Abstract:
The present invention describes a method of preparation of a permanent neutral wall coating made of thermally immobilized polysaccharides. The coating suppresses electroosmotic flow and adsorption on the wall under acidic, neutral, and basic conditions in capillary electrophoresis.

Description:
REFERENCES CITED 
     U.S. Patent Documents 
     
         
         1) Hjertén, S., Coating for electrophoresis tube. U.S. Pat. No. 4,680,201, 1987. 
         2) Novotny, M. V.; Cobb, K. A., and Dolnik, V., Suppression of electroosmosis with hydrolytically stable coatings. U.S. Pat. No. 5,074,982, 1990. 
         3) Novotny, M. V.; Cobb, K. A., and Dolnik, V., Suppression of electroosmosis with hydrolytically stable coatings. U.S. Pat. No. 5,143,753, 1991. 
         4) Dolnik, V. and Chiari, M., Compounds for molecular separations. U.S. Pat. No. 6,074,542, 2000. 
         5) Schomburg, G. and Gilges, M., Deactivation of the inner surfaces of capillaries. U.S. Pat. No. 5,502,169, 1996. 
         6) V. Dolník, W. A. Gurske, and A. Padua: Solution of galactomannans as a sieving matrix in capillary electrophoresis. U.S. Patent Application 20020049184, Sep. 5, 2001. 
         7) Whistler R., Conversion of Guar Gum to Gel-Forming Polysaccharides by the Action of Alpha-Galactosidase. U.S. Pat. No. 4,332,894, 1982. 
       
    
     Other References 
     
         
         1) Maier, H., Anderson, M., Karl, C., Magnus on, K., in: Whistler, R. L., BeMiller, J. N. (Eds.),  Industrial Gums. Polysaccharides and Their Derivatives , Academic Press, San Diego 1993, pp. 181-226. 
         2) Dolník, V., Gurske, W. A. and Padua, A.: Galactomannans as a sieving matrix in capillary electrophoresis.  Electrophoresis  2001, 22, 707-719. 
         3) Williams, B. A. and Vigh, G. Determination of accurate electroosmotic mobility and analyte effective mobility values in the presence of charged interacting agents in capillary electrophoresis.  Anal. Chem.  1997. 69, 4445-4451. 
         4) Liu, Q., Lin, F., Hartwick, R. A. Capillary zone electrophoretic separation of basic proteins and drugs using guaran as a buffer modifier.  Chromatographia  1998, 47, 219-224. 
         5) Belder, D. Deege, A., Husmann, H., Kohler, F., and Ludwig, M. Cross-linked poly(vinyl alcohol) as permanent hydrophilic column coating for capillary electrophoresis.  Electrophoresis.  2001; 22, 3813-3818. 
         6) Shen, Y. and Smith, R. D. High-resolution capillary isoelectric focusing of proteins using highly hydrophilic-substituted cellulose-coated capillaries.  J. Microcol. Sep.  2000; 12, 135-141. 
         7) Slais, K., Friedl, Z. Low-Molecular-Mass pI Markers for Isoelectric Focusing.  J. Chromatogr. A  1994, 661, 249-256. 
         8) Gilges, M., Kleemiss, M. H., Schomburg, G., Capillary zone electrophoresis separations of basic and acidic proteins using poly(vinyl alcohol) coatings in fused silica capillaries.  Anal. Chem.  1994, 66, 2038-2046. 
         9) Belder, D. Deege, A., Kohler, F., and Ludwig, M. Poly(vinyl alcohol)-coated microfluidic devices for high-performance microchip electrophoresis.  Electrophoresis.  2002; 23, 3567-3573. 
         10) Belder, D., and Ludwig, M. Surface modification in microchip electrophoresis.  Electrophoresis.  2003; 24, 3595-3606. 
         11) Becker, H., Locascio, L. E., Polymer microfluidic devices.  Talanta  2002, 56, 267-287. 
       
    
     FIELD OF THE INVENTION 
     The present invention generally relates to surface treatment, films and coatings, and more particularly coatings for chromatography and electrophoresis in capillaries and microchannels. Specifically, the invention is directed to wall coatings in capillary electrophoresis to reduce electroosmotic flow and adsorption of analytes on the wall. 
