength in the test medium will pull the particles out of suspension into an aggregate, but will not be so strong as to overly compress the aggregate. According to the present invention, a desired magnetic field strength within the magnetic field cavity of the test medium may be created by appropriately adjusting the distance between the magnet and the container. The apparatus of the invention provides means for adjusting the distance between a magnet and the container. 
     In certain preferred embodiments, two magnets may be located on the opposite sides of the container, preferably with similar magnetic poles facing each other, to distort the magnetic flux lines and generate two magnetic field gradients and two loci of magnetic force forming in one cavity. Such an arrangement is particularly useful for agitating magnetic particles, as described below. In a particularly advantageous arrangement, an assembly comprising a vertical array of magnets is positioned exterior to the container to create multiple magnetic field gradient cavities within desired cross-sections of the test medium. 
     The present invention provides two methods for agitating and mixing the magnetic particles within the test medium while maintaining the test medium substantially motionless with respect to the container. Both methods are based on changing the relative angular position between magnetic means and the aggregated particles on the inside surface of the container at an angular acceleration of at least 0.84 rad/s 2  and preferably between about 1.05 to 4.19 rads/s 2 . The first method comprises rotating the container with respect to a stationary magnet. The magnetic field gradient cavity defined by the magnet in this instance is hence stationary. At an angular acceleration of about 0.83 to about 4.19 rads/s 2 , the test medium is not agitated and rotates with the container. The second method comprises rotating a magnet about a stationary container. The magnetic field gradient cavity defined by the magnet in this instance is rotating. It may be noted that using either method causes a change in the angular position between the aggregated particles within the container and the magnet. 
     As the relative angular position between the container and the magnet is displaced at an angular acceleration of at least 0.84 rad/s 2  and preferably between about 1.05 to about 4.19 rad/s 2 , the aggregated mass of particles move with the wall of the container to a position of weaker magnetic field. At this position, the stronger magnetic field in the vicinity of the magnetic means begins to pull off the particles from the aggregated mass, the trajectories of the particles being pulled off depends on the angular position of the aggregated mass and magnet. As the particles are pulled, they move and form chains of particles, due to the induced magnetic dipole on the particles by the applied magnetic field. As the chains accelerate towards the magnet, fluid drag force causes them to break creating a cloud of magnetic particles in the fluid medium. At a constant angular acceleration of either the container or magnet, the relative angular position between the magnet and the internal surface of the container bearing the aggregated particles recedes continuously and causes the particles to move ceaselessly in angular trajectories within the test medium thereby enabling the re-suspension and mixing of magnetic particles. The parameter, angular acceleration, is important in the mixing of the magnetic particles as described in the present invention. Applicant has found that at angular accelerations below 0.84 rad/s 2 , the aggregated mass of particles on the inside wall of the container do not move sufficiently rapidly to overcome the strong magnetic field in the vicinity of the magnetic means, resulting in a rolling mass of aggregated particles. Angular acceleration between about 1.05 to 4.19 rad/s 2  permits the aggregated mass of particles to move away with the wall of the container to a position of weaker magnetic field thereby effecting mixing as described above. 
     As regards particles it should be noted that the force pulling a magnetic particle through a fluid medium is the product of its magnetic saturation and field gradient and the viscous force opposing particle motion, which is governed by Stokes Law. The displacement of particle trajectories in a continuous manner is based on the action of magnetomotive force acting at a continuously changing angle between the magnet and the paramagnetic particles which results in a mixing process without fluid turbulence. Furthermore, this mixing process significantly increases the collision frequency between the particles and target species thereby enhancing the efficiency of the affinity binding reaction. 
     A suitable angular acceleration can be calculated on the basis of radius of the container, forces of gravity, buoyancy, fluid friction and magnetic field strength. However, for a given set of parameters, the intensity of the magnetic field or fields and the appropriate angular acceleration will be modulated experimentally. It should be noted that too high acceleration will not allow the particles sufficient time to detach from the aggregated mass and particles will be spread over the circumference of the inner wall of the container. Similarly, too slow acceleration such as about 0.10 to 0.21 rads/s 2  will produce a rolling mass of the aggregated particles. In both cases, re-suspension and mixing of the particles will be prevented. The field strength in the magnetic field cavity of the test medium must also be balanced so as to allow the aggregated particles to move with the wall of the container. It will be appreciated that a fixed magnet position is inconvenient when the desired particle size may vary considerably. In such situations, it is advantageous to be able to adjust the distance between the magnet and the container to create the optimum field strength in the magnetic field cavity of the fluid medium. 
     Although angular acceleration in the sense described above can be obtained by continuous rotation and provides satisfactory mixing of magnetic particles, in certain situations it is advantageous to provide a step-wise change of a predetermined angular position. For example, the relative angular position may be changed to 90 or 180 degrees in a single step at a significantly higher angular acceleration than is useful in continuous rotation method. Such steps may be repeated more than once, provided a suitable time delay is imposed between such steps. Applicant has found that angular acceleration as high as 300 rads/s 2  may applied when the relative angular position is changed to 180° in a single step mode provided a time delay of at least 0.5 second is imposed between subsequent steps. Such repeated step-wise change of a predetermined angular position provides a very efficient mixing of magnetic particles. It should be noted that the selection of suitable angular acceleration is particularly important in the present invention with respect to the said mixing operation. In general, a specific angular acceleration, to ensure re-suspension and mixing, will depend on the size, density and magnetic susceptibility of the particles, the cross sectional diameter of the container, the density and viscosity of the fluid test medium and the strength of the magnetic field. Although a theoretical calculation of such an angular acceleration is possible, for a given set of parameters an appropriate angular acceleration will be determined experimentally. The International patent application WO 91/09308 cited earlier, discloses that rotating the magnet around the container fails to produce resuspension as the particles remain aggregated. The cited WO 91/09308 is silent on the importance of angular acceleration with regard to resuspending and mixing process of the aggregated particles. As shown in example 1 (see later), Applicant has experimentally verified this teaching of the cited WO 91/09308 and has found that at an angular acceleration at or below 0.21 rads/s 2  the particles roll over one another and remain substantially aggregated. As a consequence the affinity binding reaction between particles and target species would be seriously hindered and the isolation efficiency would be reduced to almost zero. The experiments described in Example 2 clearly demonstrate the effect of angular acceleration on the purification efficiency. Applicant has found that angular acceleration above 0.84 rad/s 2  and preferably between about 1.05 to 4.19 rad/s 2  is necessary to provide useful mixing and resuspension of magnetic particles required in affinity binding reaction for the isolation of a desired target species. By recognizing the importance of angular acceleration the present application overcomes the problem of non-mixing disclosed in the prior art cited above. 
     In certain situations, re-suspension and mixing of magnetic particles may be improved by creating a magnetic field gradient in which the magnetic flux lines are distorted by providing two magnets placed on the opposite sides of the container with similar magnetic poles facing each other as shown in  FIG. 11 a   . The magnetic field lines generated by the two magnets are mutually repulsive and the cavity is characterized by having two zones with corresponding loci of high magnetic attraction and a small region in the center (neutral zone) where there is virtually no magnetic field. Since this neutral zone is very small, the random motion of magnetic particles caused by Brownian, gravitational, thermal, and like causes will tend to push most of the magnetic particles into either of the two magnetic field cavities. In a dynamic situation where the relative angular position between the magnet and the container is continuously changing, opposing magnetic flux lines cause the magnetic particles to disperse and mix more efficiently than in the case of a single magnet. However, when two magnets are of opposite poles, as shown in  FIG. 11 b   , the magnetic field lines are mutually attractive and the cavity is characterized by having two relatively small magnetic fields with corresponding loci of high magnetic attractions and a large region in the center (neutral zone) where there is virtually no magnetic field. Such an arrangement may be of use in certain situations. 
     The separation of magnetic particles from the liquid test medium in accordance with the invention is effected by stopping the rotation of either the magnet or the container to terminate the agitation of the magnetic particles. In the stationary position between magnet and aggregated particles, the magnetic particles within the magnetic field gradient in the fluid medium are attracted to and immobilized at the inside wall of the container nearest to the magnet. 
     The need for a reliable and readily automated method for resuspending and mixing the aggregated magnetic particles without causing fluid turbulence has not been satisfactorily addressed. Applicant&#39;s invention utilizes a new principle of mixing which has allowed, for the first time, integration of a simplified mixing and separation process into a single device. 
