Background of the Invention

This invention relates to the field of electrophoresis, more specifically, to a sieving medium for electrophoretic separation of biopolymers.

Capillary electrophoresis (CE) has found widespread applications in analytical and biomedical research. The scope and sophistication of CE are rapidly increasing. CE can perform analytical separations that are often substantially better than those using established chromatographic methods such as high-performance liquid chromatography (HPLC) . The separation modes of the conventional electrophoretic methods are slow, labor-intensive, prone to relatively poor reproducibility and have limited quantitative capability. Furthermore, it has been difficult to fully automate the process. The major advantages of capillary electrophoresis are that it can be fully automated, offers high resolution, and can quantitate minute amounts of samples, as reviewed by N.A. Guzman, L. Hernandez and S. Terabe, Analytical Biotechnology, Chapter l, ed. by C. Horvath and J.G. Ni elly. ACS symposium series, ACS, Washington, DC (1990) . These capabilities lie far beyond those of traditional electrophoretic methods.

CE has recently been used in the analysis of an extremely wide variety of molecules, including organic and inorganic anions and cations, drugs, dyes and their precursors, vitamins, carbohydrates, catecholamines, amino acids, proteins and peptides, nucleic acids, nucleotides, and oligonucleotides. In comparison with gas chromatography, supercritical fluid chromatography, and liquid chromatography, CE is the best separation technique from the point of view of molecular weight range of applicability. It is possible to separate in the same column species ranging in size from free amino acids to large proteins associated with complex molecular matrices.

From the detection point of view, HPLC provides better concentration sensitivity and CE provides better mass sensitivity. However, initial attempts to resolve complex mixtures of biological macromolecules in open CE columns were disappointing. The complex protein macromolecules present a serious problem when using untreated fused-silica capillaries due to the adsorption of many proteins onto the walls of the capillary. With oligonucleotides, the unfavorable mass-to-charge ratio tends to cause comigration of larger mixture components.

A highly advantageous solution to these difficulties was the development of gel-filled capillaries. Remarkably high separation efficiency has been obtained by gel-filled CE. To accomplish size selection in electrophoretic separation of mixtures of nucleic acids and SDS-denatured proteins, a cross-linked gel matrix is employed. However, the routine preparation of homogeneous stress-free gels in capillaries is difficult due to polymerization-induced shrinkage and appearance of bubbles inside the capillaries.

As an alternative to cross-linked gels, solutions of entangled polymer, such as polyethylene glycol, linear non-cross-linked polyacrylamides or (hydroxyethyl) cellulose (HEC) have been tested as acromolecular sieving media, as reported by P.D. Grossman and D.S. Soane, Biopolymers , 3_1, 1221 (1991); P.D. Grossman and D.S. Soane, J. Chromatography, 559. 257 (1991); M. J. Bode, FEBS Lett . , .35, 56 (1991); M. Zhu, D.L. Hansen, S. Burd and F. Gannon, J. Chromatography, 480 _f 311 (1989); A.M. Chin and J.C. Colburn, Am . Biotech . Lab. , 2 _/ 16 (1989); D: Tietz, M.M. Gottlieb, J.S. Fawcett and A. Chrambach, Electrophoresis , 2, 217 (1986) . This approach provides ease of filling and flushing the separation capillary after each analysis, thus avoiding the

possible contamination from the previous analysis. However, entangled polymer solutions in the capillaries exhibit lower resolution and reproducibility than the cross-linked gels.

The resolving power of capillary electrophoresis (CE) using the prior art entangled polymer solutions as the separation media is not good for large analyte molecules, presumably as a result of the relevant time scales of the sieving polymers and analyte molecules. The residence time (or passage time) of analyte molecule in a mesh is controlled by the size and electrophoretic mobility of the analyte, mesh size of the network, and the imposed electric field strength. The life time of entanglement, i.e., mobility of strands forming the mesh, depends on network integrity, the length and concentration of macromolecules constituting the network. In order to achieve good resolution, the relaxation time of the entangled polymer solution should be orders of magnitude greater than the residence time of the analyte molecules.

