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Gradient System

A separation in which the mobile phase composition is changed during the separation process is described as a gradient elution. One example is a gradient starting at 10% methanol and ending at 90% methanol after 20 minutes. Thre are two tpye of Gradient HPLC System :

1. High Presurre Gradient System , solvent passes from each reservoir directly to a pump and then to a mixing manifold from which it passes to the sample valve and column. The pumps control the actual program and are usually driven by stepping motors. The volume delivery of each solvent is controlled by the speed of the respective pump which is precisely determined by the frequency of its power supply. The controlling frequency can be generated either by external oscillators or, if the chromatograph is computer controlled, directly from the computer itself.

2. Low Pressure Gradient System, in a low pressure programmer, the solvent from each reservoir passes to an oscillating valve, the output from which is connected to a mixing manifold. The manifold receives and mixes solvents from each of the programmed valves. The valves are electrically operated and programmed to open and close for different periods of time by adjusting the frequency and wave form of the supply. Thus, a pre-determined amount of each solvent is allowed to flow into the manifold. The valves can also be driven either by oscillators contained in a separate electronic programmer or by the chromatograph computer which modifies the wave form and frequency to control the flow of each solvent.


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