Stop Solution for AP Substrates (STOPP) is a dry powder for use with pNPP-based colorimetric substrates. To use, reconstitute with DiH2O. STOPP is compatible with our pNPP-based alkaline phosphatase substrate, pNPP 1-Component AP Microwell Substrate. This product is suitable for all endpoint ELISAs using an AP substrate for color development of an AP-conjugate.