The paper deals with approximate analysis of mass transfer problem in thermodiffusion column to create theoretical basis for determination the Soret coefficient from unsteady separation measurements. The problem consists of quasi-steady one-dimensional equation of convection and unsteady mass transfer equation. The obtained expressions are used to evaluate the thermodiffusion coefficient of ferrite nanoparticles in hydrocarbon based colloids. Positive Soret coefficients of surfacted particles are found. In the presence of an external magnetic field the theoretically predicted anisotropy of Soret effect is established.