Centrifuged density gradient columns are often analyzed by pumping or dripping the contents through a flow cell to measure and record the absorbance as a function of depth (4, 7). The absorbance is usually measured at 253.6 nq.~, since light of this wavelength is easily obtained from a low-pressure mercury vapor lamp. Density gradient columns are not destroyed by flow through a channel, provided that the path does not force a density inversion, The path must be continuously upward if the gradient is pumped out of the top of the tube, and downward if the gradient is removed from the bottom of the tube. In a smooth tube gradient columns may be repeatedly pumped up and down with little effect on the gradient or a virus zone (5).
We report here on a flow cell with a reservoir on top for collection of the centrifuged gradient column after it has been pumped through the flow cell. The flow cell fits in the sample chamber of a recording spectrophotometer. The gradient column can be pumped up and down through the flow cell several times while the absorbance is recorded at a different wavelength each time.
Methods
Viruses. Tobacco mosaic (TMV), brome mosaic (BMV) , and southern bean mosaic (SBMV) viruses were partly purified by differential centrifugation of extracts of frozen, infected leaves of tobacco (Nicotiana