WO1993013400A2

WO1993013400A2 - Washing/aspiration systems and methods for solid phase assays employing paramagnetic particles

Info

Publication number: WO1993013400A2
Application number: PCT/US1992/011132
Authority: WO
Inventors: Jeffery W. Kaul; Joseph D. Raymo; Michael R. Henry
Original assignee: Baxter Diagnostics Inc.
Priority date: 1991-12-18
Filing date: 1992-12-18
Publication date: 1993-07-08
Also published as: AU3333593A

Abstract

Washing/aspiration systems and associated methods for assays employing paramagnetic solid phase binding particles (422) remove fluid and unbound components from the test well (18) without losing the paramagnetic particles (422) in the process. In one arrangement, the surface tension of the fluid in the well (18) is altered in a preselected manner to redistribute larger volumes of fluid away from the center region of the well (18) and toward the side region of the well (18). Fluid is aspirated from the side region, freeing the center region of the well (18) for the collection and concentration of the paramagnetic particles (422) away from the aspiration flow. In another arrangement, the distribution of the paramagnetic particles (422) in the well (18) is altered by subjecting the particles to a dynamic magnetic field (602; 608) that varies the direction and intensity of the magnetic gradient as a function of time. The dynamic magnetic field (602; 608) collects and concentrates the particles (422) within a region of the test well (18) away from the aspiration probe.

Description

WASHING/ASPIRATION SYSTEMS AND METHODS

FOR SOLID PHASE ASSAYS EMPLOYING

PARAMAGNETIC PARTICLES

5 Field of the Invention

The invention generally relates to analytical systems and methods that detect and quantify the presence of targeted compounds by forming solid phase bound complexes. In a more 10 specific sense, the invention relates to assay systems and methods that employ mobile paramagnetic particles as the solid phase binding sites.

Background of the Invention

15 Many different analytical procedures are in widespread use today in diverse environments to quantify the presence of targeted materials within a given sample. For example, biological analytical procedures carry out enzyme chemistry assays, DNA

20 probe assays, im unoassays, and cellular or cell surface assays of biological materials using fluorescent, absorbance, and chemiluminescent

. techniques. Nonbiological analytical procedures detect the presence of pollutants or toxins in

25 water, air, and soil.

Each given analytical procedure follows its own prescribed protocol, which specifies a carefully timed and prescribed sequence of steps that must be closely followed. Each protocol also specifies

30. other environmental conditions, such as temperature and humidity, that must be carefully maintained to assure accurate and reproducible results.

Many analytical procedures employ so-called "solid phase" assay techniques. These techniques rely upon a solid support of sorbent material to bind the targeted compounds that are to be detected and measured. Solid supports like filter paper, plastic balls, polysaccharide beads, or the interior walls of test tubes are used for this purpose. Mobile paramagnetic particles can also be used as solid phase binding sites. Analytical procedures employing paramagnetic particles are described in U.S. Patent Application No. entitled The solid phase paramagnetic particles are typically tiny spheres measuring about three microns in diameter made of an inert plastic material in which magnetite is encapsulated. These particles are coated with protein and then dipped in a preselected chemical that serves as the binding sites. These small, spherical particles are generally added to the sample to be analyzed with one or more specified reagents, buffers, and/or diluents (which will be collectively called "reagents") . The sample/reagent mixture containing the mobile particles is typically contained in a test well itself made of an inert plastic material. The sample/reagent mixture containing the particles is incubated within the test well one or more times to form a solid phase bound complex that typically includes an enzyme label. After incubation, the sample/reagent mixture containing the particles is usually washed and aspirated one or more times to remove the "free" or otherwise nonspecifically bound components from the test well, with all excess fluid. A substrate is then usually added. The enzyme label on the complex serves as a

* catalyst that splits the substrate, in the process forming a molecule that can be detected and

._<« 5 quantitatively measured. Typically, the formed molecule fluoresces, and it is this fluorescence that is detected. Alternatively, an enzyme label that itself fluoresces without a substrate can be used. 10 During the washing/aspiration process, it is desirable to remove from each test well as much unbound components suspended in as much fluid as possible. Also, the loss of the mobile paramagnetic particles suspended in the well is to be avoided. 15 The loss of these particles reduces the signal strength of ultimate reading, causing inaccurate and invalid results.

The washing/aspiration process is most efficiently accomplished by drawing fluid from the 20 regions of the test well that contain the greatest relative fluid volumes. In prior washing/aspiration techniques, the larger volumes of fluid are found in the center region of the well. This shape favors placement of the aspiration probe in the center 25 region of the well.

To avoid the loss of the mobile paramagnetic particles while the fluid is being aspirated away, it is desirable to keep the particles within the well away from the region or 30 regions where aspiration occurs. In prior techniques, where fluid is aspirated from the center _j, region of the well, a magnetic field is applied to the sides of the test well to draw the paramagnetic particles away from the center of the well.

35 However, loss of the paramagnetic particles inevitably occurs.

The inventors have observed that the fluid volume concentrates in the center region of the well because of the surface tension of the fluid, which forms a meniscus having a generally convex shape. The inventors have also observed that, despite efforts to keep the particles away from this center region, the surface tension of the fluid has the undesirable tendency of drawing the paramagnetic particles suspended in the fluid into the outgoing aspiration stream.

The inventors have further discovered the material (typically plastic) from which the paramagnetic particles are made can exhibit an affinity for, or attraction to, the material of the test well (which is also typically plastic) . The inventors have discovered that, due to this mutual affinity, the particles tend to be distributed generally uniformly along the interior surface of the test well. Because of this mutual affinity, the particles can resist efforts made to disturb this general uniform distribution, further complicating the task of collecting and concentrating the particles into smaller, more compact groups away from the aspiration probe.

Due to these pheno enons, the inventors have discovered that the prior washing/aspiration techniques do not and cannot maximize the retention of paramagnetic particles within the well.

Summary of the Invention

The invention provides improved systems and methods for conducting large volumes of fluid from a test well containing mobile paramagnetic particles while minimizing the loss of the paramagnetic particles during the process.

According to one aspect of the invention, the surface tension of the fluid contained within the test well is altered in a preselected manner to redistribute larger volumes of fluid away from the center region and toward the side region of the well. This aspect of the invention allows fluid to be aspirated from the side region of the well, freeing the center region of the well for the collection and concentration of the paramagnetic particles.

The inventors have also discovered that this alteration of the natural surface tension also reduces the tendency of the fluid to draw the paramagnetic particles into the outgoing aspiration stream, while reducing the natural affinity between the paramagnetic particles and the adjacent side walls of the well. These attendant benefits further simplify efforts to collect and retain the particles in the center region of the well away from the aspiration flow.

In one embodiment of this aspect of the invention, the washing fluid introduced into the wells includes a preselected quantity of a material that reduces the surface tension of the fluid. The additive material is introduced in sufficient quantities to alter the meniscus of the fluid within the well from its naturally convex shape into a more concave shape, in which relatively more fluid volume is distributed away from the center region of the well and more toward the sides of the well.

In one preferred embodiment, where the wash fluid is saline based, and the paramagnetic particles and well are both made from polystyrene- based materials, the material used to alter the meniscus comprises a liquid surfactant having the attributes of Tween-20. In this embodiment, the liquid surfactant is contained in the wash fluid in a concentration of no more than about .05%. According to another aspect of the invention, the distribution of mobile paramagnetic particles suspended in the fluid is itself altered by subjecting the particles to a dynamic magnetic field that varies the direction and intensity of the magnetic gradient as a function of time. The dynamic magnetic field collects and concentrates the particles within a preselected region of the test well.

In one embodiment of this aspect of the invention, the dynamic magnetic field has two principal components. One component reaches out from the origin of the magnetic field and attracts the mobile particles vertically down toward the origin of the field. The other component gathers the downwardly attracted particles radially toward the magnetic center of the field. In this embodiment, the paramagnetic particles are exposed in an alternating fashion to the vertical reach component and to the radial reach component. In the process, the particles are progressively drawn down toward and concentrated within the desired region of the well. Preferably, the particles are commonly attracted and grouped in a region of the well spaced away from the region where fluid is to be aspirated. In one embodiment, the radial reach component is more narrowly focused upon the particles just prior to insertion of the aspiration probe.

In a preferred embodiment, the dynamic nature of the magnetic field is created by reciprocating, or "racking," the test well between two fixed magnetic fields, one in which the vertical reach component predominates and another in which the radial reach component predominates.

In a most preferred embodiment, both aspects of the invention are used in combination. In this arrangement, the surface tension of the fluid is altered to redistribute the major fluid volume toward the side region of the well, while a dynamic magnetic field is created to collect and concentrate the mobile particles in the center region of the well. Aspiration of large volumes of fluid occurs in the side region of the well, without detectable loss of paramagnetic particles.

Other features and advantages of the invention will become apparent upon considering the accompanying drawings, description, and claims.

