Tetrabromoethane, with a freezing point almost 8 degrees C. below that of bromoform, gives greater assurance that the frozen plug with the contained heavy minerals will not loosen and come out of the tube when the light fraction is removed. An additional refinement is to use a tapered rubber stopper that has a glass rod fitted in its top and place this in a tapered centrifuge tube scaling the frozen heavy liquid beneath. The tube can then be washed of any light fraction that may have adhered to the side.

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