Fused-silica capillary tubes polyimide coated

Proper cutting of the column and retention gap is critical. Square, clean cuts minimize flow disturbances. A cleaving tool for cutting polyimide-coated fused silica capillary tubing always should be used (Figure 20-33). Make the cuts after the ferrules have been installed onto the tubing. 

In 2004, Ding et al. synthesized the chiral ILs (lS,2P)-(- -)-N,N-dimethylephedrinium-bis(trifluoromethanesulfon) imidate, (lJt,2S)-(-)-dimethylephedrinium-bis(trifluoromethanesulfon) imidate, and racemic (1S,2S)-(-F)-N,N-dimethyIpseudoephedrinium-bis(trifluoromethanesuIfon) imidate for coating of fused-silica capillary tubes (250 pm inner diameter, polyimide coated) [42], All ILs showed enantioselective retention for at least four general classes of... 

One can find 1-mm glass-lined stainless steel columns or fused silica capillary columns. Fused silica is available with 75-yU.m, 100-/xm, or 320-/xm internal diameter by 20-cm, 25-cm, or 30-cm length. Fused silica is a glass tubing with an outside polyimide coating to prevent breakage. 

Nebulizer sampie tube (polyimide coated fused silica capillary)... 

The capillaries are made either of borosilicate glass or fused quartz. Pyrex glass has a cutoff at about 280 nm, so fused silica is often used. The capillary tube is coated with a polyimide to enhance flexibility by preventing the hydration of strained siloxy bonds. The columns vary in internal dia-meter from 10 to 100 pm and in length from 30 to 100 cm, with 50 to 100 cm being common. The capillary side wall is usually thick by comparison, 300 to 600 pm, to act as a heat sink. A small portion (1 to 2 mm) of the coating is burned away if a photometric detector is used. Figure 31-6 shows typical dimensions. 

The column tubing itself must be stable at the necessary temperature. The polyimide scratch-protective outer coating of fused silica capillaries is itself stable only to about 380°C. Some exceptional stationary phases can exceed this limit and may require the use of metal capillaries, which for some analytes at those temperatures must be deactivated—perhaps with an inner coating of silica, which will not yield a flexible column coil. 

Ti and To are the inner and onter radins of the fused-silica tnbe refers to the onter radins of the polyimide coating of the capillary tube Kj and are the thermal conductivities of the electrolyte solution and of the capillary wall, respectively... 

Extensive experience of the preparation of bonded-silica columns for CEC has suggested to us that all packing methods have similar problems associated with the fragility of the fused-silica tubing material, especially at the Mt and detection window, where the polyimide coating had been removed. A less commonly met effect is the production of loosely packed silica particle beds with voids, giving rise to unstable currents. Approximately 70% of capillaries could be successfully packed by an experienced operator, with approximately 80% of packed and tested columns giving satisfactory CEC [23],... 

One of the limitations with packed columns is eddy diffusion. This is a remixing of the separated compounds as they go around the particles in the column packing, much like you get whirlpools when water flows around a rock in a stream. To completely eliminate this problem requires the elimination of the column packing If you make the column diameter very small (< 1.0 mm), the inside wall of the column can act as the inert phase. A thin film (0.1 pm) of stationary phase is coated onto the inside of the tubing wall or even bonded to it. These capillary columns are made of fused silica and then coated with a polyimide polymer to reduce breakage. They are quite flexible and can be tied in a knot, but that is seldom done. Their natural tendency is to form a straight column, and they are hard to coil and keep... 

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