using ref set to 0.3. Initially, all data points were used in the fits, but it was observed that in some cases the standard errors of the fit (sE) were much larger than the errors in the measurements, and visual inspection showed a significant lack of fit to the NK model. A histogram of the sE values is shown in Fig. , with a mean error of 0.100 (k units). This indicates that the empirical NK model is not valid in some cases, especially when fitting data over a broad range of mobile phase compositions. Some examples of this lack of fit are shown in Figs. . Figure shows the fit for toluene on the 300 Å StableBond C3