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Capillary columns (also Open Tubular Columns) consist basically of a fused silica (quartz, SiO2) capillary, which is generally coated with a polyimide layer. The polyimide layer gives the capillary its flexibility and stability as well as its characteristic brownish colour. The capillary inner walls are deactivated for use in the GC. This process is necessary to cover the silanol groups located on the fused silica surface.