Topics - Flow Rate
In chromatography the flow rate normally refers to the flow of mobile phase through the column and is usually measured in ml/min. In liquid chromatography, the mobile phase is a liquid and as the compressibility of liquids is very small, the flow rate in an LC column is, therefore, sensibly constant throughout the length of the column. This is not true for gas chromatography, due to gases being highly compressible as described by Boyles Law. Thus, although the mass of gas passing though the column per unit time is constant throughout the column, the volume of gas passing through the column increases, non-linearly and continuously from the column inlet to the column exit. Thus, if the true retention volume of a solute is required, a pressure correction must be applied to the volume measured at the column exit. The true retention volume of a solute can be calculated as 1.5 times the product of the retention time and exit flow rate, times (the inlet-outlet pressure ratio squared minus 1) divided by (the inlet-outlet pressure ratio cubed minus 1).