isoelutropic mobile phase

[olivia]

Hi, can anyone tell me what the equation is to convert the organic modifier in the mobile phase from methanol to acetonitrile and keep the retention time the same
Thanks
Liv

[tom jupille]

I don't think I've actually seen the equation, but the nomogram has been around for decades:



It's only an approximation in any case (obviously, details will depend on stationary phase and analyte).

[Uwe Neue]

Unfortunately, it depends on the column, and the analytes. A reasonable rule of thumb is MeCN = 0.25 MeOH^2 + 0.65 MeOH (MeCN and MeOH in volume fractions).