Gradient average retention factor

[Austin]

Hi, everyone

Recently I read some literatures(e.g. Practial HPLC method development) about gradient LC and puzzled by a problem:

The gradient effect(or average) retention factor k* is equal to the instantaneous value of k for the band when it has migrated halfway through the column.

I doubt why to set the value at median point of the column, rather than other point(e.g. 1/3, 1/4 length of the column), should anyone give me some suggestions and related informations will be appreciated greatly.
Thanks in advance!:)

Best regards

Austin

[tom jupille]

Two reasons:

1. If you want to define an "average" value, what better reference point than the midpoint of the column (i.e., the point at which half of the stationary phase has been traversed).

2. On a more practical level, k* defined in this way behaves much the same as isocratic k', so it can be plugged in to an analogous resolution equation:
Rs = 0.25 x N*^0.5 x (a*- 1)/a x k*/(1 + k*) where N*, a*, and k* are the gradient equivalents to N, a, and k' [I'm using a for "alpha"].

If you want (much) more detail on the theoretical underpinnings, get a copy of Snyder & Dolan's later book
High-Performance Gradient Elution: The Practical Application of the Linear-Solvent-Strength Model

[Austin]

Tom,

I understand the instantaneous retention factor is relate to the peak migration rate, and during the band migrating through the column, the migrate rate(dx/dt) becomes faster as the organic modifier strength rises stronger gradually, so the migrate rate at the median point is the average rate corresponding to the isocratic migrate rate. Is it right? I hope you give me some information. Thanks

Austin

[tom jupille]

You are correct.