Converting a pump for use in supercritical fluid chromatography

Pumps used in SFC Supercritical Fluid Chromatography control the mass-flow of each component of the mobile phase, and therefore control the composition of the mobile phase through the column. Pumping compressible fluids, such as carbon dioxide (CO2), at high pressures in SFC systems while accurately controlling the flow, is more difficult than that for a liquid chromatography system. SFC systems use two pumps to deliver two different source streams into a single mobile phase flow stream. Each pump on each flow stream adds pressure, flow ripples, and variances that cause baseline noise. The two pumps also operate at different frequencies, different flow rates, and require separate compressibility compensations, further adding to the complexity of flow operations. A method for converting a pump for use in a flow stream containing a mixture of highly compressed gas, compressible liquid or supercritical fluid wherein the pump delivers a compressible fluid against a back-pressure regulator which in turn delivers a controllable flow rate downstream of the regulator without performing variable compressibility compensation adjustments on the pump. By using a pressurized source of compressible fluid combined with isocratic conditions, the delivery pressure from the pump is controlled at a rate higher than the downstream gradient with the back-pressure regulator, and an inexpensive pump may be used in place of specialized, expensive pumps and compressibility compensation systems for use in stems operating at or near supercritical fluid levels of compressible fluids. Significant capital and operating laboratory costs are saved through a simpler and cheaper system for accurately delivering compressible fluids into mobile phase flow streams.