selectivity

[olivia]

Hi, can anyone give me an acceptable value for selectivity. Is it >1.5.
Is there a quantified number anywhere
Hope someone can help
Liv

[JGK]

I attended a seminar recently on optimizing separations. Selectivity, was not assigned an optimum value but the as an effect on resolution.

The example given showed a slight increase in selectivity (1.1 to 1.2) producing a resolution change from 2.0 to 4.0.

The emphasis of the lecture was to gain maximum resolution and then to trade off some resolution modify the analysis to meet other targets.

[tom jupille]

"Acceptable" selectivity is a function of the desired resolution, the plate number, and retention.

The general relationship for two close peaks is:

Rs ≈ 0.25 * N^0.5 * (α - 1) *[k'/(1+k')]

You can usually get sufficiently high retention to make [k'/(1+k')] approximately equal to 1, so that simplifies to:

Rs ≈ 0.25 * N^0.5 *(α -1). Solve for a and you get:

α ≈ 1 + 4 * Rs /(N^0.5)

How many plates do you have ? and how much resolution do you need? Plug in the values and find out what kind of selectivity (alpha) value is required.