Description of the Drawings

Fig. 1 is a perspective view of a washing and substrate dispensing station that includes a washing/aspiration system embodying the features of the invention;

Fig. 2 is a perspective view of a portion of the washing station shown in Fig. 1; Fig. 3 is a front perspective view of an analytical system that incorporates the washing station shown in Fig. 1;

Fig. 4 is a perspective view of the interior of the processing module associated with the analytical system shown in Fig. 3, showing the location of the washing station that embodies the features of the invention;

Fig. 5 is a perspective view, with portions broken away and in section, of the shuttle member associated with the analytical system shown in Fig. 3 ;

Fig. 6 is a front perspective view of the test carrier associated with the analytical system shown in Fig. 3; Fig. 7 is a top view of the test carrier shown in Fig. 6;

Fig. 8 is a perspective view of the interface between the test carrier and the shuttle member that are associated with the analytical system shown in Fig. 3;

Figs. 9 to 12 are a series of perspective views showing the transport of the test carrier by the shuttle member to and from the washing station shown in Fig. 1; Fig. 13 is a side view of the carrier transport mechanism associated with the washing station shown in Fig. 1;

Figs. 14 to 16 are a series of side section views of the washing/aspiration sequence employed by the washing station shown in Fig. 1;

Fig. 17 is a side section view of a suspension of paramagnetic particles within a test well having a convex meniscus;

Fig. 18 is a side section view of a suspension of paramagnetic particles within a test well having a concave meniscus;

Fig. 19 is a top view of the washing station shown in Fig. 1 with portions broken away to show the associated magnetic field that embodies the invention;

Fig. 20 is a tip view of a portion of the magnetic field shown in Fig. 19;

Fig. 21 is a diagrammatic side perspective view of an asymmetrical region of the magnetic field shown in Fig. 20; Fig. 22 is a diagrammatic side perspective view of a symmetrical region of the magnetic field shown in Fig. 20;

Figs. 23 to 27 are a series of side section views showing the reciprocating or "racking" movement of the test carrier relative to the asymmetrical region of the magnetic field shown in

Fig. 20;

Figs. 28 to 34 are a series of side section views, generally corresponding to Figs. 23 to 27. respectively, showing the effect of the reciprocating movement shown in Figs. 23 to 27 upon the paramagnetic particles;

Fig. 35 is a side section view of a test well after having undergone the sequence shown in

Figs. 23 to 27, showing the congregation of paramagnetic particles in the center region of the well;

Fig. 36 is a top view, taken generally along line 36-36 in Fig. 35, of the congregation of paramagnetic particles shown in Fig. 35;

Figs. 37 to 39 are a series of side section views showing the reciprocating or "racking" movement of the test carrier relative to the symmetrical region of the magnetic field shown in

Fig. 20;

Fig. 40 is a top view, taken generally along line 40-40 in Fig. 39, showing the elongated configuration of the paramagnetic particles after undergoing the movement shown in Figs. 37 to 39;

Fig. 41 is a diagrammatic view, with portions in section, of an alternate embodiment of a magnetic field that embodies the features of the invention. Fig. 42 is a perspective view of another washing and substrate dispensing station that includes a washing/aspiration system embodying the features of the invention;

Fig. 43 is an enlarged perspective view of the substrate dispensing area of the system shown in Fig. 42;

Fig. 44 is an enlarged perspective view, with portions broken away, of the washing/aspiration area of the system shown in Fig. 42; Fig. 45 is an enlarged and partially exploded perspective view of the pipette assembly for dispensing substrate in the system shown in Fig. 42, with the associated well in its recessed position; Fig. 46 is an enlarged perspective view of the pipette assembly for dispensing substrate in the system shown in Fig. 42, with the associated well in its uplifted position;

Fig. 47 is a side section view taken generally along line 47-47 in Fig. 45;

Fig. 48 is a side section view taken generally along line 48-48 in Fig. 46;

Fig. 49 is a perspective view of the substrate dispensing pipette shown in Fig. 42 dispensing substrate into an associate test carrier;

Fig. 50 is a perspective view, with portions broken away, of the washing/aspiration pipette assembly associated the system shown in Fig. 42; Fig. 51 is an enlarged side section view of the probe sets associated with the pipette assembly shown in Fig. 50; and

Figs. 52 to 54 are a series of views depicting the probe sets associated with the pipette assembly shown in Fig. 50 while in use. Description of the Preferred Embodiments

Fig. 1 shows a washing/aspiration system 11 that incorporates the features of the invention. The washing/aspiration system 11 is applicable for use in different environments to carry out diverse types of analytical, laboratory, and clinical procedures that employ mobile paramagnetic particles as solid phase binding sites.

In the illustrated embodiment, the system 11 will be described in the specific functional context of a solid phase assay procedure for screening human blood serum/plasma for one or more significant analytes, such as Hepatitis type B surface antigen (HBs Ag) ; Hepatitis type B core antibody (HBc Ag) ; Human immuno deficient virus antibody (HIV-1) ; Human T-cell lymphotrophic virus (type 1) antibody (HTLV-I) ; and T. Pallidum (syphillis) antibody (TPA Ab) .

All the above blood assays employ a so- called "solid phase" assay technique. This technique relies upon a solid support of immunosorbent material to bind the complexes. In the illustrated embodiment, the assays employ mobile paramagnetic particles (generally designated by the numeral 422 in the drawings) as solid phase binding sites. These particles 422 are spheres measuring about three microns in diameter made of a polystyrene material containing magnetite. The particles are coated with protein and then dipped in a preselected immunochemical that serves as the binding site.

Preferred analytical procedures employing paramagnetic particles and fluorescent substrates are described in U.S. Patent Application No. entitled which is incorporated herein by reference.

The illustrated embodiment (see Figs. 3 and 4) shows the washing/aspiration system 11 as part of an integrated analytical system 10 of the type disclosed in copending U.S. Patent Application Serial No. , entitled "Systems for

Conducting Multiple Analytical Procedures," which is assigned to the same assignee as the present application. The system 10 includes a specimen carrier 12 (as Figs. 6 and 7 show) , a processing module 14, and a control module 16 (as Fig. 3 shows) .

The test carrier 12 serves to contain one or more samples of fluid for analysis by the system 11. The carrier 12 holds the samples throughout the processing procedure.

The test carrier 12 (see Figs. 6 and 7) includes a series of test wells 18 aligned in a predefined relationship. Each test well 18 retains a prescribed aliquot (or sample) of the biological fluid to be analyzed.

In the illustrated, the test carrier 12 takes the form of a tray of unitary molded construction made of an inert plastic or another lightweight inert material, such as glass. In the illustrated embodiment, the carrier 12 is molded from a polystyrene-based plastic polymer and is a single use, disposable component of relatively low cost. In the illustrated embodiment, the test wells 18 are molded in the carrier 12 in a prescribed matrix consisting of twelve linear columns Cl to C12 (extending vertically in Fig.7) and eight linear rows Cl to C8 (extending horizontally in Fig. 7) . The prescribed arrangement of test wells 18 in the matrix makes it possible to establish discrete processing sectors upon the carrier 12. Each processing sector contains one or more samples and is dedicated to the performance of one selected blood assay procedure on all the contained samples. The samples can originate from either the same or a different fluid source.

In the illustrated embodiment, each complete column on the carrier 12 defines a "processing sector", as this description uses that term. Each processing sector contains eight test wells 18, which corresponds to the number of rows in each column. For ease of description, the processing sectors will be called by their column number on the tray (Cl to C12) , and the test wells 18 will be called by their row number within their respective columns (RI to R8) . For example, the third test well 18 in the fourth processing sector will be identified as well C4, R3 (or, alternatively, well R3, C4) .

To accommodate the different blood assay procedures, the processing matrix established on the carrier 12 creates two pairs of six processing sectors (for a total of twelve processing sectors) , as follows (also see Fig. 7) :

Processing Sectors Cl and C7 : the solid phase HBs Ag assay;

Processing Sectors C2 and C8: the solid phase HBc Ag assay;

Processing Sectors C3 and C9: the assay for Glutamate pyruvate transaminase (GPT or ALT) , an enzyme chemistry that does not employ a solid phase support; Processing Sectors C4 and CIO: the solid phase HIV-1 assay;

Processing Sectors C5 and Cll: • the solid phase HTLV-I assay; and

Processing Sectors C6 and C12: the solid phase TPA Ab assay.

Up to eight samples can be contained within each processing sector, one for each row RI to R8. Therefore, in the illustrated embodiment, the carrier 12 can accommodate as many as six different blood assay procedures on as many as sixteen different source samples. A single test carrier 12 can carry out a total of ninety-six blood assay procedures.

In this arrangement, the washing/aspiration system 11 is a component part of the processing module 14 (as Fig. 2 shows) . The processing module 14 is a self-contained unit that performs all the various steps of the selected assays automatically from start to finish, almost without any operator intervention.

The processing module 14 includes within a common housing several processing stations 28 to 36 individually serviced by a shuttle member 20 (as best shown in Figs. 4 and 5) . The shuttle member 20 transports the test carrier 12 to the various processing stations 22 to 36, which in turn perform one or more prescribed processing tasks on the samples contained in the test carrier 12.

The processing stations 28 to 36 accomplish eight generic processing tasks that are, for the most part, common to all the assays performed, as follows:

(1) Processing station 22 is a carrier dispensing station for storing and dispensing the one or more test carriers 12; (2) Processing station 24 is a sample dispensing station for receiving multiple sources of biological fluids and for dispensing samples of biological fluids from these sources into the wells 18 of the test carrier 12;

(3) Processing station 26 is a reagent dispensing station for dispensing one or more reagents into the samples contained in wells 18 of the test carrier 12; (4) Processing station 28 is at least one incubation station for incubating the sample and reagent mixtures (in the illustrated embodiment, there are nine incubation stations, designated 28 A to I) ; (5) Processing station 30 is a washing station for removing unbound materials from the samples and includes the washing/aspiration system 11 that embodies the features of the invention; (6) Processing station 32 is a substrate dispensing station for adding substrate from which the fluorescent molecule is formed for detection. This station also includes aspects of the washing/aspiration system 11 embodying the features of the invention; (7) Processing station 34 is a reader station for determining the presence and concentration of the fluorescent molecule; and (8) Processing station 36 is a carrier disposal station for disposing of the test carrier 12 once all the processing steps are completed.

The washing/aspiration system 11, like the other processing stations that comprise the processing module 14, is configured as a self- contained module that can operate in a "stand alone" configuration, out of association with other processing stations shown. When in association with other stations (as in the illustrated embodiment) , the modular nature of each processing station 22 to 36 simplifies removal, repair, and replacement of each station, thereby simplifying routine maintenance, troubleshooting, and repair.

In the illustrated arrangement (as Figs. 5 and 8 show) , the shuttle member 20 is mounted on a turntable 58 and includes a shuttle platform 70 slidably mounted on a deck 62. The shuttle platform 70 can be rotated about a 360-degree arc, or increments thereof, as the turntable 58 rotates about its axis 54 (by actuating a first stepper motor 60) . The shuttle platform 70 can be moved vertically up and down along the axis 54 as the deck 62 moves up and down (by actuating a second stepper motor 68) . The shuttle platform 70 can be moved in an out radially from the axis 54 upon the deck 62 (by actuating a third stepper motor 76) . The shuttle platform 70 is movable into and out of secure engagement with the test carrier 12 for transporting the carrier 12 among the various processing stations 22 to 36. The test carrier 12, the shuttle platform 70, and the processing stations 22 to 36 are each specially configured for this purpose.

