Abstract:
Method and materials to carry out preparative-scale electrophoretic separations based on the principle of dynamically created non-co-directional effective electrophoretic mobilities are disclosed. The primary application areas of the method are in the separation, purification, enrichment, concentration or conditioning of both small and large molecular weight, weak and strong electrolyte compounds, such as pharmaceuticals, oligo- and polypeptides, proteins, oligonucleotides, etc. These objectives can be achieved based on the use of a secondary chemical equilibrium, alone or in combination with multiple protic and other secondary chemical equilibria. Though such electrophoretic operations could be achieved by other means, such as by conventional zone electrophoresis or isotachophoresis, the method disclosed here can provide greater separation power, simplicity and higher production rates.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 60/277,126 filed Mar. 19, 2001, and entitled Method for Electrophoretic Separations Using Dynamically Generated Opposite Mobilites. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to preparative-scale separation of components by electrophoresis. More particularly, method and apparatus for altering the initial composition of a feed solution by electrophoretic separations using a resolving agent to dynamically alter the sign of the effective mobility of the band of one or more selected sample components in the feed solution is provided. 
     2. Description of Related Art 
     The ability to conduct preparative-scale chromatographic enantiomer separations has improved significantly during the last decade. Once the fundamentals of nonlinear chromatography became well understood and adequate hardware became readily available, many batch-wise nonlinear chromatographic separations were converted to continuous separations, using a simulated moving bed approach, to achieve higher productivities. [Juza, M. et al.,  Trends in Biotech.  2000, 18, 108–118] Mirroring these developments, as understanding of capillary electrophoretic enantiomer separations improved, preparative-scale electrophoretic enantiomer separations were attempted. The appearance of a new, continuous, free-flow electrophoretic units, such as the “Octopus,” permitted exploration of the conversion of batch-type electrophoretic separations to continuous, preparative-scale separations that can alter the initial composition of the feed solution. These developments are more fully explained in a recent paper co-authored by the inventor [“Use of Single-Isomer, Multiply-Charged Chiral Resolving Agents for the Continuous, Preparative-Scale Electrophoretic Separation of Enantiomers Based on the Principle of Equal-But-Opposite Analyte Mobilities,”  Electrophoresis  2000, 21, 1019–1026], which is hereby incorporated by reference herein. 
     The Octopus unit is illustrated in  FIG. 1  (Prior Art). There is a continuous, laminar flow of the separation medium, orthogonal to the electric field, through shallow, rectangular electrophoresis chamber  10  of the unit. The sample is continuously fed at inlet  12 , above inlet ports  14  where the separation medium enters the unit, either at the center or at one of the sides of the chamber. The separated components, dissolved in the separation medium, are collected through sampling ports  16  as they leave the separation chamber. The sampling ports (normally including 96 ports) provide a lateral spatial resolution of about 1 mm per collection port across the 100 mm wide separation chamber. Recent studies indicated that the reproducibility and long-term stability of the separation patterns obtained in the Octopus unit were satisfactory. 
     The Octopus unit is well suited for preparative-scale, continuous isoelectric focusing separations because the well known isoelectric focusing mechanism successfully counters most of the flow-related band broadening mechanisms. The Octopus unit has been successfully used for the preparative-scale isoelectric focusing separation of the enantiomers of dansyl phenylalanine with 30 mM hydroxypropyl β-cyclodextrin as chiral resolving agent in binary Bier buffers. [P. Glukhovskiy et al.,  Anal. Chem.,  1999, 71, 3814–3820] The Octopus unit has also been used for the much more difficult, continuous free-flow and intermittent-flow electrophoretic separation of the enantiomers of methadone with non-charged hydroxypropyl β-cyclodextrin as the chiral resolving agent. [P. Hoffmann et al.,  Anal. Chem.,  1999, 71, 1840–1850] In both of these flow modes, the cationic methadone enantiomers were injected at the anodic side of the separation chamber (opposite to fraction collection ports  19 – 20 ), and were collected, partially separated, in fractions  52 – 96  (continuous flow mode) and fractions  72 – 96  (intermittent flow mode). This means that in the continuous flow mode the available separation distance was only twice as large (about 80 ports wide) as the band width of each enantiomer (about 44 ports wide) resulting in an alteration of the initial composition of the feed solution. The situation was a little better in the intermittent flow mode, where the available separation distance (about 80 ports wide) was four times as large as the band width of each enantiomer (about 20 ports wide) resulting, once again, in an alteration of the initial composition of the feed solution. Clearly, one would need a much larger separation distance (much wider separation chamber) if one wanted to completely eliminate the overlap of the enantiomer bands to not only alter the initial composition of the feed solution, but recover each enantiomer in pure form. 
