Evaluating Performance for Measurement Systems Configured for TO-15 Toxic Organics in Ambient Air and Other Bulk Gases

Oral Presentation

Prepared by E. Bramston-Cook, R. Bramston-Cook
Lotus Consulting, 5781 Campo Walk, Long Beach, CA, 90803, United States

Contact Information: edward@lotusinstruments.com; 310-486-6881


The primary approach to measuring trace toxic organics in ambient air remains the EPA Compendium Method TO-15 using Gas Chromatography and Mass Spectrometry. This protocol was last updated in 1999 with performance criteria purposefully limited to allow instruments of that era to meet all indicated requirements. Dramatic improvements in instrument capabilities have extended performance well beyond listed abilities in the method. With these enhancements, new procedures are needed to demonstrate performance accomplishments. Better detection capabilities put added pressure on blank checks to ensure that the analytical system is not contributing responses that match with target analytes, especially after repetitive cycling of adsorbent traps. Linear ranges can now extend over much wider span than the method listed range of 50, often widening to 105 or more. The conventional Cartesian plots of calibration data and associated least-squares fitting tend to skew the quality of the fit. A more useful display is a plot of response factor vs log[concentration] for each calibration level. With this extension to a wider range, carryover of high concentration analytes can severely impact subsequent runs with carryover; procedures are presented to minimize this effect. Also, discussed is the effect from sample canister pressures on the accuracy of results.

This method is often referenced for measuring concentrations well outside the specified range, and with sample matrices radically different than ambient air. Many target levels for soil gas and biogas are well outside the method calibration range. Special operations are presented to allow these samples to be properly reported, independent of the sample matrix and without severe sample manipulations to get concentrations within the operating range of the analyzer.