More particularly (as Figs. 6 and 8 best show) , the test wells 18 of the carrier 12 generally terminate along a common base plane. The test carrier 12 also includes two pairs of opposing sidewalls 78(A and B) and 80 (A and B) . The first pair of sidewalls 78A/B extends parallel to the prescribed rows of the test wells 18. The second pair of sidewalls 80A/B extends parallel to the prescribed columns of test wells 18. The first sidewalls 78A/B commonly terminate below the base plane of the wells 18, while the second sidewalls 80A/B terminate above the base plane of the wells 18. A notched keyway 82 (A and B) is formed in each first sidewall 78A/B, with the uppermost edge 84 of each keyway 82A/B extending above the base plane of the wells 18. The two keyways 82A/B are generally axially aligned with each other on their respective sidewalls 78A/B. The first sidewalls 78A/B also each terminates with a flanged bottom edge 86A/B that extends from opposite sides of the keyway 82A/B along the entire width of the carrier 12. The shuttle platform 70 includes a transverse groove 88 and an upwardly raised keyway 90 within the groove 88. The groove 88 and raised keyway 90 on the shuttle platform 70 mutually capture one flanged bottom edge and associated keyway (either 86A and 82A or 86B and 82B) on the carrier 12, depending upon the orientation of the carrier 12. This cooperation of interlocking parts secures the carrier 12 on the platform 70 for transport. The shuttle member 20 preferably includes a position sensor 98 for detecting the presence of the test carrier 12 upon the shuttle platform 70.

The different processing stations 22 to 36 are positioned around the multi-directional shuttle member 20 in an arcuately spaced and vertically stacked relationship. In this arrangement, the shuttle member 20 operates from within an enclosed, stationary center hub 100 to obtain access to all the processing stations. The processing module 14 includes several shuttle accesses (which are generally identified by reference numerals 23, 25, 27, 29, 31, and 35) associated with the processing stations 22 to 36, where the carrier 12 can be picked up and dropped off by the shuttle platform 70 operating in the central hub 100.

Access can be obtained by moving the shuttle member 20 in a variety of discontinuous rotational, axial, and radial paths about its rotational axis 54.

The master control module 16 issues the appropriate command signals to the various control mechanisms of the processing stations 22 to 36 to sequence the overall operation of the processing module 14 according to the protocol established for each analytical procedure selected to be performed. The overall operation of the various processing stations is described in detail in the above-mentioned copending U.S. Patent Application Serial No. , entitled "Systems for

Conducting Multiple Analytical Procedures," and this disclosure is incorporated herein by reference. The washing station 30 and substrate dispensing station 32 each embodies aspects of the invention. For this reason the more specific structural arrangements of the illustrated and preferred embodiments of these two stations will now be described.

Prior to arriving at the washing station 30, a test carrier 12 is dispensed by the carrier dispensing station 22 and is delivered by the shuttle member 20 to the sample dispensing station 24. There, source specimens are pipetted into the individual test wells 18 of the carrier 12. The shuttle member 20 then transports the carrier 12 (now containing the samples to be analyzed) to the reagent dispensing station 26. There, reagents are pipetted into the test wells 18. The paramagnetic particles are also pipetted into the appropriate test wells 18 at the reagent dispensing station 26. After being transported by the shuttle member 20 to an incubation station 28, and there left for a prescribed period, the carrier 12 (now containing the samples to be analyzed, the appropriate reagents, and the paramagnetic particles) is transported to the washing station 30, which occupies a center back portion of the processing module 14. The washing station 30 communicates with the hub 100 through the access 31.

Once the proper command signal is received from the control module 16, the shuttle member 20 transports the carrier 12 to the washing station 30 access from its previous work station (which is assumed to be an incubation station 28) in response to additional commands issued by the shuttle control mechanism.

The washing station 30 includes a support bed 472 for receiving and transporting the carrier 12. The bed 472 has a cutout portion 94 in the hub access 31 (see Figs. 1 and 9 to 12) . The shuttle platform 70 enters the cutout portion 94 in a drop¬ off position, with the carrier 12 positioned above the plane of the bed 472. The platform 70 lowers through the cutout portion 94 to lift the carrier 12 off the platform 70 and into the bed 472 (as Figs. 9 and 10 show) . The bed 472 includes a spaced pair of transverse grooves 96 that capture the flanged edges 86 A and B of the carrier 12 as the carrier 12 is placed onto the bed 472.

The washing station 30 also includes a transporter 474 that is movable axially along the bed 472 along one of the grooves 96 (see Figs. 1 and 13) . The transporter 474 is carried by a belt 476 that extends between a drive pulley 478 and an idler pulley 480. A stepper motor 482 actuated by the control mechanism for the washing station 30 rotates the drive pulley 478 and thereby advances the transporter 474 in opposite linear directions (either to the left or to the right in Fig. 30) along the bed 472. The transporter 474 includes two oppositely facing grab arms 484 and 486 (as Fig. 13 best shows) . Each grab arm 484 and 486 is specifically configured to capture a bottom edge of one of the carrier sidewalls 80A or B. The transporter 474 can thereby accommodate two carriers 12 at a time, one in front (engaged by the first grab arm 484) and one behind (engaged by the second grab arm 486) . The sequence of movement of the carriers 12 upon the bed 472 will be described in greater detail later. The shuttle control mechanism acts in concert with the control mechanism for the washing station 30 to move the transporter 474 (via the stepper motor 482) into the proper positions within the hub access 31 to sequentially capture the front carrier 12 with the first grab arm 484 and then capture the rear carrier 12 with the second grab arm 486, as the shuttle platform 70 arrives and sequentially deposits the carriers 12 in the hub access 31. Subsequent operation of the stepper motor 482 moves the carrier 12 or carriers 12 engaged by the grab arms 484 and 486 along the bed 472.

The independently operated transporter 474 frees the shuttle member 20 other tasks while the washing process proceeds. As Figs.l and 2 shown, the washing station 30 includes a pipette subassembly 488 to accomplish the washing and aspiration operations. The pipette subassembly 488 is arranged along a boom 490 that spans the bed 472 transversely across the path that the carrier 12 is moved by the transporter 474.

The washing station 30 performs the washing and aspiration operations simultaneously upon all the samples located within an entire designated test sector (that is, RI to R8 in each column) and, in this arrangement, sequentially upon each entire test sector in turn (that is, column by column) . The transporter 474 simplifies this sequence by advancing the carrier 12 beneath the pipette subassembly 488 in stepwise fashion, one entire test sector at a time. The pipette subassembly 488 includes a series of probe sets 492 (see Fig. 2) equal in number to the maximum number of test wells 18 within a given test sector defined on the carrier 12, which, in the illustrated embodiment, is eight (8) . The eight probe sets 492 are spaced apart a distance equal to the spacing of the wells 18 within a test sector on the carrier 12.

A stepper motor 494 actuated by the control mechanism serves to raise and lower the pipette subassembly 438 as a unit, thereby moving the probe sets 492 in unison with respect to the test sector positioned beneath them.

In the illustrated and preferred embodiment (best seen in Figs. 14 to 16) , each probe set 492 includes a pair of probes 496 and 498. In use, one probe 496 aspirates fluid from the associated test well 18, while the other probe 498 adds a washing liquid (typically a saline solution) to the contents of the well 18. A pump (not shown) conveys washing solution to the washing probe 498. The aspiration probe 496 communicates with a waste fluid reservoir (not shown) via an associated vacuum pump (also not shown) .

In the preferred arrangement (see Figs. 14 to 16) , the aspiration probe 496 extends below the washing probe 498. The aspiration probe 496 is also offset laterally from the washing probe 498. In the preferred arrangement, the washing probe 498 generally registers with the centerline of each associated well 18, and the aspiration probe 496 is offset away from the centerline closer to a sidewall of the associated well 18. It should be appreciated that the aspiration probe 496 need not be offset from the centerline of the well 18, but could be at a range of positions closer to or along the centerline of the well 18. Still, for the reasons that will be described later, it is believed that offsetting the aspiration probe 496 toward the sidewall of the well 18 serves to maximize the effectiveness of the washing operation, especially in the preferred embodiments.

In the illustrated embodiment, the bed 472 extends beyond the location of the pipette subassembly 488 a distance a sufficient distance to accommodate the two carriers 12 engaged back-to-back by the transporter 474 (as Fig. 1 shows) .

The wash station 30 includes an electrical resistance heater 502 located under the washer bed 472 (see Fig. 2) . An associated thermistor 504 maintains the heater 502 at a desired temperature, which in the illustrated embodiment is between 30 and 45 degrees centigrade and preferably about 42 degrees centigrade. When two carriers 12 are to be washed, the transporter 474 is first positioned with respect to the cutout portion 94 so that front grab arm 484 engages the first carrier 12 delivered to the hub access 31. The transporter 474 then advances and is repositioned by the stepper motor 482 so that the rear grab arm 486 engages the next carrier 12 delivered to the hub access 31.

Once both carriers 12 are engaged by the grab arms 484 and 486, the control mechanism operates the transporter 474 to advance the first (front) and second (rear) carrier 12 in sequence beneath the washing pipette subassembly 488, one test sector (i.e., one column) at a time. When a given test column advances into position beneath the pipette subassembly 488, the transporter 474 stops, and the stepper motor 494 operates to lower the pipette subassembly 488 as a unit into each associated well 18 of the column. As Figs. 14 to 16 show, the pipette subassembly 488 first lowers deep enough to position the longer aspiration probe 496 at a predetermined depth within the fluid adjacent the sidewall of the associated well 18. As can be seen in Fig. 14, the tip 497 of the aspiration probe 496 is beveled in the direction of the sidewall and is thereby directed away from the center of each test well 18. A vacuum pump continuously applies a negative pressure simultaneously to all the aspiration probes 496 to draw fluid from the wells 18. The aspiration probes 496 removes a quantity of unbound materials suspended within the fluid, leaving behind within the wells 18 the solid phase supports and the complexes bound to them. The stepper motor 494 then raises the pipette subassembly 488 so that the shorter wash probes 498 are positioned generally at the top of each well 18. The wash pump then delivers a predetermined amount of washing fluid simultaneously through each wash probe 498 into the wells 18. The washing fluid resuspends any remaining unbound material for later aspiration, as well as begins to dilute the contents of the well 18.

