Retention parameters

The easiest way to find the chromatographic retention is to measure the time between the injection point and maximum of the detector response for correspondent compound. This parameter usually called "retention time". Retention time, t_R is inversely proportional to the eluent flow rate.

The product of retention time and eluent flow rate, so called "retention volume", is more of a global retention parameter. Retention volume, V_R represent the volume of the eluent passed through the column while eluting a particular component.

Component retention volume V_R could be split into two parts:

1. Reduced retention volume is the volume of the eluent that passed through the column while the component was sitting on the surface.
2. Dead volume is the volume of the eluent that passed through the column while the component was moving with the liquid phase.

The second part is equal to the volume of the liquid phase in the column (dead volume, V_o), and it will be the same for any component eluted on this column.

Retention volume is independent of the flow parameters for the particular run, but it depend on the geometrical parameters of the column. V_R will be different for the same compound eluted on the different columns packed with the same type of adsorbent.

The more universal and fundamental retention parameter is the ratio of the retention volume and dead volume (k).

k = V_R/V_o

Historically, a slightly different retention parameter, called "capacity factor" (k') was introduced by the analogy with the liquid partitioning theory and widely accepted in chromatographic practice.

Capacity factor is dimentionless and independent on any geometrical parameters of the column or HPLC system. It could be considered to be a thermodynamic characteristic of the adsorbent-compound-eluent system.