there is disclosed a sample preparation chamber for a system for preparing samples of various compositions for assay by liquid chromatography . the sample preparation chamber is a container having a threaded cap and a threaded , lightweight , translucent plastic cup . a stirred / grinder shaft driven by a motor and connected to a propeller / grinder passes through the cap . the cap has a sloped bottom with a sump region , and a fill / empty pipe passes through the cap and has its outlet at or near the sump . a nozzle fill pipe arrangement allows the walls to be washed down as liquid is pumped into the cup . a second fill pipe with its outlet spaced up from the bottom of the cup is also used , and a sample metering valve having an inlet in said cup is present . the sample metering valve is used to isolate a known volume of the sample from the rest of the sample for release back into the cup after the remaining sample has been pumped to waste for purposes of diluting the known volume of sample to a known concentration .