ELECTROPHORESIS APPARATUS AND METHOD

Abstract: Disclosed are an apparatus, system, and method for two-dimensional electrophoresis of analytes of interest, particularly polypeptides. The apparatus includes a sample separation cavity (24) comprising (1) an electrophoresis region (26a) located along an upper portion of the cavity for performing charge and/or size-based electrophoresis in a first dimension along the upper portion, and (2) below the first electrophoresis region, second electrophoresis region (26b) for performing electrophoresis in a second dimension in a direction substantially perpendicular to the first dimension. In one preferred embodiment, the second electrophoresis region contains an isoelectric focusing region containing a pKa gradient immobilized on at least one of the major opposing surfaces of the cavity.

Description: ELECTROPHORESIS APPARATUS AND METHOD Field of the Invention The present invention relates to methods and apparatus for electrophoretically separating and characterizing sample analytes. In particular, the invention relates to multidimensional electrophoresis. References Alfonso, E.S., et al.. J. Chromatogr. 689:85-96 (1995). Anderson, N.L., et al., Electrophoresis 12:907-930 (1991). Andrews in Electrophoresis. Theory. Techniques and Biochemical and Clinical Applications. Oxford U. Press, Oxford, p. 276 (1986). Bjellqvist et al.. Biochem. Biophvs. Meth. 6:317-339 (1982). Busch, M.H.A., et al.. J. Chromatogr. 695:287-296 (1995). Bushey et al., J. Chrom. 480:301 (1989). Capelli, L., et al., J. Biochem. Biophvs. Meth. 32:109 (1996). Chen et al., J. Liq. Chrom. 15:1143 (1992). Fung, E.N., et al.. Anal. Chem. 67:1913-1919 (1995). Ganzler, K., et al.. Anal. Chem. 64:2665-2671 (1992). Gilges, M., et al., Anal. Chem. 66:2038-2046 (1994). Lauch., T., et al., J. Chromatogr. 680:375-381 (1994). Li in Capillary Electrophoresis. Principles. Practice, and Applications. Elsevier, Amsterdam, p. 160 (1992). Liao, J.L., et al.. J. Cap. Elec. 2:191 (1995). Molteni, S., et al., Electrophoresis 15:22-30 (1994). Ng, C.L., et al.. J. Chromatogr. 659:427-434 (1994). Peterson et al., EP Publ. No. 494686 Al (1992). Righetti, P.G., Immobilized pH Gradients: Theory and Methodology, Elsevier, Amsterdam (1990). Righetti, P.G., et al., in Gel Electrophoresis of Proteins. B.D. Hames and D. Rickwood, Eds., IRL Press, Oxford, p.149 (1996). Sandoval and Pesek, U.S. Pat. 5,017,540 (1991). Sandoval and Pesek, U.S. Pat. 5,326,738 (1994). Schans, M.J., et al.. J. Chromatogr. 680:511-516 (1994). Sun, P., et a . J. Microcol Sep. 6:403-407 (1994). Wang, T., et al.. J. Chromatogr. 594:325-334 (1992). Werner, W.E., et al., Anal. Biochem. 212:253-258 (1993). Wiktorowicz, J., U.S. Pat. Nos. 5,015,350 (1990) and 5,181,999 (1991). Wilkens, M.R., et al., Eds., Proteome Research: New Frontiers in Functional Genomics. Springer Verlag, Berlin, Germany (1997). Wong, S.S.. Chemistry of Protein Conjugation and Cross-Linking. CRC Press, Boca Raton, FL (1991). Zhu et al.. J. Chrom. 516:123 (1990). Background of the Invention For decades, electrophoretic separation methods have been central to identifying and characterizing chemical and biochemical samples. In the usual procedure, an electrophoresis tube or slab is filled with a fluid electrophoresis medium, and the fluid medium is covalently cross-linked or temperature-solidified to form a non-flowable, stabilized gel separation medium. A sample is loaded into one end of toe tube, or into one or more wells of the slab gel, and an electric field is generated to draw the samples through the medium. Electrophoretic separation may depend predominantly on molecular size, e.g., in the cases of nucleic acids and SDS-bathed proteins, or on a combination of size and charge, as in the case of non-denaturing gel electrophoresis of polypeptides or polysaccharides, for example. Isoelectric focusing (IEF) is an electrophoresis method based on the migration of a molecular species in a pH gradient to its isoelectric point (pi). The pH gradient is established by subjecting an ampholyte solution containing a large number of different-pi species to an electric field, usually in a crosslinked matrix. Analytes added to the equilibrated ampholyte-containing medium will migrate to their isoelectric points along the pH gradient. For complex samples, multidimensional electrophoresis methods have been employed to better separate species that comigrate when only a single electrophoresis dimension is used. The conventional approach to two dimensional electrophoresis is to perform the first dimension in a rigid, usually crosslinked matrix. For analysis of proteins, for example, the sample is usually fractionated first by IEF in a tube or strip gel to exploit the unique dependence of each protein's net charge on pH. Next, the gel containing the separated proteins is extruded from the tube, dried (these two steps can be bypassed using a strip gel) and laid horizontally along one edge of a slab gel, typically a crosslinked polyacrylamide gel containing sodium dodecylsulfate (SDS). Electrophoresis is then performed in the second dimension, perpendicular to the first, and toe proteins separate on the basis of molecular weight. Thus, proteins having similar net charges, and which are not separated well in the first (IEF) dimension, will separate according to their different masses in the second dimension. Since these two separation methods depend on independent properties (net charge and mass), toe overall resolution is approximately toe product of the resolution in each dimension. A significant drawback of traditional methods for two-dimensional electrophoresis is that two separate devices are used to accomplish electrophoresis in the two dimensions. These protocols can be very time-consuming and cumbersome to practice. Moreover, traditional methods are susceptible to significant run-to-run variation because of variability in standard IEF and SDS gels, which cannot be re-used. Accordingly, there is a need for a new multidimensional electrophoresis method that is faster and easier to use, which allows the identification and characterization of hundreds or thousands of components in complex mixtures, and which is highly reproducible. Ideally, the method will employ a single separation apparatus for electrophoresis in both dimensions. The method preferably involves a flowable (liquid-state) separation medium toat can be easily replaced with fresh media, so toat a single apparatus can be used repetitively for multiple samples. Ideally, the apparatus is adaptable for automation. Summary of the Invention The present invention is directed to methods and apparatus for conducting multidimensional electrophoresis of samples within a single apparatus, such toat sample components that have been resolved in a first electrophoretic dimension can be directly electrophoresed in a second dimension toat is substantially perpendicular to toe first, without needing to move or manipulate the sample between the first and second electrophoretic steps. In one aspect, the invention includes a two-dimensional electrophoresis system. In one embodiment, the system includes an electrophoresis plate assembly that defines (i) a sample separation cavity, (ii) a sample loading port positioned at a corner of the upper portion, for introducing a sample into the electrophoresis region, and (iii) optionally, one or more fluid passageways positioned along the lower portion of the cavity, for introducing or removing liquid from the cavity. The cavity defined by the assembly is bounded by opposing major first and second surfaces, each having a defined width and length. These major surfaces are spaced apart by an interfacial distance substantially shorter than the width and length of the cavity. The cavity further comprises (1) a first electrophoresis region located along the upper portion of toe cavity for performing charge and/or size-based electrophoresis in a first dimension along said upper portion, and (2) below toe first electrophoresis region, a second electrophoresis region for performing electrophoresis in a second dimension in a direction substantially perpendicular to the first dimension, such that the basis of migration of sample components in toe second dimension depends on sample properties that are different from the sample properties that determine the basis of migration in the first dimension. In one embodiment, the second electrophoresis region is an isoelectric focusing region containing a pKa gradient immobilized on at least one of the major opposing surfaces, for isoelectric focusing in a direction substantially perpendicular to the first dimension. The pKa gradient may span any attainable range, such as a pKa range of about 4 to 10, or 4 to 6, for example. In another embodiment, the second electrophoresis region does not contain a pKa gradient and is used to perform charge and/or size-based electrophoresis under conditions different from the electrophoretic conditions used for the first dimension. The system may further include electrode means for generating a first voltage potential across the first electrophoresis region between the loading port and the lateral portion across from the loading port, electrode means for generating a second voltage potential between the upper portion and lower portion, and an aqueous medium occupying the cavity. In one embodiment, the loading port is joined to the upper corner portion of the cavity by an elongate sample transport channel. In another embodiment, the one or more fluid passageways are provided as a single slot which passes through one of major surfaces near the lower portion. In an alternative configuration, the one or more fluid passageways comprise a plurality of openings positioned along the lower portion of the cavity. In another embodiment, toe second electrophoresis region contains a plurality of elongate separation channels, which are substantially parallel to the lateral portions of the cavity and prevent flow of the medium between adjacent separation channels. The second electrophoresis region preferably contains at least 30 separation channels, and preferably more than 100 separation channels. In another embodiment, toe plates define a fan-shaped region that abuts the upper portion or the lower portionof the separation cavity, for introducing or removing liquid from the cavity. In another embodiment, the upper edge of the first electrophoresis region and/or the lower edge of the second electrophoresis region is bounded by an ion-permeable membrane, which can be used to separate the separation cavity from an electrode reservoir. In another aspect, the invention includes an electrophoresis plate assembly, such as described above. In another aspect, toe invention includes a method for separating one or more components of a sample mixture, using an electrophoresis plate assembly or system such as those described above. In the method, a sample mixture is applied to the sample loading port. A first voltage potential is applied across the first electrophoresis region under conditions effective to cause different components in the mixture to migrate towards the opposite portion of the electrophoresis region, such that different components become separated at least partially on toe basis of size. After electrophoresis in this first dimension, a second voltage potential is applied across the upper and lower portions of the