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, wherein varying the first flow restriction occurs in response to changes in temperature produced by the first variable-restriction device.

A liquid-chromatography apparatus, comprising:

• a chromatographic column;
• means for variably restricting a first flow path, the means for variably restricting the first flow path comprising a variable-restriction component that varies restriction in response to changes in temperature;
• 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 the chromatographic column.

A liquid chromatography system, comprising:

• a pumping device pumping liquid at a flow rate of less than one microliter per minute (μL/min);
• first and second solvent reservoirs, each solvent reservoir containing a solvent;
• a flow splitter directing a first portion of the pumped liquid over a first flow path to the first solvent reservoir and a second portion of the pumped liquid over a second flow path to the second solvent reservoir;
• a first variable restrictor disposed in the first flow path between the pumping device and the first solvent reservoir to variably restrict a flow of the first portion of liquid and thereby a rate of displacement of the solvent in the first solvent reservoir by the first portion of liquid;
• and
• a second variable restrictor disposed in the second flow path between the pumping device and the second solvent reservoir to variably restrict a flow of the second portion of liquid and thereby a rate of displacement of the solvent in the second solvent reservoir by the second portion of liquid;
• a flow combiner coupled to receive and combine the solvents displaced from the first and second solvent reservoirs and to produce a solvent gradient therefrom;
• and
• a chromatographic column in fluidic communication with the flow combiner to receive the solvent gradient therefrom.