The current invention provides an improved immunoassay for the detection and determination of pyrrolidinophenone based designer drugs in hair and biological fluids (urine, blood, and oral fluid). The generic immunoassay is underpinned by novel, sub-family-specific antibodies, which display surprising sensitivity. The invention further describes substrates comprising an antibody that is specific to compounds of the pyrrolidinophenone family. Also described are the novel immunogens from which the antibodies are derived and kits incorporating the antibodies of the current invention.