The transporter 474 then advances the carrier 12 the distance between adjacent columns to place the next column beneath the pipette subassembly 488. The above described aspiration/washing cycle repeats for each test column (except test columns C3 and C9 for the ALT assay, which does not undergo washing) .

After the first and second carriers 12 are sequentially subjected to the above-described aspiration/wash cycle, the transporter 474 returns the carriers 12 back to the starting position, in which the first column of the first (front) carrier 12 is located beneath the pipette subassembly 488. The entire aspiration/washing cycle for the first and second carriers 12 is conducted again and after that repeats for a predetermined number of additional times. In the illustrated embodiment, each well 18 undergoes at least five separate aspiration/washing cycles.

After the predetermined number of aspiration/washing cycles, the transporter 474 returns the first and second carriers 12 back to the starting position, again locating the first column of the first carrier 12 in line beneath the pipette subassembly 488. The first and second carriers 12 then advance past the pipette subassembly 488 again, one column at a time, for a final aspiration cycle. In this final cycle of the washing operation, the pipette subassembly 488 lowers and raised for each column (except columns C3 and C9) only to aspirate fluid from each well 18. Unlike previous cycles, no washing fluid is added during this final cycle of the washing sequence.

It should be appreciated that the washing/aspiration sequence just described can be varied for each processing sector (carrier column) . For example, different volumes of washing solution can be introduced into different processing sectors. Alternatively, one or more processing sectors can be skipped entirely, as is done for columns C3 and C9 of the ALT assay. In another variation, after washing, an aspiration cycle can be skipped, thereby lengthening the separation time.

After undergoing the above-described washing sequence, the transporter 474 returns the second (rear) carrier 12 (engaged by the rear grab arm 486) into a pick up position on the cutout portion 94 of the washer bed 472. An appropriate "Not Busy" signal is sent in response to the next status inquiry of the control module 16, which in turn issues a command signal to move the shuttle member 20 to pick up the second carrier 12 in the manner previously described. This carrier 12 is transported to the next work station according to the protocol provided.

In the illustrated embodiment, this carrier 12 is returned by the shuttle member 20 to the reagent dispensing station 26. There, the enzyme label is added to designated processing sectors on the carrier 12 in the manner previously described. The carrier 12 is then returned by the shuttle member 20 to an incubation station 28A to I in the manner previously described. After undergoing incubation for the second time, the carrier 12 is returned by the shuttle member 20 back to the washing station 30 for a second washing sequence, which is identical with the first described sequence. In this processing sequence, the carrier 12 that is to undergo a second washing sequence ultimately becomes the first (front) carrier 12 engaged by the front grab arm 484. Immediately after the removal of the second

(rear) carrier 12 from the washing station 30, and while this carrier 12 is processed by the reagent dispensing station 26 and incubation station 28, the first (front) carrier 12 then engaged by the transporter 474 is transported to the substrate dispensing station 32 for the delivery of a substrate.

In the illustrated embodiment (as best shown in Fig. 1) , the substrate dispensing station 32 shares the same bed 472 as the washing station 30, and thereby shares the same hub access 31. For this reason, in the illustrated embodiment, the first (front) carrier 12 is not removed from the bed 472 after the aspiration/washing sequence, but remains there for subsequent transport directly to the substrate dispensing station 32. This sequence will be described in greater detail later.

It should be appreciated, however, that in a different arrangement, the substrate dispensing station 32 could be at a different hub access site. In this arrangement, the transporter 474 would advance the first (front) carrier 12 engaged by the front grab arm 484 to the cutout portion 94 of the bed 472 for pick-up by the shuttle member 20 and transport to the substrate dispensing station 32, or whatever work station was next prescribed by the protocol.

1. Addition of Material to Reduce surface Tension/Affinity

During the washing process for each well 18 containing paramagnetic particles 422, it is desirable to aspirate from each test well 18 as much unbound components suspended in as much fluid as possible, while also minimizing the loss of the mobile paramagnetic particles 422 suspended in the well 18. Loss of these particles 422 reduces the signal strength of ultimate reading, causing inaccurate and invalid results. The aspiration process is most efficiently accomplished by drawing fluid from the regions of the well 18 containing the greatest relative fluid volumes. To avoid loss of the paramagnetic particles 422, it is desirable to collect and concentrate the particles 422 within the well 18 away from the region or regions where the aspiration process is taking place.

It has been observed that the surface tension of the wash fluid in which the particles 422 are suspended during the washing process creates a meniscus 600 within the well 18 that assumes a general convex shape (see Fig. 17) . In this convex shape, larger volumes of fluid are contained in the center region of the well 18 than at the side regions. This convex shape favors placement of the aspiration probe 496 in the center region of the well 18. However, in according to the present invention, the center region of the well 18 also offers the most effective site for the collection and concentration of the particles 422. Also, the surface tension can tend to draw the particles 422 into the outgoing aspiration stream.

It has also been observed that the material from which the particles 422 are made can exhibit an affinity for, or attraction to, the material from which the carrier body is made. Due to this mutual affinity, the particles 422 tend to be distributed generally uniformly along the interior surface of the test wells 18. Because of this mutual affinity, the particles 422 can resist efforts made to disturb their general uniform distribution along the sidewalls. This affinity further complicates the task of collecting and concentrating the particles 422 into a smaller, more compact groups away from the aspiration probe. According to one aspect of the invention, the washing fluid introduced into the wells 18 through the washing probe 498 includes a preselected quantity of a material that reduces the surface tension of the fluid. The additive material is introduced in sufficient quantities to alter the meniscus 600 of the fluid within the well 18 from its naturally convex shape into a more concave shape (see Fig. 18) in which relatively more fluid volume is distributed away from the center region of the well 18 and more toward the sides of the well 18. This concave shape of the meniscus 600 favors the placement of the aspiration probe 496 in the side regions of the well 18, thereby freeing the center region of the well 18 for the collection and concentration of particles 422 away from the aspiration probe 496.

As used herein, the meniscus 600 has a "convex shape" when the contact angle alpha between the slope of the liquid and the sidewall of the test well 18 (see Fig. 17) is above about 90 degrees. The meniscus 600 has a "concave shape" when the contact angle alpha is below about 90 degrees (see Fig. 18) .

It has also been observed that the additive material also reduces the mutual affinity between the particles 422 and the interior surface of the wells 18. The tendency of the particles 422 to uniformly distribute themselves within the well 18 is thereby reduced, thereby further simplifying efforts to collect and nonuniformly concentrate the particles 422 away from the aspiration stream. Due to the reduced surface tension, it is also easier to remove the wash fluid from the well 18 without carrying with it the particles 422 sought to be left behind.

In the illustrated embodiment, which uses polystyrene-based materials and saline wash solutions, the material comprises a liquid surfactant having the attributes of Tween-20. In the illustrated embodiment, the liquid surfactant is contained in the wash fluid in a concentration of no more than about .05%.

It can be seen that the placement of the aspiration probe 496 in the illustrated and preferred embodiment (see Fig. 18) , offset from the center of the well 18 and toward these peripheral regions, takes advantage of the reduced surface tension that results from the addition of the surfactant during the washing sequence. The offset aspiration probe 496 draws fluid from the well 18 at these peripheral regions of the meniscus 600 where relatively increased volumes occur.

2. Creation of a Dynamic Magnetic Field at Washing Station According to another aspect of the invention (refer now to Figs. 19 to 22), the washing station 30 includes means for creating a dynamic magnetic field 602 that attracts the paramagnetic particles 422 toward a common preselected region of the well 18. The Specification uses the term "dynamic" to indicate that the direction and intensity of magnetic gradient to which the particles 422 are exposed varies as a function of time. The changing magnetic gradient created by the magnetic field 602 has two predominant components (as Fig. 21 shows) . One predominant component 604 constitutes the reach of the gradient generally perpendicular to the origin of the field 602. In the illustrated embodiment, the magnetic field 602 lies in a horizontal plane, and the perpendicular component 604 extends vertically from the field 602. The vertical reach component 604 attracts the particles 422 that are spaced away from the origin of the field 602, drawing them generally vertically down closer toward the origin.

The other predominant component 606 (still referring to Fig. 21) constitutes the reach of the gradient generally radially toward the center of the field 602, where the magnetic filed strength is largest. The radial reach component 606 affects the particles 422 that have been attracted closer to the origin by the vertical reach component 604, grouping these particles 422 even closer toward the center of magnetic field 602.

According to the invention, the intensity and direction of the magnetic field 602 is varied so that exposure of the particles 422 is alternated over time between the vertical reach component 604 and the radial reach component 606. The particles 422 are thus successively pulled down toward the origin of the field 602, where they are further grouped toward the magnetic center of the field 602. In the preferred embodiment, the preselected region in the well 18 toward which the particles 422 are ultimately grouped by the dynamic magnetic field 602 is spaced away from the region in the well 18 where the aspiration probe 496 is inserted.

As Figs. 19, 20, and 22 show, the washing station 30 also preferably includes means for creating a second dynamic magnetic field 608, different from the first described field 602, to further group the particles 422 collected within the preselected region into a more precisely shaped pattern which pulls the peripheral edge of the grouped particles 422 even farther away from the region where the aspiration probe 496 is inserted. The term "dynamic" is again used to indicate that the direction and intensity of magnetic gradient to which the particles 422 are exposed varies as a function of time.

The second magnetic field 608 is varied to concentrate the exposure of the particles 422 over time principally to a focused radial reach component 610. Repeated exposure to this focused radial gradient creates the desired pattern of the particles 422 within the preselected region.

In the illustrated and preferred embodiment, the aspiration probe 496 draws fluid from the peripheral regions of the well 18, where relatively larger volumes of fluid collect due to the addition of the surfactant. In this arrangement, the preselected region of particle grouping and shaping is preferably located generally toward the center region of the well 18. Relatively smaller volumes of fluid remain in this center region.