cavity, in a direction substantially perpendicular to the first dimension, under conditions effective to separate different components on toe basis of physical properties (e.g. , charge and/or size features) that are different from those used in the first dimension. In one embodiment, the second electrophoresis region is an isoelectric focusing region, and the electrophoretic conditions are effective to generate a pH gradient in toe isoelectric focusing region in a direction substantially perpendicular to the direction of the first voltage potential. Sample components migrate into the isoelectric focusing region and separate on the basis of their isoelectric points. In a second embodiment, toe second electrophoresis region does not contain a pKa gradient, and the second electrophoretic conditions are effective to perform electrophoresis under conditions different from the electrophoretic conditions used for the first dimension. After electrophoresis in the second dimension is complete, sample components can be detected and imaged to obtain information about the composition of the sample. To facilitate detection, sample components preferably contain a detectable label, such as a fluorescent label or radiolabel. In a preferred embodiment, the sample comprises one or more polypeptides to be detected. In one embodiment, after electrophoresis in the second dimension is complete, one or more sample components can be collected from the separation cavity for further characterization. Preferably, the second electrophoresis region includes a plurality of elongate separation channels as above, to facilitate collection of sample components. In another embodiment, the invention includes an electrophoresis plate, channel, or tube having a surface which is coated with a pKa gradient immobilized thereon. Preferably, the gradient spans at least one pH unit. The tube or channel preferably has a diameter the is 200 μm or less, preferably 100 μm or less, or 50 μm or less. These and other features and advantages of the present invention will become more clear from the following detailed description together with toe appended drawings. Brief Description of the Drawings Fig. 1 shows an overhead view of a plate assembly in accordance with the invention; Fig. 2 shows a perspective view of the assembly of Fig. 1; Fig. 3 illustrates another exemplary plate assembly in accordance with the invention, which additionally includes a liquid loading region, a plurality of separation channels, and an elongate sample transport channel; Fig. 4 shows a perspective view of the assembly of Fig. 3; Fig. 5 shows a cross-section of the channel region of the assembly from Figs. 3 and 4; Figs. 6-9 show exemplary modifications of the triangular region of toe device from Fig. 3; Figs. 10 and 11 show overhead and perspective views of another exemplary plate apparams in accordance with the invention; Fig. 12 shows an enlarged view of sample loading channels of the apparatus from Figs. 10-11; Fig. 13 illustrates a modification of toe apparatus of Figs. 10-12; Fig . 14 illustrates an enlarged view of an electrode reservoir from Fig . 13. Detailed Description of the Invention As noted above, the present invention is directed to methods and apparatus for conducting multidimensional electrophoretic separations of sample mixtures within a single separation cavity, such that the separation conditions in the first dimension are different from the separation conditions in the second dimension. The invention is adaptable to a variety of separation conditions, including conditions for (1) isoelectric focusing, and (2) denaturing or non-denaturing size-based separations in flowable sieving media. Moreover, since electrophoresis can be accomplished in both dimensions with flowable (i.e., liquid) media, the medium can be replenished after each sample separation without having to separate the plates. I. Apparams In one aspect, toe invention provides apparatus for conducting two-dimensional electrophoresis of selected analytes, particularly polypeptides. In general, the apparatus includes a plate assembly toat defines a cavity bounded by opposing major first and second surfaces, each having a defined width and length. These major surfaces are spaced apart by an interfacial distance substantially shorter than the width and length of the cavity. The cavity further comprises (1) a first electrophoresis region located along the upper portion of toe cavity for performing charge and/or size-based electrophoresis in a first dimension along said upper portion, and (2) below the first electrophoresis region, a second electrophoresis region for performing electrophoresis in a second dimension in a direction substantially perpendicular to the first dimension, such that the basis of migration of sample components in the second dimension depends on sample properties (e.g., molecular weight, molecular shape, hydrophobicity and/or hydrophilicity, and/or charge) toat are different from the sample properties that determine the basis of migration in the first dimension. The following discussion is directed to a first embodiment of toe invention wherein electrophoresis in toe first dimension is performed on toe basis of size and charge, resulting in different migration rates, and electrophoresis in the second dimension is performed on toe basis of isoelectric point. Other, alternative embodiments are discussed subsequently. Figs. 1 and 2 show an overhead view and perspective view, respectively, of an electrophoresis plate assembly 10 in accordance with the present invention. A pair of plates 20,22 are disposed such that inner plate surfaces 20a and 22a are juxtaposed face-to face to form an enclosed separation cavity 24, for holding a separation medium through which the sample is electrophoresed. Plate 20, which is also referred to arbitrarily as the bottom plate, defines a recessed region 26 which defines five of the six walls of cavity 24. As seen wito reference to Fig. 2, region 26 further includes a first sample separation surface, designated surface 26a, for electrophoresis along the lateral dimension of this region, and a second sample separation surface 26b, for electrophoresis in a direction perpendicular to toe first-mentioned dimension. For an IEF embodiment of the invention, surface 26b is further characterized by the presence of a plurality of buffering moieties, or immobilines, which define a pKa gradient leading away from region 26a. The properties of region 26b are discussed further below. Plate 22, which is also referred to arbitrarily as the cover plate, includes an inner surface 22a which is substantially flat, for providing the sixth wall of separation cavity 24. As seen particularly with reference to Fig. 2, plate 22 defines, in its upper left-hand corner, a sample loading port 30, at or through which sample is introduced into the separation cavity, and which also provides access to the separation cavity for a first electrode 30a (not shown) for establishing a voltage potential at that site. Electrodes can be made of any appropriate conductive material, such as platinum, nichrome, or gold, etc., with platinum being preferred. An electrode port 32 is defined in toe upper right-hand corner of plate 22, for providing a second electrode 32a (not shown) in electrical contact with toe separation cavity. The apparatus may include an optional third electrode 34a (not shown), which is electrically separate from the first and second electrodes mentioned above, and which extends laterally across the top of cavity 24. This third electrode can be admitted into the separation cavity through port 30 or 32, and can be immobilized in the cavity by being affixed, for example, to the bottom plate along the upper edge of recessed region 26. Plate 22 also contains an elongate slot 36 which extends laterally along the bottom of toe plate, for providing (i) an electrode 36a (not shown) extending along the bottom of toe separation cavity, and (ii) a passageway for ingress and egress of separation media and wash solutions into and out of the separation cavity before or after electrophoresis. Preferably, the surfaces 26a and 26b of recessed region 26, and toe inner surface 22a of cover plate 22, are substantially planar, to facilitate the creation of undistorted electric field lines and enhance sample separation during electrophoresis. The inner surface 22a of the cover plate preferably includes a surface region 40b whose dimensions are congruent with and opposite to surface region 26b in plate 20, for generating an isoelectric focusing pH gradient in the separation medium located in that region. Like region 26b, for isoelectric focusing, region 40b is preferably coated with a plurality of buffering moieties, or immobilines, which define a pKa gradient leading away from the region 26a. Plates 20,22 can be formed of any material suitable for electrophoresis of the selected sample. Preferably, at least one of the plates is formed of a material toat is transparent with respect to a type of signal that is used to visualize or locate sample components in toe separation cavity. Typically, the plates are formed out of a silicon dioxide-based glass, such as borosilicate, although other materials such as plastics, such as polycarbonate, or metals rendered non-conductive by a suitable coating, are also contemplated. Visually opaque materials such as TEFLON™ and MYLAR™, for example, can also be used, e.g., with radioactive sample detection. In a preferred embodiment, both plates are made from borosilicate glass. Glass is advantageous because it is transparent to a broad range of wavelengths, e.g. , for fluorescence detection and visual inspection, it can be cut easily, and fine features on the order of 1 to 100 μm or greater can be formed in glass by standard photolithographic etching techniques. For example, a recess region 26 having a substantially planer surface with a depth of 100 μm can be formed readily by conventional hydrofluoric acid etching. Similarly, ports 30,32 in cover plate 22 can be formed by mechanical drilling, for example. In the assembled apparams (plate assembly), plates 20 and 22 are joined together by any suitable means that are sufficient to ensure a liquid-tight seal wito respect to separation cavity 24. For example, glass plates can be fusion- welded together using methods known in the art, i.e., by holding the opposing faces of the plates together at an elevated temperature toat is below the softening point of the plates, such that the inner surfaces 20a and 22a of the plates become bonded together. Alternatively, the plates can be joined together by anodic bonding, or simply by using one or more clamps along the edges of toe plates. Figs. 3 and 4 illustrate another exemplary embodiment of an apparatus (100) of the invention which additionally includes a liquid loading region 160, a plurality of separation channels 170, and an elongate sample transport channel 180. In this example, bottom plate 120 (Fig. 4) contains a recessed region 126 which defines five of the six walls of separation cavity 124. Region 126 includes a first sample separation surface 126a which defines a first electrophoresis region 126a, for electrophoresis along the lateral dimension of this region, and a second sample separation surface 126b which defines a second electrohporesis region for electrophoresis in a second direction, such as for isoelectric