     The present invention provides many advantages over the prior art devices for affinity magnetic separation. The mixing of the present invention provides a high rate of contact between the affinity surface of the magnetic particles and the target substance, thereby enhancing the affinity bonding, without causing fluid turbulence. As a consequence, the hydrodynamic shear forces remain low and will not affect the affinity bond between particle and target substance complex or prevent denaturation, or other damage to the target substance. The process of the present invention can be used for sample volumes as little as 100 μL and can be scaled up to process sample volumes in excess of 100 mL. The present invention is particularly useful for the isolation of human rare cells required in various cell therapies as it permits a level of operating efficiency, which has not been achievable before this. 
     The purity and yield of the target substance obtained by a particular affinity magnetic separation is largely determined by the mixing process employed to promote the affinity binding reaction between the target substance and the surface of the magnetic particles. The binding reactions require a close contact between the affinity surface and the target substance. The rate of the reaction largely depends on the collision frequency between the two entities and the rate of surface renewal of the magnetic particles. The surface renewal is the process of removing the thin layer of media at the affinity surface and exchanging it with fresh media from the bulk. The hydrodynamic shear force at the affinity surface, therefore, must be carefully balanced so that it is sufficient to remove the thin layer of media without disrupting the affinity bonds. This has been difficult to achieve by past mixing methods based on agitating the fluid medium. The present invention, however, provides a high collision frequency and a substantially balanced shear force by magnetically inducing a controlled movement of the magnetic particles in an essentially motionless fluid medium. 
     In affinity magnetic separation, the particle concentration is, typically, much greater than the target substance to enhance the yield of the target substance. This is particularly important in the isolation of rare cell types such as mammalian hemopoietic cells where a particle to cell ratio of 20:1 may be required to obtain a desired isolation efficiency. In such applications, magnetic beads of uniform size distribution are required. The high cost of these beads are widely appreciated. The ability to isolate highly purified stem cells may serve in the treatment of lymphomas and leukemias as well as other neoplastic conditions. However, for the isolation of human stem cells, processing of large sample volumes is required. Such a process consumes large quantities of magnetic beads. Thus there is a need to reduce the concentration of magnetic beads without sacrificing the required high purity and yield. One embodiment of the present invention is capable of treating large sample volumes by relatively small concentrations of paramagnetic particles by combining a vertically moving magnet along the length of the container while the container is rotating. 
     The mixing and separation process of the present invention have particular utility in various laboratory and clinical procedures involving biospecific affinity binding reactions for separations. In such procedures, magnetic particles are used which have their surface coated with one member of a specific affinity binding pair, i.e. ligand or receptor, capable of specifically binding a substance of interest in the test medium. 
     Such biospecific affinity binding reactions may be employed for the determination or isolation of a wide range of target substances in biological samples. Examples of target substances are, cells, cell components, cell subpopulations (both eukaryotic and prokaryotic), bacteria, viruses, parasites, antigens, specific antibodies, nucleic acid sequences and the like. The apparatus and method of the invention may be used to carry out immunospecific cell separations for the analysis or isolation of cells including, by way of example: tumor cells from bone marrow; T-lymphocytes from peripheral blood or bone marrow; lymphocyte subsets, such as CD2, CD4, CD8, and CD34 from peripheral blood, monocytes; granulocytes and other cell types. The removal or depletion of various cell types may be carried out in a similar manner. The invention may be also be used in the separation or analysis of various bacteria or parasites from food products, culture media, body fluids and the like. Similarly, the apparatus and method of the present invention may be used in: bioassays including immunoassays and nucleic acid probe assays; isolation and detection of DNA and mRNA directly from crude cell lysate; and isolation and detection of proteins. 
     The magnetic particles preferred for the practice of the invention are noncolloidal paramagnetic or superparamagnetic particles. Such magnetic particles are typically of polymeric material containing a small amount of ferro-magnetic substance such as iron-based oxides, e.g., magnetite, transition metals, or rare earth elements, which causes them to be captured by a magnetic field. The paramagnetic particles useful for practicing the invention should provide for an adequate binding surface capacity for the adsorption or covalent coupling of one member of a specific affinity binding pair, i.e. ligand or receptor. The preferred diameter of a particle is typically in the range between 0.1 to 15 μm. Suitable paramagnetic particles are commercially available from Dynal Inc. of Lake Success, N.Y.; PerSeptive Diagnostics, Inc., of Cambridge, Mass.; and Cortex Biochem Inc., of San Leandro, Calif. Particularly preferred particles are spherical and of uniform size between about 1 and 5 μm in diameter, and contain magnetizable material evenly dispersed throughout. Such particles may be obtained from Dynal under the identification numbers M-280 and M-450 by Dynal Inc. These beads are coated with a thin shell of polystyrene, which provides a defined surface for the immobilization of various ligands or receptors. Such immobilization may be carried out by any one of many well-known techniques; techniques employing either physical adsorption or covalent coupling chemistry are preferred. 
     Depending on the radius of container, size of magnetic particle and its ferromagnetic content, and other experimental parameters, a suitable magnetic field gradient may be estimated by the magnetic circuit analysis method well known in the magnet art. Appropriate magnetic field gradients may be generated by one or more permanent magnet(s) or electromagnet(s). Permanent magnets are generally preferred for use in laboratory-scale operations and for automated devices employed in clinical diagnostics. A permanent magnet assembly may include soft iron pieces to enhance or modify the magnetic flux lines over a given area inside the container. In some situations, a magnet assembly comprising two soft iron pieces separately attached to north and south poles of the magnet provide a more thorough and uniform mixing. However, larger scale devices or automated devices such as those employed in pharmaceutical or industrial production can be more advantageously produced using electromagnets, since the field gradients can be more easily altered under automatic control to affect various processing steps. 
     Permanent magnets for practicing the invention preferably have a surface field strength sufficient to attract a majority of the magnetic particles. Permanent magnets of rare earth alloys having a surface field strength in the range of several hundred Gauss to several kilo-Gauss are preferred. High energy permanent magnets made from Neodymium-Iron-Boron or Samarium-Cobalt magnets and characterized by BHmax (maximum energy product) in the range of about 25 to 50 MGOe (megaGauss Oersted) are particularly preferred. Such magnets may be obtained from international Magnaproducts Inc., of Valparaiso, Ind., and many other commercial sources. Preferably the permanent magnets have a rectangular cross-section and may be glued or fixed by mechanical means to a nonmagnetic holding support to form a permanent magnet assembly. The assembly may include a ferromagnetic harness to house the magnet or magnets and to intensify and focus the magnetic field. The magnets are preferably oriented with their magnetic lines of force perpendicular to the vertical axis of the container. Alternate cross-sectional shapes, orientations, and magnetic pole orientation with respect to the container are also envisioned. 
     Generally the permanent magnet assembly is placed in close proximity to the container without the magnet extending to the bottom of the container. The distance between each magnet and the container shown in  FIGS. 1 through 6 and 12  is adjustable between about 1 mm to about 20 mm to create a desired magnetic field strength within the magnetic field cavity of the test medium. The apparatus shown in these figures includes a means for adjusting the distance between each magnet assembly and the container. An adjusting means is shown in  FIG. 12 . Lateral (or horizontal) movement of magnets is provided by a linear motion mechanism. Linear motion mechanisms are well known in the art. A simple linear motion mechanism comprises a slider with a rectangular notch or groove, riding on rail with corresponding rectangular shape. Such linear motion mechanisms exist in common furniture drawers. Multiple rails can be provided, as well as ball bearings and rollers if desired. A gear rack and pinion mechanism comprising of a rectangular gear teeth bar (rack) and a mating gear teeth pulley is advantageous when accuracy in the distance between magnet assembly and the container is desired. Suitable gear racks and pinions are available from Designatronics Inc., 2101 Jericho Turnpike, New Hyde Park, N.Y. 11042-5416. Lateral movement of magnet assembly can also be changed by attaching it to an electromagnetic actuator or plunger and such lateral movement may be synchronized with the rotary motion of the container or magnet assembly. Electromagnetic actuators or plungers are also well known in motion control art. While  FIG. 12  shows six cylindrical containers, obviously the number can be increased, or decreased to one  FIG. 12  further shows vertical movements of magnets driven by screw  116 . Obviously structures shown in  FIG. 1  and  FIG. 4  can be moved by  FIG. 12  mechanisms. For instance, the stationary containers of  FIGS. 2 and 3  or the stationary magnet of  FIG. 1  could be made movable using a screw mechanism, or similar mechanical means, like the one shown in  FIG. 12 . The magnet position can also be changed by fastening the magnet assembly at a desired position by various male and female fasteners. 
     Depending on the size and magnetic susceptibility of the particles and the field strength of the magnets and cross-section diameter of the container, the appropriate distance will be determined experimentally. The field strength created in the magnet field cavities should be carefully balanced so that it is sufficient to pull the particles out of suspension, aggregate the particles on the side of the container, and allow the aggregated particles to move with the wall of the container. However, the magnet may be moved closer to the container, as discussed, to increase the field strength in order to separate the particles from the liquid test medium. In certain situations involving the processing of a plurality of containers, it may be advantageous to place the permanent magnet assembly between containers or between rows of containers so that one single permanent magnet assembly can be used to generate a magnetic field cavity in the two containers in its vicinity. 