Unfortunately, this condition does not necessarily hold in typical CE applications, as demonstrated in the examples separating DNA by CE using entangled polymer solutions, reported by T. Hino, MS Thesis , University of California, Berkeley (1991) , demonstrating that the slow-mode relaxation time of polyacrylamide was 5.9 x 10^ sec (T3%, 25°C) for entangled solutions and 4.0 x 10" ³ sec (T3%, 25 ^β C) for cross-linked systems. The typical residence time of DNA can be estimated from the literature (P.D. Grossman and D.S. Soane, Biopolymers , ___! , 1221 (1991) ; P.D. Grossman and D.S. Soane, J. Chromatography, 559. 257 (1991) ; J. Sudor, F. Foret and P. Bocek, Electrophoresis , .12, 1056 (1991)) by assuming the two extremes (Ogston and biased reptation) = Residence time between

- - -

mesh size _{( }}

(electrophoretic velocity) _Qg ^,, and

DNA contour length ₍ )

(electrophoretic velocity) _nf4ttio . where subscript Ogston means that the migrating solute behaves as an undefor able particle (Ogston Model) and reptation under the influence of large electric fields, the solute becomes more elongated, and the motion mimics that of a snake threading its way through the network. The calculated residence time limits of DNA are as follows: 1.5 x 10" ⁵ to 1.8 x 10"* sec. for 30 base pairs and 1.5 x 10" ⁵ to 8.2 x 10 ^"4 sec. for 100 base pairs. The calculated residence times of 100 base pairs are extrapolated from 30 base pairs data based on a hydrodynamic diameter per base pair of 3.3A (K.S. Schmitz, An Introduction to Dynamic Light Scattering by Macromolecules, Academic Press, NY (1990) ; V.A. Branfield, Chapter 10, Dynamic Light Scattering, Plenum Press, NY (1985)). The results show clearly that the relaxation time of entangled polymer and residence time of DNA are very close. Therefore, sharp resolution cannot be expected for the high molecular weight of DNA using entangled polymers. The network imposing the sieving medium fails before the analyte moves through a mesh completely.

In summary, the conventional electrophoretic methods are slew, labor-intensive, with relatively poorly reproducibility and have limited quantitative capability. Furthermore, it is difficult to accomplish a fully automated operation. CE promises to offer a solution for those problems. However, choosing the sieving medium is difficult. Open ^" CE columns adsorb many proteins onto the walls of the capillaries. A cross-linked gel filled CE is complicated by problems in the routine preparation of homogenous stress-free gels in capillaries due to

polymerization induced shrinkage and appearance of bubbles inside the capillaries. Entangled polymer solutions in the capillaries exhibit poor resolution and reproducibility. All the above problems can also be found with slab gel electrophoresis as well as other electrophoretic configurations.

It is therefore an object of the present invention to provide an improved method and medium composition for separation of molecules, especially by capillary gel electrophoresis.

It is a further object of the present invention to provide a method and means to improve the resolution and reproducibility of entangled solutions, in combination with the ease of fill/flush.

It is another object of the present invention to provide a new class of separation media for CE and related electrophoretic technologies, including slab and annular configurations, as well as sequencers for DNA and protein, and methods of preparation and use thereof.

Summary of the invention Methods of preparing sieving media and of filling and flushing of capillaries and other electrophoretic devices have been developed which combine the advantages of gel filled and entangled-polymer- solution filled systems such as good resolution, ease of fill and flush, and reuse of the capillary or device. Two different approaches are used to make a stable system with good resolution that is easily filled and flushed for reuse of the device: chemical cross-linking and physical cross-linking. In another embodiment, the network is formed with a polyelectrolyte, with the resulting ionic medium achieving greater resolution than the uncharged medium.

Cross-linked gel particles are used as a sieving medium in chemically cross-linked systems. The properties of gels depend on the gel state represented by osmotic pressure, temperature, solvent composition, and degree of swelling. The cross-linked gel particles are prepared by inverse emulsion polymerization, precipitation polymerization, or standard suspension polymerization. The gel particles formed by these methods can be as small as in the range of 30θA - 50θA to as large as microns or tens to hundreds of microns. Optimal range of the finite cross-linked gel network size depends on the gel swelling ratio and diameter of the capillary. The aqueous medium for the gel particles can be a buffered solution of various pH and ionic strength or pure deionized water depending on the specific application. The optimal loading of gel particles in suspension for filling capillaries is determined by the viscosity of the suspension with gel particles and swelling/deswelling considerations. In general, the procedure for use includes the steps of 1) capillary filling, 2) swelling and running, and 3) deswelling particles and capillary flushing. Swelling of gel particles can be effected by a change of temperature, pH and ionic strength of solution, electric field, and ions for deswelling.

