External mass transfer in high performance liquid chromatography systems

External mass transfer in a HPLC system operated in the reversed-phase mode was studied by pulse response experiments, using a column packed with non-porous C18-silica gel spherical particles, 18 μm in diameter. The first and second moments of the elution peaks, measured under different flow velocities and temperatures, were analyzed by the moment method to determine the external mass transfer coefficient (kf). The dependence of the Sherwood number on the Reynolds and the Schmidt numbers is almost the same as that observed in previous investigations of conventional literature correlations. The exponent of the last two nondimensional parameters was derived as being in the range from 0.28 to 0.41. When the Kataoka equation is used, the mean square deviation was calculated to be 0.21 for the values of kf estimated in this study. It is concluded that conventional correlations can be used to estimate kf values, even when the particle diameter is of the order of a micrometer.