High-Pressure Mixing

In high-pressure mixing systems, individual high pressure pumps are used to provide each liquid. The outlet of each pump is either connected to a mixing connector (usually referred to as a "T" since there are normally two inlet lines and one outlet line) or to a mixing chamber. In these, the two flows are blended en route to the injector and column. In other words, mixing is accomplished on the high-pressure side of the pumps.

Most modern liquid chromatography systems utilizing this principle include pump-drive electronics which control the pumping speed. Blending is achieved through adjustment of a master controller. The flow rate which always represents the combined (total) output of both pumps is selected, then the dividing control adjusted to provide the desired ratio of the two components. These controls are commonly calibrated to read out the percentage of strong component in the mixture.

The stronger component is considered Component A; thus, the mixing control is labelled "%A in A+B". When a given %A is dialled in, the mixing electronics automatically divides the output signal between the two pumps. Pump A (strong component) receives the percentage of the signal called for in the control indicator, and Pump B (weaker component) receives the remainder of the signal, so that the total output of both pumps always equals the preselected flow rate.

Figure A shows the mixing of two mobile-phase components. It is also possible to prepare ternary mobile phases by utilizing three separate pumps. Each of these pumps will then deliver a separate solvent into the mixing chamber.

Among the advantages of high-pressure mixing is the ability to achieve precise control and repeatability of mobile phase mixtures to within 0.1% levels (only within the medium eluent composition range), rapid response to changes in concentration and the ability to utilize each pump separately if separate isocratic pumping systems are required.

There are some disadvantages for this type of gradient formation:

even special mixing chambers do not provide complete mixing;
many HPLC solvents being mixed will change the total volume, sometimes up to 20%;
mixing accuracy within the 0-10% range for component A or B is usually poor;
high pressure mixing needs two or sometimes three high pressure pumps, which increase the cost of the whole system.