There are also at least two approaches in physically cross-linked systems. First, one can copolymerize hydrophilic monomers and hydrophobic monomers. Under normal conditions, the copolymers are linear and unassociated, possibly having some entanglement. However, by changes of solvent conditions such as temperature and ionic strength of solution (or pH) , the hydrophobic region of the copolymer can be made to coalesce and the copolymer behave like a cross-linked system. In this way one can fill, run, and flush by changing temperature, pH,

and ionic strength of solution. Second, by exchanging monovalent ions and divalent ions, e.g., by electrophoresis of buffers, one can make a sieving medium containing conjugated functional groups (ionized groups) which are either cross-linked or dissociated.

In addition to these two specific examples, other members of the general class of electrorheological fluids can be used as separation media. Filling involves zero-field, hydrostatic-driven fluid flow. During analysis, the electric field is switched on, turning the fluid into an immobile solid. After the run, the field is switched off and the medium is liquid again and can be flushed out.

Brief Description of the Drawings

Figure 1 is a graph of the swelling ratio and volume fraction versus reduced temperature at various f values: 0, 0.659, 1, 2, 3, 4, 5, 6, 7, and 8.

Figures 2A, 2B, and 2C are schematics of filling (Figure 2A) , swelling (Figure 2B) , and deswelling and flushing capillaries (Figure 2C) .

Figure 3 is a graph of the viscosity of suspensions as a function of volume fraction of fillers.

Figure 4 is a graph of the gel swelling ratio ( ^v / ^v _o ) with respect to the volume fraction of acetone and ethanol at different temperatures: open circle, 25°C Acetone; closed circle, 15°C Acetone; open square, ethanol; and closed square, 15°C ethanol.

Figures 5A and 5B are schematics illustrating physically cross-linked systems. Thin lines are hydrophilic polymers and thick lines are hydrophobic • polymers. The shaded areas represent physically bonded regions, via the hydrophobic salting out phenomenon.

Figures 6A and 6B are schematics of ionic association, forming physical cross-links: 6A, dissociated state, 6B, associated state.

Figure 7 is a graph of the temperature dependent gel swelling ratio in different solutions. Open symbols (RB) are (V/V ₀ ) in buffer solution (0.025 M Tris-HCL+ 0.192 M glycine + 0.1% Sodium dodecylsulfate) . Dark symbols (PW) are (V/V ₀ ) in deionized water. Squares are pH 3; circles pH 4.5, and triangles pH 12.1.

Figure 8 is a graph of the pH dependent gel swelling ratio in different solutions. Open symbols (RB) are (V/V ₀ ) in buffer solution (0.025 M Tris-HCL + 0.192 M Glycine + 0.1% Sodium dodecylsulfate), dark symbols (PW) are (V/V ₀ ) in deionized water. Squares are 25°C, circles are 45°c, and triangles are 15°C.

Detailed Description of the Invention Cross-linked systems have been developed for use with capillary electrophoresis (CE) as well as conventional slab and other types of electrophoresis. The cross-linking is achieved either by chemical or physical means. The media is selected to collapse or shrink for ease of filling and flushing and to swell to fill all voids during running. This is achieved by selection of the buffer the gel is suspended in, either through temperature, ionic strength, pH, presence or absence of ions, solvent composition, or some combination thereof.

The media is not only easy to use but achieves high resolution. For example, the relaxation time of polymer is orders of magnitude greater than the residence time of DNA. As a result, cross-linked systems lead to much better resolution than those of entangled polymer solutions.

As discussed above, gel-filled capillaries suffer serious inconvenience in filling and reusability.

These problems are overcome by the use either of chemical cross-links or physical cross-links systems.

