Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Content or effect of a constituent of a liquid mixture
Patent
1997-02-10
1998-06-30
Williams, Hezron E.
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Content or effect of a constituent of a liquid mixture
73 6141, 73 6159, 73 191, 210544, 210651, 422 83, 422256, G01N 1305, G01N 100, B01D 1508
Patent
active
057737134
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for environmental monitoring of low concentration levels of organic compounds, and relates particularly, though not exclusively, to a method and apparatus for real-time monitoring of low concentration levels of volatile organic compounds (VOCs) in groundwater from an aquifer in situ using a diffusion cell.
Where drinking water is obtained underground from an aquifer, it is essential that the water be carefully monitored for contaminants. Contaminants of particular concern are VOCs which include such organic compounds as benzene, toluene, ethylbenzene, xylene (BTEX compounds), trichloroethene (TCE), halomethanes and other derivatives thereof. One of the potential sources of VOCs in groundwater is leakage of organic solvents and petroleum fuels for example from underground tanks which may be susceptible to leaking. Contamination of ground water with VOCs in such locations may occur and can be difficult to detect.
Recommended concentration levels of VOCs in drinking water are usually extremely low. The VOCs need, therefore, to be detected relatively accurately at low concentration levels, typically in the parts per trillion (ppt) to parts per billion (ppb) range. In many places permitted concentrations have been progressively lowered. Measurement of VOCs at such low levels may also be necessary in atmospheric monitoring. Monitoring of VOCs is often required on a continuous or semi-continuous basis to provide real-time data on the level of contamination in the environment.
A diffusion cell consists of a sealed hollow membrane containing a gas phase connected directly to a gas sensing device. The membrane, which is permeable to gases such as oxygen and volatile organic compounds but relatively impermeable to water, provides an interface between the gas phase within the cell and the medium external to the cell. A diffusion cell relies on establishment of an equilibrium between the atmosphere within the open volume and the medium external to the cell.
Diffusion cells constructed of various polymeric materials are known to absorb organic or other compounds and have been used, for example, to determine dissolved methane and dissolved oxygen concentrations in groundwater. Silicone materials, for example, preferentially allow organic compounds to permeate therethrough while rejecting water and other highly polar molecules. Steady state diffusion across a polymeric material of a diffusion cell is driven by partial pressures of the liquid and gas phase each side of the polymeric material, respectively. This diffusion at steady state can be defined by Fick's first law of diffusion.
A known monitoring apparatus comprises a polymeric diffusion cell connected on each end thereof to an upstream and downstream tube, respectively. These tubes are referred to as access lines. The access lines are usually constructed of stainless steel or other materials which are relatively impermeable to VOCs such as nylon. The diffusion cell usually comprises an elongate tube, for example constructed of silicone. The access lines and diffusion cell are purged with a carrier liquid or carrier gas, such as air. Samples of gas are taken from the downstream access line for determination of gas concentration, such as methane, using a gas chromatograph. A problem with this technique is that the detector is relatively expensive and not portable. Furthermore, gas concentrations of non-halogenated VOC's cannot be measured in the ppb range.
Another known monitoring apparatus, using a compact solid state detector for measuring selected compounds such as chlorinated hydrocarbons, has been used to provide real-time continuous analysis of such compounds. A sensor used in the monitoring apparatus comprises a permeable silicone tube in fluid communication with access lines through which a flow of carrier gas is provided. The solid state detector is connected to the downstream access line. The carrier gas flows through the access lines and the permeable silicone tube at a relatively high flow rate, for
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Barber Christopher
Davis Gregory Bruce
Patterson Bradley Mark
Power Terence Robert
CRC For Waste Management & Pollution Control Limited
Wiggins J. David
Williams Hezron E.
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