Abstract: A method for detecting a first chelator (shown in the figure as dipicolinic acid DPA (1), the method comprising the steps of providing a reagent (2) comprising a second chelator (shown as a luminescent dye (3) and a metal ion (4), contacting the reagent (2) with a sample (6) containing the DPA (1), exciting a luminescence (7) of the dye (3), and detecting the luminescence (7) emitted by the dye (3), the method being characterized in that the metal ion (4) is bound to the luminescent dye (3) within the reagent (2), the luminescence (7) of the dye (3) is altered by the metal ion (4), the metal ion (4) is capable of binding to the DPA (1), and the dye (3) and the metal ion (4) are such that the DPA (1) can compete for the metal ion (4) in competition with the dye (3), thereby re-establishing the luminescence (7) of the dye (3).