The invention comprises a vessel for solid phase micro-extraction chemical sampling. The vessel includes a container having an open end and a cap used to close the open end. The cap has a holder to hold a solid phase coated fiber within an interior of the container. There also is an inlet at a first location of the vessel and an outlet at a second location of the vessel. The inlet and the outlet are adapted to fill the vessel with a material to be exposed to the solid phase coated fiber. The vessel also includes a diffuser plate attached to the container. The plate is adapted to reduce fluid forces exerted on the solid phase coated fiber as material flows from the inlet to the outlet. The purpose of the diffuser plate is both to assure that forces are reduced and to provide laminar flow to assure rapid and complete exchange of the fluid in the vessel.