2.5.06 Water quality monitoring – new measuring procedures developed

Drinking water sources require continuous monitoring of their contaminant content – a time-consuming and expensive process. Yet monitoring is particularly important in crisis regions, where there is a risk of terrorist attacks – for example, poisoning of drinking water sources. Scientists from Germany and Israel have worked together to develop new measuring procedures, which can serve as the basis for early warning systems. These processes are based on the infrared absorption spectrum of substances.

Continuous monitoring is essential to the reliable and timely detection of pollutants and contaminants in drinking water sources. In politically sensitive regions such as Israel, there is also a risk of targeted poisoning of water sources (chemo-terrorism). As a result, there is great demand for measuring procedures that can provide continuously reliable information on water quality.

German-Israeli co-operation

Previously, groundwater testing in Israel primarily took the form of laboratory analyses. The samples had to be taken from the measurement locations before being transported to the lab for processing – a time-consuming, expensive and error-prone process. Funded by the BMBF as part of the German-Israeli scientific co-operation, the project “Compact fibre-optic infrared system for online monitoring of pesticides and other pollutants in water” was set up to develop a measurement system that would enable continuous, reliable monitoring of the water quality in real-time and across great distances. The research was conducted by the School of Physics and Astronomy of Tel Aviv University and the Fraunhofer Institute for Physical Measurement Techniques (IPM).

Analysis of radiation spectrum

The IPM had already laid the technical foundation in the form of a wide-band spectrometer, which was originally developed to monitor landfill and seepage water online as well as to control industrial processes. The analytical device is programmed to detect different organic molecules (e.g. pesticides).

The measuring process is based on the property of all substances to absorb infrared radiation in a specific spectrum. Infrared spectroscopy, a technical analysis method, can be used to identify substances via their infrared absorption spectrum. The researchers employed the technique of attenuated total reflectance (ATR) spectroscopy, which is based on an optical system. The analytical radiation spectrum provides information on the presence and concentration of contaminants. This enables identification of water contamination by most harmful chemicals and immediate radio transmission of the data to a central monitoring station.

The measurement system of the wide-band spectrometer consists of three modules: a light source, a sensor element and an infrared spectrometer. The radiated light of a miniaturised emitter is directed into the ATR sensor element where it passes through the measuring path and is recorded by the spectrometer. The individual substances in the water, and their concentrations, are then determined in the subsequent spectrum analysis.

Detection of minimal contaminant concentrations

ATR technology is based on the fact that the field of a light wave passing through a transparent medium partially extends into the surrounding medium. This so-called evanescent field is ideal for performing absorption measurements. A sensitive fibre serves as the sensor: when it is coated with a suitable polymer, the targeted molecule is enriched, thus amplifying the measured signal up to a thousandfold. The water-resistant polymer also offers protection against water, which would otherwise greatly interfere with the measurement. A movable grid allows the spectrometer to measure wavelengths between 8 and 12.5 micrometres. Other wavelength bands can be analysed by replacing the grid. Absorption measurements in the mid-infrared range enable detection of organic molecules with concentrations of less than 1 ppm.

Two measurement systems developed

Over the course of the project, which started in 2003 and was concluded in June 2006, the German-Israeli team developed two different measurement systems based on ATR technology: a large device with higher sensitivity – but which was also more expensive and not suitable for field use – as well as a handy and more cost-effective model with lower detection sensitivity. In a follow-up project, advances in the miniaturisation of commercial Fourier transform infrared (FTIR) spectrometers was utilised to create a measurement system offering virtually the same detection sensitivity as the expensive counterpart (see photo). These projects have formed the basis for the development of compact, reliable and user-friendly measurement systems. The devices have been subjected to initial field tests and are at a high stage of development. Additional field use in Germany and Israel is planned.

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