Flux chambers are mentioned in the international technical literature as a useful and real tool for estimating the flow of interstitial gases that spread in the air from contaminated sites; the Italian Guidelines on the monitoring of soil gas propose them as a technique for the direct measurements of interstitial gases.
These measurements have assumed a very important value in the risk analysis and in the subsequent decision-making processes within the remediation activities: they allow the evaluation of the “vapor intrusion”, namely the process through which the pollutant chemical substances present in the soil or in the underground water can fill the unsaturated soil thus reaching the above environment and altering indoor and/or outdoor air quality, with potential risk to human health.
A flux chamber can isolate an area from external environmental conditions: the main microclimatic parameters that are monitored inside the chamber are not bound to the weather conditions of the site, especially wind.
By means of two connecting lines positioned on the dome, clean and dry air passes inside the hood at a controlled flow rate. The air in the soil is then “pushed” out of the system and monitored continuously or by sampling to determine the concentration of pollutants. Such measurements thus offer an estimate of the parameters searched for surface unit and time unit.
For example, the main substances that flow chambers can search are aromatic, aliphatic and organ-chloride hydrocarbons, nitrogenous and halogenated hydrocarbons, sulphurates, mercury, etc.
Mérieux NutriSciences owns several flow chambers and has a consolidated experience in their installation and management. When proposing this technique, we consider the geology and hydrogeology of the site as well as the climatic data to better plan the activities. During the quick sampling of the gas leaving the hood and once the conditions of the incoming air and the internal temperature have stabilized, it is possible to proceed in different ways:
- sampling with suitable adsorbing cartridges made of activated carbon or selective resins and subsequent laboratory analysis;
- continuous analysis directly on site by means of portable analyzers in order to recognize a series of compounds in real time and, for some substances, reach very low detection limits;
- sampling of organic VOCs by Canister and subsequent laboratory analysis. The high analytical sensitivity of this technique makes the model applicable to site-specific risk analyzes.
The Canister is well known for its simplicity because it allows to operate with a reduced chain of components; moreover, it allows the detection of a considerable number of volatile compounds, it reaches much higher sensitivity than traditional methods (GC-FID) and it can be easily transported.
The Canister is a sampling system that consists mainly of a vacuum metal casing and of a further valve to regulate the filling in a certain time.
Mérieux NutriSciences’ Canisters have a capacity of 3.2 or 6.0 liters and the valves (passive samplers) are both internally coated with a special material, the SILONITE®, which makes the Canister inert and reusable over time (after cleaning and vacuuming).
With reference to calibration, a digital flow meter is used: when coupled to the Canister and the valve, it allows to prepare it according to sampling requirements. After being reclaimed and prepared for reuse, the Canister lasts about 15 days; after such period, there could be micro-infiltrations of air that could compromise sampling and tests, too.
The advantages of Canister:
- no modification of the sample compared to collection conditions;
- no need to measure sample volume during collection;
- wide range of sampling time intervals (from a few seconds to a week);
- Canister internal surface is inert if compared to the pollutants present in the air sample;
- determination of many compounds, including thermo-labile ones;
- low-temperature sample transfer from the Canister to the testing tool;
- possibility of multiple tests and their repetition;
- high capacity for compounds separation/identification;
- reduced possibility of Canister breaking (during transport and use);
- no need to know pollutants concentration in advance to identify the capacity of the Canister;
- high or low concentrations of pollutants do not affect the sampling methods.
Mérieux NutriSciences’ laboratories offer sampling by Canister and subsequent analyses; the Canister can be rented and the relevant tests of volatile organic compounds (VOCs) are also available.
The control of volatile organic compounds offers high stability in sampled compounds during sampling and storage. The test method used by Mérieux NutriSciences for the determination of VOCs is the EPA TO-15 1999 (determination of volatile organic compounds in air-environment, sampled by Canister and analyzed by gas chromatography with mass detector); it is accredited according to the standard UNI CEI EN ISO/IEC 17025: 2005.
The Canister also allows the MADEP APH (Massachusetts Department of Environmental Protection - Air Petroleum Hydrocarbons) analysis in soil-gas for which Mérieux NutriSciences adopts the method MADEP APH rev. 1 2009: to date, it is the only existing method for assessing steam hydrocarbons according to a risk-based corrective intervention approach (namely with fractional aliphatic substances and aromatic ranges).