Tapered versus constant diameter post-column restrictors in capillary SFC

A one-dimensional compressible flow model is used in the description of actual flow through ducts. This model serves well in the explanation and prediction of the characteristics towards signal distortion, noise, mass flow rate and influence on dead-time behavior of end-restrictors as used in supercritical fluid chromatography (SFC) with post-column detection. It was experimentally verified that tapered restrictors have a better performance than constant diameter restrictors at relatively low detection temperatures (200°C). The mass flow rate of a tapered restrictor is found to be linearly proportional to applied inlet pressure, implying the same linear dependence of the product of linear velocity and on-column density. A more than proportional rise of density will, therefore, lead to a decrease in linear velocity and vice versa. Since thermostat temperature fluctuations are directly translated into linear velocity variations, a very temperature stable thermostat has to be used. As will be discussed, this complicated dead-time dependence on applied pressure has a large effect on the chromatographic results