Centrifuge Indexing Seal Head Assembly And Method

Abstract

An indexing seal head assembly and method for selectively loading and unloading one or more cells in a multi-cell centrifuge rotor while the rotor is spinning.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to centrifuges and more particularly to an indexing seal head assembly and method for selectively interconnecting one or more cells of a multi-cell centrifuge rotor with at least one fluid passageway while the rotor is spinning to permit the selected rotor cell or cells to be loaded or unloaded during rotor rotation.

2. Description of the Prior Art

Commercially available centrifuges and ultracentrifuges generally employ either fixed angle, swinging bucket, or zonal rotors depending upon the nature of the experiment being conducted. Fixed angle and swinging bucket rotors are loaded and unloaded by the operator while the rotor is at rest. As the rotor accelerates and decelerates between rest and its operating speed it passes through certain critical speeds, known as low speed criticals, in which vibration occurs. During deceleration this vibration may cause undesirable mixing and loss of resolution in a sample which had been centrifugally separated. Moreover, the tubes carrying the sample normally include generally parallel side walls which produce undesirable effects because the sample is in part forced against them during centrifugation. Furthermore, the tubes must often be handled by the operator of the centrifuge which contributes to additional loss and resolution.

As for zonal rotors, heretofore it has been the practice to load or unload all of the sector-shaped compartments simultaneously. Indeed until the present invention it has been impossible to do otherwise. Among other limitations, such as the absence of a stabilizing gravity field during loading and unloading (since present zonal rotors must be loaded and unloaded at relatively low speeds) this has meant that only one sample per run could be investigated. Investigation of other samples required additional runs with obviously varying conditions making accurate comparisons difficult, if not impossible. There is, therefore, a great need for an apparatus capable of loading and unloading selected cells of a multi-cell centrifuge rotor while the rotor is spinning.

SUMMARY OF THE INVENTION

The present invention contemplates a method and apparatus for loading and unloading one or more selected cells of a multi-cell centrifuge rotor while the rotor is spinning and avoids or eliminates the aforediscussed problems and concomitant limitations associated with present loading techniques. The uninterrupted spinning of the rotor produces a stabilizing gravity field which permits a plurality of samples to be run under substantially identical conditions. Moreover, by loading and unloading the rotor at a speed above its low speed critical, mixing caused by vibration is eliminated and resolution is substantially improved.

It is in general an object of the present invention to provide a new and novel method and apparatus for loading and unloading a centrifuge rotor which avoids the problems encountered with presently used loading techniques.

Another object of the invention is to provide a method and apparatus for selectively loading and unloading one or more selected cells in a multi-cell centrifuge rotor while the rotor is spinning.

A still additional object of the present invention is to provide an apparatus including a zonal rotor having a plurality of sector shaped cells or compartments.

A further object is the provision of an indexing assembly for selectively interconnecting one or more rotor cells in a multi-cell rotor with at least one fluid passageway while the rotor is spinning.

These and other objects and features of the invention will become apparent from the following detailed description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially exploded, partially broken away, perspective view of one embodiment of an indexing seal head assembly in accordance with the principles of the present invention;

FIG. 2 is a cross-sectional view of the assembly shown in FIG. 1 taken substantially along line 2--2 with all the passages in alignment therewith;

FIG. 3 is a diagrammatic view of an alternate geometrical groove configuration which may be used in the rotary indexing sleeve 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing and more particularly to FIGS. 1 and 2 thereof, it will be observed that the indexing seal head assembly of the present invention comprises, generally speaking, a rotary indexing sleeve member 12, a stationary indexing sleeve member 13, a braking means 15 and a housing 14 surrounding these elements to provide a stationary connection to the centrifuge body. The indexing seal assembly cooperates with a multi-cell centrifuge rotor 11 which in the illustrated embodiment takes the form of a zonal rotor. Rotor 11 is spun at selected speeds through a motor -- drive shaft combination (not shown) connected to the underside of the rotor 11.