focusing. Edge region 120a surrounds region 126. Region 126 also encompasses a fan-shaped (triangular) liquid loading region 160 at the upper end of the plate, for conveniently introducing and removing separation media to and from the separation cavity before and after electrophoresis. Region 160 is also useful for forming a pKa gradient coating on the inner surfaces of the plates for isoelectric focusing, as detailed below. With continued reference to Figs. 3 and 4, surface 126b comprises a plurality of parallel separation channels 170 toat are aligned in a direction perpendicular to the bottom edge of the plate. For an IEF embodiment of the invention, the channels are each coated wito a plurality of buffering moieties, forming a pKa gradient in the vertical direction with respect to the top and bottom edges of the plates. Each channel 170 is separated by a partition 172 (Fig. 5) having a height that is flush with inner surface 120a of plate 120. This is preferred in order to form liquid-tight seals between the channels when plates 120 and 122 are assembled together. The depths of the channels are preferably the same, and are also preferably the same as the depths of regions/surfaces 126a and 160. The number of channels 170, and their dimensions, will vary depending on sample complexity and the desired resolution. Generally, sample resolution will increase as the number of channels is increased, subject to the limit of resolution achieved by electrophoresis in the first, lateral, dimension. Preferably, the channels are dimensioned so toat the channel resolution is at least twice the sample resolution in the lateral dimension of the separation medium, so toat each sample band partitions into from one to three channels. Plate 120 also contains an elongate sample transport channel 180 extending from the lower left hand area of the plate to toe upper left hand corner of surface 126a, for size-based electrophoretic separation of the sample during transit to separation cavity 124. Channel 180 preferably has a depto in plate 120 equal to the depth of recess 126. The width of channel 180 is preferably no more than about 10 times the channel's depth, and is preferably equal to or less than about 5 times the channel depth. Plate 122, which is also referred to arbitrarily as the cover plate, includes an inner surface 122a which is substantially flat, for providing the sixth wall of separation cavity 124. As seen in Fig. 3, and particularly with reference to Fig. 4, plate 122 defines, in its lower left-hand corner, a sample loading port 130, at or through which sample is introduced into the separation cavity, and which also provides access to the separation cavity for a first electrode 130a (not shown) for establishing a voltage potential at that site. An electrode port 132 is defined in the upper right-hand corner of plate 122, for providing a second electrode 132a (not shown) in electrical contact with the separation cavity. The first and second electrodes are useful for performing electrophoresis of the sample along a first dimension stretching from port 130 to port 132, to generate a series of separated sample components along the upper edge of 126a after toe first electrophoresis step is complete. Plate 122 also contains an elongate slot 140 which extends laterally along toe bottom of the plate, like slot 40 mentioned above. Port 135 in plate 122 is included for transporting separation media and wash fluids into and out of toe separation cavity, and also for forming the IEF coating gradient. To facilitate sample loading, port 130 can be accompanied by a waste port 131 defined in plate 122, such toat both ports are in fluid communication with elongate sample transport channel 180. This allows a precise amount of sample to be injected into channel 180, as discussed further below. Re rning to Fig. 4, plates 120 and 122 further define channel egress ports 136 and 138, which are located slightly above slot 140, for collecting resolved sample components individually from one or more channels after the IEF step is complete. In toe embodiment illustrated in the Figure, each plate contains an egress port for alternating channels, so that ports 136 and 138 are staggered relative to each other. The staggering of egress ports, while not required, facilitates toe connection of capillary tubes to the ports for collecting fluids from each channel. Figs. 6-9 illustrate additional embodiments that may be utilized in the devices of the present invention, particularly for maintaining a relatively uniform electrical field during electrophoresis in the first dimension. Fig. 6 shows a modification in which triangular liquid loading region 160 from Fig. 3 can be modified to include a vertical barrier 150 that extends from beneath port 135 to the top of surface 126a. A benefit of barrier 150 is that during the first dimension of electrophoresis, the electric field lines are constrained to the region bounded by surface 26a, so that band distortion may be reduced. Fig. 7 shows a modification in which one of plates 120 and 122 contains an additional port 152 which is closable with a suitable plug (not shown). After regions 126a and 126b have been filled with a desired medium or media, a low conductance medium that preferably has an ionic strength at least 5 times, and more preferably at least 10 times lower than toat of the surrounding medium, is introduced into triangular region 160 through port 152 with egress through port 135, so that a vertical "wall" or region of low conductance medium is created in region 160 between ports 152 and 135. After loading of the low conductance buffer, ports 135 and 152 are closed, and electrophoresis is performed as described herein. Field lines in the first dimension of electrophoresis are thus constrained to region 126a. In Fig. 8, toe first electrophoresis region includes a trench 154 extending across the lateral dimension of region 126 and which provides a deeper cross-section for this region relative to region 126b (e.g., twice as deep as the region 126b). After regions 126a and 126b have been filled with a desired medium or media, region 126a (trench 154) is preferably filled with a high conductance/high viscosity medium (e.g., due to toe presence of a selected polymer) via port 130 or 131 and port 132, to help constrain electrical field lines to region 126a during electrophoresis in the first dimension. By providing a greater cross- section than the cross-sections of adjacent regions 160 and 126b, trench 154 also helps limit mixing between regions 126a and 126b. Fig. 9 shows another modification in which plate 122 additionally includes a horizontal slot 156 located just above region 126a. Initially, slot 156 is filled wito a plug (not shown) which closes the slot and has an inner surface toat is flush wito toe inner surface of plate 122. After the inner surfaces of regions 126a and 126b have been prepared as described herein, and after the chamber has been filled with the desired electrophoresis medium or media, the plug is pressed further through toe slot until toe end of the plug snuggly contacts surface 126a of plate 120, thereby creating a horizontal barrier across toe top of region 126a which constrains electric field lines during the first dimension of electrophoresis. Figs. 10-12 illustrate another plate configuration in which the triangular region from Fig. 3 is moved to the other end of the device, adjacent the edge of the second electrophoresis region. Apparatus 200 includes a bottom plate 220 and top plate 222. Bottom plate 220 includes a first sample separation surface 226a, defining a first electrophoresis region 226a, for electrophoresis along the lateral dimension of this region, and a second sample separation surface 226b, for electrophoresis in a direction perpendicular to the first dimension. Plate 220 further defines a lateral channel 232a for providing fluid communication with port 232 in plate 222. Plate 220 also contains an elongate sample transport channel 280 extending along the left edge of region 226b. Channel 280 terminates with a peripheral channel 23 la which links port 230a to channel 280. Peripheral channels 231b, 231c, and 23 Id meet at junction 23 le, and are linked in fluid communication with channel 280 via channel 23 If. Channels 231b, 231c, and 23 Id place ports 230b, 230c, and 230d in fluid communication with channel 280. The operation of these channels is discussed further below. Also, it will be appreciated that any other suitable channel arrangement can be used in the apparatus to facilitate loading of sample. Optionally, surface 226b comprises a plurality of parallel separation channels 270 that are aligned in a direction perpendicular to the bottom edge of the plate, and which are separated by partitions 272 as with apparatus 100 discussed above. At the end of those channels, plate 220 further a fan-shaped (triangular) liquid loading region defined by surface 260a, for conveniently introducing and removing liquids into and from the separation cavity. Triangular region 260a is bordered at its lateral edges by channels 233a and 233b. which provide fluid communication with ports 234a and 234b, respectively, in plate 222. Port 235 in plate 222 provides a convenient site for for transporting separation media and wash fluids into and out of the separation cavity, and also for forming an IEF coating gradient in region 226b. Plates 220 and 222 further define pluralities of alternating channel egress ports 236 and 238, for collecting resolved sample components individually from one or more of channels 270, as wito apparams 100 above. The various ports, particularly ports 230a, 230b, 230c, 230d, 232, 234a, and 234b may also be provided wito electrodes in order to control movement of sample (ports 230a-d), for electrophoresis in the first dimension (ports 230a and 232), and for electrophoresis in the second dimension (ports 230a , 232 , 234a and 234b) , The dimensions of the apparams and assembly are a matter of design choice and are selected for convenience of use. For example, the separation cavity preferably has a length dimension of about 1 to 20 cm (e.g., 12 cm); a width dimension of about 1 to 50 cm (e.g., 10 cm); and a depth dimension (interfacial distance between the major opposing surfaces of plates) of about 50 to 200 μm (e.g., 100 μm); the first electrophoresis region preferably has a path width of about 0.1 to 2 cm (e.g., 0.25 cm); channels 170 preferably have a widths of about 0.25 to 1 mm (e.g., 0.67 mm), depths that are preferably the same as the above- mentioned interfacial distance, and are spaced apart by partitions having a width of about 0.1 to 0.5 mm (e.g., 0.33 mm); elongate channel 180 preferably has a length of about 0.5 to 15 cm (e.g., 12 cm), a width of about 0.2 to 1 mm (e.g., 0.5 mm), and a depth that is preferably the same as the above mentioned interfacial distance); slot 140 preferably has a length spanning all channels, and a width of about 0.5 to 3 mm (e.g., 2 cm); ports 130, 131, 132, 135 and 136,138 preferably have diameters of 0.5 to 3 mm (e.g., 1 mm); and ports 130 and 131 are spaced apart by a center-to-center distance of 0.5 to 2.5 cm (e.g., 1.2 cm). Of course, dimensions outside toe above preferred dimensions can also be used. With reference to the embodiments illustrated in the preceding figures, the inner surfaces of toe separation cavity are preferably inert with