Unusual behavior of the height equivalent to a theoretical plate of a new poroshell stationary phase at high temperatures

Abstract

The reduced HETPs of naphtho[2,3-a]pyrene (a polycyclic aromatic hydrocarbon with six condensed phenyl rings) was measured on three different RP-C 18columns packed with 5 and 3 μ m totally porous silica-B particles, and with 2.7 μ m Halo shell particles. The measurements were made at temperatures between ambient and 323 K, the retention factor being kept constant by modifying the eluent composition (water/acetonitrile mixtures). This compound was chosen because (1) it is strongly retained on RP-C 18 stationary phases and is eluted only by acetonitrile-rich mobile phases, which have a low viscosity ( ≃ 0.35 cP). As a result, the C-term of the HETP plots was measured very precisely; (2) this compound does not interact strongly with the bare silica surface. Its adsorption isotherm remains linear up to high concentrations, its UV-absorbance (at λ = 294 nm) is high, and its HETP can be measured accurately and precisely (relative error of 2%). The experimental data were fitted to a general HETP model derived recently. The B term was measured independently, using the parking method to validate the fit in the low velocity range. The kinetics parameters derived from this model allow a qualitative comparison of the performances of the three columns. The column packed with the C 18-Halo particles exhibit an unusual rate of HETP increase at high linear velocities, particularly at elevated temperatures (310 and 323 K). A comparison of the HETP data measured for naphtho[2,3-a]pyrene and for bovine serum albumin (BSA), a large protein that is excluded from the mesopore network of all three columns, demonstrates that this behavior is related not to the mass transfer kinetics in the stationary phase but to some unexpected variation of the eddy dispersion with the linear velocity at high temperatures. The coupling theory of eddy dispersion proposed by Giddings fails to describe the data obtained for the Halo column while they predict well those measured on the totally porous materials. The explanation does not reside in the width of the particle size distribution (5% for Halo and ≃ 15 % for the silica-B particles) but more likely in the roughness of the external surface of the Halo particles.