In particular, operating the orifice 43 in the critical flow regime allows the downstream pressure P2 to be adjusted independently of the flow F. As will be appreciated by those skilled in the art, P2 the pressure immediately above the SAW resonator 52 is related to the average residence time of a molecule in the chamber enclosing the SAW resonator 52: ##EQU3## where r is a unit of time, k is the