     Specifically, in order to move the stationary magnet along the vertical axis of the moving container, as shown in  FIG. 1 , the solid support  2  may be mechanically fastened to a carriage on a linear slide mechanism similar to the one ( 122 ) shown in  FIG. 12 . The apparatus and method shown in  FIG. 1  is simply a one-container variation of  FIG. 12 . 
     In the case of moving magnets as shown in  FIGS. 2 and 3  where the container(s) remains stationary, the rotating magnets may be simultaneously moved along the vertical axis of the stationary container by providing a hole in the rotatable support  22  ( FIG. 2 ), support  22  being mounted on a hollow shaft electric motor which is available from EAD Motors, 1 Progress Drive, Dover, N.H., U.S.A., and the motor itself being mounted on a motorized linear slide mechanism such as shown in  FIG. 12 . The internal diameters of the hole in support  22  ( FIG. 2 ) and the hollow shaft of the electric motor will be larger than the container  29  outer diameter. Hence the hollow shaft rotates and passes freely from one end of the container to the other. Similarly, in the case of  FIG. 3 , appropriately positioned holes may be provided on the support  35 . The internal diameters of the holes in support  35  are sufficiently large so that the support  35  rotates freely around the containers and the length of the support shafts  34 A and  34 B ( FIG. 3 ) mounted on pulleys  38 A and  38 B will be sufficiently long to accommodate the length of the containers. The entire rotation assembly as shown in  FIG. 3  may be mounted on a motorized linear slide mechanism such as shown in  FIG. 12 . 
       FIG. 1  illustrates an apparatus for mixing and separating magnetic particles according to the present invention, which includes a magnet  1  next to a container  3 . The magnet  1  is adjustably fixed to a solid support  2  without extending to the container&#39;s bottom end. The magnet  1  is preferably movable with respect to the container  3  to adjust the magnetic field strength as desired. In the preferred embodiment, the container  3  is a test tube used for holding a liquid medium  8  with magnetic particles  9  shown as small dots located in the medium. If the magnet  1  is a permanent magnet, it preferably comprises a rare earth composite type such as Neodymium-Iron-Boron or Samarium-Cobalt and has a surface field strength of about 200 Gauss to 5 kilo Gauss, which is sufficient to attract the magnetic particles in the size range of about 0.1 μm to 10 μm. The permanent magnet employed has dimensions and geometries that define a magnetic field cavity of a desired field strength having a desired cross-section within the liquid test medium  8  in the container  3 . An electromagnet of comparable field strength may be used for the magnet  1 . 
     The container  3  with the liquid medium  8  and the magnetic particles  9  is removably placed in a vertical position in a holder  5 . The holder  5  is fixed to a rotating shaft  4 , which is in turn attached to a variable speed electric motor  6 . The holder  5  has vertical slits  7  which are elastic, to receive and firmly grip the container  3  in a vertical position. The electric motor  6  rotates the container  3  causing the relative angular position of the aggregating magnetic particles  9  in the container  3  with respect to the magnet  1  to be continuously altered, thereby inducing the magnetic particles  9  to move within the cavity of the magnetic field gradient defined within the test medium  8 . 
     The motor  6  may be an electric step motor instead of a continuous rotation motor to provide a step-wise change of a predetermined distance in the relative angular position. Step movements of a predefined angle may be repeated more than once, and if desired, with time delays from a fraction of a second to tens of seconds between each step. Such step rotation would be accomplished by an electronic motor control that is well known in the art. Other means for effecting step-wise motion and time-delays well known in the electro-mechanical art could also be used. 
     The container  3  when rotated continuously is rotated from a resting position to a speed, preferably between about 50 and 200 rpm in less than one second. This speed ensures the agitation of the magnetic particles  9 , while the liquid test medium  8  inside remains relatively stationary with respect to container  3 . Switching off the electric motor  6  stops rotation of the container  3 . The magnetically-induced agitation of the magnetic particles  9  stops and the magnetic particles  9  are attracted to and immobilized at the inside wall of the container  3  closest to the magnet  1 . At this time, if desired, magnet  1  may be moved closer to container  3  to tightly aggregate the magnetic particles  9  on the vertical side of the container  3  to facilitate dean removal of the liquid test medium  8 . 
       FIG. 2  illustrates an alternate preferred embodiment for mixing and separating magnetic particles according to the present invention which includes a test tube  23  removably inserted through an opening in a test tube holder  25  without extending to a rotating support  22 . Magnet assembly  21  is adjustably fixed to rotatable support  72  without extending to the test tube&#39;s bottom end. The magnet assembly  21  may be moved or fixed at a desired distance with respect to container  23  to adjust the magnetic field strength. The magnet  21  may be either an electromagnet or a permanent magnet. If the magnet  21  is a permanent magnet, it is preferably comprised of a rare earth composite such as Neodymium-Iron-Boron with a surface field strength of about 200 Gauss to 5 kilo Gauss, sufficient to attract the magnetic particles in the size range of about 0.1 μm to 15 μm. The magnet  21  may comprise one or more magnets of suitable dimensions and geometries so as to define a magnetic field cavity of a desired field strength having a desired cross-section within the liquid test medium  28  in the test tube  23 . 
     The rotatable disc  22  is mounted to a shaft  24 , which is in turn attached to a variable speed electric motor  26 . The electric motor  26  rotates the magnet  21  orbitally around the vertical axis of the stationary test tube  23  creating an angularly moving magnetic field gradient within the test medium  28 . The test tube  23  remains motionless while the magnetic field cavity rotates continuously through the stationary test medium  28 . The motor  26  may be an electric step motor to provide a step-wise change of a predetermined distance in the relative angular position such as described above. 
     The magnet when rotated continuously is rotated from a resting position to a speed, preferably between about 50 and 200 rpm in less than one second. The angularly moving magnetic field with respect to the aggregating magnetic particles  29  induces the magnetic particles  29  to move within the magnetic field cavity through the relatively motionless liquid test medium  28 . When the electric motor  6  is switched off, the magnetically induced agitation stops. The magnetic particles  29  in the now stationary magnetic field are attracted to and immobilized on the inside wall of the test tube  23  closest to the magnet  21 . At this time, if desired, the magnet  21  may be moved closer to test tube  23  to tightly aggregate the magnetic particles  29  on the vertical side of the test tube  23  to facilitates a cleaner removal of the test medium  28 . Aggregation of the magnetic particles  28  on the vertical side of the test tube  23  facilitates removal of the test medium  28  by aspiration or other means. 
       FIG. 3  illustrates another preferred embodiment of the present invention for processing a plurality of test media simultaneously and is a variant of the embodiment of  FIG. 2 . The apparatus comprises a row of identical test tubes  33 , fixed in vertical positions by their top ends passing through corresponding openings in a fixed horizontal support plate  32 . The vertical position of the corresponding row of multiple magnets in a magnet assembly  31  is adjustably fixed without extending to the bottom ends of the test tubes  33 . The magnet assembly  31  may be moved to and fixed at a desired distance from the test tubes  33  to adjust the magnetic field strength. If permanent magnets are used, they are preferably of a rare earth type as described above, and are selected to have suitable dimensions and geometries to define a magnetic field cavity with a desired field strength having a desired cross-section within the liquid test medium  29  in each test tube  33 . 
     A support plate  35  for the magnet assembly  31  is fixed at its extremities by two shafts  34   a  and  34   b . These shafts are eccentrically attached to pulleys  38   a  and  38   b , which are, in turn, connected by a drive belt  39 . The pulley  38   a  is attached to a variable speed electric motor  36 . The motor  36  rotates the pulleys  38   a  and  38   b , thereby imparting an eccentric rotation to support plate  35 . This motion causes each magnet of the magnet assembly  31  to orbit around the vertical axes of its corresponding stationary test tube  33 , thereby creating a separate moving magnetic field gradient within the motionless test media  28  of each test tube  33 . The motor  36  may be an electric step motor to provide a step-wise change of a predetermined value in the relative angular position such as described above. 
     The magnets when rotated continuously are rotated from a resting position to a speed, preferably between about 50 and 200 rpm in less than one second. The simultaneous movement of multiple magnetic fields induces the aggregating magnetic particles  29  in each test tube  33  to move within their individual cavities of the magnetic field gradient. Stopping the electric motor  36  stops the rotation of the magnet assembly  31  and stops the magnetically induced agitation. The magnetic particles  29 , in the stationary magnetic fields are attracted to and immobilized on the inside walls of each test tube  33 . If desired, magnet assembly  31  may be moved closer to test tubes  33  to tightly aggregate the magnetic particles  29  on the vertical sides of the test tubes  33  to facilitates a cleaner removal of the test medium  28 . The separation of magnetic particles on the vertical side of the test tubes  33  facilitates removal of the supernatant liquid media by aspiration or other methods. 
       FIG. 4  illustrates another preferred embodiment of the present invention for processing a plurality of test liquid media simultaneously, and is a variant of the embodiment of  FIG. 1 . The apparatus comprises a row of multiple magnets  41 , fixed on a support plate  41   b  (not shown). The support plate is preferably adjustably mounted to align the row of magnets so each magnet corresponds with its respective test tube  43 . Support plate  41   b  also preferably provides lateral movement to adjust the distance between the magnet assembly  41  and the row of test tubes  43 . The magnets  43  thus can be moved to a desired distance from the test tubes  43  to adjust the magnetic field strength. If permanent magnets are employed, they are preferably a rare earth type as described above and have dimensions and geometries so as to define a magnetic field cavity which accommodates a desired cross-section within the liquid test medium  8  in each test tube  43 . 