In order to achieve high resolution of separation, one can vary the mesh size of the sieving medium by increasing T% and C%. However, a high gel swelling ratio is needed. The higher the T% and C%, the lower the gel swelling ratio. Accordingly, one must increase C% (or T%) to achieve high resolution, yet reduce C% (or T%) to have a high swelling ratio. This is achieved using a polyelectrolyte network. One can prepare a negatively charged gel as the sieving medium, from, for example, acrylamide in a concentration of 80%, with sodium acrylate in a concentration of 20%, plus crosslinker. The negatively charged mesh poses a more restrictive barrier for a negatively charged analyte due to electrostatic repulsion. A similar mechanism applies for positively charged analytes. Proteins and DNA are negatively charged so the use of the negatively charged polyelectrolyte (sodium acrylate) reduces the effective mesh size of the sieving medium, increasing resolution. At the same time, the ionic gels have a greater swelling ratio than those of regular gels. Chemical Cross-linked Systems

A gel is a cross-linked polymer network swollen in a liquid. Its properties depend strongly on the interaction of these two components. The liquid prevents the polymer network from collapsing into a compact mass, and the network, in turn, retains the liquid. Gels are characterized by their equilibrium, dynamic, and kinetic properties. These properties depend on the gel state represented by osmotic pressure, temperature, solvent composition, and degree of swelling. The distance of the gel state from the phase boundary crucially influences all properties. Phase transition in polymer gels has been recently

discovered. The state of the gel drastically changes based on the external conditions. The compressibility becomes infinite at a certain critical temperature; the gel can swell or shrink by a factor of as much as 500, depending on temperature. Under a fixed condition, swelling or shrinking is discontinuous, and therefore a minute change in temperature can cause a large change in gel volume. Such transitions can also be effected by altering solvent composition, pH or ionic composition.

The equation of state of gels relates osmotic pressure, polymer-network concentration, temperature and solvent composition (P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY (1953) ; K. Dusek and D. Patterson, J. Poly. Science , Part A-2, 6., 1209 (1968); and T. Tanaka et. al., Phys . Rev. Lett. , 4J5, 1636 (1980)). There are three pressures acting on the polymer network. Positive pressure of counterions, negative pressure due to the affinity among polymer strands and the rubber elasticity, which keeps the network in a moderate expansion. Phase equilibria of gel can be described using the Flory - Huggins derivation.

equation 1

;r=-

where N is Abrogadro's No.; k the Boltzmann constant; T the absolute temperature; v the molar volume of the solvent; φ the volume fraction of the polymer network; AF the free energy decrease associated with the formation of a contact between polymer segments; <ρ ₀ the network volume fraction at the condition that the constituent polymer chains have random configurations; n the number of constituent chains per unit volume at

φ = φ ₀ ; and f the number of dissociated counterions per effective polymer chain. In equilibrium, e.g. the above equation one can be expressed as: equation 2: -φ)

where τ is the reduced temperature.

Figure 1 shows the calculated equilibrium swelling ratio as a function of the reduced temperature for various degrees of ionization f. As shown in Figure 1, the hydrolysis of a small amount of acrylamide groups causes a drastic change in the collapse size upon the transition. If the term In (1- φ) in equation 2 is expanded up to the order φ ³ , Equation 2 becomes: equation 3

equation 4 where + ²

equation 5

S = .v ^. v (2f ₊ l) ⁴ = S ₀ (2f ₊ l) ⁴ Nφ

equation 6

In equation 3 , parameter S determines whether the volume change is continuous or discontinuous . The

parameter can be interpreted in the following manner. An effective polymer chain consists of freely jointed segments of radius a and persistent length b. The volume of solvent molecules is assumed to be a ³ . Under the assumption that the interaction is negligible among the polymer segments, the average end-to-end distance of the chain is R * n ^1/ b. Since R is the mean distance between neighboring cross-links, v « a ² /R ³ , and φ « n b a ³ /R ³ . Therefore, S can be expressed as:

equation 7

Equation 7 shows that the S value depends on f and the ratio of the persistent length b to the effective radius a of the polymer chain ( ^b /a) (which represents the stiffness of the chain) . Therefore, in order to have a large swelling ratio of the gel (i.e., discontinuous phase transition) , the constituent polymer chains must have sufficient stiffness or a sufficient number of ionized groups. Both the counterions to the ionized groups and the stiffness of the polymer chains increase the osmotic pressure acting to expand the polymer network resulting in a discontinuous volume change.

