Analysis of Volatile Organic Compounds in Air by Thermal Desorption

The Clean Air Act (CAA) (1) provides the U.S. Environmental Protection Agency authority to enforce regulations limiting emissions of volatile organic compounds (VOCs) and other air pollutants. The Compendium of Methods for the Determination of Toxic Compounds in Ambient Air includes a variety of sampling and analysis methods (2, 3), including use of single- and multi-sorbent tubes. Concentrating a large volume of sample onto a sorbent tube, followed by thermal desorption onto a GC column provides an efficient, cost-effective means of monitoring VOCs at parts per billion (ppb) or parts per trillion (ppt) levels.

A proficiency study was conducted to demonstrate the performance of the Master TD 4750 Thermal Desorber. Benzene and its homologues: toluene; ethyl benzene; and m-, p-, and o-xylene (BTEX), were used because benzene is a carcinogen released into the atmosphere with automobile exhaust and is on the priority pollutant list.

Experimental Conditions

Instrumentation used for this study included an OI Analytical Master TD 4750 Thermal Desorber interfaced with an Agilent 7890A GC and a 5975C Mass Spectrometer. Ten new stainless steel thermal desorption tubes packed with Tenax® TA were conditioned for 2 h each at 300 °C using the Master TD 4750 conditioning mode. Samples were prepared by spiking liquid BTEX standards onto sorbent tubes using the procedure detailed in Method TO-17, Section 9.3. Each tube was purged with 500 mL of nitrogen to remove the methanol and to simulate distribution of the analytes within the tube during air sampling in the field.

Results

A 9-point calibration curve from 1 to 200 ng (approximately 0.5 to 100 ppbv) was run using fluorobenzene as an internal standard (IS). The %RSD for all five compounds ranged from 12.0 to 19.3% and fell well within the specified 30% acceptance criteria. The Method Detection Limit (MDL) study was run by analyzing nine replicates of the low-level standard; calculated MDLs were well below the 0.5 ppbv requirement specified in Method TO-17. A Precision and Accuracy study at the low- and mid-point calibration levels produced %RSD that were less than 10% and easily within the method requirement (Figure 1).

Figure 1: EICP of the BTEX compounds and fluorobenzene (IS) in a mid-range calibration standard.

Conclusions

Results of this proficiency study met all calibration and MDL criteria specified in Methods TO-15 and TO-17 and produced a high level of accuracy and precision for low- and mid-range samples. The study demonstrated that the two-stage thermal desorption process employed by the Master TD 4750 is a reliable technique for accurate, high-sensitivity analysis of VOCs in air.

References

(1) The Clean Air Act. Public Law 108–201, 1963; 1970; 1990.

(2) Method TO-17. EPA-625/R-96/010b; U.S. Environmental Protection Agency, Office of Research and Development, 1999.

(3) Method TO-15. EPA-625/R-96/010b; U.S. Environmental Protection Agency, Office of Research and Development, 1999.

(4) OI Analytical Application Note #3269, "Analysis of Volatile Organic Compounds in Air by Thermal Desorption", 2008.

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