A gradient is a change in the proportion of the two (or more) solvents that make up the mobile phase. With a pump controlling flow rates and proportioning valves on all the solvent reservoirs, it is possible to gradually or abruptly change the mobile phase composition.
A Step Gradient is an abrupt change in mobile phase composition. An example of a step gradient is shown here (blue line), where the salt concentration in the mobile phase is changed from 0.1 M to 0.2 M at 10 min, then from 0.2 M to 0.3 M at 20 min.
A Linear Gradient is a gradual change in mobile phase composition over time, as seen in the red line in this figure. A linear gradient is used to separate proteins that have similar affinities for the stationary phase. This application is designed to wash the column with Solvent A for 10 minutes, then apply a linear gradient from 0 to 100% B between 10 and 20 minutes; the gradient elutes from the bottom of the column between 21 and 31 minutes.
Mixing ValveProportioning Valve Salt