A method is provided for disposing of a liquid within an automated analytical system for processing a fluid biological sample, wherein a contamination prevention shield including channels for pipets or pipet tips is reversibly docked to a container for liquid waste in order to reduce the risk of cross-contamination. A tip rack assembly is provided for processing a fluid sample, the assembly including a rack for pipet tips and the contamination prevention shield. An analytical system is provided using the tip rack assembly, wherein cross-contamination is avoided.