     The test tubes  43  are removably placed in vertical positions with their bottom ends resting in a row of shallow grooves on a bottom plate  42 . A portion of their top ends pass through corresponding openings in an upper plate  42   b  of the test tube rack  42 . The diameter of the openings in the upper plate  42   b  is slightly larger than the diameter of the test tubes  43  so that they can be readily inserted and freely rotated. The plates  42  and  42   b  are spaced apart so as to hold the test tubes  43  in a stable vertical orientation. 
     A drive belt  49  is mounted on two pulleys  48   b  and  48   c . Pulley  48   c  is attached to a variable speed motor  46 , and guided by two parallel rows of guidance rollers  47  mounted on the top plate  42   b . The guidance rollers  47  are positioned between the rows of openings to slightly pinch the drive belt  49  so that the drive belt  49  grips the upper ends of the test tubes  43 . Motor  46  moves the drive belt  49 . The linear sliding friction of belt  49  against the external surface of each test tube simultaneously rotates all test tubes  43  around their vertical axes. The motor  46  may be an electric step motor to provide a step-wise change of a predetermined distance in a relative angular position, such as described above. 
     At a suitable acceleration, test tubes  43  rotate, the relative angular position of the aggregating magnetic particles  9  in each one of the test tubes  43  and its corresponding magnet  41  is continuously altered. This induces the magnetic particles  9  to move within the cavity of the magnetic field gradient. The test tubes  43  are rotated from resting position to a speed, preferably between about 50 and 200 rpm in less than one second to ensure the agitation of the magnetic particles  9  while maintaining the test media  8  inside relatively stationary. Stopping the electric motor  46  stops rotation of test tubes  43  and the magnetically induced agitation. The magnetic particles  9  in each test tube  43  are now attracted to and immobilized at the inside wall closest to the magnets  41 . The aggregation of the magnetic particles  9  on the vertical side of the test tubes  43  facilitates removal of the test medium  8  by aspiration or similar methods. If desired, magnet assembly  41  may be moved closer to container  23  to tightly aggregate the magnetic particles  9  on the vertical side of the container  43  to facilitates a dean removal of the test medium  8 . 
     An instrument incorporating the above-described principles of the invention has been built and is being sold by Sigris Research, Inc., P.O. Box 968, Brea, Calif. 92622. Literature describing the operation of the instrument, specifications and actual performance statistics widely distributed since 1996 is available from Sigris Research, Inc. and is incorporated herein by reference. 
       FIG. 12  illustrates another preferred embodiment of the present invention for processing a plurality of test liquid media simultaneously. It includes a linear drive mechanism mounted on a positioning mechanism and a rotation mechanism. The three mechanisms allow vertical linear movement of a magnet assembly, adjustment of the distance between the magnet assembly and containers, and rotation of the containers. Simultaneous container rotation and linear magnet movement provides the advantage of processing large volumes of test media with a relatively small quantity of magnetic particles. 
     The apparatus of  FIG. 12  consists of two main pans, linear drive assembly  111  and base assembly  112 . Both assemblies are constructed of a nonmagnetic material, aluminum being preferred. The linear drive assembly  111  comprises a rigid frame  113  with two fixed guide rods  114  and  115  and a centrally located screw shaft  116 . The end portions of screw  116  are smooth and un-threaded and are mounted in two centrally located ends flanges (not shown). The screw  116  is freely rotatable and includes a roll nut (not shown) which moves linearly in the vertical plane, either up or down, upon rotation of screw  116 . A pulley  117  is fixed to the smooth portion of screw  116  protruding from the top plate  136  of frame  113  and is connected by a timing belt  118  to another pulley  119  fixed to the shaft of a variable speed electric motor  120  mounted on bracket  121  of frame  113 . Timing belt  118  is made of neoprene or urethane with precisely formed grooves on the inner side. The belt width and groove pitch match the dimensions of the teeth on pulleys  117  and  119  to provide positive and non-slip power transmission. Suitable timing belts and gear pulleys may be obtained from Stock Drive Products, New Hyde Park, N.Y. or from other similar vendors. 
     A carriage  122  is fixed on the roll nut of screw  116 . Its vertical motion is ensured by the accurately aligned guide rods  115  and  114 . Linear drive assembly  111  is attached to base assembly  112  by bolting the bottom plate  139  of frame  113  to a linear slide mechanism  123 . A rod with a knob  128  inserted through a center hole of the base assembly  112  is attached to the linear slide mechanism  123 . The linear slide mechanism  123  thus can be moved forward or backward by pulling or pushing the knob  28  to position it at a desired distance from the containers  124 . 
     A magnet assembly  125  with magnets  126  is removably mounted on the linear drive carriage  122  by means of three evenly spaced screws  127 . This is advantageous because magnets of varying size and geometry can be easily exchanged. The magnets  126  are aligned with the row of containers  124 . Their distance from the containers is adjusted by pulling or pushing the knob  128 . 
     The motor  120  rotates the screw  116 . The roll nut converts this wary motion to a linear motion moving magnet assembly  125  vertically. The direction of the linear movement of magnet assembly  125  is controlled by the clockwise or counter-clockwise rotation of the motor  120  by a motor controller. The movement of magnet assembly  125 , either upward or downward can thus be controlled at will and may be repeated for as many cycles as desired. 
     The position and the stroke length of the linear up and down movement of the carriage  122  may be controlled by two position sensors to control the lowest and highest extremes of travel of the carriage  125 . An electronic signal from these sensors may be used to reverse the motor rotation, thereby causing a repeated scanning for a desired length of the containers  124  by their corresponding magnets  126 . 
     Electronic motor controllers and position sensor are well known in the art and may be obtained from any one of a number of vendors. If permanent magnets are employed, they are preferably a rare earth type as described above and have suitable dimensions and geometries so as to define a magnetic field cavity of a desired field strength which provides a desired cross-section within the liquid test medium in each container. 
     The base assembly  112  includes a mechanism for rotation comprising a variable speed electric motor  129  with a gear pulley  130  fixed to its shaft. A pulley rotor  131  is attached to each one of a plurality of holder  134 . A timing belt  132  is wrapped around the gear teeth of pulley  130  and each of the rotors  131 . Although only one rotor  131  is shown next to a holder  134  for a container  124 , it should be understood that each container holder  134  has a rotor  131  associated with it, which is driven by the belt  132 . The motor  129  and rotor pulleys  130 ,  131  are secured in their precise positions by a top metal plate  133  fixed to base assembly  112 . It should be noted that the gear pulley rotors  131  are free rotating and their respective shafts protrude from corresponding holes in plate  133 . The belt width and the inner groove pitch of the timing belt  132  dimensionally match with gear teeth of the motor gear pulley  130  and the rotors  131  to provide positive and non-slip power transmission. If desired, idling rollers may be installed between the pulleys to increase the wrap around the gear teeth for a firmer non-slip power transmission. The motor  129  rotates the timing belt  132  thereby simultaneously rotating all pulley rotors  131 . 
     Holders  134  are removably mounted on the tapered end of a rotor shaft  135  protruding from corresponding holes in plate  133  and provide means for firmly holding containers  124  in a substantially vertical position. A removable holder design is advantageous as it provides a convenient means to accommodate a variety of container sizes on the apparatus by simply changing the holders to correspond to the container geometry. 
     The position of the magnet assembly  125  may be adjusted to a required distance from the row of containers  124  by pulling or pushing the knob  128 . The motor  129  rotates containers  124  around their vertical axes. At a suitable angular acceleration preferably between about 1.05 to 4.19 rads/s 2 , the relative angular position of the aggregating magnetic panicles in each container with respect to its corresponding magnet  126  is continuously altered, inducing the magnetic particles to mix within the cavity of the magnetic field gradient, as described above. While the containers  124  are rotating, motor  120  may be switched on to move the magnets  126  up and down in the vertical plane thereby moving the magnetic field cavity in alignment with the vertical axis of the containers. Upon reaching a desired length of the container, the direction of movement of magnet assembly  125  is reversed. This process is repeated for the entire duration of particle mixing necessary to bind the target species to particle surface by affinity reaction. 
     It will be recalled that the magnetic particles remain confined in the magnetic field cavity. Particle to target substance ratio therefore may be adjusted to relatively high levels within the magnetic field cavity to provide reaction conditions, which overwhelmingly favor affinity binding. By combining a linearly moving magnetic field cavity with the angular movement of particles confined within the magnetic field cavity, a simple and efficient means to process large volumes of test media without a concomitant increase in particle concentration is obtained. This was not heretofore possible. 
     The motor  129  may be an electric step motor to provide a step-wise change of a predetermined distance in the relative angular position such as described above. Similarly, motor  120  may be an electric step motor to provide a step-wise change of a predetermined distance in the vertical plane. Various combinations of continuous and step-movement for the rotation and linear movement may be utilized. In every case the optimum speed of rotation and linear movement will be determined by trial and error. 
     For separation, the linear drive motor  120  is turned off. The magnet assembly  125  is brought to a home position. The rotation drive motor  23  is turned off. The magnetic particles in the containers  124  are attracted to and immobilized at the inside wall closest to the magnets  126 . The aggregation of the magnetic particles on the vertical side of the container  124  facilitates removal of the test medium by aspiration or similar methods. If desired, magnet assembly  125  may be moved closer to containers  124  by moving knob  128 . This tightly aggregates the magnetic particles on the walls of the containers  124  to facilitate a clean removal of the test medium. 
