Protective coatings, fused-silica columns

Three steps are involved in the preparation of a fused-silica column (1) the high-temperature extrusion of the blank capillary tubing from a preform, where the capillary receives a protective outer coating in the same process (2) the deactivation of the inner surface of the column and (3) the uniform deposition of a stationary phase of a desired film thickness on the deactivated inner surface. In this section the extrusion of fused silica is described the procedures employed for the deactivation and coating of fused-silica capillaries are presented in the following section. 

Capillary Columns Capillary, or open tubular columns are constructed from fused silica coated with a protective polymer. Columns may be up to 100 m in length with an internal diameter of approximately 150-300 )J,m (Figure 12.17). Larger bore columns of 530 )J,m, called megabore columns, also are available. 

The vast majority of analyses use long, narrow open tubular columns (Figure 24-2) made of fused silica (Si02) and coated with polyimide (a plastic capable of withstanding 350°C) for support and protection from atmospheric moisture.3 As discussed in Section 23-5, open tubular columns offer higher resolution, shorter analysis time, and greater sensitivity than packed columns, hut have less sample capacity. 

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

The most commonly used columns for CEC have so far consisted of a small-diameter (usually 50-100-pm internal diameter) fused silica tubing with protective polyimide coating packed with, most often, 3-pm-diamctcr silica particles with bonded organic groups. Much attention has been paid to the factors which have a major bearing on the properties of the resulting column the means of packing the bed and the procedure used to hold the packed bed in the column. 

The column tubing itself must be stable at the necessary temperature. The poly-imide 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 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. 

The capillaries typically used in CE are made up of fused silica, with IDs of 10-100 p,m, outer diameters (ODs) of 100-375 (xm, and coated with a 10-30 xm protective layer of polyimide (PI). There are several locations relative to this capillary where detection can take place. These detection locations include on-column, end-column, whole-column, and post-column detection. 

For on-column detection, a portion of the protective PI coating is removed, either by acid application (hot HNO3) or by burning. This results in a cylindrical length of bare fused silica, which is used as the detection window. The window is generally placed as close as practicable to the outlet of the capillary in order to maximize the fraction of the capillary used for the separation. 

A third type of protective outer coating, stainless steel, for fused silica offers an alternative to aluminum-clad fused silica for elevated column temperatures. This technology is the inverse of that for polyimide-clad fused-silica capillary where a layer of fused silica is deposited on the inner surface of a stainless capillary. In Figure 3.19 scanning electron micrographs are displayed to compare the rough surface of stainless steel with the smooth surface of untreated fused silica and the surface of stainless steel after a micron meter layer of deactivated fused silica... 

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