Method for quantitatively determining a chemical parameter of a sample

In a method for the quantitative determination of at least one chemical parameter of a sample medium, in which two substances in close spatial contact are used, as a first substance a on the parameter to be determined not responsive fluorophore with an excitation and an emission spectrum and as a second substance on The substance of the sample medium to be determined by changing its absorption spectrum reacting substance, wherein the emission spectrum of the fluorophore overlaps at least partially with the absorption spectrum of the second substance is proposed to improve the long-term stability and to avoid frequent calibrations, that as the second substance influencing the fluorescence of the fluorophore Chromophore is used,whose absorption maximum is shifted depending on the parameter to be determined and that the reduction of the fluorescence decay time t caused by energy transfer between the chromophore and the fluorophore is used for the quantitative determination of the chemical parameter.