respect to the sample, to minimize adsorption of the sample to toe inner surfaces during electrophoresis. Such adsorption is generally undesirable because it can disrupt band resolution particularly in toe first dimension of electrophoresis. Additionally, materials such as silicate glasses tend to have charged groups on their surfaces toat can cause electroendosmotic flow (EOF) of the separation medium during electrophoresis. EOF is a phenomenon in which a bulk flow of the electrophoresis medium arises due to the effect of the electric field on counterions adjacent to charged surfaces of a separation cavity. In toe case of a surface toat is negatively charged, such as a silicate glass surface, there is a build-up of positive counterions (cations) in the solution adjacent to the surface. In an electric field, this shell of cations can cause the medium to migrate toward the cathodic electrode at an EOF rate dependent on the thickness of the cationic shell. The rate of EOF can provide an important variable that can be optimized to improve toe separation of two or more closely migrating species. In particular, when electrophoresis is carried out under conditions in which EOF and the migration of species to be separated are in opposite directions, the effective path length for separation can be made extremely long by making toe rate of EOF in one direction nearly equal to the electrophoretic migration rate of toe analyte attracted most strongly in the opposite direction by the electric field. In the present invention, EOF may or may not be desirable for the first dimension of electrophoresis, depending on the nature of toe sample and the degree of desired separation. However, for toe second (IEF) dimension of electrophoresis, EOF is preferably avoided so toat the uniformity of the IEF pH gradient is not disturbed. If the materials from which the plates are made are not inherently sufficiently inert towards the sample, the inner surfaces of the plates and all other inner surfaces of the separation cavity can be coated with any suitable coating material, to reduce sample adsorption to an acceptable level. Since electrophoresis is usually performed in an aqueous separation medium, adsorption of sample can usually be reduced by covering the inner surfaces of the separation cavity with a hydrophilic coating toat masks potentially adsorptive surface regions. Exemplary reagents for coating adsorptive surfaces include polyacrylamide, polyvinyl alcohol, polyethers, cellulose acetate, polyalkylene oxides, poly(vinylpyrrolidone), and other materials as are known in the art. Preferably, such coatings are attached to interior surfaces covalently, although coating by adsorption may also be suitable. Coating reagents for reducing sample adsorption can also be used to control the magnitude of EOF. For example, EOF along glass silicate surfaces can be substantially reduced by coating them with a neutral reagent that masks a substantial percentage of surface silanol groups. The magnitude of EOF can be further controlled by using coating reagents that include positively or negatively charged groups. Positively charged coatings can be used to nullify surface negative charges to give a net surface charge of zero, so toat EOF = 0. Coatings wito higher positive charge densities can be used to reverse toe direction of EOF for charged surface materials. This can be useful for slowing the net migration rates of positively charged sample species. Conversely, negatively charged coatings can be used to impart to or increase the magnitude of negative charge on surfaces, to slow the net migration rates of negatively charged species. Representative positively charged coatings include polyethyleneimine, quaternized polyethyleneimine, and chitosans, for example. Representative negatively charged coatings include carboxylate and sulfonate containing materials, such as poly(methylglutamate) and 2-acrylamido-2-methylpropanesulfonate polymers, for example. It will be recognized that charged coatings can also effectively reduce sample adsorption, especially for samples having the same charge polarity as toe coating. Sample adsorption and EOF can also be adjusted by including suitable reagents in the separation medium and running buffers. For example, negative surface charges can be masked by including a cationic additive in the medium, such as metal amine complexes, amines and polyamines such as propylamine, trietoylamine, tripropylamine, triethanolamine, putrescine, spermine, 1,3-diaminopropane, morpholine, and the like. Zwitterionic species comprising both negatively and positively charged groups that are isoelectric at the pH of electrophoresis can also be used, such as trialkylammonium propyl sulfonates, where alkyl is methyl, ethyl, propyl, etc. (Peterson et al. , 1992, Zhu et al., 1990, Bushey et al., 1989, and Chen et al. , 1992). The choice of additives in the separation medium will depend in part on the sample and the nature of the interior surfaces, as well as other factors. In some applications, it may be desirable to use both a covalent surface coating and soluble buffer agents to control sample adsorption and EOF. According to a preferred embodiment, the second electrophoresis region includes an isoelectric focusing region toat contains a pKa gradient immobiUzed on at least one of toe major opposing surfaces, for isoelectric focusing in a direction substantially perpendicular to toe first dimension. The pKa gradient is effective to produce an isoelectric focusing pH gradient when the apparatus is filled with an aqueous medium, to promote migration of sample components to locations in the gradient where the local pH is equal to the pi of each component. The immobilized pKa gradient is formed on one or both major inner surfaces by any method suitable for forming a gradient having a desired pKa range, resolution, and buffering capacity. Generally, forming the pKa gradient entails exposing plates to a solution containing a gradient of immobilines under conditions effective to promote covalent attachment of the immobilines to the plates such that the pKa gradient of toe solution is transferred to the plates. In one approach, the immobilized pKa gradient is formed by pumping a solution containing a gradient of immobiline molecules into the bottom of a vertically oriented separation cavity. The gradient can be formed using a simple binary gradient-forming assembly consisting of first reservoir and second reservoirs connected to a pump. The first reservoir contains a first solution containing low pKa immobilines, and the second reservoir contains a second solution containing high pKa immobilines. Initially, the gradient solution is drawn only from reservoir A. As filling progresses, the pump draws an increasing amount of solution from reservoir B instead of A. Although a linear gradient is preferred in most simations, it will be appreciated that curved gradients can also be formed, depending on the design chosen by the user. Also, while a mechanical pump may be most convenient for loading the gradient, other metoods such as gravity-based loading can also be used. The solution in the second reservoir preferably has a mass density greater than that of toe solution in the first reservoir, to help maintain the resolution and continuity of the gradient during the loading and attachment of the immobilines to the IEF region. This is accomplished, for example, by including glycerol or a mono- or disaccharide such as glucose in the second solution. The sample capacity of the isoelectric focusing region will depend in part on the buffering capacity of toe immobilized buffering groups on the major inner surface(s) of the plates. Buffering capacity generally increases as the density of buffering groups on the surface is increased. Thus, it is preferable to attach as high a density of buffer groups to the surface as possible, to allow the plates to accommodate higher concentrations of sample components to be separated on toe basis of pi. The buffering groups for creating the IEF gradient are attached to the plates by any suitable method known in the art. In particular, suitable compounds will include (i) buffering groups having desired pKa values and (ii) reactive groups for covalently binding to chemically complementary reactive groups on the inner surface of the isoelectric focusing region. A large number of buffering compounds have been developed over the past several decades for creating IEF gradients. Exemplary descriptions of such buffering compounds can be found in Righetti (1990), Bjellqvist et al. (1982), Andrews (1986), and Righetti et al. (1996). The buffering compounds may be coupled directly to toe IEF regions of the plates using suitably activated plates or buffering compounds, or may be attached via a crosslinking reagent (e.g., see crosslinking groups reviewed by Wong, 1991). Also, a variety of buffering compounds suitable for covalent attachment to solid phase surfaces are commercially available (e.g. , the "IMMOBILINE" compounds sold by Amer sham- Pharmacia Biotech, Uppsala, Sweden). For silicate glass plates, buffering compounds can be attached directly to surface silanol groups, as reviewed in Li (1992) or can be attached via an intermediary coating that provides other reactive groups. A variety of intermediary coating compounds have been described in the literature, such as "bind silane" (3-(trimethoxysilyl)propyl methacrylate, Sigma Chemical Co., St. Louis, MO; Capelli et al., 1996), α-glycidoxypropyl trimethoxy silane (Liao et al., 1995), chlorination followed by Grignard or organolithium reaction (Sandoval and Pesek, 1994), and allyl attachment via silane-hydride chemistry (Sandoval and Pesek, 1991, 1994). Immobilization methods that produce Si-C bonds of attachment are generally preferred, to enhance longevity of covalent attachment of the buffering groups to the surface. Conveniently, IEF buffering groups are attached to toe IEF surface region using the two-step silane-hydride chemistry taught by Sandoval and Pesek (1991 , 1994). In the first step, a solution of (EtO) 3 SiH is washed slowly over toe plate surface to deposit Si-H groups (silyl hydrides) over existing silanol groups. It is usually necessary to maintain a constant flow rate of toe hydride reagent over the plates to prevent formation of solid deposits inside the separation cavity. Next, the hydride groups are reacted with a dialkene such as allyl methacrylate in the presence of a hexachloroplatinic acid catalyst to bind allyl groups to the surface via a stable Si-C bond. Preferably, this 2-step coating procedure is performed on the entire separation cavity, to coat all free silanol groups. Also, the allyl methacrylate reaction can be repeated to increase overall yield. Exemplary reaction conditions are provided in Example 1. After the surface silanol groups have been converted to surface allyl groups, the IEF region is contacted with a selected pKa gradient solution. The orientation of the IEF gradient is selected in accordance with (i) the nature of the analytes being separated, and (ii) toe expected pH for electrophoresis in the first dimension. If the pH of the electrophoresis medium in toe first dimension is acidic (e.g., pH 2), the IEF gradient is usually oriented such that toe more acidic pKa buffering groups are proximal to the first separation region, and the more basic pKa buffering groups are distal to