In accordance with the present invention zonal rotor 11 includes a plurality of sector-shaped compartments (only one of which is illustrated) which serve as sample cells 19 for containing the sample to be centrifuged. Compartments or cells 19 are equally spaced and disposed symmetrically about the rotational axis 17 of rotor 11 and each includes a pair of radially extending side walls 21, substantially parallel top and bottom walls 22 and 23, respectively, inner wall 24 and a circumferentially extending outer edge 26. In the illustrated rotor the upper and lower walls 22, 23 are fabricated of a transparent material, such as clear plastic or glass to permit observation of the sample within the cell by suitable optical monitoring means. While for purposes of description a zonal rotor is shown, it will be appreciated that the present invention may be readily adapted to be used with all types of rotors whether they be zonal, fixed angle, or swinging bucket rotors. Also, it will be understood that the term "cell" as used herein refers generically to either a sector-shaped compartment of a zonal rotor, or individual tubes carried by swinging bucket or fixed angle rotors. Furthermore, the cells may be stacked vertically, as opposed to arranged horizontally, if desired.

Each cell 19 is associated with one pair of flow passageways 31 and 32 which carry fluid to and from the sector-shaped cell to which they are connected. Passageways 31 and 32 extend outwardly in a radial direction with the passageway 31 passing through the inner wall 24 to communicate with the inner radius of cell 19 and passageway 32 communicating with the outer edge of cell 19. Fluid is fed to passageways 31 and 32 by way of inner and outer feed lines 91 and 92, (FIG. 2) respectively. In practice density gradient solution, cushion fluid, and displacing solution are fed through outer feed line 92 and passageway 32 while the sample solution, overlay, and, after centrifugation, the centrifuged sample pass through inner line 91 and passageway 31. However, it will be appreciated that the lines through which these solutions are transmitted into and removed from the rotor cell 19 may be varied at the will of the operator.

The sample to be investigated as well as appropriate density gradient and displacing solutions and cushion fluids are introduced into feed lines 91 and 92 via tubular conduits 100 and 101, respectively, which conduits are interconnected with either conduit 102 or conduit 103 by way of four way valve 105. A suitable gradient pump 104 is inserted in conduit 102 to force gradient solution through the connecting lines and into the rotor cells. Conduit 103 communicates via valve 107 with either a sample pump 106 operatively connected to a sample reservoir (not shown) or a fraction collector (not shown) to collect and store the centrifuged sample. Instead of a sample pump the sample to be investigated may be introduced into the system by means of a commercially available syringe, if desired.

During operation, since the rotor is spun at rapid rotational speeds, a conventional upper rotating bearing and seal assembly is employed to provide liquid communication between the stationary and spinning components of the centrifuge. Generally speaking, this assembly comprises a cylindrical shaped rotating seal member 36 located beneath a cylindrical stationary seal member 86 and a resilient coil spring 97 which presses against the upper surface of stationary seal 86 to urge the lower face of stationary member 86 into sealing engagement with the upper face of rotary member 36 thereby providing a liquid-tight dynamic seal between these faces.

Rotary seal member 36 is carried by rotor 11 and a seal 43. More specifically, a U-shaped seal 43 is disposed about the wall of a similarly shaped wall 28 formed along the rotational axis 17 in the upper surface of rotor 11. Seal 43 is affixed to the wall of the well 28 to permit the rotary seal member 36 to slip relative to rotor 11 (at selected times) and includes a plurality of circumferential spaced openings 44 and 46 (one pair for each rotor cell) which communicate with the passageways 31 and 32, respectively, formed in rotor 11.

Rotary motion is imparted to seal member 36 by rotor 11 via rotary sleeve 12. In particular, as will be presently discussed in more detail, sleeve 12 is operatively connected to rotor 11 by a lug 47 and to rotary seal 36 by a pair of diametrically opposed pins 39 (only one of which is shown) each pin of which engages a one side of a corresponding slot 38 formed in the seal member 36. Thus, rotor 11 drives sleeve 12 which in turn drives seal 36.

The upper portion of rotary seal 36 projects through a counterbore 29 formed in a hub 27 integrally associated with the upper surface of rotor 11. Counterbore 29 is slightly larger than well 28 for receiving rotary sleeve 36 in a manner to be presently discussed.

