Lab Science News

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.

These applications typically require a 6-liter evacuated summa canister equipped with low volume flow controller to collect time-integrated VOC samples. To minimize contamination and ensure against false positive results, it is important that canisters and flow controllers be individually certified down to the reporting limits. For less demanding projects, where full scan reporting limits are adequate, batch certification (one per ten) of canisters is acceptable.

Using GC/MS in the SIM mode is a valuable tool for investigating situations where ultra-low level reporting limits are desired. Compound lists and reporting limits are constantly being reviewed and revised.

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 recently, investigators have begun utilizing these devices for applications involving lower levels of contaminants over longer periods of time. This usage has become feasible with the development of passive samplers containing solid sorbents that may be thermally rather than chemically desorbed . This application of diffusive sampling can be useful for risk assessors and others wishing to evaluate airborne contaminant levels over extended periods of time (e.g., weeks to months).

Previously, the absence of an established method and the fact that very little data are available for longer term applications (e.g., greater than 8 hours) has proved limiting. Earlier this year, the International Organization for Standards (ISO) published a document , which defines a sampling and analytical method and also includes sampling rates for a wide range of compounds on numerous types of sorbents.

CAS Simi Valley is currently involved in a project being conducted by a regional air pollution control agency that is looking at the feasibility of utilizing diffusive samplers to monitor sub ppb levels of benzene, methylene chloride, trichloroethene and tetrachloroethene over a 30 day period. This project has presented some unique challenges that highlight some of the parameters to consider before selecting this methodology. The goal was to select a single sorbent for which there were documented sampling rates for each of the compounds of interest (preferably based on long term studies), that was also strong sorbent and hydrophobic. Chromosorb 106 was selected because it was the best compromise based on the available data.

The use of passive diffusive samplers coupled with analysis by thermal desorption/GC/MS may be a valuable tool for situations where individuals wish to monitor low level exposures over a long duration.