Topics - Column Length

Column Length There are three methods of measuring column length. The first, and the most common, is to measure the distance between the point of connection of the column to the mobile phase supply and the connection of the column to the detector. The second, a more precise method is to measure the actual length of the column packing or, in the case of a capillary column, the actual length of the coated portion of the column. For thermodynamic measurements, the most accurate method is to measure the distance between the point of injection and the end of the column packing or coating. In gas chromatography, capillary columns can be 100m or more in length and, when operated at the optimum mobile phase velocity, can provide very high column efficiencies and consequent high resolution. Conversely, short capillary columns, a few meters in length, operated at very high mobile phase velocities, can produce relatively simple separations extremely rapidly. In liquid chromatography, due to the flow impedance of a packed bed increasing as the particle diameter is reduced, short columns (ca 3 cm long) are packed with the smallest particles (ca 3 m in diameter) and operated at very high mobile phase velocities to provide fast analyses. Conversely, columns packed with particles 20 m in diameter can be used to pack long columns (1m in length) and operated at the optimum mobile phase velocity to produce very high efficiencies and high resolution