The means for creating the dynamic magnetic fields 602 and 608 can vary. In the illustrated embodiment, the means includes a stationary magnetic field 612 (best shown in Figs. 19 and 20) that extends beneath the bed 472 of the washing station 30.

As Figs. 19 and 20 show, the magnetic field 612 is arranged in differing predetermined patterns to create three regions 614/616/618. Two regions 614 and 618 are located on either side of the pipette subassembly 488. The third region 616 is located beneath the pipette subassembly 488.

The means also includes control means for actuating the transporter 474 to move the carrier 12 in a predetermined fashion with respect to the magnetic fields beneath the bed 472. The particles 422 carried within the wells 18 are exposed to the two dynamic magnetic fields by moving the carrier 12 with respect to the differing magnetic field regions 614/616/618 established on the bed 472.

In the illustrated embodiment, the magnetic field is comprised of an array of individual permanent rare earth magnets 620 (see Figs. 21 and 22) . Each magnet 620 is generally disk-shaped, having a north magnetic pole face (N) and an opposite south magnetic pole face (S) . The magnets 620 are arranged in an array with one magnetic pole face facing generally perpendicularly toward the bed 472.

According to this aspect of the invention, the array arranges the magnetic pole faces beneath the bed in columns .and rows spaced to align with the spacing of columns and rows of test wells 18 on the carrier 12; that is, the distance between the centers of the magnetic pole faces of the array correspond to the distance between the centers of the wells 18 on the carrier 12.

In this array, the diameter of the pole face of each magnet 620 is generally equal to or slightly larger than the diameter of an individual test well 18. In the illustrated embodiment, each test well 18 has a diameter of about 0.267 inch, and the magnetic pole faces each have a diameter of about 0.330 inch.

The transporter 474 initially positions the carrier 12 upon the bed 472 of the washing station 30 with the center of each well 18 registering with a center of one underlying magnetic pole face (see Fig. 23) . In the illustrated and preferred embodiment, the wall forming the well 18 creates an interior bowl-shaped interior wall 621 and a generally planar (or flat) exterior bottom wall 622. The planar exterior wall 622 sits flat upon the bed 472 in registration with the underlying magnetic pole face. The cross sectional dimension between the interior wall 621 and exterior wall 622 is also preferable formed with a reduced thickness (compared to the wall thickness for the remainder of the well 18) to maximize as much as possible the passage of the magnetic field emanating from the underlying magnetic pole face into the interior regions of the well 18.

As best shown in Fig. 20, the pole faces located in the two end regions 614/618 of the stationary magnetic field 612 are arranged in the same fashion. In these regions 614/618, the pole faces of the magnets 620 are arranged in columns having a pair of individual like poles situated next to another pair of individual like poles, with the two pairs of poles being unlike (as Fig. 21 also shows) . This arrangement creates the alternating pattern along each row (that is, along the path of movement of the carrier 12) of complementary magnetic fields (between two unlike poles) and uncomplimentary magnetic fields (between two like poles) , as follows:

N N S S N N S S N N S S, and so on. The pattern of these two regions can be called "asymmetric" because a given pole face within each column is bounded on one side by a like or same pole face (creating a uncomplimentary magnetic field) and on the other side by an unlike or opposite pole (creating a complementary magnetic ield) . As shown in Fig. 21, in the region spanning two like pole faces, the complementary magnetic field consists principally of the vertical reach component 604. By placing two like pole faces together in close proximity, the vertical reach of the combined uncomplimentary field is amplified, compared with the vertical reach of each pole face taken individually.

Also as shown in Fig. 21, in the region spanning two unlike pole faces, the complementary magnetic field consists principally of the radial reach component 606. By placing two unlike pole faces together in close proximity, the radial reach of the complementary field is focused, extending between the centers of the unlike pole faces in the direction from the N pole face toward the S pole face.

The pole faces of the stationary magnetic field

612 in the intermediate region 616 are arranged in columns alternating individual like and unlike poles (see Figs. 20 and 22). This arrangement creates the pattern between each column (along the path of movement of the carrier 12) of complementary magnetic fields, as follows:

N S N S N S N S, and so on. The pattern of the intermediate region 616 will be called "symmetrical" because a given pole face within each column has an unlike or opposite pole in the next adjacent row on both sides.

In the illustrated embodiment (as Fig. 20 shows) , the intermediate region 616 is located in the region of the pipette subassembly 488 of the washing station 30. As shown, the intermediate region 616 spans three columns, one column located directly beneath the pipette subassembly 488 and one column proceeding and one column following this center column. The column located directly under the pipette subassembly 488 has magnets 620C with pole faces of reduced diameter less than the diameter of the well 18. The reduced diameter of the magnets 620C relative to the diameter of the well 18 more narrowly focuses the radial reach of the gradient with respect to the center of the well 18.

According to this aspect of the invention, the dynamic nature of the magnetic fields 602 and 608 is created by moving the carrier 12 relative to the magnetic fields 602 and 608, using the transporter 474, in a reciprocating, or racking, motion on the bed. During the first cycle of this reciprocating motion, the transporter 474 moves the center of each well 18 out of registry with the center of its starting "home" pole face and one-half the distance toward the center of one immediately adjacent pole face; and then in a second cycle back in the opposite direction through the center of the home pole face and one-half the distance toward the center of the other immediately adjacent pole face

(as Figs. 23, 24, and 25 show); and then back to into registry with the center of the home pole face (as- Fig. 26 shows) . The first and second cycles repeat for a predetermined number of times, at which time a third cycle occurs to move the well 18 to the next adjacent pole face, which becomes the new home pole face (as Fig. 27 shows) . The first and second cycles repeat, this time relative to the new home pole face, after which a third cycle advances the test well 18 to the next adjacent pole face, which become the new home pole face.

When the well 18 is located within one of the asymmetrical magnetic regions 614/618 (on either side of the pipette subassembly 488) , the racking motion moves the well 18 sequentially through an uncomplimentary magnetic field and through a complementary magnetic field (see Figs. 28 to 34) . The order of the sequence of course depends upon the initial home position of the well 18.

Movement of the well 18 through the uncomplimentary field moves the paramagnetic particles 422 present in the well 18 with respect to a magnetic gradient that has an amplified vertical reach component 604 (due to closeness of the adjacent like poles) . Movement through this gradient (which can also be called a "pulling gradient") attracts the paramagnetic particles 422 in suspension within the well 18 down toward the origin of the magnetic field (as shown by the movement of well W2 in Figs. 28 and 29 and by the movement if well Wl in Figs. 30 and 31). Since the particles 422 are paramagnetic (having magnetic properties that are induced by the magnetic field itself) , the pulling gradient 604 is always attractive in a downward direction toward the origin of the magnetic field (i.e., toward the magnets 620 beneath the well 18) , whether the uncomplimentary filed is between adjacent N-N pole pairs or between adjacent S-S pole pairs.

Movement of the well 18 through the complementary field moves the paramagnetic particles 422 present in the well 18 with respect to a magnetic gradient that has a relatively focused radial reach component 606 (due to the closeness of the adjacent unlike poles) . Movement through this gradient (which can also be called a "shaping gradient") attracts the paramagnetic particles 422 in suspension within the well 18 radially between the centers of the adjacent magnetic fields in the direction from the N pole face toward the S pole face (as shown by the movement of well Wl in Figs. 28 and 29 and by the movement of well W2 in Figs. 30 and 31) .

As a result of this reciprocating movement through region 602, the particles 422 are drawn into a generally circular bunch the center region of the well 18 (as Figs. 35 and 36 show) . When the well 18 is located within the intermediate symmetrical magnetic region 616 beneath the pipette subassembly 488, the well 18 is sequentially moved during each leg of the reciprocating cycle through two complementary magnetic fields. As before described (as shown in Figs. 37 to 39) , movement of the well 18 through the complementary field moves the paramagnetic particles 422 present in the well 18 with respect to two magnetic gradient each having a focused radial reach component 606 (due to the closeness of the adjacent unlike poles) . Because the pole face 62OC directly below the pipette subassembly 488 has a reduced diameter, the shaping gradients are even more narrowly focused toward the center of the well 18 than the shaping gradient 606 in the end regions 614 and 618. These more focused radial gradients 606 in the intermediate region 616 have the effect, as the well 18 moves with respect to the gradients, of "stretching" the paramagnetic particles 422 out along the radial gradients (as Fig. 40 shows) . The paramagnetic particles 422, which have already been assembled into a generally circular shape by the first shaping gradient of the preceding end region 614 (as Fig. 36 shows) , are further rearranged by the second dynamic field into more elongated, "cigar-shaped" shape extending in the direction between the unlike pole faces (as Fig. 40 shows) . This shape is longer in the direction of the radial gradient between the adjacent N pole face and S pole face than the other transverse dimension.

In the illustrated and preferred embodiment (as Fig. 40 shows) , the aspiration probe 496 is inserted into the well 18 along a sidewall that is generally perpendicular to the elongated dimension of the shaped particles 422, with the beveled tip 497 of the probe 496 facing the sidewall (as Fig. 15 shows) . In this way, the distance between the tip 497 of the aspiration probe 496 and the periphery of the shaped particles 422 is maximized to the fullest extent possible to minimize particle loss during aspiration.

According to this preferred aspect of invention, the control mechanism of the washing station 30 operates the transporter 474 to advance the carrier 12 or carriers 12 to the pipette subassembly 488 one column at a time in the manner previously described. Following aspiration and the additional of washing fluid by the pipette subassembly 488, the transporter 474 "racks" the carrier 12 a preselected number of cycles to dynamically expose the particles 422 in the wells 18 to the components 604 and 606 of the magnetic regions 614/616/618, depending upon their relative position upon the bed 472. The carrier 12 then advances one column, and the above sequence repeats itself until the final aspiration- only cycle, during which no racking preferably occurs.

It should be appreciated the dynamic magnetic fields 614/616/618 can be created in alternate ways. For example the field having two magnetic regions can itself be moved in a reciprocating path with respect to the carrier 12 (which in this arrangement would be held stationary) .

