Abstract:
Apparatus and methods for conducting electrophoretic separation concurrently in a plurality of gels with improved reproducibility among the gels.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. provisional patent application Ser. No. 60/505,051, filed Sep. 22, 2003, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to apparatus and methods for conducting electrophoretic separation concurrently in a plurality of gels. More specifically, the present invention relates to apparatus and methods for performing multiple concurrent electrophoresis experiments with increased reproducibility among the gels through incorporation in the apparatus of improved passive thermal management features and improved electric field geometries.  
       BACKGROUND OF THE INVENTION  
       [0003]     Gel electrophoresis is a common procedure for the separation of biological molecules, such as DNA, RNA, and proteins. In gel electrophoresis, the molecules are separated into bands according to the rate at which an imposed electric field causes them to migrate through a filtering gel.  
         [0004]     The basic apparatus used in this technique consists of a gel enclosed in a glass tube or sandwiched as a slab between glass or plastic plates. The gel has an open molecular network structure, defining pores which are saturated with an electrically conductive buffered solution of salt. These pores through the gel are large enough to admit passage of the migrating molecules.  
         [0005]     The gel is placed in contact with buffer solutions that make electrical contact between the gel and the cathode and anode of an electrical power supply. A sample containing the macromolecules and a tracking dye is placed on top of the gel. An electric potential is applied to the gel causing the sample macromolecules and tracking dye to migrate toward the bottom of the gel. The locations of the bands of separated macromolecules then are determined. By comparing the distance moved by particular bands in comparison to the tracking dye and macromolecules of known mobility, the mobility of sample macromolecules can be determined. Once the mobility of the sample macromolecules is determined, the size of the macromolecule can be calculated.  
         [0006]     As electrophoresis is used with increasing frequency in basic research, quality control, and in forensic and clinical diagnoses, it is increasingly important to be able to replicate all experimental conditions in multiple locations and labs.  
         [0007]     Among these experimental conditions, temperature is extremely important.  
         [0008]     The application of an electrical field to a gel results in the generation of heat. In general, higher temperatures increase the molecular kinetics, which results in faster migration of macromolecules through the separating gel. Further, a temperature increase affects the electrical conductivity of an electrolyte solution and may cause dissociation.  
         [0009]     Without temperature control or uniform electric field geometry, gels often exhibit uneven temperatures across the width of the gel resulting in “smile” or “frown” distortions. Smile distortions occur when bands migrate faster on the sides than in the middle of the gel; frown distortions occur when bands migrate faster in the middle than on the sides.  
         [0010]     Often, even a small temperature differential between the front and rear plates of the gel, if not mitigated, can cause the resulting bands to slant front to back, depending on the thickness of the gel and the heat transfer properties of the cassette plates. This challenge is particularly acute in test runs where the molecular migration rates exhibit overly temperature sensitive characteristics, as in DNA sequencing. For such runs, even a slight temperature differential, e.g. of 0.1° C., can cause the slanted bands to appear overlapping.  
         [0011]     Additionally, overheating of the gel (e.g., greater than 70° C.) can result in deleterious effects such as breakdown of the gel matrix resulting in poor resolution and band shape, alteration of the macromolecules including denaturation, alkylation or oxidation, and/or damage to the electrophoresis apparatus itself.  
         [0012]     In DNA sequencing, electrophoresis is conducted at high voltage (1200-3000 volts, 55 watts) to maintain a gel temperature of 45°-50° C. for maximum resolution of the denatured DNA strands. The temperature is controlled by the amount of power applied to the gel. Gels that run too cool (e.g., &lt;40° C.) will have bands that are blurred, perhaps due to incomplete denaturation. Gels that run too warm (e.g., &gt;60° C.) will lose resolution, perhaps due to the breakdown of the polyacrylamide.  
         [0013]     Precise temperature control is particularly critical in Single Stranded Conformational polymorphism (SSCP) analysis of DNA, where bands are extremely close together. The relative temperature differential between the front and the back surfaces of the gel therefore can have a critical effect on the resolution of the DNA bands.  
         [0014]     Various means have been used to attempt to control the temperature of the gel during electrophoresis. These include applying active or passive heat sinks to one side of the gel, regulating power to the gel, employing an enclosed heat exchanger internal to one of the buffer chambers, immersing the gels in a buffer-filled tank containing a heater/circulator, circulating the buffer through tubing immersed in an ice water bath, circulating the buffer through an external metal heat exchanger, and use of piezo thermo-electric heater/cooler controls.  
         [0015]     These means are limited in their ability to provide a compact apparatus for maintaining consistent and uniform thermal control across the area encompassing the front and back of the electrophoresis gels. The heat sinks exchange heat on only one side of the gel; the regulation of power to the gels cannot control regional hot spots and obviously limits the application of high wattage to the gels; the internal heat exchanger again exchanges heat on only one side of the gel and does not actively circulate buffer, resulting in vertical thermal gradients within the buffer chamber; immersing the gels in a heater tank is cumbersome, in that it requires a large volume of buffer and cannot cool the gels; and circulating the buffer through tubing immersed in an ice water bath is also cumbersome, and makes difficult fine control of temperature.  
         [0016]     Circulating the buffer through an external metal heat exchanger provides the most satisfactory temperature control. However, with the current electrophoresis systems, two pumps and heat exchangers would be required to assure uniformity of temperature and separation of the buffer fluids between the cathode and anode chambers. Further, with current electrophoresis systems, circulation of buffer within the chambers and across the gels is random and undirected, which may result in vertical and horizontal thermal gradients.  
         [0017]     Moreover, for electrophoretic separation, the first and second buffer solutions must be isolated from one another. To provide isolation, prior art electrophoresis systems use various methods, among which is use of a buffer core to which the gel cassettes are secured during electrophoresis. Previously known electrophoresis systems using a buffer core commonly use a buffer core subassembly containing clamps or latches that secure the gel cassettes to the buffer core. Once the cassettes are secured, the buffer core subassembly must then be loaded in the container prior to electrophoretic separation. For example, in prior art systems that use a clamping mechanism, a user generally must first construct a clamping subassembly that is then loaded into the container prior to performing electrophoresis. It would be desirable to provide a clamping device that is easier to use and does not require additional or moving parts. For example, there would be no need to configure, assemble, or adjust a clamp or other adjustable part.  
         [0018]     Various prior art patents have proposed apparatus and methods for simultaneously running multiple gels, but many potential problems exist, including ineffective temperature control on both sides of the gel cassettes, ineffective or inconvenient clamping of gel cassettes, and inability to apply a uniform electrical field to all of the gels.  
         [0019]     For example, U.S. Pat. No. 6,451,193 to Fernwood et al. (Fernwood) describes a single cell configured to receive multiple slab gels for conducting simultaneous electrophoresis experiments. The multitude of slab gels are supported vertically and parallel to one another while immersed in a buffer solution. A voltage is applied to all gels simultaneously while temperature control is achieved by circulating the buffer solution upward through the cell and cooling the circulating buffer solution with a tube heat exchanger positioned on the floor of the cell.  
         [0020]     There are several drawbacks associated with the electrophoresis system described in Fernwood, and in particular, the relative complexity of the buffer circulation and cooling mechanisms that are employed. For example, with respect to the cooling mechanism, circulation is effected by a coolant pump and chilling of the coolant prior to its return to the tank requires an external chilling or refrigeration unit. With respect to the buffer circulation mechanism, an external pump and an external circulation line are required. All of these external components make the device more cumbersome, and proper circulation of the buffer and coolant depend on proper and consistent operation of several external components.  
