Abstract: Disclosed is apparatus for distributing a solid, gel, powder or viscous test substance in an extraction fluid, the apparatus comprising a flow cell for holding the test substance and having an inlet and an outlet; an extraction chamber located between said inlet and outlet comprising a convergent nozzle; and a recirculating pump for driving extraction fluid: (i) into the flow cell via the inlet; (ii) through the extraction chamber; and (iii) back to the flow cell via the outlet, whereby a pressure differential is established across the extraction chamber such that the velocity of the extraction fluid is greater at the outlet than at the inlet; characterized in that the apparatus further comprises a tortuous path flow control valve (TPV) located at the outlet and configured to permit flow of extraction fluid and extracted test substance but to prevent or retard passage of said test substance through the outlet.