Benzoic acid, at high concentrations, has appeared in my field blanks, trip blanks, and samples using SW-846, Method 0010. Is benzoic acid real or a breakdown product of XAD-2 resin?
As you recall, XAD-2 is a cross-linked styrene-divinylbenzene organic polymer adsorbent. When used for ambient and source testing, the native XAD-2 must be certified clean prior to field application. This requires Soxhlet extraction with an organic solvent ( 10% diethyl ether in hexane or methylene chloride) to remove residual organics (i.e., benzene, toluene or the xylenes) from the surface of the polymer. Consequently, when ready for field use, the XAD-2 should come with a “Certificate of Cleanliness” from the laboratory indicating that there are no residual organics (i.e, < 4 ug/g of individual organics) on the resin bed. This is very important since the detection limits we are trying to reach are 1.0 ng/m3. Consequently, the benzoic acid should not be a contaminant on freshly extracted XAD-2 resin.
The field and trip blanks should also help one determine what is the source of the benzoic acid. When charging and recovering the Method 0010 sample train, the field blank should be exposed to the same atmosphere as the sample cartridge. This means opening up the field blank and setting it in the same area as where the Method 0010 sample train is being charged or recovered. At the end of each activity, the field blank is capped and stored with the other samples. If benzoic acid is in the atmosphere from fugitive emissions, it would effect both cartridges (sample and field blank) the same. As you recall, the trip blank is never opened. The trip blank is prepared just like a sample, but is never exposed to the atmosphere.
However, because of it’s chemical structure, XAD-2 can degrade when exposed to heat, sunlight, and oxidants. To minimize the influence of heat, Method 0010 requires that the source gas entering the resin bed be maintained to < 68 F through the use of a coiled condenser during sampling to prevent deterioration of the XAD-2 resin. In addition, after sampling, Method 0010 requires that the resin bed be maintained at
< 4 C until extraction to provide continue integrity of the resin and analytes on the resin.
To minimize the influence of ultraviolet light from breaking down the resin, we suggest that the resin cartridge always be wrapped in hexane-rinsed aluminum foil. This protects the resin from harmful ultraviolet light, once again maintaining integrity of the resin. The aluminum foil also minimizes contamination from hands on the resin cartridge.
The influence of oxidants has only recently come to light. During sampling, we are pulling large volumes of stack gas containing many oxidants (i.e., oxygen, ozone, peroxides etc) through the resin bed. It has been speculated that, during sampling, benzoic acid is produced as an artifact from the oxidation in the stack gases. Polymers like XAD-2, because of their substituted benzene ring structure, slowly degrade and give off compounds like toluene, styrene, and similar compounds that oxidize to yield the benzoic acid during sampling.
For more information associated with semi-volatile monitoring and background concentrations of the XAD-2 resin, please contact Tom Ward, US Environmental Protection Agency, MD-74B, Research Triangle Park, North Carolina 27711 (919-541-3788).