       FIGS. 5 a  through 5 f    illustrate the preferred steps in a method practiced by the preferred embodiments described above, using affinity reactive magnetic particles of about 2.8 μm for the purpose of bioassays, or for the isolation of cellular or molecular species from a sample solution or suspension of biological fluids. 
       FIG. 5 a    shows an apparatus of  FIG. 2 , in which a suspension of magnetic particles  58  in a sample solution is dispensed with a pipette  59   c  into a test tube  23  of about 10 mm diameter. A magnet  21  of about 45 MGOe is moved to a distance of about 5 mm from test tube  23  so as to a create a field of about 600 Gauss in the center of the test tube. This preferred distance is determined by measuring the magnetic field inside the tube by a magnetometer. The motor is turned on and the magnetic particles  58  are mixed by rotating the magnet  21  around the test tube  23 .  FIG. 5 b    shows the same apparatus when mixing is completed, rotation of the magnet  21  has stopped, and the magnet is moved closer to the test tube  23 . The magnetic particles  58  are immobilized against the inner wall of test tube  23  closest to the stationary magnet  21 . 
       FIG. 5 c    shows the apparatus during a washing step. In this step, an outlet tube  59   a  aspirates the supernatant test medium and an inlet tube  59   b  adds a suitable wash solution into the test tube  23 . The magnetic particles  58  are then mixed in the wash solution. The old wash solution is aspirated and new clean solution may be added. The washing step may be repeated as many times as required. 
       FIG. 5 d    shows the apparatus stopped for the addition of one or more reagent solutions by pipette  59   c  for effecting a desired analytical reaction for a bioassay or a chemical displacement reaction to elute the target substance from the magnetic particles  58 . 
       FIG. 5 e    shows the same apparatus turned on for dispersing and mixing the magnetic particles  58  for carrying out the desired reaction. 
       FIG. 5 f    shows the apparatus stopped to separate the magnetic particles  58  from the reaction medium. In the case of bioassays, the supernatant liquid may be measured by any desired measurement method, either directly in test tube  23  or by transferring it elsewhere. For the purpose of isolating a cellular or molecular species, the supernatant may be transferred to a suitable container for subsequent treatment as desired. Examples of actual separations of mRNA and protein are described in a technical brochure entitled “MixSep”, obtainable from Sigris Research, Inc., and are incorporated herein in its entirety. 
     Various preferred configurations of magnet assemblies and their position with respect to a container will now be described with reference to  FIGS. 6 through 9 .  FIG. 6  shows a perspective view of an embodiment of the magnet assembly  61  according to the invention wherein a rectangular permanent magnet  62  is fixed on a nonmagnetic base  63  and placed in proximity to a container  64  to generate a cavity of magnetic field gradient  65  in a cross-section of a liquid test medium  66 . The usable magnetic field remains mostly confined within this cavity, i.e., there is negligible field strength outside the cavity. 
       FIG. 7  shows two magnet assemblies,  71   a ,  71   b , each comprised of two rectangular permanent magnets  72   a  and  72   b  fixed on two nonmagnetic bases  73   a  and  73   b , respectively. The two magnet assemblies  71   a ,  71   b  are located on the opposite sides of a container  74  with similar magnetic poles facing each other to distort the magnetic flux lines and generate a cavity of magnetic field gradient  75  in the liquid test medium  76  and two loci of magnetic force in the cavity  75  as explained above (see  FIG. 11 a   ). Such an arrangement may be particularly effective for mixing magnetic particles. 
       FIG. 8  shows a magnet assembly  81  designed to generate multiple cavities of magnetic field gradient in a container  84 . An array of six rectangular permanent magnets  82   a  to  82   f  fixed on a nonmagnetic support frame  83  is preferred. Magnets  82   a  to  82   f  are vertically mounted on the non-magnetic support  83  wherein each magnet is substantially separated by a non-magnetic spacer and like poles over like poles so that magnetic flux lines from each magnet traversing the test medium  86  are mutually repulsive and generate a plurality of distinct magnetic field cavities. The spacing between magnets should be such as to prevent the intermixing of magnetic particles from one field cavity to other. Such spacing may be even or uneven. 
     The magnet assembly  81  is placed at a desired distance from the container  84  to generate six separate cavities of magnetic field gradient  85   a  to  85   f  in a liquid test medium  86 . Such multiple magnetic field cavities are useful for isolating a multiple of target substances from a test medium in a single operation. The affinity magnetic particles in a given cavity will specifically bind a given target substance only. Specific types of magnetic particles are added sequentially from bottom cavity to top cavity. In the first step, the container is filled with a suspending solution to the level of the first cavity, magnetic particles are then added and allowed to aggregate. This step is repeated until all cavities are filled with the desired type of magnetic particles. The suspending solution, is then removed and the container filled with the test medium. Alternatively, a test liquid sample may be layered over the test medium and the target substance allowed to settle down by gravitational force while the particles are mixing. Such a method is of particular use for isolating different cellular components in a single process. Mixing and separation are then carried out as described in connection with  FIG. 5 . 
       FIG. 9  shows two magnet assemblies  91   a  and  91   b , each comprising an array of six evenly-spaced rectangular permanent magnets  92   a  to  92   f  fixed on two nonmagnetic support frames  93   a  and  93   b , respectively. The spatial and pole arrangements of assemblies  91   a  and  91   b  are similar to the one described in  FIG. 8 . The two magnet assemblies  91   a  and  91   b  are located on the opposite sides of a container  94  with like magnetic poles facing each other. Six cavities of magnetic field gradient  95   a  to  95   f  thus generated in a test medium  96  by distorted magnetic flux lines of two operative magnetic fields in each cavity. 
     The various configurations of magnet assemblies and position as described above may be advantageously employed in the embodiments of the invention depicted in  FIGS. 1 to 4 and 12 . 
     As mentioned above, permanent magnets and electromagnets are interchangeable in most configurations of the present invention. However, those configurations that require movement of a magnet are more easily realized with permanent magnets. Electromagnets require commutators or other arrangements to conduct electricity to the moving magnets. Them are certain unique configurations in which electromagnets are greatly preferred.  FIG. 10 a    shows two electromagnet coils  101   a  and  101   b  mounted on a support frame  104  and displaced at about 180 degrees at the exterior of a container  102  with the liquid test medium and magnetic particles  103  inside.  FIG. 10 b    shows a cross-section of a single container  102  with the liquid test medium and magnetic particles  103  surrounded by a ring of individual electromagnet coils  101   a  to  101   r  mounted on a support frame  104 . 
     Here neither the container  102  nor the electromagnets  101  actually move. Instead, angular movement is induced in the magnetic particles suspended within the test medium  103  inside the container  102  by sequentially energizing the electromagnets. This sequential energization may be “binary” (i.e., on and off) or “analog,” in which a first electromagnet is gradually fully energized, and then has its power reduced, while the next electromagnet is gradually energized, and so on. It will be apparent that rate of motion of the magnetic particles  103  can be modulated by the rate of change and the degree of overlap between the sequential electromagnets. 
     The exact number of sequential electromagnets employed will depend on the size of the container  102  and other parameters.  FIG. 10 a    shows that this configuration reduces to a configuration not unlike that of  FIG. 7 , but with two opposed electromagnets rather than two permanent magnets. The angular movement from one magnet to the other in its simplest form is 180 degrees so that the magnetic particles in the test medium  103  will move in relatively straight lines back and forth across the container  102 . More variety is preferably added to the paths of the magnetic particles by modulating the polarity, as well as the power level of the electric current, thereby altering the direction of the magnetic poles with alterations of the magnetic field corresponding to those shown in  FIGS. 11 a    and  11   b.    
     It has been found that a configuration employing four electromagnets equally spaced (i.e., 90 degrees apart) around a container can produce very acceptable agitation of magnetic particles through a judicious use of sequential activation of the electromagnets and through polarity reversals, as discussed above. 
     The container defining the mixing and separation chamber includes at least one opening for the addition and removal of a test medium. The container is preferably of substantially cylindrical form and made from a magnetically permeable material such as plastic or glass. Additionally, the inside surface of the chamber may be biocompatible and, if desired, the chamber may be sterilized for aseptic processing of the test media. The volume of the container is not critical as long as an adequate magnetic field gradient can be provided to accommodate the chamber and, particularly, can accommodate the desired cross-section of the liquid test medium inside. 
     As shown in  FIGS. 1 through 9 and 12 , the container used to hold the test medium may be a test tube or an eppendorf type of tube with a conical bottom. The volumetric capacity of the test tube is preferably between 250 μl to about 18 ml as usually employed in research laboratories. The various configurations of apparatus as described above can be easily scaled up to process much larger volumes of liquid test media as may be required for clinical applications. In all cases, the size and geometry of the magnet is adjusted to generate an adequate magnetic field strength within the field cavity of the test medium inside a particular size of container. 
