Chromatography Forum

Hello,
I need clarification regarding the variation of oganic composition of mobile phase for checking the robustness of a method. In the case of isocratic method the organic composition is varied by 10% (plus or minus) from the orginal value. My question is how the same effect can by studied in the case of gradient elution say for eg: M.phase A: 0.1% TFA and M.phase B: Acetonitrile. "Whether the study of gradient robustness is enough and whether it gives the effect as varying the Organic composition".
Thank you.
Santosh Gandhi.

First of all, plus and minus 10% is a HUGE change. More typical values for a robustness study would be plus and 1 or 2% (absolute) from the specified value. Assuming small molecules with an S-value of 2.5, the gradient equivalent would be about a 10% change in gradient time.