     In general, the separation of two like-charged analyte ions of similar, but not identical effective mobilities (derived either from strong electrolytes or weak electrolytes) by electrophoresis typically requires the use of long migration distances and/or large applied electric potentials. While these requirements can be fulfilled relatively easily in capillary electrophoresis for analytical-scale separations, they are often difficult or impossible to meet in preparative-scale separation. 
     Hydrodynamic flows or electroosmotic flows have been utilized to shift the observed mobilities of at least one of the analytes to be separated, as described, e.g., in the paper by B. A. William and Gy. Vigh, “The Use of Hydrodynamic Counterflow to Improve The Resolution of the Minor Component in the Capillary Electrophoretic Analysis of Enantiomers.”  Enantiomer  1 (1996) 183. However, due to the frequently poor temporal stability of the electroosmotic flow, and the extra band broadening created by the laminar hydrodynamic flow, these approaches are not conducive to efficient preparative-scale separations. What is needed is a method for use in preparative-scale electrophoresis that can alter the initial composition of the feed solution over relatively short migration distances. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic of the Octopus continuous free-flow electrophoretic system. 
         FIG. 2  shows the effective mobility (μ eff ) of the more strongly complexing terbutaline enantiomer as a function of the heptakis-6-sulfo-β-cyclodextrin (HS) concentration of the separation medium. 
         FIG. 3  shows the effective mobilities (μ eff ) of the less strongly binding (symbol: +) and more strongly binding (symbol: filled circle) terbutaline enantiomers as a function of the HS concentration of the separation medium around the cross-over point of the effective mobilities. 
         FIG. 4  shows the effective mobilities (μ eff ) of the less strongly binding (symbol: +) and more strongly binding (symbol: filled circle) terbutaline enantiomers as a function of the isopropanol (IPA) concentration in a 30 mM HS separation medium. 
         FIG. 5  shows the electropherograms of the terbutaline enantiomers (small peaks) and nitromethane (tall peaks) in the 30 mM HS, 5% v/v IPA separation medium (bottom trace), 30 mM HS, 9% v/v IPA separation medium (middle trace), and 30 mM HS, 13% v/v IPA separation medium (top trace). 
         FIG. 6  shows the distribution of terbutaline in the fractions collected at the exit of the Octopus unit; hydrodynamic flow only, no electrophoresis. 
         FIG. 7  shows the distribution of terbutaline enantiomers in the fractions collected at the exit of the Octopus unit after electrophoresis. Solid black columns: less strongly binding enantiomer with cationic effective mobility. Hatched columns: more strongly binding enantiomer with anionic effective mobility. 
         FIG. 8  shows the cumulative purity vs. % recovery curves for the continuous free-flow electrophoretic separation shown in  FIG. 7 . Symbol +: less strongly binding enantiomer with cationic effective mobility. Symbol circle: more strongly binding enantiomer with anionic effective mobility. 
     
    
    
     SUMMARY OF THE INVENTION 
     A method for altering the composition of a feed solution containing at least two components is provided. The feed solution is placed in a separation medium-filled separation chamber of an electrophoretic apparatus and an electrical potential is applied to the electrophoretic apparatus. The separation medium contains a resolving agent that differentially interacts with the components to cause migration of the bands of the component away from each other so as to separate the bands. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The approach of this invention is based on the principle that the number of cascade stages required to effect a binary separation decreases very rapidly as the value of separation selectivity increases. Applied to electrophoretic enantiomer separations this means that if one can force the band of one of the enantiomers to migrate cationically and the other anionically, both enantiomers will migrate away, in the opposite direction, from their feed position and separation can be achieved by migration through the shortest possible distance, the sum of the two band widths. According to the predictions of the charged resolving agent migration model (CHARM model) of electrophoretic enantiomer separations [B. A. Williams and G. Vigh, J. Chromatogr., 1997, 777, 295–309], one might be able to create such a situation by using a multiply charged anionic resolving agent (such as the single-isomer heptakis-6-sulfo-β-cyclodextrin) for the separation of the enantiomers of a single-charged cation (such as protonated terbutaline) in a unit such as the Octopus unit. 
