Band broadening along gradient reversed phase columns: A potential gain in resolution factor

The advantages of using columns packed with stationary phases having a composition gradient so that retention factors increase toward the outlet and eluting them with an isocratic mobile phase may provide increased peak resolution in liquid chromatography. This approach is discussed from a theoretical viewpoint. The retention factor is assumed to increase linearly along the column. The peak width can be predicted under three different conditions: (1) the possible band compression is neglected (the Giddings model); (2) band compression is taken into account (the Poppe model); and (3) both band compression and extra-column effects are considered in the calculations. The impact of a stationary phase gradient on the resolution of a pair of compound difficult to separate (constant selectivity α = 1.05) is illustrated for a 3 mm × 100 mm silica monolithic column of the second generation (porosity ϵt = 0.85, plate height H = 6 μm) operated with a new generation of liquid chromatograph (pre- and post-column volume variances of 10 and 5 μL2, respectively). The results show that the resolution factor RS becomes optimum for a specific positive value of the retention gradient along the column. This optimum depends on the retention factor at the column inlet.