Single-pump multiple stroke proportioning for gradient elution liquid

Abstract

In a high-performance, high-pressure liquid chromatography system, gradient elution of solvent components from a plurality of component reservoirs can be accomplished according to a programmed temporally variable ratio by a single reciprocating pump. The pumping cycle comprises a plurality of fill strokes with intervening pumping strokes. During one of the fill strokes of the cycle, a proportioned mixture of solvent components is admitted to the pump chamber from the respective reservoirs by means of proportioning valves. During each of the other fill strokes of the cycle, only a single solvent component is admitted to the pump chamber. Consequently, a relatively low concentration of a particular component can be delivered by the pump with precision over a complete pumping cycle, although a comparatively higher and therefore more accurately measurable concentration of that component is admitted to the pump chamber during any given fill stroke. The opening and closing of the proportioning valves during the component proportioning fill stroke is sequenced to minimize deviations of the relative concentrations from the programmed concentration ratio. Proper sequencing of the proportioning valves reduces inaccuracies that would otherwise tend to occur, especially at the upper and lower concentration limits, due to effects related to the compressibility of residual solvent remaining in the pump chamber at the end of each pumping stroke.