A cartridge for dispensing a fluid is presented. The cartridge comprises a reservoir chamber for receiving the fluid and for receiving a ventilation gas. The reservoir chamber comprises an inlet for receiving the ventilation gas and an outlet for dispensing the fluid. At least a portion of the reservoir chamber is filled with the ventilation gas when in an operating position. The inlet is located in the portion being filled with the ventilation gas. The fluid comprises a reagent. The cartridge further comprises a baffle for restricting gas diffusion through the inlet. The reservoir chamber receives the ventilation gas via the baffle. The inlet is maintains a constant gas pressure within the portion of the reservoir chamber that is being filled with the ventilation gas.