In another alternate embodiment (shown in Fig. 41) , the means for creating a magnetic field comprises an electromagnetic field 624 located along the plane of the bed 472. In this arrangement, means 626 is provided for modulating the electromagnetic field 624 with respect to the paramagnetic particles 422 to alternatively create the first magnetic gradient 604 and the second magnetic gradient 606. In this arrangement, the carrier 12 itself would remain stationary.

It has also been observed that the addition of surfactant (as above described) in addition to the use of the dynamic magnetic field lead to optimizing the washing process in terms of minimizing the loss of paramagnetic particles 422.

In the illustrated embodiment (see Fig. 1) , where the substrate dispensing station 32 shares the same bed 472 and hub access 31 as the washing station 30, the second (rear) carrier 12 engaged by rear grab arm 486 (which is the last carrier 12 delivered to the washing station 30) is the one that is undergoing the washing sequence for the first time, after which it is picked up by the shuttle member 20 for delivery to the reagent addition station. The first (front) carrier 12 engaged by the front grab arm 484 (which is the first carrier 12 delivered to the washing station 30) is one that is undergoing the washing sequence for the second time, after which it is to be transported directly to the substrate dispensing station 32 without further use of the shuttle member 20. The substrate dispensing station 32 includes a substrate pipette subassembly 506 to accomplish the addition of substrate and a separate substrate aspiration subassembly 508 to accomplish the aspiration of the substrate. The substrate pipette subassembly 506 is arranged along a boom 510 that spans the bed 472 transversely across the path of movement of the carrier 12. In the illustrated embodiment, the substrate pipette subassembly 506 is position in the path of carrier 12 movement of the bed 472 in front of the washing pipette subassembly 488, as already described. A pump (not shown) conveys substrate to the pipette subassembly 506.

In the illustrated embodiment, the magnetic fields 614/616/618 associated with the washing station 30 begins one carrier column spacing beyond the substrate pipette subassembly 506.

The substrate dispensing station 32 adds substrate simultaneously to all the samples located within a designated test sector and, in this arrangement, sequentially upon each test sector in turn. The transporter 474 therefore advances the carrier 12 along the bed 472 beneath the substrate pipette subassembly 506 one entire test sector at a time. The pipette subassembly includes a series of substrate probes 514 (see Fig. 1) equal in number to the maximum number of test wells 18 within a given test sector defined on the carrier 12. In the illustrated embodiment, the substrate pipette subassembly 506 therefore includes a eight probes

514 spaced apart a distance equal to the spacing of the wells 18 within a test column on the carrier 12.

A stepper motor 516 actuated by the control mechanism serves to raise and lower the substrate pipette subassembly 506 as a unit into and out of the wells 18 of the test sector positioned beneath it.

In the illustrated embodiment, the aspiration subassembly 508 for the substrate dispensing station 32 employs the same aspiration probes 496 used by the washing pipette subassembly 488. It should be appreciated, however, that different arrangements could be used. For example, the substrate pipette subassembly 506 and aspiration subassembly 508 could be positioned side-by-side or back-to-back upon the same boom 510.

After the second (rear) carrier 12 (engaged by the rear grab arm 486) has been removed by the shuttle member 20, the control mechanism operates the transporter 474 to advance the first carrier 12 beneath the substrate pipette subassembly 506, one test sector (i.e., column) at a time. When a given test column advances into position beneath the substrate pipette subassembly 506, the transporter 474 stops, and the stepper motor 516 operates to lower the pipette subassembly 506 as a unit into each associated well 18 and dispense the desired amount of substrate.

The stepper motor 516 then raises the substrate pipette subassembly 506 out of the wells 18, and the transporter 474 advances the carrier 12 the distance between adjacent columns to place the next designated column beneath the pipette subassembly 506. This sequence, as just described, repeats itself until substrate has been added to the well 18 in all columns on the carrier 12 that are to receive substrate.

The transporter 474 then advances the carrier 12 to the aspiration subassembly 508 (which comprises the aspiration probes 496 of the washing pipette subassembly 488) , positioning the first column of the carrier 12 in line beneath the subassembly 508. The aspiration-only cycle of the washing sequence repeats to draw substrate fluid from the wells 18 through the previously described aspiration probe 496.

In a preferred sequence, the transporter 474 also serves to rack (that is, reciprocate back and forth) the carrier 12 for a predetermined period in the manner previously described prior to the aspiration-only cycle of the substrate addition sequence. Also in a preferred sequence, the carrier 12 is racked again after each column is aspirated during the substrate addition sequence. Following the aspiration-only cycle of the sequence, the transporter 474 advances the carrier 12 one column at a time back through the substrate pipette subassembly 506 for the addition of another aliquot of substrate into each well 18. Preferably, in the interest of saving time, the carrier 12 moves back through in the substrate pipette subassembly 506 in an opposite direction (that is, from right to left in Fig. 1) .

The above sequence of adding then aspirating wash solution (during the washing sequence) , then adding and aspirating substrate, followed by the final addition of substrate (during the substrate addition cycle) in effect constitutes a buffer exchange, during which residual wash buffer is exchanged for substrate buffer.

It should be appreciated that the substrate addition/aspiration sequence just described can be varied for each processing sector (carrier column) . For example, different volumes of substrate can be introduced into different processing sectors. Alternatively, one or more processing sectors can be skipped entirely.

After the carrier 12 advanced through the substrate pipette subassembly 506 for a second time, the transporter 474 returns the carrier 12 to a pick-up position on the cutout portion 94 of the bed 472. A "Not Busy" signal is sent in response to the next status inquiry of the control module 16, which in turn issues a command signal to move the shuttle member 20 to pick up the carrier 12 in the manner previously described. This carrier 12 is transported to the next work station according to the protocol provided, which in the illustrated embodiment is the reader station 34. At the reader station 34, the fluorescence of the particles is measured at two different times (separated by an incubation period) to derive the quantitative result of the assay.

Finally, the carrier 12 is transported to the carrier disposal station 36, where it is deposited in a waste container 582.

Figs. 42 to 54 show an alternative embodiment of a washing/aspiration system 700 that incorporates addition aspects of the invention. In many respects, the system 700 shares features with the system 11 previously described.

For example, like the system 11, the system 700 includes a support bed 702 for receiving and supporting the test carrier 12. The bed 702 includes a cutout portion 704 to allow the shuttle platform 70 to drop off and pick up the carrier 12 in the manner previously described.

Also like the system 11, the system 700 includes a transporter 706 for moving the carrier 12 along the bed 702. The transporter 706 is carried by a belt 708 that extends between a drive pulley 710 and an idler pulley 712. A stepper motor 714 rotates the drive pulley 710 to advance the transporter 706 in opposite linear directions (either to the left or to the right) along the bed 702.

As in the system 11, the transporter 706 includes two oppositely facing grab arms 716 and

718. Each grab arm 716 and 718 is specifically configured to capture a bottom edge of one of the carrier sidewalls 80A or B. The transporter 706 can thereby accommodate two carriers 12 at a time, one in front (engaged by the first grab arm 716) and one behind (engaged by the second grab arm 718) . The sequence of receiving the two carriers 12 for transport is the same as previously described with respect to system 11.

Like system 11, the system 700 includes a washing station 720 and a substrate dispensing station 722. Each station 720 and 722 includes its own pipette subassembly: one 724 for conducting aspiration and washing, and the other 726 for adding substrate. These pipette subassemblies 724 and 726 differ from those shown for system 11.

Please refer to Figs. 45 to 49, which show the details of the pipette subassembly 726 for the substrate dispensing station 722. As there shown, the pipette subassembly 726 includes an array of individual syringe pumps 728 equal in number to the number of samples located within a designated test sector on the carrier 12. In the illustrated embodiment, each designated test sector includes eight wells (that is, RI to R8 in each column) , so the illustrated array includes eight individual syringe pumps 728. Each syringe pump 728 includes a pump chamber 732 having an outlet 736 and a pump piston 734 movable within the chamber 732 toward and away from the outlet 736. Movement of the pump piston 734 away from the outlet 736 draws fluid into the associated chamber 732, while movement of the pump piston 734 toward the outlet 736 expels fluid from the associated chamber 732.

An actuator bar 738 joins each pump piston 734. The actuator bar 738 is carried on an axial screw 740. A stepper motor 742 rotates the axial screw 740 to advance the actuator bar 738 either axially up or down. The actuator bar 738 thereby moves the attached pump pistons 734 in tandem away from or toward the chamber outlets 736 to simultaneously draw fluid into all pump chambers 732 or to simultaneously expel fluid from all pump chambers 732.

The substrate pipette subassembly 726 also includes a number of fluid dispensing nozzles 744. The number of nozzles 744 can vary. In the illustrated embodiment, three dispensing nozzles 744 A, B, and C are shown. The dispensing nozzles 744 A/B/C conduct fluids into a dispensing well 746 located beneath the outlets 736 of the ganged syringe pumps 728.

In the illustrated and preferred embodiment, two of the dispensing nozzles 744 B/C also serve as a part of a liquid level sensing circuit 772 for the dispensing well 746. The two nozzles 744 B/C form a part of an electrical series circuit 772 that conducts electricity when the nozzles 744 B/C are in contact with fluid.

The dispensing well 746 is movable between a recessed position within the bed 702 (shown in Figs. 45 and 47) and an uplifted position above the bed 702 (shown in Figs. 46 and 48) . When in the recessed position, the dispensing well 746 is generally flush with the plane of the bed 702 (see Fig. 47) , allowing the test carrier 12 to travel over it (as Fig. 49 shows) . When in the uplifted position (see Fig. 48) , the dispensing well 746 encloses the tips of fluid dispensing nozzles 744 and syringe pump outlets 736 and is positioned to convey or receive fluid to and from them. Various mechanisms can be used to move the dispensing well 746. In the illustrated embodiment (shown in Figs. 47 and 48) , the bottom of the dispensing well 746 is attached to an axial screw 748. A stepper motor 750 rotates the axial screw 748 to advance the dispensing well 746 either axially up or down between its recessed and uplifted positions.

