Method for determining the anionic charge density of at least one polymer present in a sample, using the following steps: - bringing the at least one polymer present in the sample into contact with, and enabling its interaction with, a developer solution comprising lanthanide ions, - exciting the sample at an excitation wavelength of λ exc and detecting, by time-resolved photoluminescence, a signal originating from the lanthanide ions that have interacted with the at least one polymer at an emission wavelength λ em, and - determining the anionic charge density of the at least one polymer in the sample using the signal detected at the emission wavelength λ em according to the following protocol: 1) different sample series are prepared by successively diluting various stock solutions of polymers of known anionicity with water, the samples from each series are then diluted with a lanthanide developer solution and analyzed by Time-Resolved Fluorescence; the measurement parameters as well as the emission and excitation wavelengths are adjusted according to the type of lanthanide, 2) for each series, the slopes of the TRF Signal Intensity vs. Dilution Rate curves are extrapolated and a Slope vs. Anionicity calibration curve is developed, 3) a new series of samples is then prepared by successively diluting a sample X of unknown anionicity, after TRF measurement of these samples, the slope of the TRF Signal Intensity vs Dilution Rate curve is extrapolated, 4) after the extrapolated slope in 3) is correlated with the anionicity calibration line developed in, the previously unknown anionicity of the sample X is deduced. - bringing the at least one polymer present in the sample into contact with, and enabling its interaction with, a developer solution comprising lanthanide ions, - exciting the sample at an excitation wavelength of λ exc and detecting, by time-resolved photoluminescence, a signal originating from the lanthanide ions that have interacted with the at least one polymer at an emission wavelength λ em, and - determining the anionic charge density of the at least one polymer in the sample using the signal detected at the emission wavelength λ em according to the following protocol: 1) different sample series are prepared by successively diluting various stock solutions of polymers of known anionicity with water, the samples from each series are then diluted with a lanthanide developer solution and analyzed by Time-Resolved Fluorescence; the measurement parameters as well as the emission and excitation wavelengths are adjusted according to the type of lanthanide, 2) for each series, the slopes of the TRF Signal Intensity vs. Dilution Rate curves are extrapolated and a Slope vs. Anionicity calibration curve is developed, 3) a new series of samples is then prepared by successively diluting a sample X of unknown anionicity, after TRF measurement of these samples, the slope of the TRF Signal Intensity vs Dilution Rate curve is extrapolated, 4) after the extrapolated slope in 3) is correlated with the anionicity calibration line developed in, the previously unknown anionicity of the sample X is deduced. 1) different sample series are prepared by successively diluting various stock solutions of polymers of known anionicity with water, the samples from each series are then diluted with a lanthanide developer solution and analyzed by Time-Resolved Fluorescence; the measurement parameters as well as the emission and excitation wavelengths are adjusted according to the type of lanthanide, 2) for each series, the slopes of the TRF Signal Intensity vs. Dilution Rate curves are extrapolated and a Slope vs. Anionicity calibration curve is developed, 3) a new series of samples is then prepared by successively diluting a sample X of unknown anionicity, after TRF measurement of these samples, the slope of the TRF Signal Intensity vs Dilution Rate curve is extrapolated, 4) after the extrapolated slope in 3) is correlated with the anionicity calibration line developed in, the previously unknown anionicity of the sample X is deduced.