Lab Science News - Science, chemistry and environmental news from laboratory experts

Archive for October, 2003

Passive Diffusion Devices & Polyethylene Diffusion Bag (PDB) Samplers

Sunday, October 26th, 2003

Passive Diffusion SamplingPassive Diffusion Samplers are generating a lot of interest in the environmental community for four main reasons:

•Water samples are more representative of the area adjacent to the well screen or area of interest than those taken by other conventional means.
•Well purging is not required.
•They are simple to deploy.
•They are inexpensive when compared to other sampling techniques.

The principle behind passive diffusion is that compounds will migrate or diffuse through a semi-permeable membrane until a concentration equilibrium is established on either side of the membrane. The following three passive devices are currently commercially available or will be soon.


Selective Ion Monitoring (SIM) for Low Level Applications in Vapor Samples

Friday, October 3rd, 2003

Selective-Ion-Monitoring-for-Low-Level-Applications-in-Vapor-SamplesGas 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.