text: Pumps used in high-pressure chromatography such as HPLC and ion chromatography are much like small piston metering pumps.  For wear resistance and chemical resistance to solvents, etc., typically the pistons are made of artificial sapphire and the ball check valves have ruby balls and sapphire seats.  To produce good chromatograms, it is desirable to have a pumping flow rate as constant as possible.  Either a single piston pump with a quick refill is used or a double pump head with coordinated piston strokes is used to provide as constant a pumping rate as possible.