     Although embodiments of the present invention particularly suited for use in the research laboratory preferably employ readily removable and replaceable containers such as test tubes, diagnostic and other devices employing the teachings of the present invention might employ permanent flow cells or other nonremovable chambers for mixing and separation. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention wherein the affinity reactive magnetic particles are admixed with the liquid test medium in a container by effecting a relative angular movement of the magnetic particles in the liquid test medium, while the liquid remains essentially motionless. Although relative angular movement is achieved by rotating the container or orbiting the magnet, alternative mechanism will be obvious to a skilled artisan. For example, relative angular movement between the magnetic source and the aggregating magnetic particles may be effected by moving the magnet or the container by a linear motion mechanism using an appropriate linear acceleration. For this purpose, at least two magnets will be positioned diametrically opposite one another relative to the container but staggered so that the magnetic field inside the container generated by one magnet is substantially unaffected by the magnetic field generated by the other magnet. The linear acceleration of either the magnet or container will mix the magnetic particles in a manner analogous to the 180° step rotation movement described earlier. 
       FIG. 13-15  shows other preferred embodiments of the present invention which generally include a “sample plate” with apertures for holding the containers, and a “magnet plate” with at least two arrays of magnets mounted onto it, and a “base plate” with at least 2 “side plates” supporting the “sample plate” and “magnet plate”. 
     The magnet arrays in either a circular or rectangular pattern are mounted on the “magnet plate”. The change in the relative angular position between the magnetic particles in the liquid sample in a container is preferably effected in a single step movement at a suitable acceleration by an electric step motor. Either the sample plate or the magnet plate can be moved and such motion linear or rotary so that each successive movement brings the opposite lateral sides of the container in front of the staggered magnets. Such an angular change between the container and magnet brings about the re-suspension and mixing of the magnetic particles in the liquid sample in a manner analogous to the 180° step rotation movement described earlier. In the absence of such movement, the magnetic particles in the liquid sample move to lateral side inside the container nearest to the magnet and thus separated from the liquid sample. 
     The devices according to the invention are advantageous for processing large number of samples and of great utility in molecular diagnostics, forensic DNA analysis and molecular biology fields. 
       FIG. 13  A-C shows various isometric views of a device with the four side plates covering the device removed. It includes a base plate  131 , an electric motor  137 , preferably a step motor, a chuck  136  mounted on the motor shaft  137  A, a magnet plate  132  with plurality of magnets  134  mounted in a circular pattern, a round shape sample plate  133  with alias plurality of apertures for inserting containers  135  with the magnetic particles in a liquid sample. Hereafter container shall mean container with a suitable volume of a liquid sample containing magnetic particles. Disposable plastic test tubes such as Eppendorf type of tubes are preferred but other types can also be used. 
     Sample plate  131  includes 2 holes  133  B, which tightly fits into the aligning dowels  136  B on the chuck  135  to align the four holes  133  C with the four threaded holes  136  A of the chuck through which four threaded screws are used to fasten sample plate  131  to chuck  136 . Sample plate can be rotated by motor  137 , preferably at predetermined angular positions in successive steps preferably with a delay time between each step. The electric motor will be driven by appropriate electronic motion controllers, available through many vendors such as Nyden Company, Fremont, Calif. 
     The magnet plate  132  is attached to the top frame of motor  137  through four holes  135  by long fasteners screwed to the base plate  131  so that both the motor  137  and magnet plate  132  remain firmly secured. The magnet plate  132  has a plurality of magnets  134  mounted in two circular arrays, which are concentric. The circular magnet arrays are sufficiently spaced apart to allow the free rotation of the containers  135  between them. 
     Permanent magnets of rectangular shape are preferred but other shapes such as cylindrical can also be used. High-energy magnets of the NdFeB type are particularly preferred. The magnets  134  will be of appropriate size (width and length) to provide sufficient magnetic field strength over substantial part of the lateral surface area of the container  135 . The arrangement where similar poles such as north-north or south south are diametrically opposite one another relative to the container is preferred. As shown in  FIG. 13  B, the individual magnets  134  in the two circular arrays are positioned diametrically opposite one another relative to the container but staggered so that the magnetic field inside the container generated by one magnet is substantially unaffected by the magnetic field generated by the other magnet. 
     For example, in a device for Eppendorf type of container, the magnets of the size about 10×10×25.4 mm was used in two circular arrays. When assembled ( FIG. 13  A), the distance between the magnet plate  132  and the sample plate was adjusted to about 11-17 mm distance so that the container rotates without hindrance. 
     Sample plate  133  was rotated by the step motor  137  in steps of about 30°, which positions the container in successive steps in front of the magnets, which are diametrically opposite side of the container. This brings about an 180° change in the direction of magnetic field attracting magnetic particles in the liquid sample. Continuing this step-wise rotation suspended and mixed the magnetic particles. On stopping the rotation, the container remained stationary in front of the magnet resulting in the separation of the magnetic particles from the liquid sample. A 15° step move brought the container at an intermediate position between the magnets to substantially remove the effect of magnetic field on the container which was useful in some downstream application such as eluting the affinity bound nucleic acids from the magnetic particles. 
     Although rotary motion of the sample plate has been described, it is obvious that the sample plate can be fixed while the magnet plate is rotated and such a configuration may useful in certain situations. 
       FIG. 14  A-D shows various isometric views of a device with a linear array magnet mounted in assemblies arranged in of plurality of rows over the magnet plate and parallel over it a sample plate with plurality of apertures for holding the containers. 
     It includes a base plate  141  with two parallel side plates  142  A-B with rectangular grooves  142  G mounted thereon, a magnet plate  143  with nine magnet assembles mounted in equidistance rows with assembly  146  A alternating with respect assembly  146  B and a sample plate  144  fixed over the top of side plates  142  A-B and a sample plate  144  mounted over the side plates  142  A-B and has an 8×12 array of apertures  144  A for inserting the containers  145  containing magnetic particles in liquid samples. As shown in  FIG. 14  B, the left and right sides of magnet plate  143  can be inserted in the groove  142  G of the two parallel side plates  142  A-B to provide a guided linear sliding movement of the magnet plate parallel to the sample plate  144 . The magnet plate  143  can be mechanically attached to a linear electric step motor, linear actuator or other suitable motorized mechanical means to horizontally move the magnet plate within the linear guide of the two grooves  142  G. 
     The magnet assemblies  146  A-B comprises of magnets  147  fixed in evenly spaced slots of a non-magnetic harness by an adhesive or suitable mechanical means with seven magnets in  146  A and six magnets in  146  B. Permanent magnets are preferred, particularly high energy magnets of NdFeB type. The magnets  147  will be of appropriate size (width and length) to provide sufficient magnetic field strength over substantial part of the surface area of the container  146 . The arrangement where similar poles are diametrically opposite one another relative to the container is preferred. 
     As shown in  FIG. 14  C, the magnet plate  143  has a plurality of magnet assemblies mounted equally spaced and parallel rows with magnet assembly  146  A alternating magnet assembly  146  B. The separation distance between the assemblies should be sufficient to allow a hindrance free movement of magnet assemblies  146  A-B with respect to the containers  145  between them. 
       FIG. 14  D is a partial cut of the top view of the invention showing the apertures  133 A of the sample plate are positioned at the center between magnet assemblies  146  A-B. It will be recalled that the containers  145  containing magnetic particles in a liquid sample would be inserted through the apertures  133  A in the sample plate. The positions of the containers will therefore be identical to the positions of apertures. Referring to  FIG. 14  D, it can be seen that individual containers in each row are precisely positioned in front of the alternating individual magnets  147  in the magnet assembly  146  A and magnet assembly  146  B. The direction of the magnetic field acting on the lateral side of the containers alternately reverses. It should be noted that magnetic field generated by one magnet is substantially unaffected by the magnetic field generated by the other magnet. 
     When magnet plate  143  is stationary with magnets and the sample containers in their respective arrays aligned, magnetic particles inside the containers will collect on the lateral wall of the containers nearest to its respective magnets. A linear step movement of the magnet plate  143  will position the magnets at the diametrically opposite side of the containers thereby attracting the magnetic particles to a position opposite to previous. Repeated forward and backward step-wise motion of the magnet plate will cause the suspension and mixing of magnetic particles in the liquid sample and when this motion is stopped separation of magnetic particles. 
     The invention shown in  FIG. 14  A, can process 96 Eppendorf types of containers simultaneously. It contains the permanent magnets of about 10×10×25.4 mm size which are mounted in the evenly spaced of about 50.8 mm slots in the magnet assembly with seven magnets in magnet assembly  146  A and six magnets in  146  B. Nine rows of magnet assemblies were mounted on the magnet plate  143  with five of assembly  146  A alternating with four of assembly  146  B. A distance of about 12 mm separated the rows of assemblies. When assembled, the distance between the magnet plate  143  and the sample plate  144  is adjusted to so that Eppendorf sample container  145  is maximally exposed to the magnets  143  and the magnet plate  143  moves without hindrance. 
     The magnet plate was mechanically connected to a linear step motor to horizontally move the magnet plate  143  to about 25.4 mm lateral distance mm in the forward and backward directions. A delay time of about 0.5 to 60 seconds was imposed during each stroke of the linear movement. Such a motion profile was electronically controlled and repeatedly positioned the magnets at the diametrically opposite side of the container which suspended and mixed the magnetic panicles inside the Eppendorf until the motor was stopped to separate the magnetic particles from the liquid samples. 