         [0021]     Another drawback associated with the device described in Fernwood is that the coolant is only circulated in tubing at the bottom of the tank, which may result in inconsistent cooling of a vertically upright gel cassette. Moreover, the coolant traverses the floor of the tank four times before further chilling of the coolant occurs. Therefore, coolant properties may vary at different locations that the coolant traverses the floor of the tank.  
         [0022]     In view of these drawbacks of previously known systems, it would be desirable to provide apparatus and methods for conducting multiple electrophoresis experiments that employ a passive cooling mechanism to avoid the need for complex, ineffective or cumbersome active cooling mechanisms.  
         [0023]     It also would be desirable to provide apparatus and methods for conducting multiple electrophoresis experiments that uses a simple clamping mechanism, without moving parts, to secure the gel cassettes in place and provide an effective seal between anode and cathode buffer solutions.  
         [0024]     It further would be desirable to provide apparatus and methods for conducting multiple electrophoresis experiments that employ one lower buffer chamber that is common to all gel cassettes in the container.  
         [0025]     It still further would be desirable to provide apparatus and methods for conducting multiple electrophoresis experiments that consistently control the temperature of the electrophoresis gels, regardless of the number of gels being run at any given time, particularly while maintaining a uniform electric field across the width of the gel  
       SUMMARY OF THE INVENTION  
       [0026]     In view of the foregoing, the present invention provides an apparatus and methods for conducting multiple electrophoresis experiments that consistently control the temperature of the electrophoresis gels, regardless of the number of gels being run at any given time. This temperature control is achieved for electrophoretic separation concurrently in a plurality of gels, by using passive thermal management to avoid the need for complex, ineffective or cumbersome active cooling mechanisms.  
         [0027]     Furthermore, the apparatus and methods for conducting electrophoretic separation of the present invention provide homogeneous electric fields across the width of a gel. The temperature and electric field control of the present invention results in dye fronts that are within 10 mm from each other, within 5 mm from each other, or within 25%, 15%, 10%, or 5% of the length of a run.  
         [0028]     In yet another embodiment, the present invention provides an apparatus and methods for conducting electrophoretic separation concurrently in a plurality of gels using a simple clamping mechanism, without moving parts, to secure the gel cassettes in place and provide an effective seal between anode and cathode buffer solutions.  
         [0029]     In yet another embodiment, provided herein is an apparatus and methods for conducting multiple electrophoresis experiments that employ one lower buffer chamber that is common to all gel cassettes in the container.  
         [0030]     Accordingly, provided herein in a first embodiment, is an apparatus or a system for removably positioning one or more gel cassettes for electrophoresis, each gel cassette having a first face, a second face, and a gel disposed therebetween. The apparatus comprises a fluid-retaining container and means for apportioning the interior of the container into a plurality of volumes upon the positioning of one or more gel cassettes within the container. Each of the volumes is proportionate to the number of positioned cassette faces with which it is in fluid contact. Accordingly, the upper buffer volumes within buffer cores of the container are within 75% of each other, and lower buffer volumes per gel are within 75% of each other when the chamber has different numbers of gels, for example from 3 to 50 gels.  
         [0031]     In one series of embodiments, the apportioning means include means that are integral to the container and at least one means that is removably engageable within the container.  
         [0032]     In some embodiments, the apparatus further comprises means for concurrently establishing an electric field within the gel of each positioned cassette, wherein the field is substantially uniform among all of the positioned gels and substantially homogeneous across the width of each gel. Substantially uniform means that the field is within 10% among all positioned gels. In certain of these embodiments, the field establishing means include means integral to the container and at least one means removably engageable within the container. The combination of field uniformity and temperature regulation of apparatuses and methods of the present invention results in dye fronts that are within 15 mm from each other, within 10 mm from each other, within 5 mm from each other, or a traveled distance difference that is no more than 25%, 15%, 10%, 5%, 4%, 3%, or 2% of the length of a run at the end of an electrophoretic separation run. Therefore, for a 10% distance difference for a 65 mm gel length electrophoretic run, the dye fronts between different gels in the container at the end of the run are within 7 mm of each other.  
         [0033]     In certain embodiments, each of the apportioning means and the field-establishing means includes both means that are integral to the container and means that are removably engageable therein. In particularly useful embodiments, each one of the removably engageable field establishing means is integrated into one of the at least one removably engageable apportioning means to form a buffer core body.  
         [0034]     Typically, the apparatus is configured so that the plurality of apportioned volumes includes at least one first volume and a single second volume; the positioned cassettes render each of the at least one first volumes fluidly noncommunicating with the single second volume. In embodiments that include at least one buffer core, each of the at least one first volumes is internal to a buffer core.  
         [0035]     In various embodiments, the integral apportioning means include, for each buffer core potentially engageable within the container, a set of opposing first and second bulkheads.  
         [0036]     The opposing bulkheads of each set are typically configured to provide an inward pressure upon gel cassettes assembled to the buffer core body engaged therebetween.  
         [0037]     For example, in certain embodiments the bulkheads of each opposing set each comprises at least one upper protrusion, the protrusions configured to apply an inward pressure upon gel cassettes assembled to the buffer core engaged therebetween. In some embodiments, the bulkheads of each opposing set each further comprises at least one lower protrusion, the lower protrusions configured to apply an inward pressure upon gel cassettes assembled to the buffer core engaged therebetween.  
         [0038]     In embodiments particularly useful in establishing a uniform field across each of the gels within positioned cassettes, at least one of the opposing bulkheads of each set includes a plurality of lower wedge-shaped protrusions, the plurality of wedge-shaped protrusions collectively making discontinuous contact to the cassette assembled to the buffer core engaged therebetween.  
         [0039]     In typical embodiments, each of the bulkheads includes an aperture disposed through the bulkhead between its upper and lower protrusions.  
         [0040]     In some embodiments, the thickness of each of the end walls of the container is greater than that of each of the side walls of the container.  
         [0041]     In another aspect, the invention provides a container having a removable lid and a plurality of communicating chambers. Each of the plurality of chambers is configured to receive and engage a buffer core assembly. Each buffer core assembly preferably comprises a buffer core body and first and second cassettes securely coupled to front and back sides of the buffer core body. A space between the buffer core body and the first and second cassettes forms an upper buffer chamber, which is configured to receive a first buffer.  
         [0042]     Each chamber in the container preferably is formed using first and second opposing bulkheads. The first and second bulkheads each have a laterally protruding upper region, recessed central region, and an aperture disposed through the recessed central region. Further, at least one wedge-shaped member is disposed beneath the aperture in the first bulkhead, and at least one wedge-shaped member is disposed beneath the aperture in the second bulkhead.  
         [0043]     In application, each buffer core assembly is configured to be inserted between the first and second bulkheads of a desired chamber. As the buffer core assembly is inserted, the first and second gel cassettes contact the wedge-shaped members of the first and second bulkheads, respectively. This causes the first and second cassettes to be pressed inward towards the buffer core body. The pressure applied by the wedge-shaped members, along with the pressure applied by the laterally protruding upper regions of the bulkheads, provides an effective seal for the upper buffer chamber. Advantageously, since the wedge-shaped members are an integral component of the container, no moving clamping mechanisms are required to secure the gel cassettes in place and provide an effective seal between anode and cathode buffers.  
