Mass transfer resistance in narrow-bore columns packed with 1.7 μm particles in very high pressure liquid chromatography

Surprisingly, the mass transfer kinetic properties of columns packed with superficially porous particles are markedly different from those of columns packed with fully porous particles. The performances of 2.1 mm ×× 150 mm columns packed with a new type of sub-2 μm particles, the superficially porous 1.7 μm Kinetex-C18, and with the classical 1.7 μm BEH-C18 fully porous particles were measured and are discussed. The sample was naphtho[2,3-a]pyrene; the use of different mobile phase compositions allowed a comparison between data measured with retention factors of k′≃2k′≃2 and k′≃20k′≃20. The minimum reduced height equivalent to a theoretical plate (HETP) of the two columns were similar, at hmin=2.0hmin=2.0. However, this minimum HETP was observed at a markedly shorter reduced linear velocity for the column packed with totally porous particles, between 5 and 7 for BEH, than for the one packed with shell particles, between 8 and 10 for Kinetex. This result is explained by the combination of (1) a 35% smaller B term for the Kinetex column than for the BEH column, due to the 37% lower porous volume of the former; (2) a larger reduced A   term for the Kinetex column (1.6), showing a relatively poorly packed column with significant trans-column velocity biases than for the BEH column (ca. 1.0); and (3) a much lesser dependance of the efficiency on the mobile phase velocity at high velocities for the Kinetex than for the BEH column, when these columns are placed in the oven of the instrument under still-air conditions. The heat friction affects significantly more the efficiency of the BEH column than that of the Kinetex column. This unexpected result is accounted for by the three times smaller heat conductivity of the BEH bed (λBEH≃λBEH≃ 0.25 W/m/K) than that of the Kinetex bed (λKinetex≃λKinetex≃ 0.75 W/m/K).