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Definition: Capacity factor
The capacity factor (also called "capacity ratio") is symbolized by k' (USP terminology) or k (IUPAC/ASTM terminology). It is a measure of the retention of a peak that is independent of column geometry or mobile phase flow rate. 

The capacity factor is calculated as:

k' = (tR - t0)/t0  
where tR is the retention time of the peak, and t0 is the dead time of the column.

Note that the capacity factor cannot be calculated in this manner from a gradient chromatogram. The "apparent" capacity factor obtained when this is done is meaningless chromatographically.