     The CHARM model, which in a first approximation neglects the ionic strength effects, predicts that when a single-charged cationic enantiomer (either a strong base or a protonated weak base) is complexed with a single-isomer, multiply-charged anionic resolving agent, the effective electrophoretic mobility of the R enantiomer (μ eff   R ) is 
                     μ   R   eff     =         μ   0     +       μ   RCD   0     ⁡     [   CD   ]           1   +       K   RCD     ⁡     [   CD   ]                   (     Eq   .           ⁢   1     )               
and separation selectivity (α=μ eff   R/ μ eff   S ) for the enantiomer pair is
 
                   α   =           μ   0     +       μ   RCD   0     ⁢       K   RCD     ⁡     [   CD   ]               μ   0     +       μ   SCD   0     ⁢       K   SCD     ⁡     [   CD   ]             ⁢       1   +       K   SCD     ⁡     [   CD   ]           1   +       K   RCD     ⁡     [   CD   ]                     (     Eq   .           ⁢   2     )               
where μ 0 , μ 0   RCD , and μ 0   SCD  are the ionic mobilities of the free and complexed enantiomers, K RCD  and K SCD  are the respective binding constants of the enantiomers, and [CD] is the species concentration of the free resolving agent. Eq. 1 predicts that the initially high cationic effective mobilities of the enantiomers decrease toward zero then, for sufficiently strongly binding enantiomers, become anionic as the concentration of the anionic resolving agent is increased. When the binding constants of the two enantiomers and/or the ionic mobilities of the two diastereomeric analyte-resolving agent complexes or both are different from each other, the resolving agent concentrations where the effective mobilities of the enantiomers change from cationic to anionic will be different for the two enantiomers. Consequently, there will be a resolving agent concentration at which the band of one of the enantiomers will still migrate cationically, while the band of the other enantiomer will already migrate anionically, and the absolute values of their effective mobilities will be equal. This migration behavior has been verified experimentally and has been exploited for the analytical-scale separation of weak base pharmaceuticals with several single-isomer, heptasulfated and octasulfated β- and Γ-cyclodextrins as the resolving agents [Tacker et al., Electrophoresis 1999, 20, 2794–2798].
 
     This migration behavior allows one to propose a continuous, preparative-scale electrophoretic separation scheme for charged (both strong and weak electrolyte) enantiomers by (i) continuously feeding the separation chamber of an electrophoretic separator (or a portion thereof) with a separation medium in which |μ eff   R | is approximately equal or equal to |μ eff   S | and μ eff   R /μ eff   S &lt;0, (ii) continuously feeding the mixture of the enantiomers into the section of the electrophoretic separator that contains such a separation medium, (iii) applying an electric potential orthogonal to the flow direction of the separation medium and (iv) continuously collecting the pure enantiomers at the downstream end of the separation chamber. This way, the enantiomers would have to migrate maximally only as much as the sum of the band widths, yet would become clearly separated from each other. Also, the total width of the separation chamber can be much smaller than what would be needed to achieve separation of the enantiomers when both of them migrate cationically or anionically. 
     Phosphoric acid, lithium hydroxide, 2-propanol (IPA) and hydroxypropylmethyl-cellulose (HPMC, average molecular weight 86,000) were obtained from Aldrich (Milwaukee, Wis.), β-alanine from Sigma (St. Louis, Mo.) and heptakis-6-sulfo-β-cyclodextrin (HS) was synthesized as described in Vincent, et al., Anal. Chem., 1997, 69, 4419–4428. All solutions were freshly prepared using deionized water from a Millipore Q unit (Millipore, Milford, Mass.). 
     The final separation medium used in the preparative-scale enantiomer separations contained 0.2% w/w HPMC, 12.5% v/v IPA, 50 mM β-alanine and 30 mM HS. The counter-flow solution at the outlet of the Octopus unit was 0.2% HPMC in de-ionized water. The sample was 5 mM racemic terbutaline and it was dissolved in the separation medium. The highly conductive anolyte and catholyte solutions were made by adding 50 mmol β-alanine, 75 mmol methanesulfonic acid and 75 mmol sodium hydroxide to 1 L deionized water. 
