Variable Resistance Fluid Controller

A method for forming a fluid gradient in a liquid-chromatography system, comprising:

• providing a fluid-gradient controller apparatus comprising a first variable-restriction device, in communication with a first solvent reservoir containing at least a first solvent, and a second variable-restriction device, in communication with a second solvent reservoir containing at least a second solvent;
• pumping at least one pump fluid through the first and second variable-restriction devices, thereby displacing the first and second solvents;
• and
• varying a first flow restriction and a second flow restriction respectively provided by the first variable-restriction device and the second variable-restriction device to mediate a gradient of the displaced first and second solvents.

A liquid-chromatography apparatus, comprising:

• means for variably restricting a first flow path;
• means for variably restricting a second flow path;
• means, responsive to the means for variably restricting the first flow path, for displacing a first solvent;
• means, responsive to the means for variably restricting the second flow path, for displacing a second solvent;
• means for combining the displaced first and second solvents;
• and
• means for delivering the combined first and second solvents to a chromatographic column.