         [0044]     In accordance with one aspect of the present invention, a common lower buffer chamber is formed when a plurality of buffer core assemblies are placed in adjacent chambers of the container. Specifically, the common lower buffer chamber is formed as a space between a second cassette of a first buffer core assembly and a first cassette of a second buffer core assembly, a second cassette of a second buffer assembly and a first cassette of a third buffer assembly, and so forth. Therefore, when a second buffer is poured into the common lower buffer chamber, the second buffer may be placed in fluid communication with each of the gel cassettes, regardless of the number of cassettes employed.  
         [0045]     In a preferred method, each buffer core assembly to be used is inserted into a respective chamber of the container, then secured using the clamping force applied by the wedge-shaped members of the bulkheads, as described above. A predetermined volume of a first buffer then is poured into each upper buffer chamber, one at a time. In a next step, a corresponding predetermined volume of a second buffer is poured into the common lower buffer chamber at one location, then flows through various open spaces in the container to contact the outer surfaces of the gel cassettes in the container. In effect, the inner surfaces of each gel cassette are in contact with the first buffer in the upper buffer chamber, while the outer surfaces are in contact with the second buffer filling in the common lower buffer chamber.  
         [0046]     In a next step, the removable lid is placed on top of the container. The removable lid is coupled to first and second cables, which are adapted to be coupled to a power supply or charging means. The removable lid also is electrically coupled to negative and positive wires that are in electrical contact with each of the first and second buffers, respectively.  
         [0047]     When an electrical potential is applied across each of the negative and positive wires, an electric field on each of the gels in the container is developed. The electrical fields in the gels effect molecular separation of the electrophoresis samples in the gels The electrical fields in the gels effect molecular separation of the electrophoresis samples in the gels since the gels act as the only conductive path between the buffer solutions which are charged at opposite polarities.  
         [0048]     In accordance with one aspect of the present invention, passive thermal management techniques are used to control the temperatures of the gels in the cassettes. The passive thermal management techniques rely on the heat sinking capabilities of the first and second buffers to maintain a relatively equal temperature on the outer and inner plates of the cassette. According to passive thermal management provided herein, the temperature between upper buffers in separate buffer cores within a container at the end of an electrophoretic separation is within 25, 20, 15, 10, 5, 4, 3, 2, or 1° C. Furthermore, the temperature difference between an upper buffer and a lower buffer is within 25, 20, 15, 10, 5, 4, 3, 2, or 1° C. at the end of an electrophoretic separation performed using the apparatus or methods. Furthermore, according to passive thermal management provided herein, the temperature between gels at the end of an electrophoretic separation is within 25, 20, 15, 10, 5, 4, 3, 2, or 1° C. In certain illustrative examples, the temperature between upper buffer cores, between upper and lower buffers, and between gels is within 10° C. at the end of an electrophoretic separation.  
         [0049]     The heat sink principles that are used in conjunction with the present invention take into account several variables, including the specific heat of the buffers, the mass of the buffers added, the change in temperature, the current and voltage applied to the gels, and other variables. By knowing the voltage and current applied, knowing the time duration required to complete separation, knowing the specific heat of the buffer, and by calculating the mass of buffer to be added, the temperature increase of the gels can be kept below a predetermined threshold (for example, 60° C.). Furthermore, the apparatus and methods of the present invention ensure that the same temperature is maintained on the outer and inner surfaces of each gel cassette to avoid slanting of the migrating bands in a sample. The present invention also ensures that each gel in the apparatus is exposed to the same thermal environment as each of the other gels.  
         [0050]     If desired, a dam system may be used in conjunction with the apparatus of the present invention to run fewer than the maximum number of gels that the container can run. The dam interrupts flow to certain areas of the common lower buffer chamber, based on its placement in the container. For example, if the container has the capacity to run six gels simultaneously, but a user only wishes to run two gels, the dam is positioned such that flow in the lower buffer chamber is interrupted to the other four regions of the container.  
         [0051]     The dam system, which preferably is adapted to be coupled to the buffer core assembly in lieu of one of the cassettes, is configured to displace half the volume of an upper buffer chamber. Therefore, when an odd number of gels are being run, only one-half of buffer is poured into the upper buffer chamber, relative to when two cassettes are used in a buffer core assembly. Accordingly, a proportional amount of buffer is used, regardless of whether an even or odd number of gels are being run, thereby ensuring that the temperatures on the outer and inner surfaces of the cassettes will remain the same during electrophoresis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments, in which:  
         [0053]      FIGS. 1A-1B  are, respectively, an exploded view of apparatus of the present invention and a sectional view of the container of  FIG. 1A  taken along longitudinal axis A-A;  
         [0054]      FIGS. 2A-2C  are, respectively, perspective views of a buffer core assembly of the present invention with no cassettes, the buffer core assembly with one gel cassette coupled thereto, and the buffer core assembly with two gel cassettes coupled thereto;  
         [0055]      FIGS. 3A-3B  are, respectively, front and side views of the buffer core assembly of  FIGS. 2A-2C  with no cassettes shown;  
         [0056]      FIG. 4A-4B  are, respectively, a front view of a gel cassette and a cross sectional view of the gel cassette taken along line B-B of  FIG. 4A ;  
         [0057]      FIGS. 5A-5B  are, respectively, a perspective view of the apparatus of  FIG. 1A  in an assembled state and a sectional view of the apparatus in the assembled state, as taken along longitudinal axis A-A of  FIG. 1A ;  FIG. 5B  illustrates for exemplary purposes the use of six gel cassettes without a dam;  
         [0058]      FIG. 6  is an exploded view showing a removable lid that may be used in conjunction with apparatus of the present invention;  
         [0059]      FIG. 7  is a perspective view showing the removable lid of  FIG. 6  in an assembled state;  
         [0060]      FIGS. 8A-8B  are, respectively, front and rear perspective views of a dam that may be used in conjunction with apparatus of the present invention;  
         [0061]      FIG. 9  is a perspective view depicting the dam of  FIGS. 8A-8B  coupled to a buffer core assembly; and  
         [0062]      FIGS. 10A-10C  are sectional views illustrating the dam of  FIGS. 8-9  being used to block flow to various regions of the container of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0063]     Referring now to  FIGS. 1A-1B , apparatus and methods for performing multiple electrophoresis experiments in accordance with the present invention are described.  
         [0064]     As shown in  FIG. 1A , electrophoresis system  10  comprises container  20  having plurality of communicating chambers  30   a - 30   c , and further comprises plurality of buffer core assemblies  60   a - 60   c  that correspond to respective chambers  30   a - 30   c . Although three chambers and three buffer core assemblies are illustratively depicted herein, greater or fewer chambers and buffer core assemblies may be employed, as will be apparent to one skilled in the art from the following detailed description.  
         [0065]     Container  20  preferably comprises first and second side walls  21  and  22 , closed bottom  23 , and first and second end walls  24  and  26 , as shown in  FIG. 1A . Container  20  is open at the top for receiving buffer core assemblies  60   a - 60   c . Each buffer core assembly  60   a - 60   c  preferably comprises buffer core body  61  and a pair of gel cassettes  80   a  and  80   b , as will be described in greater detail hereinbelow with respect to  FIGS. 2A-2C .  
         [0066]     Container  20  further comprises negative bus bar  44  and positive bus bar  45 . Negative and positive bus bars  44  and  45  preferably are disposed atop first and second side walls  21  and  22 , respectively, as shown in  FIG. 1A . One or more screws  41 , or other means for attaching the bus bars, may be inserted into corresponding holes  42  to secure the bus bars to the side walls.  
         [0067]     Negative bus bar  44  is electrically coupled to pole conductor  48 , and further coupled to plurality of sockets  46   a - 46   c , which correspond to chambers  30   a - 30   c  of container  20 . Positive bus bar  45  is electrically coupled to pole conductor  49 , and further coupled to plurality of sockets  47   a - 47   c , which correspond to chambers  30   a - 30   c , respectively, as depicted in  FIG. 1A .  