     Although linear motion of the magnet plate is described, it is obvious that the magnet plate can be fixed while the sample plate is horizontally moved to repeatedly position the sample container between the diametrically opposed magnets. 
       FIG. 15  shows an isometric view of an apparatus for mixing and separating magnetic particles in the wells of a 96-well microplate.  FIG. 15  A is an exploded isometric view corresponding to  FIG. 15  and shows a base plate  150  with four guide rods  150  A, two magnet assemblies  151 - 152  with magnets  155  mounted in linear arrays thereon, a sample plate  153  with apertures for inserting the 96 wells  154 A of the microplate  154 . 
     The disposable plastic microplates  154  known as “skirt-less” type are widely available and have integrally formed multiple wells for holding liquid samples. As shown in  FIG. 15  A, the wells  154  A in the microplate  154  are closely spaced and arranged in an 8 by 12 array. The volumetric capacity of the wells is usually in time range of 250-350 micro liters. While a ninety-six well microplate is shown, this invention is equally applicable to standard 6, 12, 24, 48 tissue culture plates as well as 96-well microplates with a volumetric capacity of 1-2 ml. 
     The magnet  155  are preferably of cylindrical shape and the high energy magnets of NdFeB type in order to provide strong magnetic fields adjacent to wells in microplate  154 . 
     The magnet assemblies  151  and  152  are slotted in the shape of “fingers” in order to permit the unhindered independent movement of either magnet assembly in the vertical direction to place the magnets between the wells of microplate. A rectangular plate with appropriate access holes for magnet passage can also be used instead of the slotted shape for magnet assemblies. 
     The magnet assembly  151  includes guide holes  151 A and a 4×6 array of individual rod-shaped magnets mounted on the four slots of the assembly. The magnet assembly  151  is precisely positioned by inserting it through the guide holes  151  A in the two individual guide rods  150 A mounted on the right of the base plate  150 . The magnet assembly  152  include two guide holes  152 A and a 5×7 array of individual rad-shaped magnets mounted on the five slots of the assembly. The magnet assembly  152  is similarly positioned by inserting it through the guide holes  152  A in the two individual guide rods  150 A mounted on left of the base plate  150 . Both magnet assembles can be independently moved up towards the bottom of microplate  154  through the linear guide provide by the holes and rods. A suitable roller bearing may be used in the holes to provide a rigid mechanical support to assemblies as well as friction less movement. Each assembly can be mechanically attached to a linear electric step motor, linear actuator or other suitable motorized mechanical means to provide independent movement to each assembly.  FIG. 15  B shows a front view corresponding to  FIG. 15  A, illustrating the relative positions of the mounted magnets  155  in the assemblies with respect to wells micro plate. The gaps between slots of the assembly  152  (hidden) allow the positioning of mounted magnets of the assembly  151  between the well of the micro plate  154 . 
     The sample plate  153  has a plurality of apertures  154  A arranged as 8×12 array of individual apertures corresponding to 8×12 array of individual wells of the microplate through which the wells can be inserted and precisely positioned with respect to magnets  155  mounted on the two magnet assemblies  151  and  152 . The sample plate  153  is fixed at the top of four guide rods  150  A by means of threaded fasteners. The micro plate thus remains fixed and the wells precisely positioned to permit access of mounted magnets between the exterior spaces of the wells underneath the microplate  154 . Sufficient distance between the base plate  150  and the sample plate so that magnetic field of the mounted magnets  155  acting on the wells of micro plate is negligible. Magnetic field becomes active only when the magnet assembly is moved upward and the mounted magnets positioned between the wells of the micro plate  154  and near the sample plate surface underneath. 
       FIG. 15  C shows the relative positions of the 4×6 array of magnets and the 8×12 array of apertures  153  A when the magnet assembly  151  upward near the sample plate  153 . Apertures can be assumed to represent the wells, as the 8×12 array patterns of both the sample plate  153  and micro plate  154  are identical. Referring to  FIG. 15  C it can be seen that the 24 magnets are uniformly distributed within the boundaries of 96 wells and the radial magnetic field of each individual magnet essentially acts on four surrounding wells. Magnet positions in each row aid column are sufficiently apart so that the effect of their magnetic fields on the distant wells can be considered negligible. Magnetic particles in the liquid sample in each set of four wells will move and aggregate on the lateral surfaces inside the wells nearest to the magnets. 
       FIG. 15  D shows the relative positions of the 5×7 array of magnets and the 8×12 array of apertures  153  A when the magnet assembly  151  is moved down and the magnet assembly  152  is moved upward near the sample plate  153 . As previously, the apertures can be assumed to represent the wells. 
     A comparison between  FIGS. 15  C and D Figure, reveals that the magnets in the magnet assemblies  151  and  152  are positioned diametrically opposite one another relative to the wells of microplate  154  and the radial magnetic field of each individual magnet will now attract the magnetic particles in the liquid sample in each well on the diametrically opposite lateral surface inside each well. 
     Successive rapid upward movement of one magnet assembly at a time while the other magnetic assembly is rapidly moved down will bring about the mixing of magnetic particles in the liquid sample in each well in a similar manner as described earlier. For the separation of magnetic particles in the wells either one or both magnetic assemblies may be moved upward and made stationary till all the particles have separated in the wells, which permits the removal of the supernatant liquid. 
       FIG. 16  A shows an isometric view of a device for single container preferably a container of large volumetric capacity such as 50 ml. It includes a magnet assembly comprised of two hexagonal magnet plates  161  and  162 , each with a mounted magnet  166 . Rare earth high-energy magnets are preferable and appropriate dimensions to provide maximum magnetic field area to cover the lateral part of the container  165 . Both the magnet plates are fixed to the shaft  163  in parallel coaxial positions with the two magnets opposite each other and separated by an angular position of 90° and preferably by an angle of 180°. Other angular positions may be used provided that magnetic field generated by one magnet is substantially unaffected by the magnetic field generated by the other magnet. Although rectangular magnets are preferred, round or other suitable shapes for magnets can also be used. Similarly, shapes other than hexagonal can also be used for magnet plates. 
     The shaft  163  can be the shaft of a step motor to directly rotate the magnet plates or a separate cylindrical shape rod, which can be reversibly attached to a step motor shaft with a suitable shaft coupler such as Fairloc Shaft Coupling available from Stock Drive Products, N.Y. 
     The sample plate  166  includes an aperture for inserting the container  165  and is attached to a height adjustable clamp of a laboratory stand to permits the positioning of the container  165  at the center of the two magnet plates without hindering its rotary movement of the magnet plates as shown in  FIG. 16  B. When the shaft is rotated, the two coaxial magnet plates rotate simultaneously which successively brings each mounted magnet at the opposite lateral sides of the container. Step motion of a predetermined angle by step motor with an imposed delay time of about 0.5 second between each step is preferred to suspend and mix the magnetic panicles in the sample liquid of the container and by stopping this rotation and positioning one of the magnet adjacent to the container separates magnetic particles from the liquid sample on the lateral wall inside the container to allow the removal of the supernatant liquid. 
     The relative angular movement is induced in the magnetic particles by either rotating a magnetic field around a stationary container or rotating the container relative to an immobile magnetic field. The magnet creating the field is disposed outside the container and defines a cavity of magnetic field gradient within the liquid test medium. Any container configuration may be utilized, such as, for example, a doughnut-shaped container. In such a container the magnetic source may be “outside” of the container and “within” the container, if it occupies the hole of the doughnut. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 
     The following examples further describe in detail the manner and process of using the present invention. The examples are to be considered as illustrative but not as limiting of this invention. All manipulations given in the examples are at ambient temperature unless otherwise indicated. 
     EXAMPLE 1 
     The effect of angular acceleration on magnetic particle mixing was determined by using a device similar to the apparatus shown in  FIG. 12  except that the linear drive mechanism for vertical movement of the magnet assembly was switched off. The magnet assembly included six rare earth type (NdFeB) permanent magnets of about 35 MGOe. The electric motor ( FIG. 12, 129 ) was a two phase stepping motor driven by a computer programmable controller-driver. The rotary motion of the stepper motor responds to sequence of digital pulses from the controller-driver. The angular acceleration is directly related to the frequency of input pulses and the length of rotation is directly related to number of pulses applied. A 1.6 ml microcentrifuge tube having diameter of about 10.6 mm was used as a container. A 250 μl suspension of about 50 million paramagnetic beads in phosphate buffer saline, pH 7.5 containing 2.5% bovine serum albumin was transferred to a microcentrifuge tube. The beads were obtained from Dynal, Inc., and their reported physical characteristics were: spherical and uniform size of 4.5 μm, magnetic mass susceptibility of about 16×10-5 m3/kg. The microcentrifuge tube was placed in the holder ( FIG. 12, 134 ) and the magnetic field acting inside of the microcentrifuge tube was adjusted by the knob ( FIG. 12, 128 ) so that the suspended beads aggregate within 60 seconds at the inside wall closest to the magnet assembly. The magnetic field gradient measured inside the container varied from about 1.1 kGuass (closest to magnet) to about 0.4 kGuass (farthest from magnet). Once the magnetic beads had aggregated, the microcentrifuge tube was accelerated from rest to a preselected speed (rpm) within one second time. The angular acceleration was calculated from the following formula:
 
