Repeatability of the efficiency of columns packed with sub-3 μm core–shell particles: Part I. 2.6 μm Kinetex-C18 particles in 4.6 mm and 2.1 mm × 100 mm column formats

The column-to-column repeatability of the mass transfer mechanism in columns packed with sub-3 μm shell particles was investigated. The parameters of this mechanism were measured for twelve columns (six 2.1 mm × 100 mm and six 4.6 mm × 100 mm) packed with the same batch of 2.6 μm Kinetex-C18 particles (Phenomenex, CA, USA). For both series, the manufacturer provided columns at different positions in the efficiency distribution given by the quality test control. Three compounds were used, uracil, naphthalene and insulin. The reduced longitudinal diffusion term was measured with the peak parking (PP) method, the reduced solid–liquid mass transfer resistance term was given by a combination of the PP results and a model of effective diffusion in ternary composite materials (non-porous cores, concentric porous shell, and eluent matrix), validated previously. The overall eddy diffusion term was obtained by subtraction of these two HETP terms from the overall reduced HETP measured by numerical integration of the entire peak profiles. The results demonstrate that the dispersion of the column efficiencies is only due to the random nature of the packing process. At the highest reduced velocity achieved, the relative standard deviations (RSDs) of the eddy diffusion term for the 2.1 mm I.D. columns were ca. 7% and 3% for the low molecular weight compounds and for insulin, respectively. For the 4.6 mm I.D. columns, these RSDs were 15% and 5%, respectively. The larger RSDs for the 4.6 mm I.D. columns is explained by the exceptionally low value of the eddy diffusion term.