Topics - Compressibility
Fluids are employed as mobile phases in chromatography and all fluids are to some extent compressible. The compressibility of liquids, however, is very small and, thus, has little impact on liquid chromatography performance. Gases, however, have high compressibility, as described by Boyle’s law, which states that at a given temperature, the product of the pressure and volume of a gas is constant. Thus, as there is a significant pressure drop across a GC column, although the mass of gas passing a give point is constant along the column length, the volume of gas passing through the column continually increases from the start of the column to the end. This has important ramifications when attempting to measure the specific retention volume of a solute for thermodynamic analysis. It is clear that, as the volume flow will continually increase as the solute passes along the column, then a pressure correction must be applied. 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) over (the inlet-outlet pressure ratio cubed minus 1).