In the early 1990's, the Energy research Centre of the Netherlands developed a measurement system for environmental monitoring of gaseous ammonia. For some applications, e.g. diagnostic breath analysis, the analyte gas consumption of the device is too demanding. In this thesis, the next step in the development of the ammonia sensing device is described. The required amount of analyte gas is reduced from several litres to less than hundred ml by miniaturization and integration of the device on a chip. The detection limit of about 1 ppb, and the response time of about 2 minutes, is accurate enough for breath analysis.
Summary
The development of a miniaturized and integrated measurement system for gaseous ammonia is described in this thesis. The measuring principle, “AMINA”, is an indirect method for selectively measuring ammonia that makes use of pH-transitions, electrolyte conductivity detection and phase-separating membranes. It is based on an environmental ammonia monitor developed by the Energy research Centre of the Netherlands, ECN. Reduction of the analyte gas volume and reagents consumption as well as an increase in speed would offer opportunities for new applications, e.g. diagnostic breath analysis. This was accomplished by miniaturization of the commercially available “AiRRmonia”, in the project that resulted in this thesis. The key elements, a gas sampler, selector and electrolyte conductivity detectors, were realized on a chip using micro-system technology. The system dimensions were optimized using mass-transport simulations. The conductivity detector with planar interdigitated electrodes was optimized for measuring low ion concentration in a small volume. Ultimately, an integrated sensing chip was realized with an analyte gas consumption that is reduced by a factor of more than 100 and a response time that is reduced from about 20 to 1.6 minutes, using a gas flow of
50 ml/min. The sensing chip has a calculated lower ammonia detection limit of 1.1 ppb. This detection limit and the small analyte gas consumption make the sensing chip suitable for measuring breath ammonia, where lower concentration levels of about 50 ppb can be found. Under normal atmospheric conditions the selectivity of the system is sufficient to measure ammonia concentrations in the low-ppb range. The system is even sufficiently selective to be used in analyte samples that contain elevated carbon dioxide levels, like exhaled air. The characteristics of the presented sensing device could be further optimized by increasing the efficiency of the used gas sampler. Two alternatives are proposed; a direct gas-liquid mixer with an integrated degasser that is shown to effectively sample ammonia from an analyte gas and a thin micromachined membrane with a self-aligning buried channel, resulting in decreased mass transport distances.