Abstract:
An apparatus for sequentially fractionating a centrifuge tube includes a capillary tube and a means for applying positive pressure. The capillary tube has an O-ring at the lower end thereof. As the capillary tube is placed within the centrifuge tube, the O-ring forms a seal within the tube. Movement of the capillary tube within the centrifuge tube places the liquid in the centrifuge tube under pressure, thus forcing the liquid to flow up through the capillary tube and into a chamber. A chase fluid is then pumped horizontally through the chamber to force the liquid therein through an exit port and into a fraction collector. The apparatus and method of the present invention may be entirely automated and controlled by a single microprocessor.

Description:
This is a continuation-in-part of present copending application Ser. No. 724,033, filed Apr. 6, 1985, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to fractionators and more particularly to sequential fractionators. 
     BACKGROUND OF THE INVENTION 
     Centrifugation has often been employed as a separation technique. In many fields, such as genetic engineering, materials are separated by centrifugation and sedimentation within a cesium chloride or other density-type gradient. After centrifugation and sedimentation, fractions of the centrifuge tube are removed and analyzed. The density of a substance determines where within the cesium chloride gradient the substance settles. This position within the gradient can be specified in terms of a distance from the center of rotation. The density of the substance can be determined by knowing the gradient and the distance from the center of rotation at which the substance settled. Thus, not only can substances of varying densities be separated by this method, but accurate density determinations may also be made. 
     From the above discussion, it can be understood that the degree of separation achieved, or the precision within which the density of a substance can be determined, is dependent upon the degree to which fractions (or layers) can be removed from the centrifuge tube for analysis without mixing between the layers. 
     One apparatus disclosed for sequential fractionation is described by Chervenka et al in U.S. Pat. No. 4,181,700. The device include a microsyringe mounted to a movable frame and a suction means for withdrawing fluid from the centrifuge tube into the syringe. The syringe is lowered a precise distance into a centrifuge tube and this distance is read from a micrometer and recorded. Suction is then applied to the syringe tip to remove a precise volume of liquid from the top of the centrifuge tube. While the above method is tolerable for many applications, serious difficulties arise if high precision is desired. 
     As stated above, precision is related to the degree of mixing which occurs between layers. When suction is applied through the syringe, flow occurs within the centrifuge tube. Since laminar flow laws apply, it is clear that liquid at the center of the tube flows faster than liquid at the outer edges. Thus, a significant amount of mixing inherently occurs. 
     Another apparatus (U.S. Pat. No. 3,151,639 to Allington) sequentially removes layers from a centrifuge tube by forcing a dense liquid into the bottom of the centrifuge tube to raise the level of the other liquid in the tube an amount corresponding to the volume of the added dense liquid. The liquid in the centrifuge to is forced out of the tube and into a fraction collector solely by the action of the added dense liquid. Although the application of suction is avoided by this method, large amounts of laminar flow and thus mixing still occur, since each time dense fluid is added, the entire liquid mass within the centrifuge tube must move upwardly. 
     SUMMARY OF THE INVENTION 
     A general object of the invention is to overcome deficiencies in the prior art, such as indicated above. 
     It is an object of the present invention to provide for improved sequential fractionation, such as by providing a method and apparatus for sequentially fractionating a centrifuge tube into precise fractions. 
     It is another object of the present invention to provide a method and apparatus for sequentially fractionating a centrifuge tube with a minimum amount of mixing between fractions. 
     It is a further object of the present invention to provide a method and apparatus for sequentially fractionating a centrifuge tube without using a vacuum upon the centrifuge tube. 
     These and other objects are achieved by the use of a capillary tube and positive pressure. The capillary tube has an O-ring at the lower end thereof. As the capillary tube is placed within the centrifuge tube, the O-ring forms a seal within the tube. Movement of the capillary tube within the centrifuge tube places the liquid in the centrifuge tube under pressure, thus forcing the liquid to flow up through the capillary tube and into a chamber. A chase fluid is then pumped horizontally through the chamber to force the liquid therein through an exit port and into a fraction collector. The apparatus and method of the present invention may be operated by hand or may be entirely automated and controlled by a single microprocessor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a preferred embodiment of the present invention. 
     FIG. 2 schematically shows a preferred embodiment for automating the present invention. 
     FIG. 3 graphically illustrates results obtained using the present invention. 
     FIG. 4 also graphically illustrates results obtained using the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferably, the centrifuge tube used is precision made. In other words, the inner diameter of the tube should be essentially uniform. The degree of acceptable variance in this regard depends on the precision and accuracy desired. Generally, the precision of the present invention is limited to twice the variance in the centrifuge tube inner diameter consistency. 