the first separation region. Conversely, if toe pH of toe first dimension will be relatively basic (e.g., pH 10), toe orientation of toe IEF gradient is reversed, such that the more basic pKa buffering groups are proximal to the first separation region. To attach toe pKa buffering groups to the IEF region, toe plate assembly can be positioned vertically (upright) such that the IEF region of the separation cavity is at the bottom for an assembly as shown in Figs. 1 and 2. If an assembly having an upper fan- shaped loading region 160 is used, such as shown in Figs. 3 and 4, the assembly is inverted so that region 160 is at the bottom. After the pKa buffer compounds are activated (if necessary), e.g., with a radical-initiator reagent such as TEMED plus ammonium persulfate (APS), the gradient is loaded into the IEF region from below until the IEF region is filled. Preferably, glycerol is included in the second buffer reservoir to minimize mixing of the buffering compounds during loading and immobilization. Also, after the gradient solution has been loaded, the gradient solution can be "chased" wito a high-density liquid containing a visible dye, such as bromphenol blue, to ensure that the end of the gradient solution reaches toe IEF region. The assembly is then allowed to incubate at a suitable temperature until the pKa buffering compounds become bound to the surface-attached allyl groups. After attachment of the buffering compounds is complete, the reaction mixture can be replaced with water. In a separate step, which can be carried out before or after toe pKa buffering compounds are attached to the IEF region, the surface-attached allyl groups in the first separation region can be reacted with an inert coating material to reduce sample adsorption in this region. Preferably, the allyl groups are reacted with a hydrophilic material, such as linear polyacrylamide, to impart hydrophilicity to the inner surfaces in this region (Example 2). For embodiments that do not require an IEF gradient region, the inner surfaces of the cavity can be coated with a suitable coating as discussed above, to control EOF and reduce sample adsorption. The separation cavity can then be stored, preferably in water or another suitable fluid, until use. Similarly, toe plate surfaces above and/or below IEF region 126b can be coated with buffering molecules defining a selected pKa, to stabilized the pKa gradient in the IEF region. Preferably, toe pKa for the coating is selected to be outside the pKa range defined by the IEF gradient. Example 2 provides a protocol for coating the triangular region and sample transport channel of apparams 100 (Fig. 3) with Immobilines having an average pKa of about 3.5. The presence of this coating can help stabilize the anodic end of the IEF gradient. A similar procedure can be used to coat the plate surfaces at the cathodic end of the IEF gradient, if desired, using suitably basic Immobilines. II. Method and System In another aspect, the invention includes a metood for separating one or more components of a sample mixture, using an electrophoresis plate assembly or apparatus such as described above. The method is useful for identifying and characterizing a variety of samples, and for monitoring changes in sample composition over time. The sample can be any substance for which electrophoretic separation by the present invention may be useful. Preferred sample-types include polypeptides, glycopolypeptides, proteoglycans, charged polysaccharides, and synthetic polymers, for example, although other substances, especially from biological sources, are also contemplated. Also, the sample may be derived from cellular or tissue extracts (e.g., Anderson et al. , 1991), or biological fluids, such as blood, urine, semen, synovial fluid, saliva, or fractions thereof, prepared by known metoods. If necessary, toe sample components can be modified to include one or more detectable labels to facilitate detection and quantification in toe separation medium. In one approach, toe sample is labeled with a fluorescent label, such as a fluorescein, rhodamine, eosin, or "BODIPY™" group, according to methods well known in the art. The reactive functionality on the label is selected to ensure labeling of most or all of the components of interest in the sample. Preferably, the label contains a reactive functionality that reacts with a limited set of complementary reactive groups. For proteins, for example, cysteine- selective reagents are preferred, such as iodoacetamide and maleimide functionalities, since most proteins (~ 90-95%) contain at least one. Tyrosine and amine-reactive labels can also be used. Generally, hydrophilic labels are preferred, to help avoid sample precipitation. Fluorescent compounds suitable for labeling proteins and the like are well known, and are commercially available from Sigma Chemical Co. (St. Louis, MO) and Molecular Probes, Inc. (Eugene, OR). Preferred derivatized labels include functionalized eosin or "BODIPY", and monobromobimane. It will be appreciated that although fluorescent derivatization of sample components may alter the pi values of some components, such alterations are acceptable since they do not interfere with detecting and monitoring the components. A chemical labeling reaction is carried out for a time sufficient to label uniformly most or all labelable components in the sample. Unbound label can be removed by quenching with an excess amount of a scavenger substrate, such as free cysteine, followed by passing the reaction mixture through a size-exclusion gel, such as Sephadex™ G-25 or G-50 (Amersham-PharmaciaBiotech) . Alternatively, the sample may include a detectable radioisotope, such as 125 1, 32 P, 35 S, 14 C, or 3 H. Chemical and biochemical methods for introducing such isotopes into samples are well known in the art. In operation, a plate assembly having the desired dimensions is selected which contains a pKa gradient in the isoelectric focusing region spanning a desired pi range. For example, for analysis of proteins in a pi range of 4 to 9, the isoelectric focusing region contains a continuous buffer gradient spanning a pKa range of less than or equal to 4 to greater than or equal to 9. The assembly is encased in a device that includes valved inlet/outlet ports and electrodes which form liquid-tight connections with corresponding ports and slot(s) in toe cover plate. Prior to sample loading and separation, toe separation cavity of toe system is filled with one or more flowable separation media. In a preferred embodiment, the separation medium consists of two solutions, one for each separation region. For embodiments that utilize IEF in the second dimension, the medium in the second electrophoresis region preferably has a low ionic strength, in order not to interfere with toe IEF step, although soluble ampholines can also be included if desired to strength toe buffering capacity of the pi gradient. Also, for embodiments in which the first dimension of electrophoresis is performed at an acidic pH, the pH of the medium is typically more acidic than the lowest pKa of the IEF region. This ensures that sample components that have pi values within the pH range of toe IEF gradient will migrate into that region during the IEF step. Exemplary acidic buffers for the first dimension of electrophoresis include citrate, formate, and acetate, typically at a concentration of about 1 to 50 mM, and preferably about 5 to 20 mM. To reduce precipitation of sample components during electrophoresis, and particularly during isoelectric focusing, the separation medium may additionally include one or more neutrally charged denaturing agents or detergents to reduce non-covalent interactions between sample molecules and wall interactions. Exemplary denaturing agents include urea, toiourea, and dimetoylformamide (DMF). Exemplary neutral detergents include polyoxy- etoylene etoers ("tritons"), such as nonaetoylene glycol octylcyclohexyl ether ("TRITON" X- 100), polyglycol ethers, particularly poly alkylene alkyl phenyl ethers, such as nonaetoylene glycol octylphenyl ether ("NONIDET" P-40 or NP-40), polyoxyethylene sorbitan esters, such as polyoxyethylene sorbitan monolaurate ('TWEEN "-20), polyoxyethylene ethers, such as polyoxyethylene lauryl ether ("BRLT-35), polyoxyethylene esters, such as 21 stearyl ether (C^) ("BRIJ" 721), N,N-bis[3-gluconamidopropyl]cholamide ("BIG- CHAP"), decanoyl-N-methylglucamide, glucosides such as octylglucoside, and the like, neutral, zwitterionic detergents can also be used. The optimal concentration of a denaturing agent or detergent will depend on the particular detergent used. Urea is typically used at a concentration up to about 10M, for example, with a concentration of 4M to 8M being pre- ferred. Generally, the detergent concentration will range from 0.01 % to 5 % (v: v), and more typically between 0.025 and 2% , although these ranges are not limiting. The separation medium may also include soluble agents for coating the walls of the separation cavity, to help reduce endosmotic flow during electrophoresis. Such soluble coating agents include quaternary ammonium-containing polymers (Wiktorowicz (1990, 1991), methyl cellulose derivatives (Molteni et al., 1994), cellulose acetate (Busch et al., 1995), polyethylene oxide (Fung et al., 1995), chitosan (Sun et al., 1994), polyvinyl alcohol (Gilges et al., 1994), polyethylene glycol (Wang et al., 1992), polyethylenimine (Ibid.), and polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymers (Ng et al., 1994), for example. Typically, soluble coating agents can be included at concentrations of about 0.05 % to about 4% , and more preferably of about 1 % to about 2% . In a particularly preferred embodiment, the separation medium contains a polymer material (also referred to as "entangled polymer") that differentially impedes sample components on the basis of their sizes. A variety of polymer materials that promote size- based separation of analytes are known in the art, such as linear polyacrylamide (Werner et al., 1993), polyethylene oxide (Schans et al., 1994), dextran (Lauch et al. , 1993), polyethylene glycol (Ganzler et al. , 1992), and polyvinyl alcohol (Alfonso et al. , 1995). The appropriate concentration and size of the polymer material included in the medium will generally depend at least in part on the physical properties and complexity of the sample being analyzed, the properties of the selected polymer(s), and toe desired range of component molecular weights to be resolved. For example, if only components with a high molecular weight are of interest, a higher concentration of polymer is used, which allows low molecular weight components to pass through quickly while larger components migrate more slowly. Preferably, toe separation medium remains flowable, that is, substantially in liquid form, so that the medium can be easily removed from or replaced in the apparatus by moderate pressure differentials (e.g. , less than 50 psi). Further guidance regarding the choice of polymer material, size and concentration can be found in the references cited above. In a preferred embodiment, the polymer material is linear polyacrylamide, e.g., 3% w/v with an average molecular weight (MW) of 100-300 kDa. The inclusion of such polymer materials is useful for enhancing the level of sample separation in the first dimension of electrophoresis, wherein sample components can be