A reaction vessel for the extraction of an analyte in a fluid, said reaction vessel having: i. a cylindrical body including a fluid containment space defined by substantially vertical walls, the cylindrical body including a conical thinned bottom portion; ii. an opening at the top of said vertical walls; and iii. an elastic valve secured to the vertical walls to define the bottom of said containment space, the elastic valve including a conical thinned rim around its upper portion and a shoulder beneath the conical thinned rim, the conical thinned bottom portion of the cylindrical body is received in the conical thinned rim such that a bottom of said vertical walls abut said shoulder, said elastic valve being concave and semispherical at the center of the containment space, said elastic valve having a slit positioned at a lowermost portion of the bottom of the containment space and being through the bottom of the concave center portion of the elastic valve, said slit having a closed condition blocking fluid from passing through said slit and an open position defining a drain opening allowing fluid to pass through said slit, and said elastic valve adapted to support a quantity of fluid in said containment space of said reaction vessel with said slit in said closed condition, and be elastically deformed and drawn down to force said slit in said open position responsive to a selected differential between a first pressure at the top opening and a second pressure beneath the elastic valve, wherein the first pressure is greater than the second pressure, wherein said elastic valve is constructed of a material possessing elastomeric properties such that said slit is normally in said closed condition, said lowermost portion of said bottom of said containment space flexing to open said slit to said open position upon application of said selected differential and said slit closing after the selected differential ceases. i. a cylindrical body including a fluid containment space defined by substantially vertical walls, the cylindrical body including a conical thinned bottom portion; ii. an opening at the top of said vertical walls; and iii. an elastic valve secured to the vertical walls to define the bottom of said containment space, the elastic valve including a conical thinned rim around its upper portion and a shoulder beneath the conical thinned rim, the conical thinned bottom portion of the cylindrical body is received in the conical thinned rim such that a bottom of said vertical walls abut said shoulder, support a quantity of fluid in said containment space of said reaction vessel with said slit in said closed condition, and be elastically deformed and drawn down to force said slit in said open position responsive to a selected differential between a first pressure at the top opening and a second pressure beneath the elastic valve, wherein the first pressure is greater than the second pressure, wherein said elastic valve is constructed of a material possessing elastomeric properties such that said slit is normally in said closed condition, said lowermost portion of said bottom of said containment space flexing to open said slit to said open position upon application of said selected differential and said slit closing after the selected differential ceases.