     BACKGROUND OF THE INVENTION 
     Capillary electrophoresis has achieved a remarkably rapid development from its introduction in the early 1980s. This technique miniaturizes the electrophoretic process and presents significant advantages over traditional slab gel electrophoretic techniques. While fused silica capillaries are the most frequently used separation format, the method was also transferred to microchips (Belder, D., and Ludwig, M. Surface modification in microchip electrophoresis.  Electrophoresis.  2003; 24, 3595-3606, Becker, H., Locascio, L. E., Polymer microfluidic devices.  Talanta  2002, 56, 267-287.) Most of materials used to prepare separation channels or capillaries for capillary electrophoresis (CE) contain ionizable groups on their surface that are responsible for so-called electrokinetic potential or ζ-potential. This potential is a cause of electroosmotic flow (EOF). The presence of EOF and especially its uneven distribution along the electrophoretic capillary or channel causes disturbances called eddy migration and loss of resolution during electrophoretic separation. To suppress EOF, a good wall coating eliminates ζ-potential at the wall and/or increases viscosity inside the electric double layer. To reduce ζ-potential, the coating may react with charged groups incorporated in the wall (silanol groups in the case of fused silica capillary). To some extent, compounds with an opposite charge than the ionizable groups of the wall can be also used to titrate ζ-potential. 
     A number of various wall coatings have been proposed and developed to eliminate EOF and adsorption of analytes in fused silica capillaries. A vast majority of them merely reduced EOF and did not eliminate it completely. Frequently a dynamic wall coating was formed by simply adding an active ingredient to the background electrolyte. It adsorbed on the wall and reduced capillary surface charge and/or viscosity of solution in the electric double layer. Dynamic wall coatings are popular because of the simplicity of their preparation. However, they do not eliminate electroosmotic flow completely. Among many dynamic coatings, a dynamic coating based on guaran has been developed (Liu, Q., Lin, F., Hartwick, R. A. Capillary zone electrophoretic separation of basic proteins and drugs using guaran as a buffer modifier.  Chromatographia  1998, 47, 219-224). 
     To eliminate EOF completely, static wall coatings have to be applied. Typically, a static wall coating is made of two layers: an intermediate layer and a hydrophilic polymer layer. A bifunctional reagent that reacts with both the capillary surface and functional groups of the polymer molecule usually forms the intermediate layer. The first polymer used for the preparation of a static wall coating was a linear polyacrylamide attached to the fused silica capillary wall by γ-methacryloxypropyltrimethoxysilane (Hjertén, S., Coating for electrophoresis tube. U.S. Pat. No. 4,680,201). More hydrolytically stable coating was obtained when polyacrylamide was attached to the silica wall by using a Grignard reagent with an olefinic moiety, e.g., vinylmagnesium bromide after activating silanol groups by a reaction with thionyl chloride (Novotny, M. V.; Cobb, K. A., and Dolnik, V., Suppression of electroosmosis with hydrolytically stable coatings. U.S. Pat. No. 5,074,982; Novotny, M. V.; Cobb, K. A., and Dolnik, V., Suppression of electroosmosis with hydrolytically stable coatings. U.S. Pat. No. 5,143,753). Polyacrylamide is, however, hydrolytically unstable at high pH and hydrolyzes forming poly(acrylic acid). The presence of carboxylic groups leads to generation of ζ-potential on the wall and to an increase of EOF. A more stable wall coating is usually obtained if acrylamide is replaced with its derivative having some substituents on nitrogen (Dolnik, V. and Chiari, M., Compounds for molecular separations. U.S. Pat. No. 6,074,542). 
