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

An apparatus for determining the concentration of at least one substance in a liquid, the apparatus comprising:

• a fixture configured to hold a test strip containing at least one test zone;
• at least two light sources providing light of different wave lengths for irradiating the test strip surface, the two light sources being controllable in such a manner that the test strip surface is irradiated alternately with light from each of the light sources;
• an optical sensor arrangement being moveable step-by-step in a first direction relative to the surface of the fixture for detecting the light reflected from the test strip surface by each of the light sources to provide a measurement signal for each of the light sources;
• and
• an analyzing unit connected to the optical sensor arrangement for analyzing the measurement signals provided by the optical sensor arrangement by each of the light sources, the analyzing unit comparing the measurement signals to each other and providing a difference curve.