Abstract: A device for treating samples, for example collection of fractions from a chromatographic separation process, comprises a series of test containers (11), a treatment unit (5) and an apparatus (4) for creating relative motion between the test containers and the treatment unit. The treatment unit passes the test containers in sequence and subjects the same to one or more treatment steps. According to the invention the test tubes are detected and their position indicated by a sensor (9) which is arranged to trigger the treatment step or steps when contacting a given area on the respective test container (11), especially by the resilient biasing of a spring (13).