Stationary seal member 86 includes two side-by-side passageways 88 and 89, the former of which is formed along the rotational axis 17, communicating with inner and outer feed lines 91 and 92, respectively, while rotary seal 36 also includes two side-by-side passageways 41 and 42 communicating with passageways 88 and 89, respectively, each of which projects in a L-shaped fashion to terminate at the outer wall of rotational member 36 at openings 41a and 42a, respectively. In order to provide continuous communication between the passageway 89 and passageway 42, in the rotatable seal 36, an annular groove 86a is formed below passageway 89 in the stationary seal 86 and the inlet to passageway 42 is always in communication with the groove 86a regardless of the rotational position of the sleeve 36. Openings 41a and 42a are selectively aligned (in a manner to be presently described) with one set of passageways 44 and 46 formed in seal member 43 to provide a pair of continuous paths to transmit fluid into and out of a rotor cell 19. More specifically, a first continuous fluid path is defined by inner feed line 91, passageway 88, passageway 41, opening 41a, passageways 44 and 31 while a second continuous fluid path is defined by outer feed line 92, passageway 89, passageway 42, opening 42a and passageways 46 and 32. Although in the preferred embodiment only one set of openings 41a and 42a are illustrated, it will be appreciated that more than one set of openings may be provided, if desired. Each of the sets of openings may be provided, if desired. Each of the sets of openings would be connected to passageways 41 and 42, respectively, and, thus, would enable the operator to selectively load and unload two or more of the cells 19 simultaneously.

As briefly mentioned above, the indexing seal head assembly comprises a rotary indexing sleeve member 12, a stationary indexing sleeve member 13, a braking means 15 and an outer housing 14. Cylindrical rotary indexing sleeve 12 fits over and about the rotating seal assembly (consisting of rotary seal 36 and stationary seal 86) and rests in counterbore 29 filling the remaining space between the outer surface of rotary seal 36 and the inner wall of the counterbore 29. Rotary indexing sleeve 12 is fixedly secured to rotary seal member 36 so as to rotate with it by a pair of diametrically opposed pins 39 (only one of which is shown) carried by the indexing sleeve 12. In the preferred embodiment each pin 39 is threadably secured in a small hole drilled through the wall of sleeve 12. While indexing sleeve 12 is prevented from rotational movement with respect to the rotary seal 36, these two elements are still permitted limited relative axial (vertical) movement with respect to each other. More specifically, the inner surface of rotary seal member 36 includes a pair of diametrically opposed (only one of which is shown) short, vertical extending grooves 38 (FIG. 2) into which the pins 39 extend. Consequently, the indexing sleeve 12 may be moved up and down in a vertical direction relative to the rotary seal member 36 a distance equal to the length of grooves 38. Thus, sleeve 12 at all times rotates with rotary seal member 36, but, may be selectively moved relative thereto in a vertical direction in a manner to be presently discussed.

To index and hold the rotary seal 36 and indexing sleeve 12 combination in a plurality of selected rotational positions with respect to the rotor 11 so that openings 41a and 42a may cooperate with the passageways communicating with different cells 19 in rotor 11, there is provided a lug member 47 carried by the hub 27 of rotor 11 and a generally circumferentially extending groove 48 formed in the outer surface of the indexing sleeve 12. The lug 47 projects radially from the inner wall of hub 27 and slidably engages the groove 48.

Groove 48 is formed in the outer surface of the indexing sleeve 12 by a conventional process such as milling. It takes the form of a plurality of alternate upper and lower circumferentially extending segments 48a and a plurality of axially (vertical) extending segments 48b interconnecting the ends of the adjacent upper and lower circumferentially extending segments to provide one continuous up and down stepped groove 48. Groove 48 also includes at least one axially extending segment 48c which opens through the lower edge of the surface of the sleeve 12 to provide a means of ingress and egress for the lug member 47 thereby permitting the rotary indexing sleeve 12 to be inserted into and removed from counterbore 29.

Alternatively, the groove 48 may take other forms, such as the step-ladder configuration illustrated in FIG. 3.

