Retentiefactor

Retentiefactor, in Dutch chromato­graphy terminology often called retentiefactor or capaciteit factor (k'), is a dimensionless quantity that expresses how strongly a compound is retained by the stationary phase relative to the mobile phase. It is defined by the formula k' = (tR − tM) / tM, where tR is the measured retention time of the analyte and tM (or t0) is the dead time, i.e., the time for an unretained solute to pass through the column.

Interpretation and use. A higher retentiefactor indicates stronger interaction with the stationary phase and a longer elution time. In typical isocratic liquid chromatography, useful k' values commonly lie between about 1 and 10, enabling effective separation without excessive analysis time. The retentiefactor is widely used to compare retention behavior across analytes, solvents, and column chemistries, and to guide method development and transfer.

Determination and calculation. To compute k', a chromatogram is analyzed to determine tR for the analyte. tM is determined from a non-retained reference compound or from column specifications (the void time). Then k' is calculated with the formula above. In practice, tR is often taken as the time to the peak center, and tM as the elution time of an unretained species under the same conditions.

Factors and limitations. Retentiefactor depends on column type (normal- or reversed-phase), mobile phase composition, temperature, pH, and flow rate. It can change under gradient elution, where its meaning is limited to the specific conditions used for calculation. It is related to, but distinct from, retention indices and the broader concept of selectivity.

See also: retention time, dead time, selectivity, capacity factor.