Getting Started in HPLC

Section 2B. HPLC Pumps

The pump can be used to mix or blend the mobile phase as well as to deliver it to the system. Usually, the mobile phase is a mixture of two or more solvents. We refer to 2-solvent mobile phases as "Binary", 3-solvent mobile phases as "Ternary", and 4-solvent mobile phases as "Quaternary" mixtures. Commercial LC pumps are available that can automatically blend 2, 3, or 4 solvents into a final mobile phase. We can formulate or mix a mobile phase in one of three ways:
  • manual (without a pump)
  • with low-pressure mixing by the pump
  • with high-pressure mixing by the pump

Manual mixing of mobile phases is the simplest and the most precise. If we need a large volume of mobile phase for the analysis of a large number of samples by the same procedure, manual mixing is probably the best approach. On the other hand, if the mobile phase composition is varied during unattended operation of the LC system, then "on-line" mixing will be required.

Many HPLC pumping systems allow the mobile phase composition to be controlled by "on-line mixing". Some systems blend the output of two independent pumps; these are referred to as "two-pump" or "high-pressure mixing" systems. Others use a proportioning valve to control the input to a single pump; these are referred to as "one-pump" or "low-pressure mixing" systems. On-line mixing systems can be used for either isocratic HPLC (in which the mobile phase composition is constant during a run) or gradient HPLC (in which mobile phase composition is changed during a run). Although on-line mixing is not required for isocratic LC, it makes mobile phase preparation more convenient.


A high-pressure mixing (two-pump) system blends the output of two high-pressure pumps to control the mobile phase composition.

A low-pressure mixing (one-pump) system uses a proportioning valve mounted upstream of a single high-pressure pump to control the mobile phase composition.


The Waters system includes a high-pressure mixing ("2-pump") solvent delivery system.

The TSP module is a low-pressure mixing ("1-pump") solvent delivery system.



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Last revised: April 02, 2001.