Method and apparatus for determining a concentration of a substance in a liquid medium

An apparatus for determining a concentration of a substance in a liquid medium of a test sample, the apparatus comprising:

• a light source for directing a light beam of a first wavelength range towards the test sample;
• a first detector for collecting irradiation emitted from the liquid medium of the test sample as a result of the light beam directed towards the test sample, the irradiation being collected at one or more second wavelengths that are characteristic for the liquid medium, and for measuring an intensity of the irradiation collected at the one or more second wavelengths;
• a second detector for collecting irradiation emitted from the substance of the test sample as a result of the light beam directed towards the test sample, the irradiation being collected at one or more third wavelengths that are characteristic for the substance, and for measuring an intensity of the irradiation collected at the one or more third wavelengths,
• a determining unit for determining the concentration of the substance in the liquid medium of the test sample based on the measured intensity of the irradiation collected at the one or more third wavelengths and the measured intensity of the irradiation collected at the one or more second wavelengths, the apparatus arranged such that the irradiation collected by the first detector is emitted from a first surface area of the test sample and the irradiation collected by the second detector is emitted from a second surface area of the test sample, which first surface area and second surface area are at least partly covered by a surface area of the test sample illuminated by the light beam of the light source;
• and further comprising an indicator for indicating when the measured intensity of the irradiation collected at the one or more second wavelengths is below a certain value.