The D-500 is controlled by a PDP-8 computer which controls an electrical pump with a piston made of synthetic ruby to facilitate a very precise flow of hydraulic fluid to drive a mechanical ram assembly - this assembly has eight syringes, four on each side, each filled with a different pH liquid buffer. The buffer solutions were then washed across the sample holder and into another resin bead filled tube which had an inside diameter of the order of fifty micrometres (about the diameter of a human hair).