The left-hand edge of each circumferentially extending segment 48a corresponds to a cell 19 in rotor 11. Thus, by rotating indexing sleeve 12 through successive segments 48a relative to stationary lug 47 openings 41a and 42a are brought into registry with successive rotor cells 19. To so move the indexing sleeve-rotary seal combination, indexing sleeve 12 must first be moved up or down in a vertical direction, depending upon whether lug 47 is located in an upper or lower segment 48a, and then rotated circumferentially with respect to rotor 11.

Vertical motion is translated to rotary indexing sleeve 12 through a cylindrical bearing 51 which rotates about its vertical axis and is interposed between rotary indexing sleeve 12 and stationary indexing sleeve 13. Bearing 51 is journaled between a shoulder 52 formed about the outer surface of the sleeve 12 and upon which bearing 51 rests and a lock ring 53 held in an annular groove 54 formed in the outer surface about the top of the sleeve 12 to constrain the bearing against axial movement relative to the rotary indexing sleeve 12. Similarly, bearing 51 is secured to and constrained against axial movement relative to the indexing sleeve 13 by journalling the bearing 51 between an annular rib 56 formed on the upper edge of the inner wall of the indexing sleeve and a lock ring 57 disposed in the annular groove 58 formed about the lower edge of sleeve 13. Accordingly, stationary sleeve 13, bearing 51, and rotary indexing sleeve 12 are moved as a single unit in a vertical direction and any vertical (up or down) motion of stationary indexing sleeve 13 manifests itself in similar movement of rotary indexing sleeve 12. At this point it should be noted that while sleeve 13 may be selectively moved both vertically and rotationally, it is referred to herein as a stationary sleeve since it does not rotate with the rotor like rotary sleeve 12.

Stationary indexing sleeve 13 is slideably and rotatably mounted within a housing 14. Housing 14 includes a cylindrical side wall 61 and a generally planar top wall 62 having a pair of small apertures 94 and 96 through which feed lines 91 and 92 extend. Portions of the side and top walls are cut away, as indicated at 63 and 64, to facilitate access to the stationary indexing sleeve 13. Housing 14 serves as the cover for the indexing sleeves 12 and 13 and braking means 15 and is removably secured to the outer centrifuge housing by a pair of diagonally opposed L-shaped slots 66 each of which engages a suitable shaped yoke (not shown) mounted on the centrifuge body.

Since stationary indexing sleeve 13 may be easily reached by the operator through the openings 63 and 64 in housing 14, vertical movement is translated to rotary indexing sleeve 12 by merely manually pulling up or pushing down on stationary sleeve 13. To this end the top edge of stationary sleeve 13 is knurled (FIG. 1) to facilitate gripping of this member.

Stationary indexing sleeve 13 also includes a groove 68 in the outer surface thereof having a geometrical configuration identical to and, when resting in a rotor cell position, corresponding in position with that of groove 48. That is, groove 68 is made up of alternate upper and lower circumferentially extending segments 68a connected together by axially extending segments 68b with all of the segments 68a and 68b corresponding in position to similarly orientated segments 48a and 48b of groove 48. It follows that stationary sleeve 13 may be made to track the position of the rotary indexing sleeve 12. Thus, by inscribing each cell position with a number 1, 2, 3, etc. about the upper edge of sleeve 13, the operator may easily identify which cell is being loaded or unloaded.

A spherical ball 69 constrained between the sleeve 13 and housing 14 travels along the groove 68. To ensure that the spherical ball 69 is retained within and travels along groove 68, the groove has a depth on the order of one half the diameter of the ball and the ball is also held in a small socket formed on the inner surface of the housing 14. A set screw 71 cooperates with the surface of spherical ball 69 by way of an internally threaded bore 72 formed in housing 14. Set screw 71 provides means for selectively adjusting the pressure on the ball 69 and hence the ease of movement of the indexing sleeve 13.