         [0068]     In a particularly useful embodiment of the present invention, black and red polarity tabs  37  and  38  are affixed to container  20  on opposing lateral sides of the container, as depicted in  FIG. 1A , to visually facilitate proper electrical attachments of buffer core assemblies  60  and lid  50  (see  FIG. 6 ). As can be seen in  FIG. 1A , each buffer core assembly  60   a - 60   c  preferably comprises corresponding polarity tabs  37  and  38  to visually facilitate proper insertion of the buffer core assemblies into container  20 .  
         [0069]     Referring now to  FIG. 1B , a sectional view of container  20  of  FIG. 1A  is illustrated to describe various internal features of electrophoresis system  10 .  
         [0070]     Container  20  has a plurality of chambers  30   a - 30   c , which are adapted to receive buffer core assemblies  60   a - 60   c , respectively. In a preferred embodiment, each chamber  30  is formed by first and second opposing bulkheads  110   a  and  110   b . Each bulkhead  110  preferably comprises a laterally protruding (i.e., protruding along the X axis, see  FIG. 1A ) upper region  112  and a recessed central region  111  having an aperture  113  disposed therethrough, as depicted in  FIG. 1B .  
         [0071]     Each bulkhead  110  further preferably comprises at least one wedge-shaped member  115  disposed beneath apertures  113 . The wedge-shaped member preferably is manufactured using a suitable substantially noncompliant compound, such as plastic.  
         [0072]     First and second bulkheads  110   a  and  110   b  have substantially identical configurations, with the main exception that laterally protruding upper region  112   a  of first bulkhead  110   a  is situated slightly higher with respect to the side walls of container  20  than laterally protruding upper region  112   b  of second bulkhead  110   b . The slight height differential facilitates insertion of buffer core assemblies  60   a - 60   c , because the buffer assemblies may be initially inserted at a slight vertical angle. The slight vertical angle allows the buffer core assemblies to slide into their respective chambers with little or no frictional interference, until each buffer core assembly contacts the wedge-shaped members at the bottom of the chamber. When each buffer core assembly contacts the wedge-shaped members, the wedge-shaped members force a vertical positioning of the buffer core assembly, as described in greater detail hereinbelow with respect to  FIGS. 5A-5B , due to the clamping action between the two opposing lower wedges  115  and also between the two opposing upper protrusions  112 .  
         [0073]     Referring now to  FIGS. 2-4 , preferred features of buffer core assembly  60  and gel cassettes  80  are described in greater detail. Each buffer core assembly  60  preferably comprises buffer core body  61  and a pair of gel cassettes  80   a  and  80   b , as shown in  FIG. 2C .  
         [0074]     Buffer core body  61  comprises upraised side walls  62  and  63 , and lower base  64  disposed between the side walls, as shown in  FIG. 2A . Buffer core body  61  further comprises handle  68  and horizontal beam  67  disposed between upraised side walls  62  and  63 . First and second male conductors  102  and  103  are securely coupled to outer regions  68   a  and  68   b  of handle  68 , respectively, as shown in  FIG. 3A .  
         [0075]     First male conductor  102  is coupled to wire  104 . A portion of wire  104  runs in groove  75 , which is formed in a lateral surface of side wall  62 , as shown in  FIG. 3B . Wire  104  continues to run underneath buffer core body  61  via base channel  76 . Wire  104  preferably spans a substantial portion of base channel  76 , and is coupled to lower base  64  using a loop attachment to the underside of lower base  64 . In this manner, wire  104  may be exposed to a buffer that is disposed on the exterior of side wall  62  and underneath buffer core body  61 , as will be described in detail hereinbelow.  
         [0076]     Second male conductor  103  is coupled to wire  105 . Wire  105  runs through first aperture  69   a  of horizontal beam  67 , and continues to extend through second aperture  69   b  at the other end of beam  67 , as depicted in  FIG. 3A . Wire  105  is coupled to horizontal beam  67  of buffer core body  61 , preferably using a loop attachment.  
         [0077]     Buffer core assembly  60  further comprises first and second recesses  73   a  and  73   b , which are disposed in side walls  62  and  63 , respectively. Recesses  73   a  and  73   b  are disposed on front side  71  of buffer core body  61 , as shown in  FIGS. 2A and 3A , and also are disposed in side walls  62  and  63  on back side  72  of buffer core body  61 .  
         [0078]     In application, first gel cassette  80   a  is placed in the recesses that are disposed on back side  72  of buffer core body  61 , as depicted in  FIG. 2B . Second gel cassette  80   b  then is placed in recesses  73   a  and  73   b  on front side  71  of buffer core body  61 , as depicted in  FIG. 2C . Each gel cassette rests upon base supports  79 , which are provided on front and back sides  71  and  72  of buffer core body  61 .  
         [0079]     Upper buffer chamber  130  is formed between first gel cassette  80   a , second gel cassette  80   b , and side walls  62  and  63  of buffer core body  61 , as depicted in  FIG. 2C . Upper buffer chamber  130  is configured to receive a first buffer, such that the first buffer is placed into submerged contact with wire  105  to provide a charged buffer, as described in greater detail hereinbelow.  
         [0080]     Referring back to  FIG. 2A , front and back sides  71  and  72  of buffer core body  61  preferably are provided with U-shaped grooves  77 , which are configured for fitting and holding one or more resilient strips  78  as a fluidic seal between gel cassettes  80   b  and  80   a , respectively, and buffer core body  61 . The seal provided by resilient strips  78  ensures electrical and fluidic isolation of the first buffer disposed in upper chamber  130  with a second buffer that is disposed in a lower chamber, as described in detail hereinbelow.  
         [0081]     Referring now to  FIGS. 4A-4B , features of gel cassettes  80  are described in greater detail. Each gel cassette  80   a  and  80   b  is substantially identical, and has an outer surface  81   a  and an inner surface  81   b . It includes a pair of plates that are of thin wall construction. The plates are commonly referred to as the divider or divider plate  82  and retainer or retainer plate  84 . Retainer plate  84  is slightly shorter in height than the divider plate  82 .  
         [0082]     Divider  82  is affixed to peripheral ridge  86  along the lateral sides and the bottom periphery of retainer  84  to define an internal gel compartment  88  for holding an electrophoresis gel  90 . As shown in  FIG. 4B , gel compartment  88  has a top or comb opening  92  at the top portion of the cassette for receiving a sample to be electrophoretically separated.  
         [0083]     Located along the lower portion of divider plate  82  and traversing the width of cassette  80  is a slot or opening  96  that opens gel compartment  88  to the exterior of cassette  80  and hence allows a direct electrical coupling with the charged buffer solution.  
         [0084]     Gel cassettes suitable for the present invention are known in the art. In a typical gel cassette, the gel is pre-filled within the internal gel compartment for ease of handling. Top opening  92  is closed with a comb (not shown), and slot  96  is masked closed with a removable tape (not shown). An example of the gel cassettes that are suitable for this application are the 12% Tris-glycine gels sold by INVITROGEN CORPORATION of Carlsbad, Calif., under catalog No. EC6005. Gel cassettes of similar types also are commercially available from other firms.  
