24mm Volatile Organic Chemical sampling vial white pp cap Volatile Organic Chemicals (VOCs) Sampling Guide. Sampling Procedures: 1. Obtain the proper sample containers for the VOC or EPA 524, 624, or 8260 method. MTA uses three 40 ml voa vials preserved with HCl. 2. Tap each vial in the upright position to drain the preservative from the cap.