separated on the basis of a combination of their sizes and net charges. Such polymer materials may also be useful for reducing convection currents and EOF in the separation medium during and after electrophoresis. A further advantage is toat these polymers do not interfere with the isoelectric focusing step. The invention will now be further illustrated with respect to an embodiment wherein electrophoresis in the first dimension is based on charge and size of toe sample components, and electrophoresis in the second dimension is based on pi (by IEF). With reference to the assembly illustrated in Figs. 3 and 4, a low-ionic strength solution, preferably containing one or more denaturing reagents such as urea and thiourea, is loaded into the separation cavity via ports 130 and 135, with egress through elongate slot 138 until residual air bubbles have been removed from the cavity. Slot 138 is then closed, and valves at ports 135, 130 and 132 are opened to admit a second electrophoresis solution via port 135 into regions 160 and 126a and elongate channel 180. This second solution is preferably a low pH, flowable entangled polymer solution as above, which preferably includes a denaturant, for effecting size-based separation in the first dimension. Some diffusion between the two solutions in the cavity may occur, but this should not significantly affect performance. For electrophoresis, the plate assembly is preferably oriented horizontally, to minimize convection currents in separation medium. To control temperature, the plates can be placed on a constant-temperature heating/cooling device, such as a Peltier device, to maintain the separation medium at a selected temperature (e.g, 0 to 40°C) and prevent overheating. The device preferably includes an elongate sample transport channel, such as channel 180 in Figs. 3 and 4 or channel 280 in Figs. 10-12, to lengthen the migration distance in toe first dimension, and to increase spacing between bands. Wito the plate equilibrated with the appropriate solutions, sample injection can be accomplished by hydrodynamic or electrophoretic means. Referring to the embodiment in Figs. 3 and 4 for illustrative purposes, hydrodynamic injection is performed by closing all ports and slot(s) except ports 130 and 131, and pumping a selected volume of sample through the injection chamber (i.e., the portion of channel 180 located between ports 130 and 131). The sample can be moved into the portion of channel 180 beyond port 131 by closing port 131, opening port 132, and pumping the appropriate volume of buffer solution through port 130. The injection chamber can then be purged of residual sample by closing port 132 and opening port 131 again, to wash a selected amount of solution through toe injection chamber. Electrophoretic sample injection can be accomplished by filling toe injection chamber with a selected amount of sample via ports 130 and 131 as above, closing those ports, and then applying an electric field between ports 130 and 132 for a selected time (e.g., 5 kV for 1 to 5 seconds) so that a small aliquot of positively charged sample (components wito pi values greater than toe pH of the medium) migrates into the separation channel upstream of port 131, and toen optionally removing any residual sample via ports 130 and 131. Note that if desired, sample migration can be momtored, for example, by fluorescence detection. The advantages of hydrodynamic over electrophoretic injection schemes are well- known and are mainly concerned with toe oversampling of faster migrating components in electrophoretic injection. Hydrodynamic injection does not suffer from this shortcoming since all components are injected within the solution. Higher sensitivity may be experienced wito electrophoretic injection, however, since only sample molecules (not buffer or water) enter the separation path, and sample components are more highly concentrated at the start. However, sensitivity can be improved for hydrodynamic injection by using a sample buffer toat has lower ionic strength than the buffer solution in channel 180, or by interposing a volume of low ionic strength buffer (preferably having an ionic strength at least 5 times, and more preferably at least 10 times lower than that of the surrounding medium) between the sample and the separation buffer, to promote sample stacking. The amount of sample injected for analysis will vary according to the complexity of the sample, the type of detection, etc. By way of illustration, a sample volume may consist of 40 nL of a 100 μg/mL sample mixture of 1 ,000-10,000 fluorescently labeled polypeptides. Wito reference to the embodiment in Figs. 10-12, the configuration of ports 230a- 230d and channels 231a-231d and 23 If is useful for loading samples by different metoods. In one approach (T-injector mode), sample solution is pumped into port 230b, with egress out of port 230d, so that sample is placed at junction 23 le. The sample at junction 23 le can then be moved into channel 180 by imposing an electric field between ports 230c and 232. After toe sample has reached channel 180, the electric field between ports 230c and 232 can be replaced with a field between ports 230a and 232, to reduce leakage into channel 180 from channels 231b-231d. Preferably, channel 23 If is preloaded with a low conductance buffer (conductance lower than toat of the surrounding buffer) to promote sample stacking immediately downfield of toe low conductance buffer when the sample reaches channel 180. In a second approach, sample is pumped into port 230b, with egress out of port 230c, in order to fill channel 231c with a selected amount of sample. Channel 231b is optionally purged of residual sample by pumping buffer into port 230d, with egress out of port 230b. An electric field is then applied between port 230c and port 232 to transport the sample into channel 180. Again, channel 23 If is preferably preloaded with a low conductance buffer to promote sample stacking immediately downfield of the low conductance buffer when the sample reaches channel 180. After sample loading is complete, electrophoresis is performed across region 126a (first dimension) by applying an electric field between ports 130 and 132 (e.g., 5 to 30 kV), so that injected sample components migrate through channel 180 and into region 126a towards a catoodic electrode at port 132. This process may be monitored in real-time, e.g. , by fluorescence or chemiluminescence detection. A constant field or pulsed field can be used, depending on the sample and the desired resolution. When the fastest migrating component reaches port 132, or the desired amount of separation has occurred, the field is turned off, and a new field is applied across region 126 in a direction substantially perpendicular to the first dimension. This field can be generated by balancing the electric potentials at ports 130 and 132 to establish a substantially uniform field vertically across regions 126a and 126b towards slot 140. In another approach, this field is generated using an elongate wire electrode which (i) is electrically isolated from point electrodes located at ports 130 and 132, (ii) enters region 126a via port 132, and (iii) spans the upper edge of region 126a. In a third approach, the field is generated using electrodes located at port 135 and slot 140. The first dimension of electrophoresis is usually completed within a few minutes, depending on the magnitude of the field. For example, cationic polypeptides can migrate approximately 20 cm within 10 minutes in a field of 250 V/cm. Longer electrophoresis times in the first dimension, or lower concentrations of entangled polymers, can be used to select for slower-migrating components. Focusing in the second dimension is typically complete in less than 10 minutes in a field of 500 V/cm (5 kV field over 10 cm). Two- dimensional separations can thus be performed well within one hour. The maximum field permissible is dependent on toe ability of the device to dissipate Joule heat, which is typically facilitated by contact-cooling (e.g. , using a Peltier device) or by convective cooling (e.g., high Reynolds number air-flow). Figs. 13 and 14 illustrate another embodiment wherein the first electrophoresis region is bordered by a membrane toat segregates toe first electrophoresis region from an external electrode for use in the second electrophoresis step. For example, apparatus 200 from Figs. 10-12 can be modified so that the upper edges of region 226a are bordered by a membrane 290 which defines the upper surface of region 226a. Membrane 290 separates region 226a from an electrode reservoir 292 that contains an electrode 294 which is linkable to a voltage source 296. The membrane is preferably permeable to small ions but not to the sample components of interest. Thus, the membrane preferably has a molecular weight cutoff (pore size) that is smaller than the smallest sample component of interest. Preferably toe molecular weight cutoff is less than or equal to 3000 MW, and more preferably less than or equal to 1000 MW. Any appropriate membrane material can be used, such as a cellulose or cellulose acetate, for example, and many such membranes are available commercially. For electrophoresis in the first dimension, regions 226a and 226b are filled with appropriate separation media, and electrode reservoir 292 is filled with a sufficient amount of buffer to keep membrane 290 wetted while keeping electrode 294 dry or otherwise electrically insulated from toe separation chamber. After electrophoresis in the first dimension is complete (e.g., by imposing an electric field between ports 230a and 232), more buffer is added to reservoir 292 so toat electrode 294 is submerged in the buffer. Electrophoresis in the second dimension can then be performed by applying an electic field between electrode 294 and an electrode located at port 235 or between ports 234a and 234b. Conveniently, plates can be manufactured for this embodiment by cutting off toe upper ends of plates 220 and 222 using a rotory saw, to produce relatively smooto, flush ends for contacting membrane 290. If desired, gaskets, such as gasket 291 in Fig. 14, can be included between toe membrane surfaces and the plate ends and reservoir 292, to help ensure a liquid-tight seal therebetween. A similar membrane/electrode/reservoir structure can be included at toe lower end of the second electrophoresis separation region, to provide an external electrode at that end . Other two-dimensional embodiments encompassed by toe invention include the following first-dimension/second-dimensioncombinations : (1) Non-SDS Denaturing/Non-SDS Denaturing. In this embodiment, the separation cavity contains different media in the first and second electrophoresis regions such that the basis of sample migration in the second dimension is different from that of the first dimension (i.e., sample migration in the second dimension depends on sample features different from those in toe first dimension). For example, the first separation region can contain a medium with an acidic pH (e.g., 2.5) and a low concentration of sieving components (e.g., 2% linear acrylamide), and toe second region can contain a medium with a more basic pH (e.g. , 8) and a higher concentration of sieving components (e.g. , 4% linear acrylamide). (2) IEF/SDS-Electrophoresis. In this embodiment, toe first electrophoresis region contains an immobilized