A braking means 15 cooperates with the upper surface of indexing sleeve 13 for momentarily retarding the rotation of this sleeve and hence rotary seal member 36, relative to the rotor 11 thereby permitting the openings 41a and 42a to index to different cells 19 in the rotor 11. This braking means comprises semi-annular shaped brake shoe 76, fabricated of a semi-rigid material, disposed about the upper portion of the indexing sleeve 12. A small gap 77 is formed at one point along the shoe 76 to permit movement between expanded and contracted positions. The outer portion of the brake shoe 76 includes a radially extending flange 79 which is slidably mounted in a groove 70 formed in the inner surface of stationary indexing sleeve 13. Thus, the braking means 15 and the indexing sleeve 12 may be moved up and down together in a vertical direction. In the preferred embodiment, the diameters of the rotatable indexing sleeve 12, the brake shoe 76, and flange 79 are chosen such that the brake shoe can be compressed to engage the sleeve 12 without disengaging the flange portion from the groove 70.

A pair of diametrically opposed button assemblies 81 are threadably connected to opposite sides of brake shoe 76 for compressing the brake shoe 76 to engage the sleeve 12. That is, the operator by momentarily pushing on button assemblies 81 may compress brake shoe 76. A resilient spring member 83 is disposed intermediate the side wall 61 of housing 14 and button 81 to provide means for yieldably urging the brake shoe 76 out of contact with the indexing sleeve 12.

In operation, while the rotor is spinning at a very low speed, the indexing assembly including rotary sleeve 12, stationary sleeve 13, braking means 15, and housing 14, is mounted on the centrifuge body by inserting rotary seal 36 into well 28, groove 48c in sleeve 12 over lug member 47, and manually twisting housing 14 clockwise a short distance to seat the associated mounting yokes (on the centrifuge body) into L-shaped slots 46. Naturally, it may happen that groove 48c does not at first line up with lug 47, in which case, brake actuating buttons 81 are momentarily pressed to cause sleeve 12 (and rotary seal 36) to rotate relative to rotor 11 until groove 48c is aligned with lug 47.

After alignment, lug member 47 is brought into registry with upper circumferentially extending segment 48a by pushing downward on stationary sleeve 13 and then brake buttons 81 are again momentarily depressed to cause rotary sleeve 12 and rotary seal 36 to slip counterclockwise (assuming the rotor is spinning clockwise) with respect to rotor 11 until lug 47 rests against the left-hand edge of upper segment 48a. In this position, openings 41a and 42a (rotary seal 36) register with passageways 31 and 32 to provide fluid communication with one of the cells 19 in the rotor 11. at this time the rotary sleeve 12 is spinning together with rotary seal 36 and rotor 11 while stationary sleeve 13, braking means 15, and housing 14 remain stationary.

While the rotor is spinning (at some speed slightly above low speed critical) the first cell 19 is loaded by introducing in a predetermined and well known sequence, gradient solution and cushion fluid through outer feed line 92 and passageway 32 and sample solution through inner feed line 91 and passageway 31. When the first cell 19 has been fully loaded, the rotary indexing assembly may be then indexed to another of its discrete positions to prepare for loading a second cell 19.

Indexing of the rotary seal assembly from the first to the second position is achieved by first manually pulling up on stationary indexing sleeve 13 which upward movement is translated through bearing 51 to rotary indexing sleeve 12 (in the manner previously discussed) to cause lug member 47 to travel downwardly in segment 48b of groove 48 until it reaches the next succeeding lower circumferentially extending segment 48a. Simultaneously, as previously noted, spherical ball 69 travels downward (relatively speaking) within an axial extending segment 68b of groove 68 formed in stationary sleeve 13. Once lug member 47 is aligned with lower circumferentially extending segment 48a (and spherical ball 69 with lower circumferentially extending segment 68a) stationary sleeve 13 is manually turned in a counterclockwise direction relative to housing 14 to cause spherical ball 69 to move along circumferentially extending segment 68a until it comes to rest against the left-hand edge of the segment. This position of sleeve 13 corresponds to the second rotor cell 19 location and is marked as such.

