A pre-heater assembly (90) for pre-heating a fluid, in particular in a fluid separation apparatus (10), wherein the pre-heater assembly (90) comprises a capillary (200) having a lumen and being configured for conducting the fluid, and a thermal coupling body (202) contacting at least part of the capillary (200), having a value of the thermal conductivity in a range between 8 W/(m K) and 100 W/(m K) and being arrangable so that heat generated by a heat source (80) is supplied to the capillary (200) via at least part of the thermal coupling body (202).