A pre-heater assembly for pre-heating a fluid, in particular in a fluid separation apparatus, wherein the pre-heater assembly comprises a capillary having a lumen and being configured for conducting the fluid, and a thermal coupling body contacting at least part of the capillary, having a value of 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 is supplied to the capillary via at least part of the thermal coupling body.