The final step is to momentarily press brake buttons 81 causing brake shoes 76 to frictionally engage rotary sleeve 12 which causes sleeve 12 (and rotary seal 36) to slip relative to rotor 11 until lug member 47 engages the left-hand edge of the lower extending segment 48a in which it is traveling. The rotary seal 36 has now been indexed to its next succeeding position and openings 41a and 42a thereof are in communication (by way of openings 44 and 46 in sleeve 12) with passageways 31 and 32, respectively, of the next rotor cell 19.

When all of the cells in the rotor 11 have been loaded, the indexing assembly is removed and the rotor is capped to close the well 28. The speed of the rotor is then increased to its normal operating speed and the sample solutions are centrifuged.

Once centrifugation is completed, the cells may be selectively unloaded by slowing down the rotor to a speed slightly greater than its low speed critical, uncapping well 28, and replacing the index assembly on the rotor in the manner previously described. Each cell 19 is then unloaded by successively indexing rotary seal 36 into communication with each cell 19 in the manner previously discussed and transmitting displacing solution through outer feed line 92 and passageway 32 to force the centrifuge sample out through passageway 31 and inner feed line 91.

While the invention has been described with respect to a preferred physical embodiment constructed in accordance therewith, it will be apparent to those skilled in the art that numerous modifications and improvements may be made without departing from the inventive concept of the invention. Accordingly, the invention is to be construed as limited only by the spirit and scope of the appended claims.

Claims

What is claimed is:

1. In a centrifuge including a multi-cell rotor having inlet and outlet passages connecting with each of its cells, the improvement comprising: means rotatable with said rotor and having at least a pair of fluid carrying passages positioned to communicate with the inlet and outlet passages of each cell, said means being movable to a multitude of discrete positions for aligning its passages with said inlet and outlet passages of said rotor cells to selectively load and unload one rotor cell at a time while the rotor is spinning.

2. In a centrifuge including a multi-cell rotor having a plurality of inlet and outlet passageways communicating with each of the rotor cells, the improvement comprising: means rotatable with said rotor and having at least a pair of fluid carrying passages positioned to communicate with the inlet and outlet passages of each cell, said means being movable to a multitude of discrete positions for aligning its passages with said inlet and outlet passages of said rotor cells for selectively loading and unloading one rotor cell to the exclusion of the remaining cells while the rotor is spinning.

3. In a centrifuge including a multi-cell rotor having inlet and outlet passages connecting with each of its cells, and a fluid carrying conduit outside the rotor the improvement comprising: means rotatable with said rotor and having at least a pair of fluid carrying passages positioned to communicate with the inlet and outlet passages of each cell, said means being movable to a multitude of positions for aligning its passages with said inlet and outlet passages of said rotor cells for selectively connecting in sequence one rotor cell at a time in fluid communication with the fluid carrying conduit while the rotor is spinning whereby the rotor may be loaded and unloaded during rotor rotation.

4. In a centrifuge including a multi-cell rotor having inlet and outlet passages connecting with each of its cells, and a fluid conduit located outside of the rotor, the improvement comprising: means coupled to and cooperating with the rotor, said means being rotatable with said rotor and having at least a pair of fluid carrying passages positioned to communicate with the inlet and outlet passages of each cell, said means being movable to a multitude of positions for aligning its passages with said inlet and outlet passages of said rotor cells for selectively interconnecting in sequence each of the rotor cells to the fluid conduit outside the rotor while the rotor is spinning whereby each of the rotor cells may be loaded and unloaded without bringing the rotor to rest.

5. In a centrifuge including a multi-cell rotor, a rotary seal assembly including a rotary seal member having at least one fluid carrying passageway carried by and normally rotatable with the rotor; the improvement comprising: an indexing assembly means connected to the rotary seal member for selectively interconnecting the fluid carrying passageway in the rotary seal member with one rotor cell at a time while the rotor is spinning whereby each cell may be loaded and unloaded without bringing the rotor to rest.

6. In a centrifuge including a multi-cell rotor, each cell communicating with at least one fluid carrying passageway formed in the rotor, a rotary seal assembly including a rotary seal member normally rotatable with the rotor and having at least one fluid carrying passageway; the improvement comprising: a first member slideably secured to and rotatable with said rotary seal for interconnecting the fluid carrying passageway in the rotary seal with a rotor passageway associated with a rotor cell; and indexing means connected to said first member for indexing the first member and the rotary seal to register the rotary seal passageway in fluid communication with successive rotor cells while the rotor is spinning whereby the rotor cells may be loaded and unloaded without bringing the rotor to rest.