Reversible swelling and contraction of ionic gels can also be induced by changes in pH and ion composition. Such behavior can be explained by the theoretical curves shown in Figure 1: the f value changes with pH. Changes in the ionic strength also induce the phase transition.

Both gel filled CE and CE using entangled polymer solutions have disadvantages in either filling/flushing or resolution reproducibility, capillary reusing. These problems can be overcome by preparing the cross-linked gel in the form of submicron or greater size spheres using inverse

e ulsion polymerization, precipitation polymerization, or standard suspension polymerization.

Inverse Emulsion Polymerization.

The term 'inverse' emulsion polymerization is used to imply a heterogeneous polymerization system in which the monomer is readily soluble in water, but only sparingly soluble, if at all, in non-polar liquids. Thus, in the inverse emulsion polymerization an aqueous solution of a hydrophilic monomer is dispersed in a continuous hydrophobic medium using a surface-active substance which promotes the formation of water-in-oil emulsions. The polymerization is then initiated with either oil-soluble or water-soluble initiators. Dibenzoyl and dilauroyl peroxides are typical oil-soluble initiators. Sorbitan monostearate is a water-in-oil e ulsifier which is suitable for this application.

The inverse emulsion polymerization is as follows. The emulsions are formed by dissolving the emulsifier in o-Xylene or a suitable organic medium such as toluene and adding the aqueous monomer solution with stirring. The crude emulsions are homogenized to decrease the average droplet size and increase the emulsion stability. The emulsions are heated with stirring at 40°C to 70°C to effect polymerization. The time required for complete conversion varies from a few minutes to several hours. The particles formed by this method can be as small as 300 A depending on the amount of surfactant added. The particles are centrifuged several times with deionized water to be cleaned.

Precipitation Polymerization.

For precipitation polymerization, one needs to know the phase diagram of polymer solutions. For example, the lower critical solution temperature (LCST) of poly-N-isopropylacrylamide poly(NIPAM) in an aqueous medium is about 32°C. Thus it will be

precipitated if the polymerization temperature is above 32°C during polymerization. Table 1 shows other similar systems such as poly-NIPAM in aqueous medium whose LCSTs are known.

Table 1: List of LCSTs of aqueous polymers

Polymer and Copolymers LCST (°C) poly (N-methylacrylamide) 95 poly (N-ethylacrylamide) 80 poly (N-n-butylarcylamide) 25 N-isopropylacrylamide - co-N-isopropylmethacrylamide

23 - 40 poly (N-n-propylacrylamide) 16 - 19 poly (N-n-propylmethacrylamide) 22 - 29 poly (N-isopropylacrylamide) 32 poly (N-isopropylmethacrylamide) 40 poly (N-ethylmethacrylamide) 54 - 57 poly (N-acroylpiperidine) 4 - 6 poly (N-methacroylpiperidine) 18 - 42 poly (N-pyrolichylmethylacrylamide) 53 poly (N-piperidylmethylacrylamide) 42 poly (N,N ^, -diethylacrylamide) 30 - 32

The method is generally as follows. Water soluble monomers are dissolved in water in an Erlen eyler flask equipped with a condenser, a nitrogen inlet, and a stirrer. Nitrogen is bubbled into the solution and the temperature is controlled at 70 ^β C. Potassium persulfate can be used as an initiator. The reaction is then continued for 24 hrs under mild stirring. The resulting microspheres are centrifuged to separate them from the suspending medium. They are redispersed in water and then are centrifuged several times to increase purity. The size of microspheres can be as small as 500 k or greater depending on stirring conditions, i.e., the

faster the mixture is stirred, the smaller the microspheres.

