Improvement in precision of sedimentation-equilibrium experiments with an on-line absorption scanner

An on-line computerized optical absorption scanner for the analytical ultracentrifuge has been developed and tested. It makes direct use of the instantaneous photomultiplier output, which is digitized and averaged by the computer. It incorporates a stepping motor to drive the photomultiplier, two analog-to-digital converters, and a device which monitors precessional movement of the rotor. The scanner has been employed chiefly in sedimentation equilibrium experiments, and techniques have been devised to correct for the adverse effects of inhomogeneities of apparent absorbance which arise from the cell windows. Correction for rotor movements which occur during the scan is also possible. The completed apparatus is a successful one, and provides a significant improvement in precision, speed and convenience over the commercially available scanners. The limitations on precision inherent in the present design are examined, and it is concluded that the combined effects of window inhomogeneities and rotor movement lead to uncertainty of the measured absorbance which is unlikely to be reduced below +/- 0.001 A.