Volatile Organic Compounds from Paired Tenax®/Tenax®-Charcoal Tubes (VOST)
These methods describe the sampling (0030), desorption (5041A) and analysis (8260B) of volatile POHCs (principal organic hazardous constituents) collected from stack gas effluents of hazardous waste incinerators using volatile organic sampling train (VOST) methodology. Samples are analyzed by thermal desorption of the sorbent tubes followed by GC/MS analysis. This method is specifically referenced for the determination of destruction and removal efficiencies (DRE) of 99.99% for volatile POHCs. Volatile POHCs are loosely defined as those compounds with boiling points less than 100°C. If a compound's boiling point is less than 30°C then the POCH may breakthrough the sorbent tubes during sample collection. Compounds with boiling points above 100°C can be efficiently collected and analyzed using these methods. Method 5041A states that compounds with boiling points above 120°C are not to be used for DRE determination.
Samples are collected using a VOST sampling train consisting of a probe, water cooled condensers, Tenax®/Tenax®-charcoal sorbent tubes, an empty impinger for condensate collection, and a silica gel drying tube. Isokinetic conditions are not required for sample collection since the compounds of interest are in the vapor phase at the point of sample collection. 20L is the recommended sampling volume for collection and there are two recommended sampling rates referred to as FAST-VOST and SLO-VOST. The sampling rate is 1 L/min. by the FAST-VOST method and 0.25 L/min. for 20 min. or 0.5 L/min. for 40 min. for the SLO-VOST method. The goal of either method is to collect a total number of sorbent tube sets to encompass a total sampling time of 2 hours (one ‘run’): i.e., if a sampling rate of 1L/min for 20 minutes is used, a total of six sorbent tube sets will be collected in 2 hours of sampling.
When sampling for a compliance project, 3 runs as described above are collected and the Tenax® and Tenax®-charcoal tubes are analyzed separately to check for breakthrough. Breakthrough is defined as recovering less than 30% of the POCH on the Tenax®-charcoal (back-half) tube as was found on the Tenax® (front-half) tube if greater than 75 ng is detected on the Tenax®-charcoal tube. Analysis of the condensate collected in the empty impinger of the sampling train is also typically required for compliance projects. In non-compliance projects, the Tenax® and Tenax®-charcoal tubes are usually analyzed in tandem and the condensates are archived.
The method also requires the collection and analysis of a field blank and trip blank per sampling event. A trip blank consisting of a single pair of Tenax® and Tenax®-charcoal tubes from the same lot number and kept under the same conditions from the time the tubes leave the laboratory until they return to check for contamination from shipping and handling.
High moisture sources may cause problems with the analyses as the moisture will dampen the Tenax sorbent. It is difficult to purge the POHCs from wet Tenax resulting in poor desorption efficiencies.
NOTE: SW-846 Method 5040 has been replaced by SW-846 Method 5041A, and SW-846 Method 8240 has been replaced by SW-846 Method 8260B.