         [0085]     Prior to use of cassette  80 , the comb (not shown) and the tape (not shown) disposed over top opening  92  and slot  96 , respectively, are removed. The sample to be analyzed is introduced into gel compartment  88  through comb opening  92  by an appropriate means, such as a pipette. The cassettes with their retainer plates  84  proximal to buffer core body  61  are held to rest within side recesses  73  and base supports  79 , as described hereinabove with respect to  FIGS. 2A-2C . One or more buffer core assemblies  60  then are slidably inserted into a desired chamber  30 , i.e., one of chambers  30   a - 30   c , as depicted in  FIG. 1A .  
         [0086]     Referring now to  FIGS. 5A-5B , plurality of buffer core assemblies  60   a - 60   c  are shown securely disposed in container  20  of  FIGS. 1A-1B . During insertion of buffer core assemblies  60   a - 60   c  into chambers  30   a - 30   c , laterally protruding upper regions  112   a  and  112   b  of opposing bulkheads  110   a  and  110   b , respectively, apply an inward pressure against first and second cassettes  80   a  and  80   b  of each buffer core assembly. In effect, laterally protruding upper regions  112   a  and  112   b  serve to guide the buffer core assemblies into their respective chambers.  
         [0087]     As each buffer core assembly further is inserted into its respective chamber  30 , each gel cassette  80  is urged in an inward direction, i.e., towards buffer core body  61 , by a force applied by wedge-shaped members  115 , as shown in  FIG. 5B . At this time, each gel cassette  80  is pressed firmly against resilient strips  78  (see  FIGS. 2A-2C ). In particular, first cassette  80   a  is pressed firmly against strips  78  by forces applied by wedge-shaped members  115  and laterally protruding upper region  112   a , while second cassette  80  is pressed firmly against strips  78  by forces applied by wedge-shaped members  115  and laterally protruding upper region  112   b.    
         [0088]     The forces applied by wedge-shaped members  115  against gel cassettes  80   a  and  80   b  ensure fluidic and electrical isolation between a second buffer present in common lower buffer chamber  140  and a first buffer present in each of the individual upper buffer chambers  130   a - 130   c . Fluidic and electrical isolation of first and second buffers reduces the risk of electrical grounding of the power supply or other sensitive instruments used in connection with the electrophoresis.  
         [0089]     At about the same time that each buffer core assembly is securely wedged into its chamber, male conductors  102  of buffer core assemblies  60   a - 60   c  engage respective sockets  47   a - 47   c  (see  FIG. 1A ) of positive bus bar  45 . Similarly, male conductors  103  of buffer core assemblies  60   a - 60   c  engage respective sockets  46   a - 46   c  of negative bus bar  44 .  
         [0090]     It should be noted that both male conductors  102  and  103  are disposed on front portion  119  of buffer core body  61 , as depicted in  FIG. 5A . This allows male conductors  102  to align with sockets  47   a - 47   c , and male conductors  103  to align with sockets  46   a - 46   c , but not vice versa. Therefore, each buffer core assembly  60   a - 60   c  cannot be wedged into chambers  30   a - 30   c  unless buffer core assemblies  60   a - 60   c  are properly oriented, thereby ensuring proper electrical connections. As noted above, black and red polarity tabs  37  and  38  may be positioned on container  20  and buffer core assemblies  60   a - 60   c , as depicted, to further facilitate proper alignment of the buffer core assemblies by appropriate visual cues.  
         [0091]     Referring to  FIG. 5B , when a plurality of buffer core assemblies  60  are securely placed in container  20 , a common lower buffer chamber  140  is formed. Specifically, common lower buffer chamber  140  is formed between second cassette  80   b  of first buffer core assembly  60   a  and first cassette  80   a  of second buffer core assembly  60   b , as depicted in  FIG. 5B . Common lower buffer chamber  140  also is formed between second cassette  80   b  of second buffer core assembly  60   b  and first cassette  80   a  of third buffer core assembly  60   c . Further, common lower buffer chamber  140  is formed between first cassette  80   a  of buffer core assembly  60   a  and end wall  26 , and between outer cassette  80   b  of buffer core assembly  60   c  and end wall  24 , as depicted in  FIG. 5B .  
         [0092]     In accordance with one aspect of the present invention, lower buffer chamber  140  allows a second buffer (not shown) to be placed in contact with each buffer core assembly  60   a - 60   c . When the second buffer is poured into any region of lower buffer chamber  140 , the second buffer will be distributed in a substantially equal fashion to the other regions of lower buffer chamber  140 . Specifically, the second buffer will flow through apertures  113  in bulkheads  110   a  and  110   b  (see  FIG. 1B ), between wedge-shaped members  115  via channels  116  (see also  FIG. 1B ), underneath lower buffer core base  64  via channel  74  (see  FIG. 3A ), and around buffer core side walls  62  and  63  via side channels  66  (see  FIG. 3A ). It should be noted that side walls  62  and  63  of buffer core body  61  preferably comprise spacers  65   a  and  65   b , as shown in  FIG. 3A , that are configured to contact a side wall of container  20 . Therefore, when buffer core assembly  60  is disposed in chamber  30 , channel  66  is formed between the side walls of the buffer core body and the side walls of the container to permit flow of the second buffer therebetween.  
         [0093]     Referring still to  FIGS. 5A-5B , in application buffer core assemblies  60   a - 60   c  are first secured within container  20  in the manner as described above. A predetermined volume of a first buffer (not shown) is then typically dispensed separately into each upper buffer chamber  130   a - 130   c  above the comb openings  92  of the cassettes to establish fluid contact with gel  90  in the gel compartments.  
         [0094]     A corresponding, predetermined volume of a second buffer (not shown) then is introduced into lower buffer chamber  140  of container  20 . Pouring the predetermined volume of the second buffer into any region of lower buffer chamber  140  will cause the second buffer to be distributed substantially equally throughout chamber  140 . It should be noted that, in alternative embodiments, the second buffer may be added before the first buffer is added.  
         [0095]     Container  20  is configured such that the volumes between assemblies  60   a  and  60   b , and between  60   b  and  60   c  are approximately twice as great as the volumes between cassette  80   a  of assembly  60   a  and end wall  26 , and between cassette  80   b  of assembly  60   c  and end wall  24 . Therefore, when the second buffer poured into lower buffer chamber  140  settles to a height h, approximately twice as much second buffer will settle between the adjacent buffer core assemblies as will settle between assembly  60   a  and end wall  26 , and assembly  60   c  and end wall  24 .  
         [0096]     For example, if 600 mL of the second buffer is poured into lower buffer chamber  140 , then after the buffer settles in container  20 , approximately 100 mL of the second buffer will settle between first cassette  80   a  of assembly  60   a  and end wall  26 , approximately 200 mL of the second buffer will settle between assemblies  60   a  and  60   b , approximately 200 mL will settle between assemblies  60   b  and  60   c , and approximately 100 mL will settle between second cassette  80   b  of assembly  60   c  and end wall  24 . Therefore, each outer surface of each cassette  80  will have approximately 100 mL of second buffer devoted as a heat sink disposed adjacent the outer surface.  
         [0097]     In a preferred embodiment of this aspect of the present invention, components of container  20  are dimensioned so that equal volumes of second and first buffers are devoted as heat sinks for the outer and inner surfaces  81   a  and  81   b  of each gel cassette  80   a . Therefore, as an example, if 600 mL of second buffer is poured into common lower buffer chamber  140 , as described above, then 200 mL of first buffer should be poured into each upper buffer chamber  60   a - 60   c . Since there are six gel cassettes in container  20 , and two cassettes per upper buffer chamber, then the inner surfaces of each of the six cassettes will have approximately 100 mL of first buffer devoted as a heat sink to the inner surfaces of the cassettes.  