The monomers that can be used in the above methods include N-adamentylacrylamide, N- Benzylacrylamide, N-Benzylmethacrylamide, N- cyclohexylacrylamide, N,N ^, -diethylacrylamide, N- dodecylmethacrylamide, N-isobonylacrylamide, N- methymethacrylamide, diacetone acrylamide, N-[3- (dimethylamino) propyl] acrylamide, ethacrylamide, and (1-naphthyl methyl) methacrylamide. The following can be used as cross-linkers: N,N' -bis (1,2- ethylene) dimethacrylamide, NN'-ethylenebisacrylamide, N,N'-hexamethyl bisacrylamide, NN'- methylenebisacrylamide, N,N'- methylenebismethacrylamide, NN ' - nonomethylenebisacrylammide, NN'- octamethylenebisacrylamide, NN'-(isopropylidene) bisacrylamide, NN'-trimethylenebisacrylamide, piperazine diacrylamide, N,N'-bisacrylylcystamine, and N,N'-diallyltartardiamide. At this time, the most preferred monomers are acrylamide and sodium acrylate. Other monomers and crosslinkers are known to those skilled in the art and can be used in place of any of those listed above.

Loading of Capillaries.

After preparing microspheres of cross-linked gels, the microspheres can be injected, or drawn in by vacuum through a filter of less than one micron size, into capillaries, as shown in Figures 2A. Note that during injection the particles are in a collapsed state. Optimal range of the finite cross-linked gel network size depends on the gel swelling ratio and diameter of the capillaries. The diameter of the capillaries usually varies from 50 μm to 100 μm. From gel swelling equilibria data, one can predict the number density of microspheres in aqueous medium. This aqueous medium can be a buffer solution or

deionized water, depending on the specific particles in suspension for filling capillaries, which is determined by the viscosity of the suspension (with particles) and swelling/deswelling considerations.

Figure 3 shows a typical viscosity curve of suspension with fillers. The viscosity can be expressed by

equation 8

In π _ J KEEΦ Φ

Φ max where n is the viscosity of aggregate-filled suspension, φ the volume fraction of fillers, n, solvent viscosity, and K _E is a geometric factor. If one assumes the particles are spherical, K _E = 2.5 (Einstein's equation) .

Variation in Gel Swelling at each stage of process.

At the first stage, the capillary filling stage, the temperature should be controlled at the above-the- discontinuous-volume-change phase region, so that the particles are in a collapsed state. By knowing the swelling ratios of gels, the number density and size of particles can be determined. At the end of the capillary, one can put a filter whose pore size is less than the collapsed particles so that only the suspending liquid can pass through, as shown in Figure 2A.

At the second stage, the swelling and running stage, the temperature should be raised above the discontinuous-volume-change-region so that the particles are swollen. As shown in Figure 2B, the total particle volume is such that the particles butt up against one another to become a continuous phase. As a result, the capillary is filled with particles only and contains no voids.

In the third stage, deswelling particles and capillary flushing, the temperature is changed to the temperature used in the capillary filling stage so that the particles collapse.

Deswelling of gels can be accomplished by several methods other than changing temperature. Reversible swelling and contraction of ionic gels can be effected by changes of pH and ion compositions consistent with the essential role of ionization in phase transition. When the pH varies or salt ions are added to the solvent, the effective number of counterions varies, as does the ionic osmotic pressure. The higher the pH or ion concentration, the more the gel swells. Changes in the ionic composition can also induce phase transition. The concentration needed for the transition differs by four orders of magnitude between monovalent NaCl and divalent MgCl ₂ . It is possible to use a combination of parameters synergistically to achieve deswelling. For example, one can change the temperature to a point at which the gel spheres shrink and then pass a buffer solution through low pH or low ion concentration by electrophoresis. Thus, the gel spheres shrink further, and can be easily flushed out of the separation device (capillary slab, or annular) , as shown in Figure 2C.

Alternatively, as shown in Figure 4, with the acrylamide monomers referenced above, a ketone or low alcohol such as acetone, methanol or ethanol is injected into the capillary so the particles are collapsed to a minimum for easy flushing.

Physical Cross-linked systems.

There are at least two different approaches in physically cross-linked systems. First, by copolymerizing hydrophilic and hydrophobic monomers into blocky structures, the hydrophobic region can be physically bonded and released by changing temperature and ionic strength of the buffer solution. Second,

exchanging monovalent ions for divalent ions by electrophoresis can make a sieving medium which contains conjugate functional groups cross-link or dissociate.

