The preparative dynamic density gradient method

In this work the development of a new ultracentrifugation method is described. The preparative density gradient method (PDDG) allows the fractionation of colloidal systems according to the density of their components. The fractions—collected separately—can be subjected to further investigations with any appropriate physical or analytical method. The particle density of a colloidal system is a very valuable information that can be used in many ways. Here for the first time a method is presented that could make the access to that information much easier for laboratories throughout the world. The preparative dynamic density gradient is performed in a preparative ultracentrifugation tube by layering heavy water under a sample that is dispersed or dissolved in water. The water and heavy water molecules undergo a fast diffusion process finally leading to a homogeneous solvent mixture. While reaching this equilibrium the water–heavy water concentration varies over the cell radius. Thus a density gradient is built up. Fractions of the sample will move to the radial position where their densities are matched. They can then be collected separately. In this work basic experiments and results that lead to the development of the PDDG will be presented. Calculations based on basic ultracentrifugation theory confirm the described experimental findings. An example will demonstrate the problem solving capability of this new method.