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Mobile phase
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In a modern chromatographic column, the retention of a solute is determined by its equilibrium distribution between the mobile and stationary phase, respectively. For a particular solute it is the combination of column and mobile phase properties which determines its retention. A proper choice of mobile phase is therefore of key importance for efficient chromatography.

 

The mobile phase composition is relatively easy in practice and can be varied infinitely. In RP chromatography the mobile phase usually is a mixture of water/methanol or water/ acetonitrile. In the chromatographic nomencalture the methanol, acetonitrile or any other organic sovent are usually called the organic modifier.

 

For a given column, it is found empirically that the retention factor for neutral solutes usually decreases exponentially with the volume fraction of the organic modifier. i.e.

 

log kf = log k0 – S*f

where kf is the retention factor when the volume fraction organic modifier is f, k0 the retention factor with modifier concentration zero, S is a constant which is characteristic for each solute and f is the concentration of organic midifier in the mobile phase.

 

For ionizable solutes the retention strongly depends on the column properties, the mobile phase and its state of ionization. The apparent pH and ionic strength of the mobile phase are therefore additional parameters in this case.

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