A Single-Method Approach for the Analysis of Volatile and Semivolatile Organic Compounds in Air Using Thermal Desorption Coupled with GC–MS

Oct 01, 2014

This article describes a new, single method to replace the two-method approach using United States Environmental Protection Agency (EPA) Methods TO-15 and TO-13A for the analysis of both volatile organic compounds (VOC) and semivolatile organic compounds (SVOC) in air. The study presents evidence that these components can be determined using one analytical method that follows EPA TO-17 capturing the volatile compounds in addition to recovering the heaviest components, such as benzo[ ghi ]perylene. The investigation also takes a look at some of the 50 additional compounds that are expected to be added to this list in the near future.



There is an increasing need to determine both volatile organic compounds (VOC) and semivolatile organic compounds (SVOC) in air because many sites across the United States are being mandated to measure them to fully understand their impact on human health. Currently, the analysis of these compounds requires two analytical methods from the Environmental Protection Agency (EPA): EPA TO-15 (1) or TO-17 (2) for volatiles and EPA TO-13A (3) for semivolatile components. To enable the analysis of both volatile and semivolatile components in one air sample, a new thermal desorption (TD) sample tube was required. The purpose of this study was to investigate whether one analytical method could be used following EPA Method TO-17 to monitor both the volatile and semivolatile target list mandated by regulatory agencies, including 1,3-butadiene, benzene, toluene, ethyl benzene, and xylenes (BTEX compounds) as well as 16 EPA-regulated polycyclic aromatic hydrocarbons (PAHs) up to benzo[ghi]perylene. One of the benefits of using TO-17 is that it is a performance-based method, meaning it can be used to analyze any compound as long as that compound meets the method criteria. This is emphasized in section 2.5 of the method, which states (2), ". . . this method provides performance criteria to demonstrate acceptable performance of the method (or modifications of the method) for monitoring a compound or set of compounds." Furthermore, by using this method only one air sample needs to be collected and analyzed instead of two, which significantly reduces sampling costs, enhances laboratory productivity, and improves safety, resulting in a more environmentally friendly analysis (4).

Discussion

Method TO-15, the conventional way of measuring VOCs in air, uses a large electropolished stainless steel vessel (Summa canister) that collects approximately 6 L of air. A fraction of this sample volume, typically 500 mL, is withdrawn from the canister and sent to a concentrator sorbent trap. The sample is then desorbed from the trap and focused onto a gas chromatography (GC) analytical column to be separated and analyzed by mass spectrometry (MS).

There are several limitations of the TO-15 approach. This method only reliably recovers up to naphthalene (C12) while several regulatory directives require measurements up to at least C13. In addition, many air samples might contain higher boiling substances, which can adsorb onto the sides of the canister and condense. Other challenges include analyzing air samples with high moisture content and the requirement for a greater number of analytes (in some cases up to C40) over a wide range of concentrations. Method TO-17 overcomes many of these challenges, by using a thermal desorption tube instead of a Summa canister to collect the sample. The thermal desorption process utilizes a sorbent tube, which contains adsorbent material specifically selected to trap the range of analytes of interest. A known volume of air is sampled through the tube, where the contents are then desorbed onto a secondary trap into the analytical column to be analyzed by GC–MS.

Method TO-13A for determining semivolatile organic compounds in air requires the use of a much higher volume sampling technique to acquire sufficient sample for analysis, mainly because of the relatively low levels of PAHs in the environment. Unfortunately, the volatility of certain PAHs prevents efficient collection on filter media alone, so the method necessitates both a filter and a backup sorbent cartridge to provide for efficient collection of the common PAHs.

Additionally, the filter medium and sorbent material cartridges have to be cleaned in the laboratory for up to 16 h with a suitable solvent (typically, methylene chloride) and vacuum dried, before sampling can be carried out. Approximately 1000 m3 of air is drawn using a high-volume flow rate air sampler. The filter and sorbent cartridge are then sent to the analytical laboratory for analysis, where the compounds trapped on the filter and cartridge are removed by a 16-h Soxhlet extraction with methylene chloride. The extract is then concentrated and cleaned using column chromatography to remove potential interferences before analysis by GC–MS. Because Method TO-17 does not require the extraction, evaporation, and concentration of large volumes of methylene chloride it is much safer for the operator, as well as being a far more environmentally friendly approach.


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