     Thermal immobilization of a polymer on a capillary wall is another way how to anchor a polymer on the capillary wall. Schomburg and coworkers proposed a poly(vinyl alcohol) (PVA) coating fixed thermally to the wall by heating the capillary at 140.degree.C. They assumed that formation of a permanent PVA coating was based on PVA insolubility in water after the thermal treatment and expected PVA to form semi-crystalline highly associated structures, which were not covalently bound to the fused silica capillary. PVA molecules became more strongly associated by hydrogen bridges and water molecules could not penetrate microcrystalline domains. The authors expressed their opinion that this was a unique property of PVA. In the pH range of 5-9, the PVA coating did not, however, completely eliminate EOF and the coated capillaries exhibited a pH-independent electroosmotic mobility of 1.2×10 −9  m 2 V −1 s −1  as measured in 20 mM sodium phosphate (Schomburg, G. and Gilges, M., Deactivation of the inner surfaces of capillaries. U.S. Pat. No. 5,502,169). The procedure was further modified to make a PVA wall coating on a glass microchip. A newly introduced PVA coating crosslinked with glutaraldehyde should improve the stability of the coating. No heating is necessary, just drying is sufficient to provide a stable wall coating (Belder, D., Deege, A., Husmann, H., Kohler, F., and Ludwig, M. Cross-linked poly(vinyl alcohol) as permanent hydrophilic column coating for capillary electrophoresis.  Electrophoresis.  2001; 22, 3813-3818). Thermal immobilization was also applied to hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC) (Shen, Y. and Smith, R. D. High-resolution capillary isoelectric focusing of proteins using highly hydrophilic-substituted cellulose-coated capillaries.  J. Microcol. Sep.  2000; 12, 135-141). The authors found that the wall coating was stable if the silica capillaries were heated at 140° C. for 20 min rather than just being dried at room temperature for 4 days. From this observation they concluded that a chemical reaction must have occurred between cellulose derivatives and fused silica capillary inner wall. 
     There are several types of galactomannans, a class of linear polysaccharides with 1,4 linked β-D-mannopyranosyl units and 1,6-linked α-D-galactopyranosyl side groups (Dolník, V., Gurske, W. A. and Padua, A.: Galactomannans as a sieving matrix in capillary electrophoresis.  Electrophoresis  2001, 22, 707-719). The four most important galactomannans are locust bean gum, tara gum, guar gum (guaran), and fenugreek gum, which differ by the frequency of galactosyl side group attachment to the polymannose. The ratio of D-mannosyl to D-galactosyl units is approximately 3.8:1 for locust bean gum, 3:1 for tara gum, 1.8:1 for guar gum, and about 1:1 for fenugreek gum. Locust bean gum, guar gum, and tara gum are commercially produced and have various applications in the food industry and as an additive to fracturing fluids in the petroleum industry. Guar gum is obtained from the endosperm portion of the legume seed ( Cyamopsis tetragonoloba ) that grows mainly on the Indian subcontinent and in some parts of Texas and Oklahoma. Typical guar gum contains 75-85% of galactomannan, 8-14% water, 5-6% proteins, 2-3% fiber, and 0.5-1% ash. Guar gum shows an excellent resistance to shear degradation (Maier, H., Anderson, M., Karl, C., Magnus on, K., in: Whistler, R. L., BeMiller, J. N. (Eds.),  Industrial Gums. Polysaccharides and Their Derivatives , Academic Press, San Diego 1993, pp. 181-226). 
     Solubility of polysaccharides and viscosity of their solutions significantly depends on branching of polysaccharide chains. Whereas cellulose is insoluble in water, as numerous hydrogen bridges between closely placed linear polysaccharide chains prevent its solubilization, highly branched polysaccharides, such as dextran, can be easily and quickly dissolved in water. When cellulose is derivatized with hydroxyalkyl groups, their presence keep polysaccharide chains apart and cellulose derivative becomes soluble in water (Whistler, R. L., BeMiller, J. N. (Eds.),  Industrial Gums. Polysaccharides and Their Derivatives , Academic Press, San Diego 1993). Similarly, locust bean gum, where polymannose backbone fibers are kept apart only by 1 galactose side group per about 3.8 mannose units, is less soluble in water than, e.g., guar gum, where polymannose backbone fibers are kept apart by 1 galactose side group per about 1.8 mannose units (Maier, H., Anderson, M., Karl, C., Magnus on, K., in: Whistler, R. L., BeMiller, J. N. (Eds.),  Industrial Gums. Polysaccharides and Their Derivatives , Academic Press, San Diego 1993, pp. 181-226). Solubility of polysaccharides can be reduced by enzymatic reaction, when side chains and side groups are hydrolytically cleaved from a polysaccharide backbone (Whistler R., Conversion of Guar Gum to Gel-Forming Polysaccharides by the Action of Alpha-Galactosidase. U.S. Pat. No. 4,332,894, 1982). 
