Heat sinks for use in assemblies for preheating liquid samples prior to their introduction into an analytical stream. The heater assemblies have a heat sink having a metallic heater block and a heating element for heating the block. The block has a basin for receiving a liquid sample and allowing heat to be transmitted from the block to the sample. A drain at the lower end of the basin allows sample in the basin to flow out of the basin. A passageway extends through the block from the drain to an exterior surface of the block and provides a conduit for sample flowing from the drain, A temperature sensor is mounted in a cavity in the block and extends into the basin to measure the temperature of sample in the basin. A temperature sensor is mounted in a cavity in the block and measures the temperature of the block.