Following his arrest after a valid buy-bust operation, Tommy was convicted of violation of Section 5, Republic Act 9165. On appeal, Tommy questioned the admissibility of the evidence because the police officers who conducted the buy-bust operation failed to observe the requisite "chain of custody" of the evidence confiscated and/or seized from him. What is the "chain of custody" requirement in drug offenses? What is its rationale? What is the effect of failure to observe the requirement? (3%)