         [0098]     As will be described in greater detail hereinbelow, the actual volumes of first and second buffers may be selected to ensure adequate heat sinking during electrophoresis to keep the temperature of gel  90  below a predetermined threshold.  
         [0099]     Referring now to  FIG. 6 , removable lid  50  is positioned above the top portion of container  20  such that female electric plugs  56  and  58  are aligned with pole conductors  48  and  49 , respectively. As the lid is lowered onto container  20 , the female plugs are coupled with the pole conductors, thereby securing the lid to seat upon the top portion of container  20 .  
         [0100]     Asymmetric mating of removable lid  50  with container  20  preferably is employed to ensure a proper electrical connection. Specifically, in one embodiment, lid  50  will only fit onto container  20  when slot  53  can fit over short tab  25 , and slot  54  can fit over long tab  27 , as illustrated in  FIG. 7 . Thus, as lid  50  is lowered, female electric plug  56  must be aligned with pole conductor  48 , and female electric plug  58  with pole conductor  49 , but not vice versa, to ensure proper electrical connections. In a preferred embodiment of the present invention, lid  50  is transparent to facilitate viewing and evaluation of the gels as they are being run, as described hereinbelow.  
         [0101]     After lid  50  is seated, conductor cables  57  and  59  are coupled to a power supply system or charging means for delivering an appropriate electrical potential to the electrophoresis system. In one embodiment of the present invention, cable  57  is coupled to the power supply to deliver a negative potential, and cable  59  to deliver a positive potential. In practice, the polarity of the electrical potential can be reversibly applied to the buffers, as a matter of choice.  
         [0102]     As a negative electrical potential is applied across pole conductor  48 , the electrical charge also is applied across each wire  105  (see  FIG. 3A ), since each wire  105  is coupled to a male conductor  103 , and each male conductor  103  is electrically coupled to a socket  46   a - 46   c  of negative bus bar  44 . Similarly, a positive electrical potential applied across pole conductor  49  also is applied across each wire  104  (see  FIG. 3B ), since each wire  104  is coupled to a male conductor  102 , and each male conductor  102  is electrically coupled to a socket  47   a - 47   c  of positive bus bar  45 .  
         [0103]     This in turn imposes an electrical potential difference between the first buffer, which is in contact with wire  105 , and the second buffer, which is in contact with wire  104 . Accordingly, the first buffer is negatively charged, while the second buffer is positively charged.  
         [0104]     As discussed hereinabove, gel  90  of cassettes  80   a  and  80   b  is in contact with the first buffers (in upper buffer chambers  130   a - 130   c ), and gel  90  is also in contact with the second buffer in common lower buffer chamber  140 . Therefore, the electrically charged buffers will result in an electrical field in gel  90  between top opening  92  and slot  96  to effect molecular separation of analytes in the sample.  
         [0105]     For optimally reproducible results among gels run concurrently, the electric field provided to each gel should be substantially identical; and for optimal separation within a gel, the electric field should be homogeneous across the gel (i.e., in the direction perpendicular to the direction of analyte migration).  
         [0106]     The apparatus of the present invention provides advantages with respect to both of these parameters in part by the design of container  20 , and in part by the placement of wires  105 , which span the length of the underside of buffer core body  61  (in direction y; as described hereinabove).  
         [0107]     In particular embodiments of container  20 , at least one of opposing bulkheads  112   a  and  112   b  of each set includes a plurality of lower wedge-shaped protrusions  115 , rather than a single wedge-shaped protrusion  115  that extends across the width of bulkhead  112 . The plurality of wedge-shaped protrusions  115  collectively make discontinuous contact with the cassette assembled to the buffer core engaged between the bulkheads, creating channels  116  (see  FIG. 1B ). Channels  116  facilitate the reconvergence of the electric field at the level of cassette slot  96 , facilitating homogeneity across the gel.  
         [0108]     By spanning the underside of buffer core body  61 , wires  105  provide a uniform electric field across the gel cassettes in direction y. Moreover, wires  105  are situated within container  20  such that they provide a substantially uniform electric field to all gel cassettes.  
         [0109]     As mentioned hereinabove, heat is generated during electrophoretic molecular separation within gel  90 , thus creating uneven temperature gradients on the surfaces of the gel, as well as across its thickness. Such problem is effectively mitigated by controlling the surface temperature of the gel cassettes.  
         [0110]     Unlike previously-known apparatus and methods that actively circulate a coolant to control temperature, the present invention employs passive thermal management techniques to effect temperature control of the surface temperatures of gel cassettes  80 . In particular, the dimensions of the apparatus are configured to permit first and second buffers to serve as heat sinks during electrophoresis, when the first and second buffers are disposed in upper buffer chambers  130   a - 130   c  and common lower buffer chamber  140 , respectively. This temperature control is achieved for electrophoretic separation concurrently in a plurality of gels, by using passive thermal management to avoid the need for complex, ineffective or cumbersome active cooling mechanisms. According to passive thermal management provided herein, the temperature between upper buffers in separate buffer cores within a container at the end of an electrophoretic separation is within 25, 20, 15, 10, 5, 4, 3, 2, or 1° C. Furthermore, the temperature difference between an upper buffer and a lower buffer is within 25, 20, 15, 10, 5, 4, 3, 2, or 1° C. at the end of an electrophoretic separation performed using the apparatus or methods provided herein. Since this is typically the maximum temperature difference, the difference during an electrophoresis run is not as great. In one illustrative example, the temperature difference between an upper buffer and a lower buffer, and the temperature between upper buffers of separate buffer cores in the same container, is within 10° C. at the end of an electrophoretic separation performed using the apparatus or methods provided herein. The temperature of the lower buffer can be measured between buffer cores, but in certain illustrative aspects is measured in front of, or in back of, the buffer cores. The front and back lower buffer regions are expected to have a greater temperature differential with the upper buffer than the lower buffer between buffer cores.  
         [0111]     The heat sink principles that are used to select dimensions of the apparatus of the present invention rely primarily on the heat transfer principle that the amount of heat added (“Q”) is equal to the product of specific heat of a substance (“c”), the mass of the substance(“m”) and the change in temperature (“ΔT”, or “T final −T initial ”)  
         [0112]     With respect to the present invention, the amount of heat added Q to the gels can be approximated by determining the product of the current (“i”) and voltage (“V”) that are applied. Therefore, since current i and voltage V are known quantities, the approximate amount of heat added Q to each of the gels can be determined.  
         [0113]     The approximate amount of heat added Q then is set equal to the product of specific heat of the buffer c, mass of the buffer m, and change in temperature ΔT (T final −T initial ). Since the specific heat of the buffer c is known, and the change in temperature is ascertainable (i.e., the initial temperature is known, and the final temperature is selected by the user), then the mass of the buffer to be added can be calculated.  
         [0114]     Therefore, a user can determine how much first and second buffer should be added to keep the temperature increase of gels  90  below a predetermined threshold (i.e., T final , such as 60° C.). Accordingly, in an other embodiment of the present invention, a method is provided for determining a volume of buffer to add to a cathode buffer reservoir or upper buffer reservoir, and the volume of buffer to add to an anode buffer reservoir, or lower buffer reservoir. The method includes selecting a target final temperature for a buffer and identifying an initial temperature for the buffer, and calculating a volume of buffer to add using a change in temperature between the target final temperature and the initial temperature and a specific heat of the buffer.  