α=(ω 1 −ω 0 )/ t =rad/s 2  
 
     Where α angular acceleration (rad/s 2 ), ω 1  is angular velocity in radians per second after one second, ω0 angular velocity in radians per second at rest which in this instance is zero, and t is the time in second which in this instance is one second. The mixing efficiency of the beads at various acceleration rate was estimated by observing the mass of beads remaining aggregated as well as the cloudiness of the suspension (scale of + to 5+). Continuous rotation was used for speeds between 5 to 200 rpm. The effect of acceleration on mixing efficiency could be clearly observed when tube was accelerated from rest to an angular position of 180° in a single step and stopped. Although a microscope may be used but a visual examination was also adequate. The results are shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Speed 
                   
                   
                   
               
               
                 (rpm) 
                 ω 1  (rads/s) 
                 α (rads/s 2 ) 
                 Mixing Efficiency 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 5 
                 0.52 
                 0.10 
                 No Mixing, beads remain 
               
               
                   
                   
                   
                 substantially aggregated 
               
               
                 10 
                 1.05 
                 0.21 
                 No Mixing, beads remain 
               
               
                   
                   
                   
                 substantially aggregated 
               
               
                 40 
                 4.19 
                 0.84 
                 Cloudiness +, about 90% 
               
               
                   
                   
                   
                 beads remain aggregated 
               
               
                 50 
                 5.24 
                 1.05 
                 Cloudiness ++, about 80% 
               
               
                   
                   
                   
                 beads remain aggregated 
               
               
                 100 
                 10.47 
                 2.09 
                 Cloudiness +++, 
               
               
                   
                   
                   
                 about 30% beads remain aggregated 
               
               
                 200 
                 20.94 
                 4.19 
                 Cloudiness +++++, 
               
               
                   
                   
                   
                 less than 5% beads remain aggregated 
               
               
                 180° Step 
                 209.44 
                 41.89 
                 100% mixing 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2 
     The effect of angular acceleration on purification efficiency was determined by isolating genomic DNA from human whole blood. The basic experimental set up was as described in Example 1 and angular acceleration of 0.1, 0.21 and 4.19 rads/s2 was used. The experiments consisted of three identical isolations using EDTA anticoagulated blood and magnetic beads from Dynabeads DNA Direct kit (commercially available from Dynal, Inc., Lake Success, N.Y. 11042). The process of DNA isolation in this kit relies upon cell lysis to release the DNA, which is then adsorbed at the surface of the beads. It was assumed that mixing efficiency would be directly reflected by comparing the yields of DNA isolated at the three angular acceleration. A 200 μl suspension of beads from the kit was pipetted in a siliconized microcentrifuge tube. The microcentrifuge tube was then placed in the tube holder of the apparatus and the magnetic field acting inside of the tube was adjusted by the knob ( FIG. 12, 128 ) so that the suspended beads aggregate within 60 seconds at the inside wall closest to the magnet assembly. Once the beads had aggregated, a 10 μl of EDTA anticoagulated blood was added to the clear solution inside the tube. The microcentrifuge tube was accelerated from rest to a preselected speed (rpm) and the rotation of the tube was continued for about five minutes. During this rotation, the beads, if mixing, would adsorb DNA. The rotation was then stopped and the magnetic field acting inside of the tube was increased to maximum by bringing the magnet assembly closest to the tube holder by adjusting the knob ( FIG. 12, 128 ). The beads aggregated at the inside wall closest to the magnet assembly and the supernatant was withdrawn. The beads were then washed twice with 200 μl of the wash buffer of the kit. During each washing the beads were mixed in the washing buffer by rotating the tube for two minutes at the angular acceleration used for DNA adsorption. The steps for the separation of beads and removal of supernatant wash buffer were as described earlier. The tube was then removed from the apparatus and bead/DNA complex resuspended in 50 μl of resuspension buffer of the kit (10 mM Tris HCl, pH 8.0) by pipetting up and down 30-40 times until the suspension is homogenous. DNA was then eluted by incubating the tube at 65 Co for five minutes. The tube was then placed back on the apparatus to separate the beads and the supernatant containing the eluted DNA was transferred to a dean tube. The DNA content in the supernatant was then determined by measuring optical density (OD) at 260 and 280 nm. The ratio OD 260 /OD 280  of 1.7 indicated that the isolated DNA was pure. The OD260 was then used to calculate the concentration of DNA. This technique for the determination of DNA is well known and widely used in the molecular biology art. The yields of genomic DNA isolated at angular acceleration of 0.1, 0.21 and 4.19 rads/s2 are shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Speed (rpm) 
                 α (rads/s 2 ) 
                 Mixing Efficiency 
                 DNA Yield ng 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 5 
                 0.10 
                 Aggregated beads roll 
                 Un-detectable 
               
               
                   
                   
                 over one another 
               
               
                 10 
                 0.21 
                 Aggregated beads roll 
                 Un-detectable 
               
               
                   
                   
                 over one another 
               
               
                 200 
                 4.19 
                 Beads are dispersed 
                 250 
               
               
                   
                   
                 and mixed