The maximum headspace permissible in a VOA vial is not to exceed 5 -6mm or 1/4” - roughly the size of a pea. Sample Containers . Three (3) VOA vials are required for all volatile samples collected: 40mL Sodium Thiosulfate Preserved Glass VOA Vial – Trihalomethanes only. 40mL Hydrochloric Acid Preserved Glass VOA Vial – Full List VOCs