An assembly, comprising: a reaction vessel constituted by a large upper compartment with a funnel shape, an elongate lower compartment with a substantially constant cross-section (5, 6) and a closed base, wherein the upper compartment comprises a first part having a constant cross section and a first funnel-shaped part, wherein the size of a cross section of the first part is larger than the size of a cross section of the elongate lower compartment (5, 6), wherein the longitudinal axis of the first part is parallel to but spaced apart from the longitudinal axis of the lower compartment (5, 6), wherein the lower compartment is constituted by one medium compartment connected on one side to the first funnel-shaped part, and one bottom compartment, which is connected on one side to the medium compartment and on another side to said closed base; and a device for extracting possible molecules of interest from a fluid to which is added a suspension of magnetic or paramagnetic particles contained in the reaction vessel and capable to bind the molecules of interest comprising: at least one separating station to capture the magnetic particles present in the large upper compartment of the reaction vessel, at least one washing station to mix said magnetic particles present in the medium compartment of the reaction vessel, at least one concentrating station to mix said magnetic particles present in the bottom compartment of the reaction vessel, and at least one pipetting means to dispense and/or to remove part or all of the fluid, the washing liquid and output buffer. a reaction vessel constituted by a large upper compartment with a funnel shape, an elongate lower compartment with a substantially constant cross-section (5, 6) and a closed base, wherein the upper compartment comprises a first part having a constant cross section and a first funnel-shaped part, wherein the size of a cross section of the first part is larger than the size of a cross section of the elongate lower compartment (5, 6), wherein the longitudinal axis of the first part is parallel to but spaced apart from the longitudinal axis of the lower compartment (5, 6), wherein the lower compartment is constituted by one medium compartment connected on one side to the first funnel-shaped part, and one bottom compartment, which is connected on one side to the medium compartment and on another side to said closed base; and a device for extracting possible molecules of interest from a fluid to which is added a suspension of magnetic or paramagnetic particles contained in the reaction vessel and capable to bind the molecules of interest comprising: at least one separating station to capture the magnetic particles present in the large upper compartment of the reaction vessel, at least one washing station to mix said magnetic particles present in the medium compartment of the reaction vessel, at least one concentrating station to mix said magnetic particles present in the bottom compartment of the reaction vessel, and at least one pipetting means to dispense and/or to remove part or all of the fluid, the washing liquid and output buffer. at least one separating station to capture the magnetic particles present in the large upper compartment of the reaction vessel, at least one washing station to mix said magnetic particles present in the medium compartment of the reaction vessel, at least one concentrating station to mix said magnetic particles present in the bottom compartment of the reaction vessel, and at least one pipetting means to dispense and/or to remove part or all of the fluid, the washing liquid and output buffer.