pKa gradient within region 26a, with an appropriate low-ionic strength buffer, toe second electrophoresis region contains an SDS-containing buffer (but not a pKa gradient), and plate 122 additionally contains a lateral slot 190 (not shown) in loading region 160, just above region 26a, which is similar in dimensions to lateral slot 140. After electrophoresis in the first dimension, an SDS-containing buffer is loaded into region 160 via port 135, with egress through slot 190. An electric field is then applied between slot 190 and slot 140 so that SDS molecules diffuse into region 26a. The SDS molecules associate with, and impart negative charge to, the neutrally charged sample components in region 26a, so that toe components are drawn into region 26b for size-based separation. It will be appreciated toat the invention can be adapted to other combinations of separation modes, according to the needs of the user. After electrophoresis has been performed for a selected time, separated bands can be detected, localized, and/or quantified in a variety of ways, including photography, confocal fluorescence scanning, phosphor imaging plates (for radioactivity detection), and by CCD (charge-coupled detection), depending on the namre of the signal being detected. According to one advantage of the invention, signal acquisition can be performed while the electric field is maintained across the IEF region, to allow integration of signal over time while bands are maintained in position by the field. The components are preferably indexed and recorded using an automated recording device, such as a computer-controlled digital imaging device (e.g. , CCD) linked to a low-power microscope. Optical absorbance densitometry techniques can also be used. If both plates are optically transparent, transmission-type signal detection can be used, as in the case of fluorescence detection involving excitation or illumination from beneath, and detection above, the plates. Alternatively, signal generation (if necessary) and detection can take place above a single plate surface, e.g., for confocal fluorescence detection or radiolabel detection. For embodiments involving separation channels in the second electrophoresis region, selected sample lanes can be collected through one or more egress ports. For each selected channel, a small-diameter capillary tube is inserted into the egress port, and fluid is witodrawn through the capillary by positive pressure, vacuum, piezoelectric pumping, or electroosmotic pumping. A diode detector can be used at the end of the capillary tube to monitor sample bands entering the tube. The witodrawn sample can be transferred to collection vials or membrane if desired. For configurations having channels that terminate wito open ends at toe lower ends of the plates, samples may be collected on a membrane by dragging the membrane across the ends of the plate assembly, or by attaching tubes to the channel termini to withdraw sample. III. Utility The present invention provides methods and apparatus for characterizing, detecting and/or identifying hundreds or thousands of components in a sample on toe basis of different charge and mass properties. The invention therefore has utility in a number of applications, including "fingerprinting" samples, e.g., for differential display to facilitate identification and differentiation of samples; detecting and/or monitoring compositions of normal and diseased cells and tissues; diagnosing or monitoring disease; characterizing or monitoring molecular expression levels of gene products; characterizing the effects of the addition, mutation, deletion or truncation of genes; detecting, identifying, distinguishing, or otherwise characterizing viruses, bacteria, fungi, and other microbes, or components or products thereof; monitoring analyte levels over time as a function of environmental change, life cycle, or exposure to exogenous chemicals or stimuli; toxicity testing; and testing drug candidates for therapeutic efficacy. The method is particularly of interest in studying and characterizing protein components of biological samples, and therefore is useful in proteome research (Wilkens et al. , 1997) . From the foregoing, it can be seen how the objectives and features of toe invention are met. The invention provides a method that permits two-dimensional electrophoresis in a single apparatus. The apparatus is simple to use and can generate analytical results more rapidly than previous two-dimensional metoods. The method permits characterization of samples containing hundreds or thousands of components under a variety of different separation conditions. The method does not require a crosslinked matrix, and therefore is easily refilled with the same or different media for separating additional samples. By using a chemically stable pKa gradient that is immobilized on one or more interior surfaces of the separation cavity, the apparams can be reused with high reproducibility, especially with respect to the IEF dimension. Thus, toere is no need to form a new pKa gradient for each sample. Furthermore, toe use of separation channels in the second dimension facilitates collecting selected lanes or individual analytes for further characterization, after the pattern of analytes has been imaged and indexed. The invention can be further understood in light of the following examples, which are not intended to limit toe scope of the invention. Materials and Methods All solvents were obtained in he highest purity available and then passed through a 0.22 μm filter. Triethoxy silane (TES, Huls America, Bristol, PA), hydrogen hexachloroplatinate IV (Aldrich Chemical), and allyl methacrylate (Aldrich Chemical) were used as received from the manufacmrer. Immobilines (pKa = 3.6, 4.6, 6.2, 7.0, 8.5 and 9.3) and TEMED were used as received from the manufacturer (Amersham-Pharmacia Biotech). Water was purified with a Milli-Q System and filtered through a 0.45 μm filter. Example 1 Isoelectric Focusing Region A plate assembly made from etched borosilicate glass having a configuration substantially in accordance with Figs. 1 and 2 was used, wito a separation cavity having dimensions of 10 cm x 10 cm x 100 μm. Prior to derivatization with immobilines, the channel region was preconditioned at room temperature by treatment with an approx. 6 mM ammonia solution, pH 10. After rinsing with deionized water and flushing with 0.1 M HCl to remove adsorbed ammonia, the channels were rinsed with deionized water, and the separation cavity was dried wito nitrogen at 100°C for 20 hours or more. Attachment of immobilines was accomplished as follows. First, allyl methacrylate moieties were attached to the entire separation cavity in a two-step process described below. Both steps were conducted in a gas chromatography heating oven (Hewlett Packard) under nitrogen atmosphere. Solvents and chemical reagents were delivered to the bottom ends of the channels using nitrogen pressure (50 psi). Solvents and reagents were removed by vacuum. The reactions in the two derivatization steps is described by the following reaction schemes: (1) -Si-OH + (EtO) 3 Si-H - Si-O-Si-H + 3 EtOH (2) -Si-H + CH 2 =CH-R → Si-CH 2 -CH 2 -R where R = -OC(=O)CH 2 C(CH 3 )=CH 2 In the first derivatization step, a silicon-hydride monolayer was formed by washing the channels with dioxane, and then treating toem with a steady flow of 1.0 M solution of triethoxysilane (TES) in dioxane at 90°C for 60 minutes. Constant flow was necessary to prevent reagent deposition and clogging of toe channels. The hydride-modified surface was toen washed sequentially at room temperature with several volumes of dioxane, tetra- hydrofuran (THF), and then toluene. Toluene was maintained over toe channels for toe next step. The hydride-modified surface was washed with dry toluene. A mixture of 25 mL pure allyl methacrylate plus 25 mL toluene was mixed with 70 μL of 10 mM Spiers catalyst (hexachloroplatinic acid in 2-propanol) and heated to 60-70°C for one hour before being flushed through the channels at 100°C for 5 to 24 hours. This allyl methacrylate treatment procedure was repeated one more time, after which the surface was washed successively with toluene and THF. Immobilines were attached to the surface allyl methacrylate groups in the IEF region of the separation cavity as follows. A gradient of immobilines was delivered to the coated region from the bottom of the electrophoresis apparams via a quaternary HPLC pump connected to the inlet of a fan-shaped delivery element. The gradient was generated using two solutions, A and B. Solution A Amount (μL) Immobiline pK 3.6 — Immobiline pK 4.6 105 Immobiline pK 6.2 46 Immobiline pK 7.0 449 Immobiline pK 8.5 144 Immobiline pK 9.3 330 acrylamide (10% w/v) 433 water 3496 glycerol 750 Solution B Amount (μL) Immobiline pK 3.6 413 Immobiline pK 4.6 — Immobiline pK 6.2 171 Immobiline pK 7.0 34 Immobiline pK 8.5 126 Immobiline pK 9.3 — acrylamide (10% w/v) 216 water 1771 glycerol 144 Solutions A and B were degassed and the pH was adjusted to 7.0 by adding 2 drops of 0.1 M HCl to solution A and 2 drops of 0.1 M NaOH to solution B. The electrophoresis apparatus was first filled from the bottom with water at a flow rate of 0.1 mL/min, to minimize capillary action. Next, 5.5 μL each of TEMED and APS were added to solution A, and 3 μL each of TEMED and APS were added to solution B. The gradient was then started, going from 0% to 100% A over 2.3 min. at a flow rate of 0.3 mL/min. At the end of the gradient, flow was stopped briefly, after which a solution of 40% glycerol/0.05% bromphenol blue (gradient-chasing solution) was pumped in at 0.1 mL/min. When the front of the dye solution reached the bottom of the plates, the flow was stopped, and the immobiline coating reaction was allowed to proceed for 5 hours with the plates in an upright position. The immobiline mixture was then removed, and the apparatus was filled with water. The resulting immobiline gradient was sufficient to create a substantially linear pH gradient spanning pH 3 to 9, from the top to the bottom of the channel region. Example 2 Coating Method with Acrylamide Capping Step The following procedure can be used to create a separation chamber coated with a continuous pKa gradient such that residual activated regions are capped with acrylamide. The plates (see Fig. 3) are activated with 0.1 M NaOH (overnight), then washed with water, 0.1 M HCl, and again water followed by washing with acetone and drying 1 hour under N 2 at 90 °C. The TES solution is loaded into the plates using a peristaltic pump equipped with PEEK tubing, Teflon fittings, VITON O-rings (DuPont), and a bubble trap between the pump and the plates. Preferably, prior to loading the plates, PEEK tubing containing the TES solution is heated in an oven at 90°C, to minimize temperature differences. The TES reaction is carried out for 90 minutes at 90 °C. The plate is then washed with dioxane and emptied. After the VITON O-rings are changed, the reaction with allyl methacrylate (10 mL allyl methacrylate + 10 mL toluene + 280 μL of 10 mM catalyst) is performed at 90°C for 48 hours. After completion of the reaction, the plate is washed with toluene (using a washing bottle and vacuum) followed by dioxane, acetone, and drying under N 2 for 1 day or more at 50 °C. For creating a pKa gradient, solutions A and B are made as in Example 1 , except that solution A is modified