     The background electrolyte (BE) used for the capillary electrophoretic CE purity analysis of the collected fractions was made by adding 10 mmol HS to a 1 L volumetric flask and filling it to the mark with a stock solution of 25 mM H 3 PO 4  titrated to pH 2.5 with LiOH. 
     The analytical-scale CE separations were completed on a P/ACE 5510 CE unit (Beckman-Coulter Instruments, Fullerton, Calif.). Its UV detector was operated at 214 nm. All CE separations were carried out in L d =19 cm, L t =26 cm, 25 μm I.D., 150 μm O.D. uncoated fused silica capillaries (Polymicro Technologies, Phoenix, Ariz.) at 10 kV. The cartridge coolant of the P/ACE was thermostated at 25° C. 
     All preparative-scale enantiomer separations were completed in the Octopus continuous free-flow electrophoretic unit (Dr. Weber GmbH, Kirchheim-Heimstetten, Germany). The schematic of the unit is shown in  FIG. 1 . The chamber coolant was thermostated at 10° C. The Octopus unit has a pair of anolyte recirculating ports (not shown in  FIG. 1 ), a pair of catholyte recirculating ports (not shown), seven separation medium feed ports  14 , central sample feed port  12 , and a counter-flow feed port (not shown) at the top of the chamber above the 96 collection ports  16 . Alternatively, sample feed port  12  may be omitted and a sample may be injected into chamber  10  through at least one of separation medium feed ports  14 . A multichannel peristaltic pump (not shown) was used to feed all solutions to the ports. The separated fractions were collected through ninety six sample collection ports  16  at the exit end of the separation chamber, offering a lateral resolution of 1.04 mm/collection port. A 215 μm-thick zone electrophoresis separation chamber gasket was used in each experiment to form chamber  10 . 
     The effective mobilities of the enantiomers were determined by conventional CE measurements as described in Vincent et al., Anal. Chem., 1997, 69, 4419–4428. The preparative, continuous free-flow enantiomer separations were completed by first filling the Octopus separation chamber with deionized water and removing all air. The separation medium was introduced through the inlet ports at a total flow rate of 35.0 mL/h. The sample was introduced through the central sample port at a rate of 2.5 mL/h. The counter-flow was pumped at 90.0 mL/h. Once stable flows were established, a separation potential of 350 V was applied across the 10 cm wide separation chamber. Fractions of 1.63 mL each were then collected in deep, 96-well titer plates and analyzed by CE for enantiomeric purity. 
     First, the effective mobilities of the terbutaline enantiomers were determined by CE at 25° C., as described in Vincent et al., Anal. Chem., 1997, 69, 4419–4428, by adding increasingly higher amounts of HS to a 25 mM H 3 PO 4  solution that was adjusted to pH 2.5 with LiOH. The effective mobility of the enantiomer that had a lower cationic mobility in the 0.5 mM HS BE is shown in  FIG. 2  for the entire HS concentration range studied. As predicted by the CHARM model, the initially cationic effective mobilities decrease, go through zero, and become increasingly anionic as the concentration of HS is increased. Since the effective mobility cross-over occurred between 1 and 2 mM HS concentration, the measurements were repeated, as shown in  FIG. 3 , with small increments in the 0.5 to 3 mM HS concentration range. The effective mobilities of the two enantiomers are equal in magnitude, but opposite in sign when the HS concentration is about 1.45 mM. Thus, this BE could be used for the proposed preparative separation scheme, except that (i) the sample load would have to be kept low, in line with the low HS concentration at which the mobility cross-over occurs, and (ii) the conductivity of the acidic buffer (11.6 mScm −1 ) would be prohibitively high for the Octopus unit. 
