Passive Air Sampling Tubes for Refinery Fenceline Monitoring (FLM)

Supelco's FLM™ Carbopack™ X Thermal Desorption Tubes (see figure) are designed for the new US EPA Method 325. This method is specific for monitoring the air concentration of volatile organic compounds that are emitted from petroleum refineries. The target compounds are a number of Hazardous Air Pollutants (HAPs) including benzene, toluene, and xylenes. This method uses thermal desorption sampling tubes to passively collect air samples along the perimeter of the property. The air concentration is determined by analysing the tubes using thermal desorption and gas chromatography.

The method requires the sampling tubes to be placed in a non-emitting shelter to protect the tubes from weather elements. The shelters are permanently installed along the fenceline (property line) of the refinery, and the sampling tubes are replaced with new ones every 14 days. The required quantity and placement of the air sampling tubes is based on the size of the property and prevailing winds. The specifics relating to tube placement are detailed within the EPA method.

The physical design of the stainless steel tube has been in used for well over 25 years, and is well accepted in the marketplace for collecting both passive and active air samples. This particular tube size fits into a number of thermal desorbers made by different manufacturers, such as PerkinElmer®, Markes International®, DANI™, and Shimadzu™.

The FLM Carbopack X passive sampling tubes work by collecting the air sample without the need of an air sampling pump to physically pull the air sample through the adsorbent. The tube relies on the natural movement (of the HAPs) across a concentration gradient inside the inlet of the tube called the 'air gap'. The volatile HAPs travel through the air gap and are adsorbed by the Carbopack X contained inside the tube.

Carbopack X is a granular graphitized carbon black (GCB) adsorbent with a surface area of 240-m²/g. This particular material contains mesopores that allow it to retain a broader range of volatile analytes than other GCBs, or porous polymers like Tenax® TA. Carbopack X is hydrophobic, so no appreciable amount of water vapour is retained when sampling in humid conditions. The sampling tubes are packed with 40/60 mesh size material, which packs uniformly within the tube and prevents channeling along the walls of the tube from occurring during sampling.

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