Free PNP < 100ppm
For assay of acid and alkaline phosphatase: O.H. Lowry, Meth. Enzymol., IV, 371 (1957); J-P. Bretaudiere and T. Spillman in "Methods of Enzymatic Analysis" 3rd Ed., vol. 4, 76 (1984) (H.U. Bergemeyer, ed), Verlag Chemie, Weinheim.