The invention allows determining a local change of an admixture concentration in a fluid flow at an entrance to a measurement cell. The change of the admixture concentration in time inside the measurement cell is first determined for a fluid containing the admixture, the change of concentration of which in time at the entrance to the measurement cell is known. Then, an impulse response of the cell is found applying the deconvolution method. The change of the admixture concentration inside the measurement cell is then determined for a fluid being studied with an unknown concentration of the admixture at the entrance. The unknown concentration is determined using the impulse response of the measurement cell and the change of the admixture concentration inside the cell.