An electrical resistance heater 752 surrounds the fluid dispensing well 746. An associated thermistor (not shown) maintains the heater 752 at a desired temperature to heat the fluid contents of the well 746. In the illustrated embodiment, the desired temperature is between 30 and 45 degrees centigrade and is preferably about 42 degrees centigrade.

Before it is time to dispense substrate, an appropriate control signal raises the dispensing well 746 to its upraised position (as Figs. 46 anά 48 show) . A pump 754 (see Fig. 42) delivers substrate through the nozzles 774 into the well 746. The conduction of electricity between the two nozzles 774 A/B generates a signal that terminates the operation of the pump 754. The sensing circuit 772 assures that the nozzles 774 consistently convey the desired aliquot of substrate into the well 746. Once filled, the well 746 then lowers to its recessed position, while the heater 752 heats the delivered substrate to the desired temperature.

When it is time to dispense substrate, an appropriate control signal again raises the dispensing well 746 to its upraised position. This time, the stepper motor 742 is actuated to draw a desired aliquot of heated substrate into each pump chamber 732. Because a stepper motor 742 is used, the movement of the pistons 734 can be closely controlled to draw precise aliquots.

The well 746 is again lowered to its recessed position. The transporter 706 advances the carrier 12 along the bed 702 beneath the pump outlets 736, one entire test sector at a time, as Fig. 49 shows. The stepper motor 742 is sequentially actuated to expel a desired amount of heated substrate into each well 18. Because a stepper motor 742 is used, the movement of the pistons 734 can be closely controlled to expel equal, precise aliquots into the wells 18.

Please refer now to Figs. 50 to 51, which show the details of the pipette assembly 724 for the washing station 720. As there shown, the pipette subassembly 724 includes an array of individual probe sets 756 equal in number to the number of samples located within a designated test sector on the carrier 12. In the illustrated embodiment, the array includes eight probe sets 756. Like the system 11, the probe sets 756 are carried by a boom 758 which a stepper motor 760 raises and lowers as a unit relative to the carrier 12 positioned beneath them.

Unlike the system 11, each probe set 756 includes three probes 762/764/766 (see Fig. 51) . The probes 762/764/766 are arranged in a linear fashion one next to the other. In use, the two end probes 762 and 766 aspirate fluid from the associated test well 18. The middle probe 764 adds the washing liquid to the contents of the well 18. As can be seen in Fig. 51, the two aspiration probes 762 and 766 extend an equal distance below the washing probe 764.

As Fig. 51 also shows, the two aspiration probes 762 and 766 are spaced apart a predetermined distance with respect to the dimensions of the associated well 18. When lowered into the associate well 18, the tips of the two aspiration probes 762 and 766 are in close proximity to sidewall of the well 18 where the curvature of the bottom of the well 18 begins. The washing probe 764 is located equidistant between the two aspiration probes 762 and 766.

When the tips of the aspiration probes 762 and 766 are positioned in close proximity to the well sidewall, the washing probe 764 extends into the well 18 about half the distance that the aspiration probes 762 and 766 extend into the well 18.

This arrangement assures that, in use, the washing probe 764 of each probe set 756 is centered with respect to the well 18.

As Figs. 52 to 54 show, the pipette subassembly 724 first lowers enough to position the longer aspiration probes 762 and 766 within the associated well 18 near the point where the curvature of the well begins. At this depth, the ends of the aspiration probes 762 and 766 enter the fluid within the well 18, while the washing probe 764 is outside the fluid. A vacuum pump continuously applies a negative pressure simultaneously to the aspiration probes 762 and 766 to draw fluid from the wells 18 through tube 774.

The stepper motor 760 then raises the pipette subassembly 724 so that the shorter wash probes 764 are positioned generally at the top of each well 18. A wash pump 768 delivers a desired aliquot of washing fluid simultaneously through each wash probe 764 into the wells 18 through a fluid manifold assembly 776. As in system 11, the transporter 706 advances the carrier 12 stepwise one column at a time beneath the pipette subassembly 726 through the appropriate aspiration/washing cycles, as already described with respect to the system 11. In the illustrated and preferred embodiment, the system 700 includes an auxiliary well 770 for the washing pipette subassembly 724. The auxiliary well 770 extends across the bed 702 and is carried by the transporter 706 between the two grab arms 716 and 718. During rest periods, when no washing or _- 5 _ςn0 --

substrate dispensing functions are being performed, the transporter 706 moves the auxiliary well 770 beneath the pipette subassembly 724. The subassembly 724 lowers to place the probe sets 756 into the well. A desired aliquot of washing fluid is conveyed through the wash probes 764 into the well 770. The probe sets 756 remain soaking in the washing fluid during rest periods to keep them open and free of particulates between periods of use. The advantages and benefits of the various aspects of the invention just described are set forth in the following claims.

Claims

CLAIMSWe claim:

1. A system for concentrating paramagnetic particles in suspension comprising a surface for supporting the suspension of paramagnetic particles in a generally horizontal plane, means for creating a magnetic field along the plane of the surface including a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the origin of the gradient and a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient, and means for alternating the exposure of the suspended paramagnetic particles over time between the first magnetic gradient and the second magnetic gradient.

2. A system according to claim 1 wherein the means for creating a magnetic field comprises a stationary field located along the plane of the bed including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates, and wherein the alternating means includes means for moving the suspended paramagnetic particles between the two regions of the stationary field.

3. A system according to claim 2 wherein the magnetic field comprises a first region adjacent to a second region, and wherein the alternating means includes means for moving the suspended paramagnetic particles in a reciprocating path between the adjacent first and second regions.

4. A system according to claim 3 wherein the stationary field comprises an array of alternating first and second regions.

5. A system according to claim 1 wherein the means for creating a magnetic field comprises a movable field located along the plane of the bed including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates, and wherein the alternating means includes means for moving the magnetic field with respect to the paramagnetic particles between the two regions.

6. A system according to claim 5 wherein the magnetic field comprises a first region adjacent to the second region, and wherein the alternating means includes means for moving the magnetic field in a reciprocating path between adjacent first and second regions.

7. A system according to claim 6 wherein the stationary field comprises an array of alternating first and second regions.

8. A system according to claim 1 wherein the means for creating a magnetic field comprises an electromagnetic field located along the plane of the bed, and wherein the alternating means includes means for modulating the electromagnetic field with respect to the paramagnetic particles to alternatively create the first magnetic gradient and the second magnetic gradient.

9. A system according to claim 2 or 5 wherein the magnetic field includes an array of magnets each having a north magnetic pole face (N pole face) and a south magnetic pole face (S pole face) , and wherein the first region includes a first group of magnets having either all N pole faces or all S pole faces directed toward the support surface, and wherein the second region includes a second group of magnets having poles ends that are directed toward the support surface and that all have the same N or S pole face different than the pole faces of the first group.

10. A system according to claim 9 wherein the magnetic field includes a first group of magnets adjacent to a second group of magnets.

11. A system according to claim 9 wherein the magnetic field includes an array of alternating first groups of magnets and second groups of magnets.

12. A system according to claim 9 wherein the first group of magnetics comprises two adjacent magnetic pole faces.

13. A system according to claim 12 wherein the second group of magnets comprises two adjacent magnetic pole faces.

14. A system according to claim 13 wherein the magnetic field includes an array of alternating first groups of magnets and second groups of magnets.

15. A system for concentrating paramagnetic particles in suspension comprising a surface for supporting the suspension of paramagnetic particles in a generally horizontal plane, means for creating a magnetic field along the plane of the surface including an array of magnets each having a north magnetic pole face (N pole face) and a south magnetic pole face (S pole face) , the magnetic field comprising a first region includes a first group of two adjacent magnets both having either a N pole face or S pole face directed toward the support surface, and a second region adjacent to the first region and including a second group of two adjacent magnets having poles ends that are directed toward the support surface and that both have the same N or S pole face different than the pole faces of the first group, and means for alternating the exposure of the suspended paramagnetic particles over time between the first region and the second region by affective relative movement between the suspended paramagnetic particles and the magnetic field in a reciprocating path in a first cycle between two adjacent magnetic pole faces in one direction, then in a second cycle between two adjacent magnetic pole faces in the opposite direction.

16. A system according to claim 15 wherein the alternating means includes means for affecting relative movement between the paramagnetic particles and the magnetic field in a third cycle, following the second cycle, advancing the paramagnetic particles in either one direction or the other direction to the next adjacent magnetic pole face.

17. A system according to claim 16 wherein the alternating means repeats the sequence of the first cycle and the second cycle for a predetermined number of them, and proceeds to the third cycle at least twice in succession.

18. A system according to claim 15 wherein the magnetic field is stationary, and wherein the alternating means includes means for moving the paramagnetic particles relative to the stationary magnetic field.

19. A system according to claim 15 wherein the magnetic field is movable, and wherein the alternating means includes means for moving the magnetic field relative to the paramagnetic particles.

20. A system for concentrating paramagnetic particles in suspension comprising a surface for supporting the suspension of paramagnetic particles in a generally horizontal plane, means for creating a first magnetic field along the plane of the surface including a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the origin of the gradient and a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient, means for creating a second magnetic field along the plane of the surface adjacent to the first magnetic field having a magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient, means for advancing the paramagnetic particles across the first magnetic field while alternating the exposure of the suspended paramagnetic particles over time between the first magnetic gradient and the second magnetic gradient of the first magnetic field, and means for advancing the paramagnetic particles from the first magnetic field to the second magnetic field.

21. A system for removing undesired materials from a suspension of paramagnetic particles comprising a surface for supporting the suspension of paramagnetic particles in a generally horizontal plane, pipetting means mounted relative to the surface for aspirating fluid from the suspension of paramagnetic particles, transport means on the surface for moving the suspension into operative alignment with the pipetting means, means for creating a magnetic field along the plane of the surface in the path of movement of the transport means, the created magnetic field including a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the origin of the gradient and a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient, and means for alternating the exposure of the suspended paramagnetic particles over time between the first magnetic gradient and the second magnetic gradient while the suspension is being moved toward the pipetting means.

22. A system according to claim 21 and further including means for creating a second magnetic field along the plane of the surface in operative alignment with the pipetting means, the second magnetic field having a magnetic gradient having predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient.

