Low-Pressure Gradient System

In low-pressure gradient systems, the same pump delivers all partial flows.

Simple systems generate the required solvent mixing ratio in the mixing chamber. Then, the mixture is sucked from the chamber and carried to the column. The solvent composition is controlled via different valve closing times in the feed lines of the individual components. As the mixing chamber volume represents a dead volume, producing the correct mixing ratio on the column is delayed.

Systems that are more complex mix the individual partial flow during the suction period in the piston. This requires the calculation of how much volume can be sucked. (The value is not constant as delivery is not always against the same external pressure; that is, small volumes are pumped if the external pressure is high, larger volumes are delivered if the external pressure is low. In addition, it is necessary to calculate the time of how long each valve must be opened for the individual partial flows to reach the exact mixing ratio.

Despite these mechanical and electronic procedures, the gradient profile produced by low-pressure gradient systems is less precise than High-Pressure Gradient Systems, however, it is the less expensive alternative.

 Tip:

Due to solvent compressibility and differing dead volumes, the chromatographic conditions of high-pressure and low-pressure gradient systems are not interchangeable.