The photoelectric scanning attachment to the analytical unltracentrifuge offers several advantages over methods that depend on changes in refractive index (1). Because the technique does not depend on bending of the light path, data are less affected by Weiner skewing or window distortion at high speeds. The sample can be illuminated with light of different wavelengths and, since the extinction coefficient changes with wavelength, samples can be observed over a wide concentration range. In some cases sedimentation velocity can even be measured at catalytic concentrations by sedimenting the enzyme through an assay mixture and observing substrate disappearance or product formation (2,3). Finally, results are obtained immediately, on a recorder, without the necessity of developing photographic plates.
Despite these advantages, interference and schlieren methods are still the most commonly used, principally because the scanner is less precise than direct optical methods. Even under ideal conditions very little light gets through the cell (a 21/O sector passes only 0.7% of the light shining on it) and this amount of light is further reduced by the photomultiplier entrance slit, which must be kept small for good resolution. The result of these two restrictions is that, under less than ideal conditions, the recorded signal is quite noisy and difficult to read. Complications also arise because the cell image is not recorded directly, as it is in photographic methods. Instead, there is considerable mechanical and electrical interfacing between the photomultiplier and the final chart paper and each link is a possible source of error. Photographic methods also have the advantage that they record the entire cell simultaneously while the scanner records different parts of the cell at different times-as a result, rotor precession gives fuzzy images on photographic plates but in scanner operation it makes radial position uncertain.
In an attempt to combine the advantages of scanner operation with increased precision, a stepping motor has been attached to the ultracentrifuge in such a way that the photomultiplier can be made to scan