Measurement method · US EPA
EPA Method 325A/B: passive sorbent-tube sampling of fenceline VOCs
EPA Methods 325A and 325B, in Appendix A to 40 CFR Part 63, are the reference method for measuring volatile organic compounds at a facility fenceline with passive diffusive sorbent tubes. Method 325A covers where and how the samplers are deployed around the property boundary; Method 325B covers preparing the tubes and analyzing them by thermal desorption and gas chromatography in a laboratory. The tubes are exposed for a sampling period, typically 14 days, and the result is an average concentration for that period at each location.
Who uses it
Who Uses It
Petroleum refineries under the Refinery Sector Rule (40 CFR 63.658) for benzene, and chemical plants under the 2024 HON fenceline requirement (40 CFR 63.184) for benzene, 1,3-butadiene, chloroprene, and ethylene dichloride. Both rules compute Δc (the highest minus the lowest fenceline result per period) from Method 325 samples, and both let an approved alternative, such as open-path monitoring, replace samplers if it gives equal or better spatial coverage.
Monitoring
How It Relates to Continuous Monitoring
Method 325 gives a laboratory-quality average over two weeks, not a time series: it shows that concentrations were elevated, not when or from which direction. Continuous boundary monitoring with wind data fills that gap, which is why the fenceline rules point facilities to real-time techniques when a root cause cannot be found from the passive samples alone.
Solutions
Solutions That Rely on EPA Method 325
Oil & Gas / Petrochemical
Fence-line VOC, H₂S, and methane leak detection
Article Library
Articles on EPA Method 325
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Questions
EPA Method 325 FAQ
What is EPA Method 325A/B?
The reference method for fenceline VOCs with passive diffusive sorbent tubes: 325A for deploying the samplers around the boundary, 325B for preparing and analyzing them in a laboratory.
How long are Method 325 samplers exposed?
Under the refinery and HON fenceline rules, for contiguous 14-day sampling periods.
Which rules use Method 325?
The EPA Refinery Sector Rule for benzene at petroleum refineries, and the 2024 HON fenceline requirement for benzene, 1,3-butadiene, chloroprene, and ethylene dichloride at chemical plants.
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