     Therefore, a second BE containing 50 mM β-alanine and 30 mM HS (pH 6.04) was prepared and the effective mobilities of the terbutaline enantiomers were again measured at 25° C. As expected, strongly anionic effective mobilities were observed. Next, increasing amounts of isopropanol (IPA) were added to this stock BE and the effective mobilities were re-measured as shown in  FIG. 4 . The extent of complexation between HS and the terbutaline enantiomers decreased as the concentration of IPA was increased. Complexation became so weak above an IPA concentration of 11% v/v that the effective mobilities of both enantiomers became cationic. Thus, by adding enough IPA to the BE, effective mobilities similar to those around the mobility cross-over point in  FIG. 3  could be obtained with HS concentrations as high 30 mM. The electropherograms containing the terbutaline enantiomers and nitromethane (a very weakly bound neutral marker) in three different BEs are shown in  FIG. 5 . In each electropherogram, the two small peaks correspond to the terbutaline enantiomers, the tall peak to nitromethane. Clearly, the bands of both enantiomers migrate cationically in the BE that contains 13% v/v IPA. The enantiomer bands are spaced about evenly on the cationic and anionic sides of nitromethane when there is 9% v/v IPA in the BE. The bands of both enantiomers migrate anionically when the IPA concentration is 5% v/v. These electropherograms indicate that the proposed preparative separation scheme is feasible. 
     The Octopus unit has to be operated at or below 10° C. so as not to overheat (and damage) its PLEXIGLAS top plate. Since the P/ACE unit that was available for the mobility measurements cannot reach such a low temperature, the IPA concentration at which equal-but-opposite enantiomer mobilities occur in the 30 mM HS separation medium (see above) had to be adjusted slightly, as follows. First, one can determine the actual fraction number in the Octopus that corresponds to the sample feed position by continuously injecting the racemic terbutaline feed stream, without turning on the separation potential, and determining the concentration distribution of terbutaline in the collected fractions. The centroid of the terbutaline band indicates the feed position in terms of fraction numbers. As shown in  FIG. 6 , our feed band centroid was at fraction  46  and the band was eight fractions wide. Next, one can repeat the same experiment, but with the separation potential turned on, and look for the change in the concentration distribution of terbutaline in the collected fractions. The IPA concentration at which the spread remains symmetric around the original centroid position (in our case, fraction  46 ), indicates the point where the enantiomers have equal-but-opposite mobilities. In our case, this IPA concentration was 12.5% v/v. The conductivity of this separation buffer at 10° C. was 7 mScm −1 , a more favorable value than that of the phosphoric acid-based separation buffer. 
     The results of a 1.25 hour long preparative run are shown in  FIG. 7 . The collected fractions were analyzed by CE to determine the amounts of the terbutaline enantiomers in each fraction. The concentration distribution obtained is plotted in  FIG. 7 . The concentration maxima of the two enantiomer bands are separated from each other by about three fractions. Each enantiomer band is only ten fractions wide, which means that the width of the electrophoresed band increased only by about 20% over that of the non-electrophoresed band (compare  FIGS. 6 and 7 ). This represents a significant improvement in band width compared to what was observed either in intermittent-flow mode zone electrophoretic separation of enantiomers (a band width of twenty fractions, see  FIG. 7  in Hoffman et al., Anal. Chem., 1999, 71, 1840–1850) or the continuous flow mode zone electrophoretic separation of enantiomers (a band width of forty fractions, see  FIG. 6  in the same paper). We believe that the main reason for the significantly improved band width is the fact that when the equal-but-opposite analyte mobility principle disclosed here is utilized, the bands do not have to travel across such a long section of the separation chamber as when they both migrate cationically or anionically and experience all the undesirable distortion effects such as discussed in Rhodes et al., App. Theor. Electrophor. 1991, 2, 87–98. 
     Purity vs. cumulative recovery curves are calculated by pooling the fractions from the opposite sides of the enantiomer bands (e.g., beginning to pool from fraction  36  toward  48  for one of the enantiomers and from fraction  52  toward  40  for the other enantiomer in  FIG. 7 ). These purity vs. cumulative recovery curves are shown in  FIG. 8 . The curves indicate that about 50% of both enantiomers can be recovered with a purity of better than 97%, or 90% of each enantiomer can be recovered with a purity of better than 80%. 
     One of the greatest advantages of the proposed preparative continuous free-flow electrophoretic separation scheme described here is that when the anionic and cationic effective mobilities of the enantiomer bands are equal in magnitude but opposite in sign, the two enantiomers will migrate away from each other, in opposite direction. Thus, in principle, a separation channel as narrow as a few times the width of the sample feed stream is all that is required to achieve the separation. Consequently, the potential applied across the separation channel can be very low, even at reasonably high field strength values. This scheme is radically different from the regular free-solution preparative electrophoretic separation systems where a significant migration distance must be covered before the two enantiomers pull away from each other. In other words, by operating around the zero mobility point, the common, non-separative part of the effective mobilities of the enantiomers can be eliminated and the useful, separative Δμ eff  part retained. 
