The invention relates to a method for analysing a sample comprising at least a first and a second scale inhibitor, which scale inhibitors are synthetic organic compounds comprising at least one ionised group. The method comprises optionally diluting and/or purifying the sample, and allowing the sample interact with a reagent comprising lanthanide(III) ion. The sample is excited at a first excitation wavelength and a sample signal deriving from the lanthanide(III) ion is detected at a signal wavelength by using time-resolved luminescence measurement. The total concentration of the first and the second scale inhibitor is determined by using the detected sample signal, and the concentration of the first scale inhibitor in the sample is determined. The concentration of the second scale inhibitor is determined mathematically by using the obtained results for the total concentration and for the first scale inhibitor concentration.