Apparatus for exposing a chemical, biological or biochemical sample to radiation comprises a sample passage (42) for conveying a sample in liquid phase along a sample path and at least one generator or source (48) for generating electromagnetic radiation and directing it at said sample path. The apparatus further includes at least one detector (46) for detecting at least one of reflected emitted and transmitted radiation from at least one point along said sample path and a controller (58) for controlling at least one of said generator or source and said detector. There is also a device (514) for dividing the liquid sample into two or more segments which pass in sequence along said sample path (804) in use.