System and Method of Measuring Contaminants in a Substantially Translucent Material, Such as Water

A system for sensing analyte in at least partly translucent material, including one or more radiation sources configured for successively providing radiation at a first and a second wavelength, respectively, two or more waveguides for simultaneously transmitting the radiation at each wavelength provided by the radiation source, a first waveguide being a reference waveguide and a second being a sensing waveguide; and measuring means for measuring a phase difference between the radiation waves from the reference waveguide and the measuring waveguide, resp. The present method can be used for measuring contaminants such as Fe, Sn, and/or Pb in oil related products, such as carburant or lubricant.