(0.10tG + tM + tD) < tR < (tG + tM + tD) This was done to ensure that the solute elutes during the gradient (tR < (tG + tM + tD)), otherwise a fit to Eq. ( ) would not be valid, and to check to see if the solute eluted at the very beginning of the gradient ((0.10tG + tM + tD) < tR), which we judged would not be as effective in informing a fit, as the solute experiences only a small range of organic compositions. If the condition of Eq. ( ) is met for the two initial gradients, four additional gradients were run using the same two gradients (i.e., same I and ), but at tG = 15tM and 35tM. This yields six experiments with six retention times that can be fit to Eq. ( ) as a function of I, , and tG.