Gas chromatography/mass spectrometry (GC/MS) is the method of choice for the identification of volatile organic compounds (VOCs) in vapor samples (e.g. EPA methods TO-14A and TO-15). As various state and federal agencies more frequently require facilities to address risk-based concentrations, such as the low level preliminary remediation goals (PRGs), they find that the standard method is not able to reach the ultra-low levels needed. To address these requirements, CAS’ Simi Valley, California lab has developed a method using selective ion monitoring (SIM) to measure the compounds. SIM is a sensitivity enhancement technique, where the mass spectrometer is programmed to scan for only those ions that are pertinent to the compounds of interest (2-3 mass ions scanned per compound) while ignoring non-essential ions. The mass spectrometer becomes a highly sensitive compound-specific detector.

The driving force for the lower limits has been health risk assessment activities in the indoor and ambient air arena. The exposure criteria for many compounds are being re-evaluated constantly. A recent symposium sponsored by the Groundwater Resources Association (GRA) on subsurface vapor intrusion to indoor air has recommended that the SIM analytical technique be used. For example, trichloroethene (TCE) will have a reporting limit of 1.0 mg/m3 (0.19 ppbv) using the standard full scan method. In contrast, the reporting limit of 0.05 mg/m3 (0.0093 ppbv) for TCE will be achieved with the SIM technique. This meets or exceeds most risk-based concentration criteria. Lower limits are occasionally requested and are reviewed on a case-by-case basis.

(more…)

Diffusive sampling has been a popular approach for the evaluation of workplace exposures to airborne contaminants, such as volatile organic compounds (VOCs) for some time. Typically, these “badge-type” samplers (e.g., 3M, SKC) have been used to evaluate exposures in the high part per billion (ppb) to part per million (ppm) range over an 8-hour period. A popular option, these samplers are easy to use, small in size and don’t require a sampling pump.

This approach involves the passive collection of an analyte on a solid sorbent via adsorption or chemical reaction. The sampling rate (e.g., diffusive uptake rate) is a function of the diffusive coefficient, which is compound and sorbent specific, and the geometry of the sampler used. Other factors that affect performance include temperature, pressure, humidity, air velocity and transient changes in contaminant concentrations. Samples are chemically desorbed and analyzed by gas chromatography with flame ionization detection (GC/FID) or by other appropriate instrumentation.

(more…)