     SUMMARY OF THE INVENTION 
     The present invention is useful as a capillary coating that satisfies the above objectives. The polysaccharide coating described here eliminates electroosmotic flow and reduces adsorption on the capillary wall. The coating is stable for at least 48 hours at pH 10.3. 
     The present invention is also a method of preparing said coating and using this coating to cover the interior of a capillary tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents the idealized structure of guaran. 
         FIG. 2  shows the idealized structure of locust bean gum. 
         FIG. 3  is an electropherogram of a model acidic protein mixture in guaran-coated capillary. Capillary: 1(total)=335 mm, 1(effective)=250 mm, ID=50 μm, OD=360 μm. BGE: 100 mM Tris-HEPES; voltage: −10 kV, injection: 30 mbar 3 s. Sample: 10 g/L polyglutamate, 4 g/L trypsin inhibitor, 8 g/L .alpha.-lactoglobulin, 1 g/L .beta.-lactalbumin. 
         FIG. 4  is an electropherogram of a model basic protein mixture in guaran-coated capillary. Capillary: 1(total)=335 mm, 1(effective)=250 mm, ID=75 μm, OD=360 μm. BGE: 100 mM .beta.-alanine-citric acid; voltage: −10 kV, injection: 30 mbar 3 s. Sample: 2 g/L polylysine, lysozyme, cytochrome c, trypsinogen, and .alpha.-lactalbumin. 
         FIG. 5  shows isoelectric focusing of synthetic pI standards in capillary coated with guaran. Capillary: 1(total)=335 mm, 1(effective)=250 mm, ID=75 μm, OD=360 μm. Anolyte: 20 mM citric acid, catholyte: 40 mM NaOH. Capillary filled with 1% Ampholines 3.5-10, 0.1% purified Synergel, 25 mM BisTris Propane. Focusing: 20 kV 8 min, mobilization: 100 mbar, 20 kV. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Here we disclose a separation column for electrophoretic separation of molecules, realized as a channel in a body; wherein
         said body made as a capillary or block of an electrically insulating material selected from the group consisting of fused silica, glass, poly(methyl methacrylate), polycarbonate, poly(tetrafluoroethylene), and cyclic polyolefins;   said separation column having a polysaccharide coating permanently attached to the interior surface of said separation column;   said polysaccharide coating comprising at least one permanent layer made of one or more polysaccharides selected from the group of polysaccharides consisting of: locust bean gum, tara gum, guar gum, hydroxyisopropyl guaran, fenugreek gum, konjac, pullulan, pustullan, curdlan, laminaran, tragacanth gum, amylose, schyzophyllan, nigeran, and scleroglucan;   said polysaccharide coating produced by thermal immobilization of one or more said polysaccharides at temperature between about 100° C. and about 160° C.       

     Further we disclose the separation column for electrophoretic separation of molecules contains a polysaccharide coating that is thermally immobilized in a protective atmosphere of dry gas selected form the group consisting of: nitrogen, helium, and argon. 
     Further we disclose the he separation column for electrophoretic separation of molecules, wherein said polysaccharide coating is thermally immobilized at pressure between about 0 ton and about 700 torr. 
     Further we disclose the he separation column for electrophoretic separation of molecules, wherein said polysaccharide coating is thermally immobilized at pressure between about 0 ton and about 200 torr. 
     Further we disclose the he separation column for electrophoretic separation of molecules, wherein said polysaccharide coating is thermally immobilized at pressure between about 0 ton and about 2 torr. 