     The capillary tube used preferably has a small inner diameter and a volume of no more than about twice that of the desired sample size so that the area in which flow can occur is as small as possible. The end of the capillary tube which is to be inserted into the centrifuge tube is outwardly flared, preferably at an angle of about 30°60° from the vertical axis. The outward flare or conical configuration help to minimize the removal of liquid from the center portion of the centrifuge tube at a faster rate than from the sides of the centrifuge tube, and thereby serves to minimize undesirable mixing of adjacent horizontal layers of liquid. 
     Both the capillary tube and the centrifuge tube should be supported so that they stand along the same vertical axis. The capillary tube is vertically lowered, or the upright centrifuge tube raised, so that the capillary tube is inserted, flared end down, into the upright centrifuge tube by a suitable means for precision movement. The means for precision movement need only move the centrifuge tube along the vertical axis relative to the capillary tube. Thus, either the capillary tube, the centrifuge tube or both may actually be moved. A starting point is obtained and a measurement of the vertical distance moved by the capillary tube or centrifuge tube is taken by any well-known suitable means, such as a calibrated micrometer directly connected to the means for lowering the capillary tube. 
     As the capillary tube is inserted into the centrifuge tube, an O-ring on the capillary tube, positioned just above the flared end, sealingly engages the inside surface of the centrifuge tube and provides positive pressure upon the liquid therein. As the capillary tube is further inserted and its depth within the centrifuge tube increased, this positive pressure forces the surface fraction of liquid into and through the capillary tube and finally into a chamber connected to the non-flared upper end of the capillary tube. In addition to an opening connecting the chamber to the capillary tube, the chamber has an exit port and an entrance port providing for the horizontal movement (transverse to the vertical axis) of fluid from the entrance port to the exit port. The entrance port is connected to a pump for applying horizontal fluid pressure within the chamber. This horizontal fluid pressure forces any liquid within the chamber through the exit port. The exit port is connected to a standard fraction collector. 
     From the above description, it can be seen that little or no mixing of flow occurs in the centrifugal tube during the removal of fractions. Of course, significant flow and mixing does occur in the capillary tube. Nevertheless, because of the relatively small diameter and small volume of the capillary tube relative to the desired sample size, the effect of this mixing on precision and accuracy are almost negligible. To this end, the ratio of the cross-sectional area of the chamber to the internal cross-sectional area of the capillary tube is preferably at least about 10:1. Obviously, larger ratios of cross-sectional areas may be used, depending on the degree to which the fraction is to be diluted with chase fluid. 
     In a preferred embodiment 10, as shown in FIG. 1, the means to move the centrifuge tube 11 (preferably a high precision quartz tube) or capillary tube relatively closer to each other along a vertical axis is a precision screw drive 12 coupled by means of a transmission (not shown) to a stepping motor (52 in FIG. 2). Using this apparatus, elevation of the centrifuge tube can be controlled to ±0.0003 cm. 
     A stationary fluid removal port 14 consists of two sections joined as illustrated in FIG. 1. The upper section of the port is a block 16 (suitably formed of Lucite, Plexiglas or other machinable rigid plastic, preferably transparent) containing a chamber 17 defined by a horizontal capillary 18 of 1 mm diameter between two opposing fittings 20, 22 for the connection of external tubing. The lower section of the port is a vertically mounted stainless steel cylinder 24, desirably of stainless steel, of 3.1 mm OD, housing a capillary 25 of 0.3 mm diameter along the cylindrical axis. An O-ring 26 seated at the bottom end of the cylinder provides a gas- and liquid-tight seal when the cylinder is inserted into the mouth of a miniature quartz centrfuge tube 11. An outwardly flared (preferably about 45°) aperture 28 at the bottom end of the cylinder 24 guides tube contents to the capillary 25. The upper end of the cylinder 24 is fixed into the Lucite block 16 so that the vertical capillary 25 exiting from the upper end of the cylinder 24 enters perpendicularly into the horizontal capillary 18, forming a T-connection. 
     In order to operate the device, a peristaltic or repeating syringe pump (56 and 58 in FIG. 2), capable of delivering 2-3 ml of liquid in a few seconds on demand, is connected via tubing to fitting 20, and a fraction collector (66 in FIG. 2) is connected via tubing fitting 22. A receptacle 30, for holding the centrifuge tube 11, is moved to the lower limit of its travel, and the quartz centrifuge tube 11 containing the solution to be fractionated placed therewithin. 
