The invention relates to detecting a composition of a sample or contamination in liquids by detecting corresponding changes in their thermal properties. In a disclosed arrangement, an apparatus is provided comprising a first probe element configured to provide a first surface in direct contact with the sample and a second surface that is not in direct contact with the sample. A measurement system measures a rate of heat transfer through the first surface. A processing unit analyses the measured rate of heat transfer in order to detect a heat transfer characteristic of the sample that is indicative of a composition of the sample.