An arrangement for determining concentration of substances in a fluid comprising a light source (2) for generating primary light pulses within a wavelength interval, a light pulse splitter (5) adapted to split up the primary light pulses into a predetermined number of secondary light pulses to be transmitted through the fluid, the secondary light pulses being separated in time as well as wavelength to be differently absorbed upon passage of the fluid depending on the concentration of the substances, a detector (13) for detecting intensity of the differently absorbed secondary light pulses, and a comparator (14) for comparing the intensities of the differently absorbed secondary light pulses with different reference intensities corresponding to different substances to thereby determine the concentration of the substances in the fluid.