     The centrifuge tube 11 is then elevated by means of the screw drive 12 until the lower end of the fluid removal port 14 enters the mouth of the stainless steel capillary 24, 25. Insertion of the port 14 is facilitated by prior application of a small amount of silicone grease to the O-ring 26. The centrifuge tube 11 is then further elevated slowly until solution at the meniscus enters the stainless steel capillary 24, 25 and a small amount of liquid is subsequently observed to enter the horizontal capillary 18 within the Lucite block 16. 
     At this stage a starting point is obtained, and the micrometer is set to zero, or preferably control of the apparatus is transferred to a microcomputer (50 in FIG. 2). The user enters the desired increment of radial distance corresponding to an individual fraction and the desired number of fractions. 
     The following procedure is then performed repetitively without manual intervention until the desired number of fractions have been collected: (1) The centrifuge tube is elevated by the designated distance. (2) That amount of solution driven into the horizontal capillary upon elevation is flushed with 2 to 3 ml of carrier fluid into a collecting vial mounted in the fraction collector. (3) The fraction collector is advanced to the next vial. 
     One use of the present invention is to measure concentration gradients of radiolabeled solutes subjected to prior application of centrifugal force. The carrier fluid used may be scintillation fluid, and the collecting vessels may be glass vials which, after fractionation, are placed in a scintillation counter for measurement of the amount(s) of one or more radiolabeled species in each fraction. However, quantitation of concentratoin gradients is not limited to radiolabeled solutes: in principle, any chemical or physical assay of the requisite sensitivity may be utilized, as, for example, an assay of enzyme activity to measure the amount of enzyme in each fraction. 
     FIG. 2 schematically illustrates an automatic fractionator according to the present invention. 
     Microcomputer 50 signals stepping motor 52 to raise receptacle 30 with centrifuge tube 11 thereon by turning screw drive 12. Receptacle 30 activates position sensor 54, thus send a signal to microcomputer 50 and establishing a reference point. The microcomputer is programmed to raise receptacle 30 in increments sufficient to raise a volume of solution equal to the selected sample volume into capillary 25. After the sample flows into capillary 25, it flows into chamber 17 and microcomputer 50 sends a signal to automatic pipetter 56, which draws fluid from the reservoir of chase fluid through line 60 and pumps the fluid through line 62 into chamber 17, thus chasing the sample into line 64 and finally to the fraction collector 66, which is also controlled by microcomputer 50 and collects fractions in an ordered manner according to fraction number. 
     By way of example, the microcomputer 50 may be an Epson HX-20, the automatic pipetter may be an Oxford automatic pipetter, and the fraction collector may be a Gilson 201B fraction collector. 
     EXAMPLES 
     Having fully described the invention above, the following examples are given solely for illustrative purposes and are not intended to limit the scope of the invention in any manner. 
     FIGS. 3 and 4 show results obtained from fraction of solutions of  131  I- labeled bovine serum albumin centrifuged under two different sets of conditions. 
     In FIG. 3 the relative protein concentration in an aliquot, expressed as counts per minute, is plotted as a function of the radial position of the aliquot during centrifugation, measured at the conclusion of a sedimentation velocity experiment. Approximately 150 ul of 0.04 mg/ml protein solution were required to perform this measurement. Resolution of the data is 10 points/nms or radial distance. The vertical line to the left of the plot indicates the position of the solution meniscus (upper boundary), and the vertical line to the right indicates the weight-average position of the trailing boundary of sedimenting protein, as calculated from the data. The sedimentation coefficient calculated from these data is in good agreement with published values. 
     In FIG. 4 the natural logarithm of the relative protein concentration in an aliquot, expressed as 1n (counts per minute), is plotted as a function of the square of the radial position of the aliquot during centrifugation, measured at the conclusion of a sedimentation equilibrium experiment. Approximately 40 ul of a 0.02 mg/ml protein solution were required to perform this measurement. Sedimentation theory predicts that this plot should be linear for a homogeneous species at sedimentation equilibrium. The molecular weight of the protein, calculated from the slope of this plot, is in good agreement with published values. 
     It is to be understood that the present invention is not limited to the embodiments disclosed which are illustratively offered and that modifications may be made without departing from the invention. For example, the present invention can be substantially increased in size, always keeping the volume of the small diameter tube (even though larger than capillary size) less than about twice the volume of the desired sample size, to perform various separation functions.