The main objective of the joint research programme was the development, testing and application of a real time, remote mobile sensing platform, combining Raman, fluorescence, and absorption spectroscopy with current optical fibre technology, for the identification of hazardous chemical species in water. Techniques to enhance sensitivity and analyte selectivity, such as solid phase micro-extraction and neural network post processing, were to be evaluated. Areas of particular concern are the contamination of potable and agricultural water resources; sea and river water contamination from coastal and inland petroleum installations; the monitoring of effluent dispersal from industrial processes and the contamination of the water table arising from chemical leakage of ageing storage facilities.

Specific goals

1. Realisation of a real time, in situ and chemically specific technique combining both Raman fluorescence and absorption spectroscopy in a single instrument, suitable for detecting multi-component mixtures having low signal to noise ratios, and colouration in water.

2. Evaluation and combination of novel, high light gathering optical probes with current optical fibre technology coupled to high powered diode laser sources for the Raman technique and high powered broad band sources for fluorescence and absorption measurements. The multiplexing of many probes to the single detection platform.

3. Investigation of polymer concentrators (such as the polydimethylsiloxanes) and reagent loaded sol-gels for the enhancement of pollutant selectivity; pollutants of particular interest are the polycyclic aromatic hydrocarbons (PAHs), the BTEX compounds and heavy metals (Pb, Cu, Fe).

4. The use of neural network post processing software to provide accurate information on multi- component sample composition for signal-to-noise ratios of less than one.

5. Identification of pre-set site parameters for periodic / continuous in situ measurements using the mobile sensor and the generation of an extensive data base from which trends in water quality changes can be used to predict ambient pollutant concentrations.

6. Training of young scientist in various aspects of optical spectroscopy, design of remote spectroscopic probes, computer control of instrumentation, data collection and presentation of data.

7. Demonstration of the system to potential users.

Raman Instrumentation

One of the major aims of the project was to research and develop an instrumental platform that would support Raman, fluorescence and absorption spectroscopy operating via optical fibre probes to enable remote sensing of chemical species in water for in situ environmental pollution studies. A Raman system was available to one of the partners, which enabled initial investigations on the detection of well known hydrocarbons such as Benzene, Toluene and Xylene (meta- and ortho- in dichluoromethane solvent). The set-up comprised a Krypton laser and a N₂ cooled CCD camera. Results showed very well resolved spectra but a very poor detection limit (~1000ppm) which is typical for Raman spectroscopy. Crude oils were also tested but there was no observable Raman signal detected. This result was not unexpected and in the proposal we had planned to overcome the lack of sensitivity by developing specially prepared surfaces to enhance the Raman scattering. There have been many reports in the literature of the sensitivity of enhanced Raman scattering being increased by factors between 10³and 10⁶. Unfortunately this enhancement factor can be greatly reduced by contamination of low levels of pollutants found in water resources, this combined with the extremely high cost of purchasing a suitable portable Raman system lead us to concentrate on the development of systems exploiting fluorescence and absorption.

Fluorescence Instrumentation

Two different fluorescence spectrometer systems were purchased, initially for laboratory use in order to obtain representative spectra of low concentrations of pollutants in water. These systems will also be used in field trails in Cyprus and the UK. The optical source for both systems is based on a pulsed N₂ laser that oscillates at a fixed frequency. To increase the range of pollutants the can be detected a dye laser and a frequency doubler pumped by the N2 laser were also purchased UK allowing measurements to be made with input wavelengths in the range 250nm to 650nm. The fluorescence signal is recovered with a spectrometer that can be tuned to a specific emission wavelength and a strobed photo-multiplier. Light from the laser is transported to the sample with a large core UV fibre and collected at an 180⁰ scattering angle with a similar optical fibre. The complete system operates under computer control allowing both the variation of the fluorescent signal with excitation and emission wavelengths to be measured as well as the lifetime of the fluorescence.

Direct Absorption Spectroscopy

The micro-optic probe is designed such that different optical paths can be used. This allows the system to operate over large absorption ranges. The absorption and fluorescence probes can in principle be used to simultaneously to analyse a sample.

Neural Nets

Neural Net programmes have been developed and trained with data produced with the absorption and fluorescence spectrometers to analyse mixtures with low concentrations of pollutants, typical results are given in the figures below.