Gradients generated at high pressure

Many high-pressure liquid chromatographs include a gradient elution system. With the exception of the Siemens and Spectra-Physics systems, only two-.solvent gradient can be made with these devices, All systems that are able to give a continuous gradient also permit operation under isocratic conditions with a mixture of the two solvents of constant composition. 

Generally, these systems use independently selected streams of the two solvents which either are allowed to flow through the column alternately with a period 

The only exception to the above is the Tracer Chromatec Model 930 gradient 

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