         [0115]     In application, it is desirable to maintain approximately the same temperature on outer and inner surfaces  81   a  and  81   b  of cassettes  80  (+/−25, 20, 15, 10, or 5° C. during a run to avoid slanting of the migrating bands in a sample. In a preferred embodiment of the present invention, the specific heat of the first and second buffers are within 25%, 20%, 15%, 10%, 5%, substantially identical, or identical. Therefore, to maintain approximately the same temperature on both sides of the cassette, the volume of first buffer devoted as a heat sink to each inner surface  81   b  is 50% to 150%, 75% to 125%, 85% to 115%, or 90% to 110% of the volume of second buffer devoted as a heat sink to each outer surface  81   a.    
         [0116]     Also, since heat is transferred to the effective heat sinks through faces of the cassettes, the inner and outer faces of the cassettes preferably are equal in area. Therefore, the heat flux out of one face is equal to the heat flux out of the other face, so long as the heat sink temperatures are equal.  
         [0117]     In a preferred embodiment of the present invention, end walls  24  and  26  of container  20  each comprise thickness t 1 , as depicted in  FIG. 5B , which is greater than a structural thickness required to support the lid and contain the lower buffer in lower buffer chamber  140 . The enhanced thickness t 1  of end walls  24  and  26  serves to insulate the lower buffer in lower buffer chamber  140  from convective or radiant heat loss due to lower temperatures present outside of the container.  
         [0118]     In particular, enhanced thickness t 1  of end walls  24  and  26  serves to insulate the lower buffer present between end wall  26  and first cassette  80   a  of buffer core assembly  60   a , and between end wall  24  and second cassette  80   b  of buffer core assembly  60   c ; these end volumes of buffer have greater exposure to a wall of container  20  than do volumes defined further internal to container  20 . By appropriately increasing the thickness of the end walls, increasing their insulating capacity, the temperature of the lower buffer present in the vicinity of end walls  24  and  26  is within 25° C. to the temperature of the lower buffer present in interior regions of container  20 , thereby facilitating consistent runs for all gels in the container.  
         [0119]     Similarly, side walls  21  and  22  of container  20  may have a chosen thickness designed to reduce radiant or convective heat loss through the side walls. However, if desired, side walls  21  and  22  may have a reduced thickness that allows for some heat loss through the side walls. In such cases, the heat loss may be accounted for in thermal calculations to ensure that a desired buffer temperature is achieved. Because side walls  21  and  22  are common to all chambers (or apportioned volumes), the lower buffer present in lower buffer chamber  140  will still have a temperature throughout all regions of container  20  that is within 35° C., 25° C., 15° C., 10° C., or 5° C., thereby facilitating relatively consistent electrophoretic conditions regardless of the number of gels being run.  
         [0120]     As described hereinabove, container  20  is configured such that the volumes between assemblies  60   a  and  60   b , and  60   b  and  60   c  are approximately twice as great as the volumes between cassette  80   a  of assembly  60   a  and end wall  26 , and cassette  80   b  of assembly  60   c  and end wall  24 . Therefore, when the second buffer poured into lower buffer chamber  140  settles to a height h, approximately twice as much second buffer volume will settle between the adjacent buffer core assemblies as will settle between assembly  60   a  and end wall  26 , and assembly  60   c  and end wall  24 . In the example described hereinabove, if 600 mL of the second buffer is poured into lower buffer chamber  140 , then after the buffer settles in container  20 , approximately 100 mL of the second buffer will settle between first cassette  80   a  of assembly  60   a  and end wall  26 , approximately 200 mL of the second buffer will settle between assemblies  60   a  and  60   b , approximately 200 mL will settle between assemblies  60   b  and  60   c , and approximately 100 mL will settle between second cassette  80   b  of assembly  60   c  and end wall  24 . Therefore, each outer surface  81   a  of each cassette  80  will have approximately 100 mL of second buffer devoted as a heat sink disposed adjacent the outer surface.  
         [0121]     Since the apparatus of the present invention is configured to simultaneously run any number of gels, temperature control is scalable to the number of gels being run. Advantageously, by placing a dam into the system to seal off the unused regions, as described hereinbelow with respect to  FIGS. 8-10 , a proportional volume of the second buffer can always be poured into lower buffer chamber  140 , regardless of the number of gels being run, to maintain a proper heat sink on the outer surface of each cassette being run. By “proportional volume” or “proportionate volume,” is meant that a volume of buffer is added to a buffer chamber such that the volume per gel is maintained within 75% of each other. In other words, if 100 milliliters of a lower buffer is used when two gels are included within an apparatus disclosed herein, then no less than 75 milliliters per gel of lower buffer would be used when three or more gels are present within the apparatus. In another aspect, a volume of buffer is added to a buffer chamber such that the volume per gel is maintained within 80%, 85%, 90%, 95%, or 99% of each other. In certain illustrative examples, when 6 gels are present within the apparatus, 640-700 milliliters of lower buffer is used, when 5 gels are present within the apparatus 550-610 milliliters of lower buffer are used, when 4 gels are present within the apparatus 480-520 milliliters of buffer are used, when 5 gels are present within the apparatus 340-380 milliliters of buffer are used. In another illustrative embodiment, between 75 and 150 milliliters of lower buffer are used per gel in the apparatus, between 100 and 135 milliliters, between 110 and 130 milliliters per gel, or in certain illustrative embodiments, between 112 and 125 milliliters per gel. In the illustrative examples discussed above, when 2 gels are present within a buffer core of the apparatus, between 225 and 275, for example 250 mLs of upper buffer are used. When 1 gel is present within a buffer core of the apparatus, between 150 and 180 mLs, for example 165 mLs, of upper buffer are used.  
         [0122]     In one example, if only two gels are being run, as described in  FIG. 10B  hereinbelow, then 225 mL of second buffer can be poured into lower buffer chamber  140 . If three gels are being run, then 360 mL of second buffer can be poured into lower buffer chamber  140 . Since the dam described hereinbelow prevents the flow of second buffer into the unused regions of the container, the level of the buffer will still rise to a level that is close to, or exactly at ‘h’. Therefore, the outer surface of each cassette will always have approximately 125 mL +/−25% of second buffer devoted as a heat sink, regardless of the number of gels being run.  
         [0123]     Referring now to  FIGS. 8-10 , a dam system that may be used in conjunction with electrophoresis system  10  of  FIGS. 1-7  is described. The dam system is used to control the volume of buffer used as a heat sink for the upper buffer chamber and lower buffer chamber. For example, when a dam is used between 30% and 80%, 40% and 75%, 40% and 70%, 50% and 70%, or 60% and 70% of the volume of the first buffer are poured into the upper buffer chamber in the presence versus absence of the dam. Container  20  can run a maximum number of gels  90  simultaneously. In the embodiments described hereinabove, container  20  is depicted as having the capability of running a maximum of six gels simultaneously, although it will be apparent to one skilled in the art that the maximum capacity may be greater or fewer than six gels. When a user wishes to run fewer gels than the maximum capacity, flow to other regions of the container must be interrupted to ensure that the proper volume of second buffer in lower buffer chamber  140  is devoted as a heat sink to each of the gel cassettes that are actually being used.  
         [0124]     Referring now to  FIGS. 8A-8B , dam  200 , which may be employed to interrupt flow to unused regions of container  20 , preferably comprises central section  202 , protruding front section  204 , and rear section  206 . Dam  200  is configured to engage buffer core body  61  such that outer portion  203  of central section  202  is positioned in recesses  73   a  and  73   b  (see  FIG. 3A ) of buffer core body  61 . Outer portion  203  is positioned against resilient strips  78  of  FIG. 2A  in a manner similar to the positioning of gel cassettes  80   a  and  80   b , as described hereinabove. When outer portion  203  is positioned in recesses  73   a  and  73   b , and rests upon base support  79 , protruding front section  204  extends approximately halfway into upper buffer chamber  230  of buffer core assembly  160 , as depicted in  FIGS. 10A-10C  hereinbelow. Therefore, upper buffer chamber  230  of buffer core assembly  160  has only half the volume as upper buffer chamber  130  of buffer core assembly  60 , which employs two cassettes.  
