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The National Air Emission Monitoring Study (NAEMS) project measured gas emissions using automated semi-continuous gas analyzers. A more practical and cost effective gas sampling technique is to use portable systems to fill 50 L gas sample bags over 24 h sampling periods and measure gas concentrations later in a laboratory. For this technique, a gas sampling bag that can retain gases over 24 h is needed. The main objective of the study was to assess the impact of initial gas concentrations (low, medium, and high) and storage times (2, 6, 12, 24, 36, 48h) on hydrogen sulfide, total reduced sulfur, ammonia, methane, and nitrous oxide stability in Tedlar and FlexFoil bags. Bag reusability and background contamination were also investigated. Although it is not perfect, the FlexFoil bag was recommended for 48 h storage of gases. Percent recovery from FlexFoil bags was above 75% except for total reduced sulfur at high concentration levels. No gas generation/emission was observed from new FlexFoil bags. FlexFoil bags were more durable to mechanical stress and ageing effects. There was no need to use a cover to protect samples from sunlight. The cost of FlexFoil bags was lower than that of Tedlar bags.