A measure of the time the sample component resides in the stationary phase relative to the time it resides in the mobile phase; it expresses how much longer a sample
component is retarded by the stationary phase than it would take to travel through the column with the velocity of the mobile phase. Mathematically, it is the ratio of the
adjusted retention volume (time) and the
hold-up volume (time):
If the
distribution constant is independent of sample component concentration, then the retention factor is also equal to the ratio of the
amounts of a sample component in the stationary and mobile phases respectively, at equilibrium:
If the fraction of the sample
component in the mobile phase is
, then
the fraction in the stationary phase is
; thus
In former
nomenclatures and in the literature one may find the expressions
partition ratio, capacity ratio, capacity factor or
mass distribution ratio to describe this term. In the literature the symbol
is often used for the retention factor, particularly in liquid
chromatography. The original reason for this was to clearly distinguish it from the
partition coefficient (
distribution constant) for which the symbol K had been utilized. Since, however, the distribution constants are all identified with a
subscript, there is no reason to add the prime sign to this symbol. It should be emphasized that all the recognized nomenclatures (IUPAC, BS, ASTM) have always clearly identified the
capacity factor with the symbol
and not
. The logarithm of the retention factor is equivalent to the
value used in
planar
chromatography. The symbol
is suggested to express
Source:
PAC, 1993, 65, 819 (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 843