The rationale for the optimum efficiency of columns packed with new 1.9 μm fully porous Titan-C18 particles—A detailed investigation of the intra-particle diffusivity

• The rationale for the unusually low minimum HETP of Titan-C18 columns was investigated.
• HETP at low speed and peak parking data show that it is due to a small intra-particle diffusivity.
• The low analyte intra-particle diffusivity is mostly due to a low internal obstruction factor.
• The unusual obstruction factor is due to constriction effects along the pore diffusion path.
• These constriction effects may result from the large RSD of the pore size distribution of Titan.

In a previous report, it was reported that columns packed with fully porous 1.9 μm Titan-C18 particles provided a minimum reduced plate height as small as 1.7 for the most retained compound (n-octanophenone) under RPLC conditions. These particles are characterized by a relatively narrow size distribution with a relative standard deviation (RSD) of only 10%. A column packed with classical 5 μm Symmetry-C18 particles, used as a reference RPLC column, generated a minimum reduced plate height of 2.1 for the same retained compound. This work demonstrates that this was due to an unusually low intra-particle diffusivity across these particles, which leads to a small longitudinal diffusion coefficient along the column. The demonstration is based on the combination of accurate measurements of the height equivalent to a theoretical plate (HETP), inverse size exclusion chromatography (ISEC), peak parking (PP), and minor disturbance method (MDM) experiments.The experimental results show that the reduced eddy dispersion HETP term (A = 0.8 for a reduced velocity of 5), the internal particle porosity (ϵp = 0.35), and the enrichment of acetonitrile in the pore volume (75% acetonitrile in the bulk, 85% inside the mesoporous volume) are identical on both the Titan-C18 and Symmetry-C18 columns. The difference between the internal structures of these two brands of RPLC-C18 fully porous particles lies in the values of the internal obstruction factor γp, which is 0.42 for the Symmetry-C18 but only 0.26 for the Titan-C18 particles. This is in part related to the diffusion hindrance due to the small average pore size of the Titan-C18 particles, around 59 Å versus 77 Å for Symmetry-C18 particles. A simple model of constriction along diffusion paths having the shape of a truncated cone suggests that the width of the pore size distribution (RSD of 30% and 20% for Titan-C18 and Symmetry-C18 particles) is mostly responsible for the difference in their obstruction factors.