Spectrophotometry is extensiveley used in off-line characterisation of chemical processes and environmental and safety parameters; on-line measurements are currently prohibitively expensive. An economical rugged field compatible instrument will extend the applicability of these measurements significantly offering a substantially widened market place for spectrophotometric measurements. Applications of specific interest to the consortium include groundwater monitoring, land fill control and air monitoring in mining.
The aim of this project is to develop on-line spectroscopic measurement systems for use in remote locations such as mining, nuclear and petrochemical plants using a small, compact, portable, rugged and inexpensive spectrophotometer design. The principle instrument uses evanescent wave technology developed for optical communication purposes as the basis for a solid state, low weight, inexpensive, electronically tunable spectral measurement instrument (MSP). Three different probes were to be developed nl. pH, CO2 and CH4.
The project has established the design and fabrication procedures for the optical system and established the tuning capabilities, selectivity and sensitivity of the instrument and of the optical probes. Due to several problems such as internal stresses introduced during the deposition process of the triple thin layer structure and distortion in the optical spectrum, the inherent costs and yield problems with the multiple layer deposition process would probably make for an expensive device. Therefore in the context of pH measurement the MSP has been overtaken by the availability of good quality very compact spectral measurement devices which came recently on the market.
The monitoring of pH over a wide range (2-10) in solutions has been assessed with immobilisation procedures and technical performances of a variety of pH sensitive dyes. For instance Bromophenol blue and modified N,N-dimethal aniline gave very stable and sensitive results in the pH range of 2-10 in solutions varying over three orders of magnitude in ionic strength. Furthermore methyl red is a very good candidate as a single dye sensitive in the pH range of 2-9.
In gain enhanced mode the MSP is combined with an optical active medium where this concept shows considerable promise offering very narrow band tunability. Further attention to the cavity design and the stabilisation electronics produced a single mode system with linewidth of below one MHz, typically 100 kHz locked to an external low finesse cavity simulating a gas cell. A transmissive methane gas detection cell demonstrated detection thresholds in the region of ppm which is more than adequate for the safety system market which requires to alert at the lower explosive limit (5% by volume methane).
Furthermore a coated carbon dioxide membrane evaluation showed that the apparent response curves varied widely with humidity. This appears to offer the possibility for a humidity insensitive carbon dioxide sensor for gaseous CO2.
Finally the multiplexing study reported that the sensitivity of the gain enhanced MSP allows to address an array of detector cells.
Funding SchemeCSC - Cost-sharing contracts
TQ6 0EB Kingswear
TQ2 7AY Torquay