A method for validating the functioning of an apparatus for producing nanoparticles, said apparatus comprising a static mixing device for mixing two liquid streams, said static mixing device comprising a first fluid inlet, a second fluid inlet, a mixing zone and a fluid outlet, said method comprising the steps of: (a) providing a first specimen of said apparatus; (b) providing a first stream of a first liquid composition, wherein the first liquid composition comprises an organic solution of a polymer selected from PLA, PGA, PLGA and combinations thereof; (c) providing a second stream of a second liquid composition, wherein the second liquid composition is an aqueous solution; (d) simultaneously injecting the first stream at a first flow rate through the first fluid inlet, and the second stream at a second flow rate through the second fluid inlet into the static mixing device of a first specimen of the apparatus such as to allow the first liquid composition and the second liquid composition to mix in the mixing zone and to form polymer nanoparticles; (e) measuring the average particle size of the nanoparticles obtained in step (d); characterised in that the method comprises the additional steps of: (f) repeating steps (a) to (e) using essentially the same liquid compositions and the same flow rates but a second specimen of the apparatus; (g) determining whether the difference between the average particle size obtained when step (e) was repeated and the average particle size obtained when step (e) was performed for the first time is larger than a predefined tolerance value; and (h) if said difference is not larger than the predefined tolerance value, using the second specimen of the apparatus for preparing nanoparticles by mixing two liquid streams that do not comprise the first liquid composition. (a) providing a first specimen of said apparatus; (b) providing a first stream of a first liquid composition, wherein the first liquid composition comprises an organic solution of a polymer selected from PLA, PGA, PLGA and combinations thereof; (c) providing a second stream of a second liquid composition, wherein the second liquid composition is an aqueous solution; (d) simultaneously injecting the first stream at a first flow rate through the first fluid inlet, and the second stream at a second flow rate through the second fluid inlet into the static mixing device of a first specimen of the apparatus such as to allow the first liquid composition and the second liquid composition to mix in the mixing zone and to form polymer nanoparticles; (e) measuring the average particle size of the nanoparticles obtained in step (d); (f) repeating steps (a) to (e) using essentially the same liquid compositions and the same flow rates but a second specimen of the apparatus; (g) determining whether the difference between the average particle size obtained when step (e) was repeated and the average particle size obtained when step (e) was performed for the first time is larger than a predefined tolerance value; and (h) if said difference is not larger than the predefined tolerance value, using the second specimen of the apparatus for preparing nanoparticles by mixing two liquid streams that do not comprise the first liquid composition. <EOS>