FIGURE 4.1 - Desalting a DNA solution. A Millipore 0.025 µm filter is floated shiny-side up on 10% PEG in a Petri dish. Using a pipettor with a large-orifice pipet tip, the DNA sample is placed in the center of the filter. Salts in the DNA solution diffuse through the filter into the PEG solution. Additionally, the volume of the DNA solution decreases as water moves by osmosis across the semi-permeable membrane. After about 90 minutes, a pipet is used to remove the now desalted DNA solution from the filter.