Methods and related apparatuses and mixtures are described for spectroscopic detection of hydrogen sulfide in a fluid, for example a formation fluid downhole. A reagent mixture is combined with the fluid. The reagent mixture includes metal ions for reacting with hydrogen sulfide forming a metal sulfide, and a capping agent that limits growth of the insoluble metal sulfide species by electrosteric or steric stabilization. The particle growth is one of chemical reaction or significant aggregation, and the capping agent further functionalizes the reagent mixture to exhibit properties outside the natural characteristics of the metal sulfide species to allow for spectroscopic detection of the metal sulfide species. The combined mixture and fluid is then spectroscopically interrogated to detect the presence of the metal sulfide thereby indicating the presence of hydrogen sulfide in the fluid. The mixture also includes chelating ligands for sustaining thermal endurance of the mixture under downhole conditions.