Figure 5 is a schematic illustration of one physically cross-linked system. In this Figure, thin solid lines denote hydrophilic regions and thick solid lines represent hydrophobic region. Under normal conditions, the copolymers are linear (see Figure 5A) , however, by changing the solvent conditions, such as temperature and ionic strength (or pH) , the hydrophobic regions may coalesce and the copolymer behave like a cross-linked system, as shown in Figure 5B. In this way, the capillary can be filled at a certain temperature, ionic strength and pH at which the polymer molecules are separate and can easily flow. Then the system can be physically cross-linked by a change of temperature and/or ionic strength and pH. After the analysis, the capillary can be easily flushed by changing these conditions back to the conditions at which the capillary was filled.

The second method for a physically cross-linked system utilizes exchange of monovalent ions and divalent ions, for example, by electrophoresis of buffers, to make a sieving medium containing conjugate functional groups cross-link or dissociate. Figures 6A and 6B illustrate this process.

In Figure 6A, the polymers are not cross-linked. In Figure 6B, a buffer containing zinc divalent ions has been added, displacing the sodium salts and crosslinking the polymers.

In a variation of this, the polymer contains metal containing compounds or metal chelating compounds. Crosslinking is achieved by introduction of chelators or the metal ions, respectively.

The present invention will be further understood by reference to the following non-limiting examples.

Example l: Preparation of Cross-linked gel particles.

A. Inverse Emulsion Polymerization

In inverse emulsion polymerization an aqueous solution of a hydrophilic monomer is dispersed in a continuous hydrophobic oil medium using a surface- active substance which promotes the formation of water-in-oil emulsions. The polymerization is then initiated with either oil-soluble or water-soluble initiators. As far as the end-product of the reaction is concerned, it is clear that inverse lattices are less stratified or flocculate more readily. Continuous and gentle agitation is needed to maintain these lattices as colloidal dispersion indefinitely.

Acrylamide was used as the water soluble monomer and N,N'-methylene-bis-acrylamide as a cross-linker to prepare a T ₁₀ Cj solution, where

%T = grams of acrylamide + grams of cross-linker x 100 Total volume

%C = grams of cross-linker x 100 grams of acrylamide + grams of crosslinker

Sorbitan monostearate (SMS) was used as an emulsifier. The emulsions were formed by dissolving 0.875 g of SMS in 7 ml of o-Xylene and adding T ₁₀ ^~ _ solution (the aqueous monomer solution) with stirring for about 3 to 4 hrs. The temperature is controlled at about 50°C during polymerization. The crude emulsions are homogenized to decrease the average droplet size and to increase the emulsion stability.

After complete polymerization, the final gel particles can be cleaned by centrifuging several times with deionized water. The gel particles are not monodisperse, however one can filter them to a certain size range, for example, using a 1 μ filter to collect particles less than 1 μ.

B. Precipitation Polymerization

As stated above, precipitation polymerization is carried out above the LCST of the system. The following reagents were combined:

N-isopropylacrylamide: 4.9 g

N,N'-methylene-bis-acrylamide: 0.1 g

Potassium persulfate: 0.2 g deionized water: 200 ml The system temperature was controlled at about 70°C.

The final gel particles are reasonably monodisperse.

Example 2: Changing of gel swelling equilibria by changing ambient conditions.

The gel swelling equilibria can be changed by changing ambient conditions, i.e. temperature, pH, solvent. Gel disks were prepared by casting in glass tubes (ID = 6.4mm) . The gel disk dimensions were about 6 mm in diameter and about 13 mm in thickness. After preparing the gel disks, each sample was blotted with laboratory tissue to remove surface water, and weighed. The swelling capacity was determined as the mass ratio of swollen gel to the gel prepared. The volume of the gel as cast (not the dry gel) is denoted by v ₀ .

In each swelling capacity measurement, the diameter and the length of each gel disk was also measured. The properties for acrylamide/N,N'- methylene-bis-acrylamide gels prepared with 10%T and 2% C are shown in Figures 4, 7, and 8.

Figure 4 shows that the volume ratio (V/V ₀ ) of the gel particles can be decreased with increasing concentrations of ethanol or acetone can be reduced from 1.2-1.4 to 0.2-0.3.

Figures 7 and 8 shew that the volume ratio (V/V ₀ ) can be decreased as a function of pH and temperature.