Abstract: This invention relates to a spin filter assembly and its usage to facilitate the isolation and analyses of molecules and compounds such as proteins present in various biological and non-biological solutions. The spin filter assembly of this invention comprise of a tube having an open top end, a bottom end opposite the open top end, a body of a first length, an inner diameter and a first outer diameter, a filter means at the bottom end, the tube insertable to an upper portion of a centrifuge holder having a closed lower end, an open upper end, a length longer than the first length of the tube, and a second inner diameter greater than the first outer diameter of the tube; and, a container closure means that fits on top of the open end of the tube. The spin filter assembly can be a component of a sample preparation or assay kit, for example, a kit for analyzing proteins by immunoprecipitation. A process for analyzing proteins by immunoprecipitation using the spin filter assembly is described.