Retention distance, or ''R_D'', is a concept in thin layer chromatography, designed for quantitative measurement of ''equal-spreading'' of the spots on the chromatographic plate and one of the Chromatographic response functions. It is calculated from the following formula:

R_D = \Bigg n+1)^ \prod^n_

where ''n'' is the number of compounds separated, ''R_{f (1...n)}'' are the Retention factor of the compounds sorted in non-descending order, ''R_f0'' = 0 and ''R_f(n+1)'' = 1.

Theoretical considerations

The coefficient lies always in range <0,1> and 0 indicates worst case of separation (all R_f values equal to 0 or 1), value 1 indicates ideal equal-spreading of the spots, for example (0.25,0.5,0.75) for three solutes, or (0.2,0.4,0.6,0.8) for four solutes. This coefficient was proposed as an alternative to earlier approaches, such as delta-Rf, delta-Rf product or MRF (Multispot Response Function). Besides its stable range, the advantage is a stable distribution as a random variable, regardless of compounds investigated. In contrast to the similar concept called Retention uniformity, ''R_d'' is sensitive to ''R_f'' values close to 0 or 1, or close to themselves. If two values are not separated, it is equal to 0. For example, the ''R_f'' values (0,0.2,0.2,0.3) (two compounds not separated at 0.2 and one at the start ) result in ''R_D'' equal to 0, but ''R_U'' equal to 0.3609. When some distance from 0 and spots occurs, the value is larger, for example ''R_f'' values (0.1,0.2,0.25,0.3) give ''R_D'' = 0.4835, ''R_U'' = 0.4066.

References

* Komsta Ł., Markowski W., Misztal G., A proposal for new R_F equal-spread criteria with stable distribution parameters as a random variable. J. Planar Chromatogr. 2007 (20) 27-37. Chromatography {{Chromatography-stub

Theoretical considerations

See also

References