When it comes to the sampling and analysis of volatile organic compounds (VOCs) in ambient/indoor air, and those entrained in contaminated soils/underground water table, EPA Compendium Method TO-15 is the most used analytical method in the United States. TO-15 is the EPA’s Selected Analytical Method (SAM) for air samples for environmental remediation and recovery.
“TO” in TO-15 stands for toxic organics. This method documents sampling and analytical procedures for the measurement of subsets of the 97 VOCs that are included in the 189 hazardous air pollutants (HAPs) listed in Title III of the Clean Air Act Amendments of 1990. Full list of the 97 VOCs may be found in Table 1 of the Method TO-15.
Analytical Instrumentation for TO-15 analysis
Laboratory analysis of the samples involves use of a pre-concentrator to focus small amounts of VOCs from large volumes of air. A gas chromatograph is then used to separate the individual VOC components and a mass spectrometer is used to identify and quantify each individual component in the sample.
TO-15 Scan vs TO-15 SIM Analysis
The mass spectrometer most widely used for TO-15 analysis is a single-quadrupole mass spectrometer which can be operated in two modes – Scan mode and SIM (selected ion monitoring) mode. With a few compromises, SIM mode allows the mass spectrometer to achieve detection limits in the parts per trillion (ppt) concentration range, as opposed to the parts per billion (ppb) levels offered by the scan mode. For high-risk VOCs, where toxicity has been re-evaluated and lower risk levels have been indicated, TO-15 SIM analysis may be required. Table 1 in the Method TO-15 Supplement provides a list of such VOCs for which SIM analysis may be required.
Does every lab offer analysis for all 97 VOCs in the TO-15 Target Compound List?
Typically, a lab will test only a subset of the 97 VOCs in the TO-15 target compound list (TCL). A lab’s choice of a subset is often dictated by commercially available certified reference standards of VOCs mixtures because labs must use reference standards to calibrate their analytical system for each individual compound. Moreover, some VOCs are challenging to analyze with the currently available analytical instrumentation and require extra resources. Labs tend to offer those challenging VOCs as a separate subset if requested. If you are dealing with unknowns, which is most often the case in environmental analysis, it is quite easy to miss the VOCs which may be present in the air sample but not listed in the subset of VOCs offered by the lab you have contracted with.
What are tentatively Identified compounds (TICs) and do you need TO-15 TICs Analysis?
TO-15 TICs analysis is a useful tool that can aid in environmental concerns, clean-up or treatment decisions by identifying compounds that might otherwise be missed at the site. Thanks to the ability of the mass spectrometer to produce reproducible and characteristic fragmentation patterns for any given compound. Mass spectral libraries have been developed that allow the lab to identify TICs in the samples by performing a library search when reference standards are unavailable. The concentration estimates for TICs are provided based on their responses in the analytical system compared to concentrations and responses of the internal standards used in the TO-15 method. However, a TIC identification is not considered absolute or confirmed until the sample has been re-analyzed with a known reference standard for the same compound. TICs analysis may not be available with SIM analysis.
How Important is Sampling for TO-15 analysis?
Sampling is a very important step for any analytical method and proper sample collection is critical to ensure reliable TO-15 data. The old saying goes “Garbage in, Garbage out”, and poor sample collection will almost certainly result in bad analytical data. Bad analytical data can then lead to bad decision making, bad investment, and continued exposure to VOCs and liabilities.
Canister-based sampling for TO-15
A specially treated canister to provide inert storage space for the VOCs is at the core of TO-15 sampling. Summa and silonite coatings are the two most prominent types of canisters available in the market for TO-15 sampling, with silonite being the preferred canister treatment due to its inertness and robustness. Various canister sizes ranging from 15L to 100 mL are commercially available for TO-15 sampling. The choice of canister size is usually dictated by the sampling duration, required sample size and sampling convenience.
How full should the canister be?
Sampling in canisters is done in both sub-ambient pressure and pressurized modes. Both modes use a canister evacuated to <50 mTorr. Sub-ambient sampling uses the vacuum in the canister to provide the driving force for sample flow into the canister. Pressurized sampling requires an external pump to provide positive pressure in the canister. Sub-ambient sampling is the preferred mode of sampling as pressurization in the sample may lead to condensation in the canister and/or contamination from the sample pump. It is recommended that 1.5-3 psig of residual vacuum is left in the canister when performing sub-ambient sampling.
At the end of the sampling event a residual vacuum or positive pressure should be recorded on the chain of custody form. Labs verify those recorded values to ensure that the sampling canister did not leak during transportation. Canister leaks, while uncommon, may result in severe contamination of the sample during transportation to the lab. Canister sampling to ambient pressure should be avoided as it provides no mechanism to verify the integrity of the sample during transportation.