     Special free-flow electrophoretic systems, designed around this single-stage, binary separation principle, can be used to achieve the facile electrophoretic separation of weak or strong electrolyte enantiomers. In addition, the same principle can also be used for the preparative-scale separation of any other, nonchiral analyte pair (a pair of ionic analytes, or a pair consisting of one neutral and one ionic analyte), as long as an interacting agent having greater and opposite charge than the analyte and capable of bringing about opposite analyte mobilities is used. 
     In general, when one of the components of the feed solution is entity A, and the other component is entity B, which interact with resolving agent R according to equilibrium reactions A+R           AR and B+R         BR, the effective electrophoretic mobilities of the bands of the two components, μ eff   A  and μ eff   B , are
                     μ   A   eff     =         μ   A   0     +       μ   AR   0     ⁡     [   R   ]           1   +       K   AR     ⁡     [   R   ]                   (     Eq   .           ⁢   3     )               
and
 
                     μ   B   eff     =         μ   B   0     +       μ   BR   0     ⁡     [   R   ]           1   +       K   BR     ⁡     [   R   ]                   (     Eq   .           ⁢   4     )               
and separation selectivity (α=μ eff   A /μ eff   B ) for the enantiomer pair is
 
                   α   =           μ   A   0     +       μ   AR   0     ⁢       K   AR     ⁡     [   R   ]               μ   B   0     +       μ   BR   0     ⁢       K   BR     ⁡     [   R   ]             ⁢       1   +       K   BR     ⁡     [   R   ]           1   +       K   AR     ⁡     [   R   ]                     (     Eq   .           ⁢   5     )               
where μ 0   A , μ 0   B , μ 0   AR , and μ 0   BR  are the ionic mobilities of entities A, B, AR and BR, K AR  and K BR  are the respective equilibrium coefficients characterizing the interactions of entities A and R in forming entity AR and B and R in forming entity BR, and [R] is the species concentration of the free resolving agent R. When separation selectivity, α is less than zero, the bands of A and B migrate away from each other in opposite direction, when the value of α is equal to zero, the band of A or B migrate away from the other band that has zero effective mobility. Thus, the bands can be separated from each other by migrating a distance as short as the sum of the band widths (when α=0) or the sum of the two half widths (when α&lt;0).
 
     If the analyte is a chiral compound, i.e., if the analytes are a pair of enantiomers, then the resolving agent has to be an enantiomer that interacts with the two analyte enantiomers differently. If the analytes are two different proteins, then the resolving agent has to be a compound that binds to those two proteins differently. Any compound that interacts with an analyte and changes the sign of its effective mobility can act as a resolving agent for the analyte. There is a complementarity between the analyte and its resolving agent. One can select analyte and resolving agent pairs either based on available complexation data, separation data, or one can select them by experimentation. 
     Preferably, the electrical conductivity of the separation medium in the separation chamber is low to minimize excess heat production. Preferably, the composition of the separation medium is selected to minimize electromigration dispersion of the analyte bands. 
     The analytes are caused to move in opposite directions according to the methods of this invention. In a preferred embodiment the mobilities in opposite directions are equal. In another embodiment the movement of analytes is in opposite directions and the absolute values of their mobilities are different. In yet another embodiment the band of one of the analytes does not move while the other band moves. 
     Compounds such as isopropanol may be used as a “competing agent” to affect the chemical equilibrium constant between the resolving agent and an analyte. Other well known solvents or competing compounds may also be used. 
     By using this invention, fast, efficient, preparative-scale electrophoretic separations can be obtained for weak or strong electrolyte analytes that have very similar effective mobilities, because the analytes have to migrate only a distance that is equal to the sum of the respective band widths. The principle opens an entirely new application field for preparative-scale electrophoretic separations. Multiple units can be operated in parallel to scale the method and apparatus to a desired capacity for separations. In a preferred embodiment, the width of the separation channel is barely wider than the sum total of the two band widths of the analytes. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. The scope of the invention is intended to be defined in the following claims when viewed in their proper perspective.