Fused silica capillaries modifications

Extracolumn dispersion is a major problem for the packed fused silica capillary columns with internal diameters less than 0.35 mm. Peak standeunl deviations will be in the submicroliter range and extensive equipment modification is required for operation under optimum conditions. A reasonable compromise is to esploy injection voluMs of a few hundred nanoliters or less with detector volumes of a similar or preferably smaller size. This demands considerable ingenuity on behalf of the analyst since, as... 

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

The efficacy of CE separation depends considerably on the type of capillary. Fused-silica capillaries without pretreatment are used most frequently. Its outside is coated with a polymer layer to make it flexible and to lessen the occurrence of breakage. The polymer coating has to be dissolved with acid or burned away at the detection point. Capillaries with an optically transparent outer coating have also found application in CE. The objectives of the development of chemically modified capillary walls were the elimination of electro-osmotic flow and the prevention of adsorption on the inner wall of the capillary. Another method to prevent the adsorption of cationic analyses and proteins is the use of mobile phase additives. The modification of the pH of the buffer, the addition of salts, amines and polymers have all been successfully employed for the improvement of separation. 

Capillary open tubular fused silica capillary with/without wall modifications capillary packed with re-versed-phase or mixedmode sorbent materials capillary packed with re-versed-phase or mixedmode sorbent materials... 

IV. MODIFICATIONS OF FUSED SILICA CAPILLARIES A. Surface Modification of Capillaries... 

Some information on various buffer additives was given in Section III.C. A more detailed discussion of dynamic modification of fused silica capillaries is provided here. This entails transitory modification of the capillary wall by utilizing certain buffer additives. Some examples are... 

Desportes et al. (2000,2001) used capillary electrophoresis on fused-silica capillaries with 25 mM, pH 2.5 phosphoric acid as running buffer, to verify the purity of collected peaks after separation by HPLC. Both methods and some others described for the analysis of wine proteins, such as that described by Luguera et al. (1997), with slight modifications, could also be used for the analysis of medium molecular weight peptides. 

Chemical modification of the inner wall of fused-silica capillaries and the surfaces of porous silica supports for HPLC utilize the same reactions. The most common method is based on organosilanization. Within this general reaction scheme, there are two possible... 

Capillaries with chiral polymer coatings have been applied in CE for resolution of enantiomers. Possibly because of its inclusive effect, cyclodextrin seems to be an effective chiral selective agent when bonded to a fused-silica capillary surface. In this case, the purpose of the modification is to induce interactions with the chiral material on the surface. Certainly, the cyclodextrin moiety lowers EOF like other wall modifications because it diminishes the number of silanols. The lower EOF allows for slower migration of the solute through the column and, hence, more time for interaction with the chiral selector. The diminished number of silanols also results in less nonspecific interactions with the fused-silica surface, which would tend to degrade the enantiomeric separation. 

A modification of the FAB technique is continuous flow FAB (CFFAB). In this approach, the sample in solution is introduced into the mass spectrometer through a fused silica capillary. The tip of the capillary is the target. The solution is bombarded by fast atoms produced as described earlier. Solvent is flowing continuously and the liquid sample is introduced by continuous flow injection (Fig. 9.13). The mass spectrum produced has the same characteristics as that from conventional FAB, but with low background. Typically, the solvent used is 95% water and 5% glycerol. The ability to inject aqueous samples is an enormous advantage in biological and environmental studies. 

Courtois, J., Szumski, M., Bystroem, E., Iwasiewicz, A., Shchukarev, A., and hgum, K., A study of surface modification and anchoring techniques used in the preparation of monohthic microcolumns in fused silica capillaries, J. Sep. Sci., 29, 14, 2006. 

Lipid Composition. After the behavior experiments, the rats were killed by decapitation. Tissue samples including brain and retina were removed and stored at -80°C. The total lipid extracts of tissues were prepared according to the method of Folch et al. (39). Butylated hydroxytoluene (300 qg/sample) was added in methanol to each sample to minimize lipid oxidation. The total lipid extracts were trans-methylated with 14% BFj/methanol at 100°C for 60 min by a modification (40) of the method of Morrison and Smith (41). Fatty acid methyl esters were then analyzed on a Hewlett-Packard 5890 gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a flame ionization detector and fused silica capillary column (DB-FFAP, 30 m X 0.25 mm x 0.25 m, J W, Folsom, CA) with carrier gas (hydrogen) at a linear velocity of 50 cm/s. Injector and detector temperature were set to 250°C and oven temperature program was as follows 130 to 175°C at4°C/min, 175 to 210°C at l°C/min, and then to 245 at 30°C/min, with a final... 

As glass and quartz exhibit the same surface property as fused-silica capillary, the monolithic materials could be conveniently prepared in a glass- or quartz-based microfluidic device via the same way of monoliths in the capillary. However, glass/quartz devices are rather expensive, and the need for specialized facilities for their fabrication with conventional photolithography technology hinders any rapid modification of the chip architecture. An attractive alternative is using a variety of polymeric materials, such as poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), polycarbonate (PC), and cyclic olefin copolymer (COC), to fabricate microchips for their mechanical and chemical properties, low cost, ease of fabrication, and high flexibility. 

Research stiU continues into the development of enhanced electrokinetic injection techniques for p-chips. Modifications made to the outlet end of the separation channel, such as its connection to a length of fused silica capillary to facilitate chip interfacing with off-chip detection systems for example, can be adopted equally well for chip inlets and sample introduction. The use of conventional capillary to interface between macroscale sample reservoirs and p-chip platforms has been successfully... 

---