Abstract: The invention relates to a measuring apparatus comprising an apparatus for forming a liquid optical waveguide having a substrate (1) having an at least partially curved closed microchannel (2) having a low-refractive coating (13), whereby there is formed in the substrate (1) at least one feed line (6) for supplying liquid, and whereby there is provided at least at one end of the closed microchannel (2) an apparatus for coupling light axially into the closed microchannel and/or for coupling light axially out of the closed microchannel (2), further comprising a light source (4), a light detector (5), and a first liquid pump (9) which supplies a sample liquid (7) to the closed microchannel (2) via the at least one feed line (6, 6a).