Retention uniformity, or R_U, is a concept in thin layer chromatography. It is designed for the quantitative measurement of equal-spreading of the spots on the chromatographic plate and is one of the Chromatographic response functions.

Formula

Retention uniformity is calculated from the following formula:

${\displaystyle R_{U}=1-{\sqrt {{\frac {6(n+1)}{n(2n+1)}}\sum _{i=1}^{n}{\left(R_{Fi}-{\frac {i}{n+1}}\right)^{2}}}}}$

where n is the number of compounds separated, R_{f (1...n)} are the Retention factor of the compounds sorted in non-descending order.

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 D (separation response), I_p (performance index) or S_m (informational entropy). 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 distance, R_u is insensitive to R_f values close to 0 or 1, or close to themselves. If two values are not separated, it still indicates some uniformity of chromatographic system. 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_U equal to 0.3609.

See also

• Chromatographic response function

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.