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combination of dilution, heat, and high vacuum. After complet- ing the cleaning steps, 10% of the canisters are certified each day. Canisters are certified for approximately 60 VOCs using GUMS. The 10% certification process requires that target compound concentrations be below 0.2 ppbv using GUMS analysis. Alternatively, the 100% certification (i.e., individual certification) process is appropriate for ambient and indoor air h� applications driven by risk assessment or litigation that require pptv (parts per trillion by volume) sensitivity. Similar to the 10% uA certification, the 100% certification also begins with the canister cleaning process. The difference with the 100% certification is that canisters are individually certified for a client-specific list of. target compounds using GUMS. The 100% certified canisters are shipped with analytical documentation demonstrating that they are free of the target compounds down to the project �� reporting limits. When sampling with certified media it is important to note that all media is certified as a train and must be sampled as such (ie. a particular flow controller goes with a particular canister). b Specify whether your project requires 10% or 100% canister cleaning certification. 2.1.E Canister Hold Time Media Hold Time : Canister sampling differs considerably from collecting a water sample in a VOA vial or a soil sample in an amber jar in that the container(valued at over $450) is cleaned and reused. Air Toxics Ltd. requires that our canisters be returned within 14 days of receipt to effectively manage our inventory. Once a canister is cleaned, certified, and evacuated we recommend the canister be used. for sample collection within 30 days. Over time, low-level (pptv) concentrations of typical VOCs may off-gas from the canister surface resulting in potential artifacts in the sample results. Sample Hold Time : Although 30 days is the most commonly cited hold time for a canister sample, the hold time is compound specific. For example, compounds such as chloroform, benzene, and vinyl chloride are stable in a canister for at least 30 days. In fact, FPA Method TO-15 states: "Fortunately, under conditions of normal usage for sampling ambient air, most VOCs can be recovered from canisters near their original concentrations for after storage times of up to thirty days". However, some VOCs such as bis(Chloromethyl)ether degrade quickly and demonstrate low recovery even after 7 days. The standard VOC list reported by Air Toxics is stable up to 30 days after sample collection. Some projects require a more rigorous 14 day hold time. 2.2 Associated Canister Hardware AIR TOXICS LTD. 5