Experimental evidence of the influence of the surface chemistry of the packing material on the column pressure drop in reverse-phase liquid chromatography

The permeabilities of six columns packed with different packing materials (neat silica, C1 endcapped silica at 3.92 μ mol/m2, C18 bonded and endcapped silica with 0.42, 1.01, 2.03, and 3.15 μ mol/m2 of C18 bonded chains) were measured. All these materials derive from the same batch of spherical particles, 5 μ m in diameter. The columns have the same tube inner diameter (ϕ=0.460±0.003ϕ=0.460±0.003  cm) and length (L=15.000±0.003L=15.000±0.003  cm). The experimental conditions were the same, flow-rate (Fv=1.000±0.003Fv=1.000±0.003  mL/min) and temperature (295 K). Nevertheless, it was found that the column permeability decreases significantly, by about 25%, from the neat silica column to the one packed with the highest density of C18-bonded silica (3.15 μ mol/m2). The results measured on two duplicate columns were very reproducible. Accurate (±0.5±0.5 %) measurements of the hold-volumes with concentrated and dilute solutions of NO3− showed that the columns had all nearly the same external porosity. The result cannot be explained by the error made on the volume of the column tube either as it was measured accurately for all the columns. The residual explanation is that the interstitial velocity distribution between the packed particles depends on the chemical nature of the external surface of these particles.