Visualization of bed compression in an axial compression liquid chromatography column

The consolidation of a packed bed undergoing axial compression was studied in glass columns using an on-column visualization process. In this visualization process the refractive indices of the mobile phase (carbon tetrachloride) and the stationary phase (YMC C ₁₈ silica) matched perfectly, hence the otherwise opaque stationary phase became transparent to the eye. Alumina layers, which have a different refractive index, were placed at regular intervals along the column bed. These layers were therefore visible and their movement could be tracked during the axial compression of the bed. Consequently, the Young's modulus could be measured at three radial locations and at four bed depths below the head fitting. The results showed that the bed was heterogeneous in packing density, both radially (with the bed density increasing from the column center toward the wall) and axially (with the density increasing from the column top toward its center). Furthermore, the bed was shown to be non-symmetrical about the column axis. This was thought to be due to the column inlet head fitting making contact with the packing material on one side of the column first, rather than making contact with the entire cross section of the packing simultaneously.