to contain 8% glycerol (use 3784 μL water and 461 μL glycerol) and solution B is modified to contain 13% glycerol (use 1542 μL water and 373 μL glycerol). A third solution, solution D, is prepared containing 40% glycerol in a solution of Coomassie Brilliant Blue, Sigma Catalog #B7920. Solutions A and B are degassed and the pH is adjusted to 7.0 by adding 2-3 drops of 1 M HCl acid to solution A and 2 crystals of Tris to solution B. Then 1.0 mL of each solution (A and B) is placed in separate tubes and 1.0 μL of APS (ammonium persulfate) and TEMED are added to each tube. The gradient is carried out from 100% solution A to 100% solution B, using an HPLC pump with a total gradient volume of 650 μL and the following time line: The gradient is stopped when the blue 40% glycerol solution reached the channels (about 10 mm). Total run time is 26 minutes. The plate is left in a vertical position overnight. In the morning the plate is emptied by vacuum and washed with deionized water. The next step is to modify the fan-shaped region and sample transport channel with low pH (-3.30) Immobiline solution. The solution is prepared as follows: in 2 mL volumetric flask add. The pH of the solution is adjusted to pH = 7.0 with solid Tris. 2.0 μL of APS (ammonium persulfate) and TEMED are added to 2 mL of the above solution. With the plates oriented vertically with the fan-shaped region at the bottom, the fan-shaped region and the region for electrophoresis in the first dimension are filled by syringe with water and 0.1 % Triton X-100 (to wet the interior surfaces of the plates). Next, the sample transport channel is filled from distal port of the channel by syringe with Immobiline solution. Using vacuum and argon pressure the fan-shaped region is filled with toe Immobiline solution through the port at the apex of the fan-shaped region. The plate is left overnight at room temperature (RT). In the morning it is emptied and washed with water. Then the plate is endcapped by flowing continuously a solution of 0.1 % NN- dimethylacrylamide (DMA) (including 1 μL each of TEMED and APS per mL of DMA solution) through the plates, with the plated heated to 50° C in a gas chromatography oven for 2 hours with a flow rate of 0.1 to 0.5 mL/min (e.g., 0.2 mL/min). During this procedure, the container from which the endcapping solution is delivered to the plates is kept in an ice bath to reduce polymerization. Optionally, the capping step is repeated using a solution containing a higher amount of acrylamide, e.g. , 1 to 2% acrylamide monomer, to promote capping further. In a further embodiment, the gradient solution can be followed directly by a low pKa Immobiline solution (which may contain a high glycerol concentration). Likewise a high pKa Immobiline solution can precede the gradient-forming solution, to provide a high pH barrier at the end of the pi (IEF) gradient zone. Finally, the plate is emptied by vacuum, washed with deionized water, and stored. Although the invention has been described with respect to particular embodiments and examples for purposes of illustration, it will be appreciated that various modifications can be made without departing from the scope and spirit of the invention.

Claims: Claims: 1. A two-dimensional electrophoresis system comprising (a) an electrophoresis plate assembly toat defines (i) a sample separation cavity bounded by opposing major first and second surfaces, each having a defined width and length, and which are spaced apart by an interfacial distance substantially shorter than said width and length, said cavity having an upper portion, a lower portion, and two lateral portions, said cavity further comprising (1) an electrophoresis region located along the upper portion of the cavity for performing charge and/or size-based electrophoresis in a first dimension along the upper portion, and (2) below the first said electrophoresis region, a second electrophoresis region for performing electrophoresis in a second dimension in a direction substantially perpendicular to the first dimension, (ii) a sample loading port positioned at a corner of said upper portion, for introducing a sample into the first electrophoresis region, and (iii) optionally, one or more fluid passageways positioned along the lower portion of the cavity, for introducing or removing liquid from the cavity, (b) electrode means for generating a first voltage potential across the first said electrophoresis region between the loading port and the lateral portion across from the loading port, (c) electrode means for generating a second voltage potential between said upper portion and lower portion, and (d) occupying said cavity, an aqueous medium that may be the same or different in toe first electrophoresis region and the second electrophoresis region, such that the basis of migration of sample components in the second dimension depends on one or more sample properties that are different from the sample properties that determine sample migration behavior in the first dimension. 2. The electrophoresis system of claim 1 , wherein said loading port is joined to said upper corner portion of the cavity by an elongate sample transport channel. 3. The electrophoresis system of claim 1 , wherein said second electrophoresis region contains a plurality of elongate, substantially parallel separation channels. 4. The electrophoresis system of claim 1, where the second electrophoresis region contains a pKa gradient immobilized on at least one of the major opposing surfaces. 5. The electrophoresis system of claim 4, wherein the second electrophoresis region is bounded by a region that contains a high pH or low pH coating region adjacent the second electrophoresis region, and that is immobilized on at least one of the major opposing surfaces. 6. The electrophoresis system of claim 4, which contains at least 30 separation channels. 7. The electrophoresis system of claim 1, wherein said one or more fluid passageways are provided as a single slot which passes through one of said surfaces at or adjacent to said lower portion. 8. The electrophoresis system of claim 1, wherein said one or more fluid passageways are provided as a plurality of openings positioned along the lower portion of the cavity. 9. The electrophoresis system of claim 1, wherein said medium includes a plurality of linear acrylamide molecules. 10. The electrophoresis system of claim 1, wherein said plates define a fan-shaped region toat abuts said separation cavity, for introducing or removing liquid from the cavity. 11. The electrophoresis system of claim 1 , wherein the upper edge of the first said electrophoresis region is bounded by an ion-permeable membrane . 12. A method for separating one or more components of a sample mixture, said metood comprising providing an electrophoresis system as recited in claim 1 , applying a sample mixture to said loading port, applying a first voltage potential across the first said electrophoresis region under conditions effective to cause different components in the mixture to migrate towards the opposite portion of the electrophoresis region, such that different components become separated at least partially on toe basis of size, after said separating, applying a second voltage potential across the upper and lower portions of the cavity under conditions effective to separate sample components on the basis of sample properties toat are different from the sample properties that determine sample migration behavior in the first dimension. 13. The method of claim 12, wherein the sample components are polypeptides. 14. The metood of claim 12, wherein the sample components include a detectable label. 15. The metood of claim 12, wherein said label is a fluorescent label. 16. The method of claim 12, which further includes detecting the locations of components in the cavity. 17. The metood of claim 12, wherein said loading port is joined to said upper corner portion of the cavity by an elongate transport channel. 18. The metood of claim 12, wherein said second electrophoresis region contains a plurality of elongate, substantially parallel separation channels. 19. The method of claim 18, wherein after isoelectric focusing, at least one sample component is collected from one or more channels via said one or more fluid passageways. 20. The method of claim 12, wherein the second electrophoresis region contains an isoelectric focusing region containing a pKa gradient immobilized on at least one of said major opposing surfaces, such that a pH gradient is generated in the isoelectric focusing region in a direction substantially perpendicular to the direction of toe first said voltage potential, so that sample components migrate into toe isoelectric focusing region and separate on the basis of toeir isoelectric points. 21. The method of claim 12, wherein the pH gradient defines a pH range of about 4 to 10. 22. The method of claim 12, wherein said medium includes a plurality of linear acrylamide molecules. 23. An electrophoresis plate assembly comprising (i) a sample separation cavity bounded by opposing major first and second surfaces, each having a defined width and length, and which are spaced apart by an interfacial distance substantially shorter than said width and length, said cavity having an upper portion, a lower portion, and two lateral portions, said cavity further comprising (1) an electrophoresis region located along the upper portion of the cavity for performing charge and/or size-based electrophoresis in a first dimension along the upper portion, and (2) below the first said electrophoresis region, an isoelectric focusing region containing a pKa gradient immobilized on at least one of said major opposing surfaces, for isoelectric focusing in a direction substantially perpendicular to the first dimension, (ii) a sample loading port positioned at a corner of said upper portion, for introducing a sample into said electrophoresis region, and (iii) optionally, one or more fluid passageways positioned along toe lower portion of toe cavity, for introducing or removing liquid from the cavity. 24. The assembly of claim 23, wherein said loading port is joined to said upper corner of the cavity by an elongate transport channel . 25. The assembly of claim 23, wherein said isoelectric focusing region contains a plurality of elongate focusing channels defined by the plate, which are parallel to the lateral portions of the cavity and prevent flow of said medium between the channels. 26. The assembly of claim 25, which contains at least 30 focusing channels. 27. The assembly of claim 23, which further includes electrode means for generating a first voltage potential across the first said electrophoresis region between the loading port and toe lateral portion across from toe loading port, and electrode means for generating a second voltage potential between said upper portion and lower portion. 28. An electrophoresis plate or tube having a surface which is coated with a pKa gradient immobilized thereon. 29. The electrophoresis plate or tube of claim 28, which is a plate, wherein the gradient spans at least one pH unit.

Inventors: RAYSBERG YEFIM M, WIKTOROWICZ JOHN E, RAYSBERG, YEFIM, M., WIKTOROWICZ, JOHN, E.
Applicants: LYNX THERAPEUTICS INC, RAYSBERG YEFIM M, WIKTOROWICZ JOHN E, LYNX THERAPEUTICS, INC., RAYSBERG, YEFIM, M., WIKTOROWICZ, JOHN, E.
Application year: 1999
Main classification: G01N27
IPC codes: G01N27