23. A washing system for removing nonspecifically bound materials from a fluid suspension retained in a test well, the fluid suspension containing paramagnetic particles having sorbent material serving as the solid phase binding sites for the targeted compound during an analytical procedure, the system comprising means for supporting the test well in a generally horizontal plane, means for creating a magnetic field along the horizonal plane of the surface beneath the test well to attract the paramagnetic particles suspended within the fluid generally toward the center region of the well, and pipetting means for drawing fluid from the side region of the well.

24. A washing system according to claim 23, wherein the means for creating the magnetic field includes means for creating a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the bottom region of the test well, means for creating a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the center region of the test well, and means for alternating the exposure of the suspended paramagnetic particles within the test well over time between the first magnetic gradient and the second magnetic gradient prior to drawing fluid from the well using the pipetting means.

25. A washing system according to claim 24 and further including means for creating a second magnetic field along the plane of the surface beneath the pipetting means, the second magnetic field having a magnetic gradient having predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the center region of the well.

26. A washing system according to claim 24 wherein the means for creating the magnetic field comprises a stationary field located along the plane of the bed including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates, and wherein the alternating means includes means for moving the test well between the two regions of the stationary field.

27. A washing system according to claim 26 wherein the magnetic field comprises a first region adjacent to a second region, and wherein the alternating means includes means for moving the test well in a reciprocating path between the adjacent first and second regions.

28. A washing system according to claim 27 wherein the stationary field comprises an array of alternating first and second regions.

29. A washing system according to claim 26 wherein the magnetic field includes an array of magnets each having a north magnetic pole face (N pole face) and a south magnetic pole face (S pole face) , and wherein the first region includes a first group of magnets having either all N pole faces or all S pole faces directed toward the support surface, and wherein the second region includes a second group of magnets having poles ends that are directed toward the support surface and that all have the same N or S pole face different than the pole faces of the first group.

30. A washing system according to claim 29 wherein the magnetic field includes a first group of magnets adjacent to a second group of magnets.

31. A washing system according to claim 29 wherein the magnetic field includes an array of alternating first groups of magnets and second groups of magnets.

32. A washing system according to claim 29 wherein the first group of magnetics comprises two adjacent magnetic pole faces.

33. A washing system according to claim 32 wherein the second group of magnets comprises two adjacent magnetic pole faces.

34. A washing system according to claim 33 wherein the alternating means alternates th exposure of the test well over time between the first region and the second region by moving the test well in a reciprocating path in a first cycle between two adjacent magnetic pole faces in one direction, then in a second cycle between two adjacent magnetic pole faces in the opposite direction.

35. A washing system according to claim 34 wherein the alternating means includes means for affecting moving the test well in a third cycle, following the second cycle, advancing the paramagnetic particles in either one direction or the other direction to the next adjacent magnetic pole face.

36. A washing system according to claim 35 wherein the alternating means repeats the sequence of the first cycle and the second cycle for a predetermined number of times, and then proceeds to the third cycle at least twice in succession prior to operation of the pipetting means.

37. A washing system according to claim 24 wherein the means for creating a magnetic field comprises a movable field located along the plane of the bed including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates, and wherein the alternating means includes means for moving the magnetic field with respect to the test well between the two regions.

38. A washing system according to claim 24 wherein the means for creating a magnetic field comprises an electromagnetic field located

'•* along the plane of the bed, and 5 wherein the alternating means includes means for modulating the electromagnetic field with respect to the test well to alternatively create the first magnetic gradient and the second magnetic gradient.

39. A test carrier for retaining at least one sample for an analytical procedure using paramagnetic particles as solid phase binding sites for the targeted compound, the test carrier

5 comprising a main body portion, and wall means forming at least one test well in the main body portion having an open top portion and a closed bottom portion, the wall means forming

10 in the closed bottom portion a generally bowl-shaped interior surface for retaining the test sample containing the paramagnetic particles and a generally planar exterior surface for resting against an external magnetic field, the wall means

15 between the interior surface and the exterior surface being free of a material that is significantly effected by the external magnetic field and having a reduced cross sectional dimension, compared to the wall means forming the

20 rest of the test well, for better transmission of the external magnetic field to attract the paramagnetic particles toward the closed bottom portion of the test well.

40. A method of washing nonspecifically bound materials from a fluid suspension containing paramagnetic particles having sorbent material serving as the solid phase binding sites for a targeted compound during an analytical procedure, the method comprising the steps of retaining the fluid suspension in a generally horizontal plane in a well, creating a magnetic field along the horizonal plane beneath the well to attract the paramagnetic particles suspended within the fluid generally toward the center region of the well, and aspirating fluid from a region of the well away from the center region where the paramagnetic particles have been attracted.

41. A method according to claim 40 wherein, in the fluid aspiration step, the fluid is aspirated from a side region of the well.

42. A method according to claim 40 and further including the step, prior to the aspiration step, of introducing a material into the well to reduce the surface tension of the luid suspension.

43. A method according to claim 42 wherein, in the step of introducing a material to reduce the surface tension of the fluid suspension, the material reduces the contact angle between the fluid suspension and the interior surface of the well to below about 90 degrees.

44. A method according to claim 42 wherein, in the step of introducing a material to reduce the surface tension of the fluid suspension, the material alters the distribution of the fluid suspension within the well to distribute more fluid volume toward the sides of the well and away from the center of the well.

45. A method according to claim 44 wherein, in the fluid aspiration step, the fluid is aspirated from a side region of the well.

46. A method of washing nonspecifically bound materials from a fluid suspension containing paramagnetic particles having sorbent material serving as the solid phase binding sites for a targeted compound during an analytical procedure, the method comprising the steps of retaining the fluid suspension in a in a well, introducing a material into the well to reduce the surface tension of the fluid suspension within the well and alter the distribution of the fluid suspension within the well to distribute more fluid volume toward the sides of the well and away from the center of the well, and aspirating fluid from a side region of the well where the larger fluid volumes have been distributed.

47. A method according to claim 46 wherein, in the step of introducing a material to reduce the surface tension of the fluid suspension, the material reduces the contact angle between the fluid suspension and the interior surface of the well to below about 90 degrees.

48. A method for concentrating paramagnetic particles in suspension comprising the steps of retaining the suspension of paramagnetic particles in a generally horizontal plane in a well, generating a magnetic field beneath the well to create a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the origin of the gradient and a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the magnetic center of the gradient, and alternating the exposure of the suspended paramagnetic particles over time between the first magnetic gradient and the second magnetic gradient.

49. A method according to claim 48 wherein the step for generating the magnetic field comprises creating a stationary field located beneath the well including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates , and wherein, in the alternating step, the well retaining the suspended paramagnetic particles is moved between the two regions of the stationary field.

50. A system according to claim 48 wherein the step for generating the magnetic field comprises creating a movable field located beneath the well including a first region where the first magnetic gradient predominates and a second region where the second magnetic gradient predominates, and wherein, in the alternating step, the magnetic field is moved with respect to the well retaining the paramagnetic particles.

51. A system according to claim 48 wherein the step for generating the magnetic field comprises creating an electromagnetic field located beneath the well, and wherein, in the alternating step, the electromagnetic field is modulated with respect to the well retaining the paramagnetic particles to alternatively create the first magnetic gradient and the second magnetic gradient.

52. A method for removing undesired materials from a suspension of paramagnetic particles comprising the steps of supporting the suspension of paramagnetic particles on a generally horizontal surface that leads to a pipetting station, generating a magnetic field beneath the surface to create a first magnetic gradient having a predominant vertical reach component that attracts the suspended paramagnetic particles exposed to the component generally vertically toward the bottom of the well and a second magnetic gradient having a predominant radial reach component that attracts the suspended paramagnetic particles exposed to the component generally toward the center of the well, transporting the well along the surface toward the pipetting station while alternating the exposure of the suspended paramagnetic particles over time between the first magnetic gradient and the second magnetic gradient, and aspirating fluid from the well at the pipetting station.

53. A method according to claim 52 and further including the step of generating a magnetic field beneath the pipetting station to create a magnetic field having a magnetic gradient with predominant radial reach component that further attracts the suspended paramagnetic particles exposed to the component generally toward the center of the well.

54. A method according to claim 52 wherein, in the step of generating the magnetic field beneath the pipetting station, the magnetic field there generated has a more focused 5 radial reach component than the radial reach component of the second magnetic field beneath the bed.

55. A method according to claim 52 or 53 wherein, in the fluid aspiration step, the fluid is aspirated from a side region of the well away from the center region where the paramagnetic 5 particles have been attracted.

56. A method according to claim 52 or 53 and further including the step, prior to the aspiration step, of introducing a material into the well to reduce the surface tension of the fluid 5 suspension.

57. A method according to claim 56 wherein, in the step of introducing a material to reduce the surface tension of the fluid suspension, the material reduces the contact angle 5 between the fluid suspension and the interior surface of the well to below about 90 degrees.

58. A method according to claim 56 wherein, in the step of introducing a material to reduce the surface tension of the fluid suspension, the material alters the distribution of 5 the fluid suspension within the well to distribute more fluid volume toward the sides of the well and away from the center of the well.

59. A method according to claim 58 wherein, in the fluid aspiration step, the fluid is aspirated from a side region of the well where the greater fluid volumes are distributed.

60. A system for removing undesired materials from a suspension of paramagnetic particles comprising a support bed for retaining the fluid i suspension in a generally horizontal plane in a well , means creating a magnetic field along the horizonal plane beneath the well to attract the paramagnetic particles suspended within the fluid generally toward the center region of the well, and means for aspirating fluid from a region of the well away from the center region where the paramagnetic particles have been attracted comprising a first probe having an interior bore for aspirating fluid from within the well, a second probe spaced from the second probe and having an interior bore for aspirating fluid from within the well, a third probe positioned between the first and second probes and having an interior bore for dispensing dilution fluid into the well, and the first and second aspiration probes being longer than the third probe and positioned to lie in close proximity along the sidewall of the well to aspirate fluid from the side regions of the well.

61. A system according to claim 60 wherein the third probe is located generally equidistant between the first and second probes.