Theoretical study of the gradient elution profiles obtained with syringe-type pumps in liquid chromatography

Abstract

Because of the different viscosities of the two solvents used, a gradient run in liquid chromatography results in either a large change in the flow-rate (constant-pressure pumps) or a large change in the inlet pressure (constant flow-rate pumps). Because of the compressibility of liquids and the large volume of their reservoirs compared with the usual values of the flow-rate, the pressure change observed with a syringe pump makes the actual concentrations profile of the eluent very different from the set profile, most often a linear one, and the flow-rate during the gradient different from the constant set value. The deviation from linearity and also the importance of the transitory change in total flow-rate are smaller when the second solvent, with the highest eluting strength, is less viscous than the first solvent. These deviations are also reduced if check valves are used on the solvent line and if the second solvent is pressurized to the column inlet pressure before the beginning of the gradient run. The correct use of pressure controllers or of feedback control of the flow-rate could in practice make negligible the consequences of these effects. A theory is suggested for the prediction of the pressure and flow-rate profiles and calculation of the concentration profile as a function of time when the variation of the solvent mixture viscosity with its composition is known.