     Further we disclose the a method for preparation of a separation column, comprising steps:
         a) flushing said separation column with about 0.1 mL thionyl chloride;   b) flushing said separation column with distilled water;   c) filling said separation column with a polysaccharide solution comprising from about 0.5 g/L to about 20 g/L polysaccharide selected from the group consisting of: locust bean gum, tara gum, guar gum, hydroxyisopropyl guaran, fenugreek gum, konjac, pullulan, pustullan, curdlan, laminaran, tragacanth gum, amylose, schyzophyllan, nigeran, and scleroglucan;   d) incubating said separation column filled with said polysaccharide solution at temperature between about 100° C. and about 160° C. for the time interval between about 5 minutes and about 30 minutes, at pressure in the range between about 0 ton and about 2 ton.       

     We also disclose the separation column for electrophoretic separation of molecules, wherein side branches and side carbohydrate units of said polysaccharide in said branched polysaccharide coating are cleaved from the polysaccharide backbone by a glycolytic enzyme. 
     Further we disclose the separation column for electrophoretic separation of molecules, wherein said branched polysaccharide is selected from the group consisting of locust bean gum, tara gum, guar gum, and fenugreek gum; and said glycolytic enzyme is α-galactosidase. 
     Further we disclose the separation column for electrophoretic separation of molecules, wherein said polysaccharide is selected from the group consisting of amylose, nigeran, and pullulan; and said glycolytic enzyme is α-glucosidase. 
     Further we disclose the separation column for electrophoretic separation of molecules, wherein said polysaccharide is selected from the group consisting of scleroglucan, schyzophyllan, pustullan, laminaran, and curdlan; and said glycolytic enzyme is β-glucosidase. 
     EXAMPLES 
     Example 1 
     Wall Coating Preparation by Thermal Immobilization of Guaran 
     3 m piece of fused silica capillary 75 μm ID, 360 μm OD was flushed with 0.1 mL thionyl chloride under pressure of 500 psi to clean the inner surface of the capillary. Then the capillary was filled with 1 g/L guaran at pressure of 1000 psi. Guaran ( FIG. 1 ) solution was prepared by dissolving a purified guaran (Jaguar  2229 , Rhodia, Hercules, Pa.) in deionized water and filtered through 0.2 μm nylon syringe filter. Crude guaran was previously purified by treatment with ion exchanger Source 30Q (Pharmacia, Uppsala, Sweden) and Amberlite MB-150 (Sigma, St. Louis, Mo.) followed by ion exchange treatment on Source 30S (Pharmacia, Uppsala, Sweden) and by precipitation with acetone (V. Dolnik, W. A. Gurske, and A. Padua: Solution of galactomannans as a sieving matrix in capillary electrophoresis. U.S. Patent Application 20020049184, Sep. 5, 2001). The solution of guaran was left to flow through the capillary for 20 minutes. Then the capillary was flushed with nitrogen filtered through 0.2 μm nylon syringe filter at 1000 psi and after the capillary was emptied it was dried by flowing nitrogen through it at 100 psi. After a few minutes, the nitrogen flow was reduced to 20 psi and the capillary was placed in the oven heated to 102-105° C. for 30 minutes. After 30 minutes the capillary was cooled to room temperature and the nitrogen flow is stopped. The process can be repeated (without thionyl chloride treatment) providing multiple layer capillary coating. 
     Similarly other polysaccharides can be used to make a hydrophilic neutral wall coating including locust bean gum ( FIG. 2 ), tara gum, fenugreek gum, scleroglucan, pullulan, and konjac, to name a few. In this way a hydrophilic film is prepared that may have other applications including homogenous lower layer for attachment of nucleic acid or proteins in preparation of microarrays. 
