Gradient

Usually, gradients are used in HPLC/IC and GC to accelerate the separation of substance mixtures. Due to the different separation mechanisms in HPLC/IC on the one hand and GC on the other, different gradient types can be used.

HPLC/IC: Two different gradient types

A change in the composition of the mobile phase during the analysis is referred to as a gradient (more exactly as %-Gradient). A change in the flow rate is referred to as a Flow Gradient. However, this gradient type is quite unusual. Chromeleon lets you generate flow and %-gradients. Both gradient types can be performed at the same time (!).

 Tips:

Performing both gradients at the same time results in non-linear partial flow alterations in the total flow. For High-Pressure Gradient Systems with more than one pump, it will result in non-linear partial flow alterations for each pump. Some chromatography pumps do not support this option.

The chromatographic conditions for high-pressure and low-pressure gradients are not transferable due to the different properties in solvent compressibility and dead volume!

GC: Three different gradient types

Usually, temperature gradients are used in GC. In addition, some GCs can run flow or pressure gradients. With GC gradients, the flow (or the temperature, or the pressure) usually increases during the chromatogram. Changing one of the three variables automatically changes the remaining two according to the equitation of state for ideal gases and the Hagen-Poiseuille law. Nevertheless, it is also possible in GC to run a temperature and a flow or pressure gradient simultaneously. In this case, the flow or pressure change that results from the temperature increase is considered for the flow or pressure settings.

Independently of the gradient types described above, the gradient forms can be different. Distinguish Ramps and Step Gradients. For example, if you run a %-gradient as a ramp, the solvent composition continually changes until the desired composition is finally reached at the desired end. The composition for a step gradient is set immediately. (Nevertheless, it may take some time until the desired composition reaches the column, depending on the column, the solvent change, and the flow)