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There are two column types, which differ in their performance: packed columns, which are being progressively abandoned, and capillary, or open tube, columns, which have a very small diameter (Figure 2.7). Their performance levels are different. For packed columns, the stationary phase is deposited by impregnation or by covalent bonds between the stationary phase and the porous support. For capillary columns, a thin layer of stationary phase is deposited onto, or grafted to, the inner surface of the column.