     Example 2 
     Electroosmotic Mobility of Guaran Wall Coating and 
     Electroosmotic mobility (μ EEO ) of the prepared wall coating was measured in a 335 mm long capillary with effective length of 250 mm using equimolar solution of tris(hydroxymethyl)aminomethane (Tris) and N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES) as ackground electrolyte (BGE) and 100 mM nicotinamide or 1 g/L hydroxyethyl methacrylate as a neutral marker. First a band of the marker was injected as a 2 s pulse at 50 mbar to the capillary inlet and was moved forward by pumping BGE electrolyte into capillary for 35 s at 50 mbar. Then another 2-s pulse of the neutral marker was injected at 50 mbar and moved forward into capillary by pumping BGE at 50 mbar for 35 s. The −10 kV voltage was applied for 180 s and a 3 rd  pulse of the neutral marker (2 s at 50 mbar) was introduced into the capillary Then the all three bands of the neutral marker were pumped through the capillary at pressure of 50 mbar, while measuring absorption at 214 nm. Migration times of these three peaks were measured and used to calculate electroosmotic mobility (Williams, B. A. and Vigh, G., Determination of accurate electroosmotic mobility and analyte effective mobility values in the presence of charged interacting agents in capillary electrophoresis.  Anal. Chem.  1997, 69, 4445-4451). 
     By using this method we measured electroosmotic mobility of the prepared capillaries. For high-quality coating that significantly suppresses EOF (μ EEO ) below 10−9 m 2 V −1 s −1 ), it is necessary to apply high voltage during measurement for extended period of time to achieve good reproducibility. 
     Example 3 
     Capillary Zone Electrophoresis of Acid Protein Mixture 
     Quality of the prepared guaran wall coating was tested by CZE of model acid proteins in the guaran-coated capillary. The total length of the capillary was 335 mm, the effective length of the capillary was 250 mm. The capillary had ID 50 μm and OD 360 μm. For a CE separation of acid proteins, background electrolyte containing 100 mM Tris and 100 mM HEPES, pH 8.1 was used and a constant voltage of −10 kV was applied. The sample containing 10 g/L polyGlu, 4 g/L trypsin inhibitor, 8 g/L .beta.-lactoglobulin, 1 g/L .alpha.-lactalbumin in water was injected hydrodynamically applying pressure of 30 mbar for 3 s. The peaks were detected by measuring UV absorption at 214 nm. All five model proteins were separated in less than 25 minutes ( FIG. 3 ). 
     Example 4 
     Capillary Zone Electrophoresis of Basic Protein Mixture 
     Quality of the prepared guaran wall coating was tested by CZE of model basic proteins in a guaran-coated capillary. The total length of the capillary was 335 mm, the effective length of the capillary was 250 mm. The capillary had ID 75 μm and OD 360 μm. For a CE separation of acid proteins, background electrolyte containing 100 mM .beta.-alanine and 100 mM citric acid, pH 3.3 was used and a constant voltage of −10 kV was applied. The sample containing 2 g/L polylysine, lysozyme, cytochrome c, trypsinogen, and .alpha.-lactalbumin in water was injected hydrodynamically applying pressure of 30 mbar for 3 s. The peaks were detected by measuring UV absorption at 214 nm ( FIG. 4 ). 
     Example 5 
     Capillary Isoelectric Focusing of pI Standards in Guaran-Coated Capillary 
     Utility of guaran-coated capillary for capillary isoelectric focusing was tested by isoelectric focusing of colored synthetic pI markers ( FIG. 5 ). The capillary had ID 50 μm and OD 360 μm. For the focusing step, anolyte containing 20 mM citric acid and catholyte containing 40 mM NaOH were used. The capillary was filled with 1% Ampholines (pI range 3.5-10), 0.1% purified Synergel, and 25 mM BisTris Propane. In the focusing step 20 kV was applied for 8 min, then the focused zones were mobilized by applying pressure of 100 mbar, at 20 kV. As a sample, a mixture of six synthetic pI markers having pI 4.0, 5.3, 6.4, 7.5, 8.5, and 10.0 ({hacek over (S)}lais, K., Friedl, Z. Low-Molecular-Mass pI Markers for Isoelectric Focusing.  Journal of Chromatography A  1994, 661, 249-256.) were injected. Migration times of the pI standards were linearly proportional to pI. In this particular case the relationship could have been expressed by the equation
 
 t   m =7.50+0.51 pI  
 
where t m  is migration time and pI isoelectric point.