Determination of the average volumetric flow rate in supercritical fluid chromatography

This work reviews and discusses controversies and errors made in the determination of the average volumetric flow rate of a compressible mobile phase forced to flow through a chromatographic column. Proper estimates of the volumetric flow rate, which obviously changes along the column, are keys to understanding the retention mechanism that takes place inside the column and to achieve repeatable and reproducible separations. Each step of the calculation process will be discussed in detail, including how to estimate the variations of the pressure and the temperature along the column. The determination of the average volumetric flow rate requires the knowledge of the average density of the mobile phase and of its mass flow rate. The calculations were carried out under various experimental conditions, including different column temperatures and inlet pressures. The estimated values of the volumetric flow rate are validated by the conversion of the retention times to the retention volumes of nitrous oxide peaks, which is valid since this compound is assumed to be non retained, which makes it a hold-up time marker.