A new colorimetric method is presented that allows for simple and effective discrimination between (iso)quinoline nuclei pertinent to many alkaloid species. The method is based on the reversible incorporation of Zn²⁺ ions into a bis-salphen chromophore under aqueous conditions. The metalation process is a function of the stability of the Zn-N (acceptor-donor) interaction and is accompanied by a clear colorimetric response.