We then elected to characterize the overall retentive behavior of the given solute on the given column, as weak, case 1 (log10 k0.3 < t1), moderate, case 2 (t1 < log10 k0.3 < t2) or strong, case 3 (log10 k0.3 > t2), where t1 and t2 are thresholds chosen empirically based on the present dataset; these may or may not be appropriate for other datasets, but are certainly a reasonable starting point. Based on this classification, we run a pair of gradients with a tG = 4tM, as outlined in the flow chart at the top of Figure , selected from four gradients spanning the full organic range, i.e., Gradient 1, 5-35 %; Gradient 2, 25-55 %; Gradient 3, 45-75 %;