         [0125]     At this time, rear section  206  of dam  200  faces away from upper buffer chamber  230 . Rear section  206  preferably has a U-shaped slot  210  configured to receive and hold resilient strip  211 , as shown in  FIG. 9 . As will be described in further detail hereinbelow, resilient strip  211  is configured to engage bulkhead  110   b  such that flow to aperture  113  of the bulkhead is inhibited.  
         [0126]     Red and black polarity tabs  37  and  38  may be disposed on opposing lateral sides of dam  200  to facilitate coupling of dam  200  to buffer core body  61  in a proper orientation, as depicted in  FIG. 9 .  
         [0127]     Referring now to  FIGS. 10A-10C , illustrative uses of dam  200  in container  20  are described. In  FIG. 10A , an arrangement is described whereby a user can run only one gel in container  20 . Buffer core assembly  160  has first gel cassette  80   a  coupled to front side  71  of buffer core body  61 , and dam  200  coupled to back side  72  of buffer core body  61 . Buffer core assembly  160  is inserted into chamber  30   a  of container  20  as described hereinabove. Specifically, buffer core assembly  160  is inserted between laterally protruding regions  112   a  and  112   b  of bulkheads  110   a  and  110   b , respectively, and then urged downward. Wedge-shaped members  115  then urge cassette  80   a  and dam  200  in an inward direction against resilient strips  78 , thereby securing buffer core assembly  160  within chamber  30   a  of container  20 .  
         [0128]     In  FIG. 10A , since only one gel is being run, dam  200  is employed to block flow to the rest of container  20 . In accordance with one aspect of the present invention, U-shaped strip  211  of dam  200  helps ensure that the second buffer in lower buffer chamber  140  does not flow into chambers  30   b  and  30   c , which would compromise the heat sinking ability of the second buffer when only one gel is run.  
         [0129]     In a next step, a first buffer (not shown) then is poured into upper buffer chamber  230   a , and a second buffer (not shown) is poured into lower buffer chamber  140 . Since only one gel is being run in buffer core assembly  160 , only one-half of the volume of the first buffer is required in upper buffer chamber  230   a , relative to using two gel cassettes in the buffer core assembly. This is because front section  204  of dam  200  protrudes halfway into upper buffer chamber  230   a , as shown in FIB.  10 A.  
         [0130]     For example, when 100 mL of first buffer is poured into upper buffer chamber  230   a , 100 mL of second buffer is poured into lower buffer chamber  140  between the outer surface of cassette  80   a  and end wall  26 . Therefore, 100 mL of first and second buffers are devoted as heat sinks for the inner and outer surfaces of cassette  80   a . Accordingly, the temperature on outer and inner surfaces  81   a  and  81   b  of cassette  80   a  will be within 25° C., 15° C., 10° C., or 5° C. during electrophoresis.  
         [0131]     Protruding front section  204  of dam  200  preferably is configured to reduce radiant or convective heat loss through the dam. For example, a sufficient thickness associated with protruding front section  204  may be selected to reduce heat loss through the dam. This approach is similar to the that described hereinabove for reducing heat loss through end walls  24  and  26  of container  20 . Like the end walls, heat loss through dam  200  may be reduced by varying the thickness of section  204  to facilitate consistent temperature properties during electrophoresis runs, regardless of the number of gels being run.  
         [0132]     Referring now to  FIG. 10B , an arrangement is described whereby a user can run only two gels in container  20  simultaneously. Buffer core assembly  60   c  having first and second gel cassettes  80   a  and  80   b  is inserted into and secured within chamber  30   c  of container  20  as described hereinabove. Then, buffer core assembly  160  having dam  200  is inserted into chamber  30   b , as shown in  FIG. 10B .  
         [0133]     In the arrangement of  FIG. 10B , U-shaped strip  211  of dam  200  is configured to block flow through aperture  113  of second bulkhead lob of chamber  30   b . Therefore, U-shaped strip  211  helps ensure that the second buffer in lower buffer chamber  140  does not flow into chambers  30   a  and  30   b.    
         [0134]     A first buffer (not shown) is poured into upper buffer chamber  130   c , and a proportional amount of a second buffer (not shown) is poured into lower buffer chamber  140 . For example, when 200 mL of first buffer is poured into upper buffer chamber  130   c , and 200 mL of second buffer is poured into lower buffer chamber  140 , then 100 mL of first buffer is devoted as a heat sink for each of the inner surfaces of cassettes  80   a  and  80   b , and 100 mL of second buffer is devoted as a heat sink for each of the outer surfaces of cassettes  80   a  and  80   b . Accordingly, the temperature on the outer and inner surfaces  81   a  and  81   b  of cassette  80   a  will be approximately the same, assuming the specific heat of the buffers are substantially identical.  
         [0135]     Referring now to  FIG. 10C ., an arrangement is described whereby a user can run only three gels in container  20  simultaneously. Buffer core assembly  60   a  having first and second gel cassettes  80   a  and  80   b  is inserted into and secured within chamber  30   a  of container  20 , as described hereinabove. Then, buffer core assembly  160  having first cassette  80   a  and dam  200  is inserted into chamber  30   b , as shown in  FIG. 10C .  
         [0136]     In the arrangement shown in  FIG. 10C , U-shaped strip  211  of dam  200  is configured to block flow through aperture  113  of second bulkhead  110   b  of chamber  30   b . Therefore, U-shaped strip  211  helps ensure that the second buffer in lower buffer chamber  140  does not flow into chamber  30   c.    
         [0137]     A first buffer (not shown) is poured into upper buffer chamber  130   a . Then, one-half of the first buffer volume poured into chamber  130   a  is poured into chamber  230   b . A proportional amount of a second buffer (not shown) then is poured into one of the regions of lower buffer chamber  140  shown in  FIG. 10C . For example, when 200 mL of first buffer is poured into upper buffer chamber  130   a , then 100 mL of first buffer is poured into upper buffer chamber  230   b , and 300 mL of second buffer is poured into common lower buffer chamber  140 . In effect, both outer and inner surfaces  81   a  and  81   b  of the three cassettes being run will have 100 mL of second and first buffer, respectively, devoted as a heat sink to the outer and inner cassette surfaces. Accordingly, the temperature on the outer and inner surfaces  81   a  and  81   b  of each cassette will be the same, assuming the specific heat of the buffers are substantially identical.  
         [0138]     As will be apparent to one skilled in the art, four or five gels also may be run simultaneously by further varying the location of dam  200  within container  20  and varying the number of cassettes employed. Moreover, it will be apparent to one skilled in the art that greater than six gels may be run simultaneously by providing additional chambers  30 . Advantageously, dam  200  can block flow to regions of container  20  so that any number of gels can be run simultaneously. The user simply needs to adjust the volume of first and second buffers in a proportional manner, as illustratively described hereinabove, to maintain proper thermal management in the system.  
         [0139]     All patents and publications cited in this specification are herein incorporated by reference as if each had specifically and individually been incorporated by reference herein. Although the foregoing invention has been described in some detail by way of illustration and example, it will be readily apparent to those of ordinary skill in the art, in light of the teachings herein, that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims, which, along with their full range of equivalents, alone define the scope of invention.