{{Short description|Concept in chromatography}}

{{technical|date=June 2012}}

'''Retention distance''', or ''R<sub>D</sub>'', 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:

<math> R_D = \Bigg[(n+1)^{(n+1)} \prod^n_{i=0}{(R_{F(i+1)}-R_{Fi})\Bigg]^{\frac{1}{n}}} </math>

where ''n'' is the number of compounds separated, ''R<sub>f (1...n)</sub>'' are the Retention factor of the compounds sorted in non-descending order, ''R<sub>f0</sub>'' = 0 and ''R<sub>f(n+1)</sub>'' = 1.

== Theoretical considerations ==

The coefficient lies always in range <0,1> and 0 indicates worst case of separation (all R<sub>f</sub> 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<sub>d</sub>'' is sensitive to ''R<sub>f</sub>'' 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<sub>f</sub>'' values (0,0.2,0.2,0.3) (two compounds not separated at 0.2 and one at the start ) result in ''R<sub>D</sub>'' equal to 0, but ''R<sub>U</sub>'' equal to 0.3609. When some distance from 0 and spots occurs, the value is larger, for example ''R<sub>f</sub>'' values (0.1,0.2,0.25,0.3) give ''R<sub>D</sub>'' = 0.4835, ''R<sub>U</sub>'' = 0.4066.

== See also ==

* Chromatographic response function

== References ==

* Komsta Ł., Markowski W., Misztal G., A proposal for new R<sub>F</sub> equal-spread criteria with stable distribution parameters as a random variable. J. Planar Chromatogr. 2007 (20) 27-37.

Category:Chromatography

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