Infrared Hollow Waveguide Gas Sensors

Research project P 14121 Infrared Hollow Waveguide Gas Sensors Boris MIZAIKOFF 24.01.2000 Monitoring of gases or of gaseous pollutants in an industrial environment is a relevant and up to now largely unsolved problem in analytical chemistry. Standard methods are, although very sensitive and flexible, usually limited to laboratory conditions, time-consuming and require the collection of discrete samples. Chemical sensors are currently available for a limited number of analytes only and frequently suffer from insufficient specificity. Therefore, a new chemical sensor system capable of on-line, real-time analysis of a range of gases would be of considerable commercial interest. This project proposes the development of a novel gas system based on hollow waveguides (HWG) transmitting infrared radiation. This will enable continuous real-time monitoring of various analytes in the gas phase under process conditions and/or in remote locations. The waveguide will act as a multi-pass gas cuvette and provide the necessary sensitivity while the use of a miniaturised FT-IR spectrometer will provide the required molecular specificity. Based upon the same sensor head IR sensor modules using non-dispersive IR light sources will be designed for target analysis. The project is organised in three work-packages. The goals of WP 1 are the development of a HWG sensor cell, its characterisation with a laboratory FT-IR spectrometer and finally the development of an industrially applicable HWG sensor cell module. WP 2 will deal with the development of a miniaturised and rugged FT-IR module with tailored performance characteristics for industrial applications. The combination of this FT-IR module and the HWG module will result in a versatile tool for the analysis of multi-component mixtures and of gases with varying components. WP 3 will deal with the development of non-dispersive light source modeles for target analysis. It is proposed to combine a HWG cell module with mid-IR quantum cascade lasers. This will result in a highly sensitive trace-gas analyser. Considering the wide field of possible applications and the considerable interest expressed by well-known industrial partners, this family of sensors has the potential to become a most valuable tool for industrial gas analysis and process control. It might also be extended to address problems of environmental analysis in a later stage. For these reasons, leading intermational partners, both academic and commercial, have agreed to co-operate and complement our own expertise in the field of chemical IR sensors.

The goal of this project was the development of a new generation of infrared (IR) gas sensing systems for broadband multicomponent analysis of volatile organic compounds directly in the gas phase or after extraction form the liquid phase. Several prototype systems have been developed, established and optimized. The developed systems have been tested in the laboratory and under harsh industrial and environmental conditions, demonstrating the real-world applicability of the sensor systems in process analysis and for environmental monitoring. The sensing system for process analysis is based on novel mid-infrared transparent silica glass fibers with coaxial hollow core internally coated with an IR-reflective layer. The hollow core accommodates gaseous sample streams delivered from a miniaturized capillary membrane sampler, enabling the permeation of analyte molecules from aqueous solution to the gas phase. A controlled inert gas flow carries the permeated gas sample from the membrane sampling module to the core of the hollow waveguide, which serves as waveguide for the launched infrared radiation to the detector, and simultaneously as miniaturized gas cell based on the principle of multiple internal reflections utilized in multi-path cells. The key feature of hollow fibers for gas sensing is their ability to accommodate sample gases in a small volume, enabling rapid response of a capillary flow cell based sensor and enhancing the sensitivity by increasing the optical path length. Thus, intimate contact between the gas sample and IR radiation is ensured. The membrane sampler is a thin-walled capillary silicon membrane mounted on a stainless steel rod and designed for equilibrium sampling from complex aqueous solutions via permeation. The probe can be utilised in a flask or a vessel filled with the analyte, in a flow-cell powered by a pump, or directly at a by-pass of an industrial process stream. Following extraction the sample is directly transported into the hollow waveguide sensing module for immediate spectroscopic analysis. Simultaneous detection of benzene, toluene, m-xylene, p-xylene, o-xylene in water at low ppb concentration levels demonstrates the possibility of performing multicomponent analysis. Furthermore, successful investigation of aqueous solutions of alcohols in the ppm concentration range has been achieved. Chlorinated hydrocarbons in water were analyzed with detection limits down to 750 ppt. 1,4-dioxane was detected in the low ppm range and established the limits of permeation of the silicon membrane towards low volatile organic compounds. Continuous monitoring of methanol concentrations in process effluents at COGNIS GmbH (Düsseldorf, Germany) has been performed and validated, demonstrating the applicability of the developed sensor systems for continuous analysis at industrial sites. The IR hollow waveguide gas sensor system for environmental analysis combines this new device concept with sorption tube sampling and thermal desorption for trace detection of atmosphereic compounds. Sensitivity of approx. 1.1 ppb for measuring ambient atmospheric ethane levels has been achieved. Validated field measurements campaigns have been performed with the developed prototype at alpine sites in Tyrol/Austria and impressively demonstrated the capabilities of this novel analyzer concept. It is envisaged that further improvement of the measurement methodology, the extraction system and the enrichment module will provide access for this novel generation of IR gas sensing systems to a wide variety of volatile organic compounds at trace concentration levels relevant to process control, industrial effluent analysis and environmental atmospheric monitoring.