Method for measuring the absorbance of a substance in a solution and a measuring device therefor

A measuring device for measuring the absorbance of a substance in a solution in a measuring cell of the measuring device, wherein said measuring device comprises:

• a light source transmitting a first light beam;
• a beam splitter provided such that the first light beam is divided by the beam splitter into a signal light ray and a reference light ray;
• the measuring cell positioned such that the signal light ray will pass through the measuring cell;
• a first signal modulation device arranged to modulate the signal light ray;
• and
• a second signal modulation device arranged to modulate the reference light ray;
• and
• a detector arranged to detect the signal light ray when it has been modulated and passed the measuring cell and also detect the reference light ray when it has been modulated, wherein the detector comprises or is connected to a processing device which is configured for performing synchronous detection of the detected signal in order to reconstruct the intensities of the signal light ray and the reference light ray from the combined signal detected by the detector, said synchronous detection being based on the modulation performed to the signal light ray and the reference light ray in order to enable measurement of absorbance changes over time in a flowing sample.