An on-line interferometer for the XL-A ultracentrifuge

Abstract

As a result of the renewed interest in analytical ultracentrifugation, the Beckman XL-A was released recently. This instrument automates spectrophotometric measurements of the concentration distribution of molecules in a gravitational field. Presented here is the design and the performance characteristics of a Rayleigh interferometer for the XL-A ultracentrifuge. The interferometer consists of a laser diode light source, imaging optics providing a 1.4-fold magnification of the cell, and a solid-state television camera detector. The source and detector are interfaced to a personal computer which synchronizes data acquisition for up to four cells, performs data reduction, and allows data analysis. About 1700 dat points, at a radial spacing of ∼ 9 µm, are acquired over a standard double sector cell image. Data are then stored on the disk and presented as a graph on the screen. The complete sequence of data acquisition, storage, and presentation requires about 15 s. The interformeter has a precision of about ± 0.003 fringe, and can be used for both equilibrium and velocity sedimentation. The design can be adapted for Schlieren detection.