7. An improvement as defined in claim 6 wherein said indexing means comprises means for momentarily retarding the rotational movement of the first member and the rotary seal member with respect to that of the rotor.

8. An improvement as defined in claim 6 wherein said indexing means comprises means for frictionally engaging the first member to momentarily retard the rotational movement of the first member and the rotary seal to cause the position of these latter elements to slip relative to the rotor.

9. An improvement as defined in claim 6 wherein said indexing means comprises a braking means for frictionally engaging the first member to momentarily retard the rotational movement of the first member and the rotary seal member causing the position of these latter elements to slip in a rotational direction relative to that of the rotor and means for vertically moving said first member with respect to said rotary seal member.

10. An improvement as defined in claim 9 wherein said vertically moving means comprises a second member movable vertically up and down and bearing means connected between said first and second members for translating vertical movement of said second member to said first member.

11. An improvement as defined in claim 10 wherein said first member comprises a first cylindrical shaped sleeve having a groove formed about the surface thereof and comprising a plurality of spaced upper and lower circumferentially extending segments, the ends of alternate ones of the circumferentially extending segments being interconnected by vertically extending segments to form a continuous groove with one edge of each circumferentially extending segment corresponding to a rotor cell position and comprising in addition a lug member carried by a rotor housing and extending radially into the groove formed in the first member whereby upon activation of said braking means said lug member is caused to advance between adjacent vertically extending segments of said groove.

12. An improvement as defined in claim 11 wherein said second member comprises a second cylindrical shaped sleeve having a groove formed about the surface thereof, the geometrical configuration of the groove being identical with that of the groove formed in the first cylindrical sleeve so that one edge of each circumferentially extending segment corresponds to a rotor cell position, and said second cylindrical sleeve being selectively movable in a rotational direction to track the position of said first cylindrical sleeve.

13. An improvement as defined in claim 12 comprising in addition a stationary housing disposed about said second cylindrical shaped sleeve and a spherical ball bearing carried by the inside wall of said housing, said ball bearing cooperating with the groove formed in said second cylindrical sleeve to enable the latter sleeve to move both in rotational and vertical directions relative to said housing.

14. An improvement as defined in claim 13 wherein said housing includes at least one opening therein exposing a portion of said second cylindrical sleeve to provide easy access thereto.

15. An improvement as defined in claim 13 wherein said braking means comprises a brake shoe disposed intermediate said stationary housing and said first cylindrical sleeve; resilient means normally urging said brake shoe out of contact with said first sleeve; and, actuator means connected to said brake shoe and extending outside of said housing for momentarily urging said brake shoe into frictional engagement with said first sleeve.

16. An improvement as defined in claim 12 comprising in addition means connected to said rotary seal fluid passageway for transmitting sample through said passageway whereby selected rotor cells may be loaded while the rotor is spinning.

17. A method of loading a multi-cell centrifuge rotor with the sample to be investigated comprising the steps of (1) spinning the rotor at a rotational speed above its low speed critical, (2) registering a cell selection mechanism in communication with one rotor cell only, (3) introducing sample through the cell selection mechanism into the selected cell while the rotor is spinning, and (4) indexing the cell selection mechanism into registry with the next successive cell without interrupting rotor rotation to permit such cell to be loaded with sample.

18. A method as defined in claim 17 wherein the indexing step comprises retarding the normal rotational movement of the cell selection mechanism relative to that of the rotor.

19. A method as defined in claim 17 comprising in addition repeating steps (2), (3) and (4) until all of the cells in the rotor are loaded.

20. A method of selecting one cell of a multi-cell centrifuge rotor for loading with sample while the rotor is spinning comprising the steps of spinning the rotor at a rotational speed above its low speed critical and registering a cell selection assembly which rotates with the rotor in communication with one rotor cell only whereby sample may be introduced into the selected cell while the rotor is rotating.