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Emissions of ammonia into the environment are mainly caused by agriculture, but also by combustion processes in waste and by road traffic. Even at low concentrations, this substance is not only an odour nuisance, but also a substance with ecological and climatic relevance. Therefore, BAM tested commercial electrochemical, and metal oxide based sensors, which have limited suitability for measuring in the environmental molar fraction range. Alternatively, own developments for the detection of ammonia in the trace range were implemented, wherein the analyte is measured by changing the fluorescence of a BODIPY dye at 550 nm by means of a portable fluorescence sensor directly from the gas phase. For the calibration of ammonia sensors and measuring instruments, a stationary system based on the mixture of certified test gases from pressure cylinders with calibrated mass flow controllers is available. A test gas generator was developed for on-site calibration and testing of sensors and measuring devices. The generation of ammonia-containing gases in the environmental relevant range of levels below 1000 nmol/mol is carried out by the permeation method according to ISO 6145-10. For the traceability of ammonia, standards are provided and further developed by the National Metrological Institutes and designated institutes. Metrological standards are based on SI units and are a basis for traceability of sensors or gas analysers.
Ammonia and its conversion product ammonium have a strong negative impact on human health and ecosystems. Most ammonia measurements in ambient air are performed in the molar fraction range (0.5 to 500) nmol/mol. There is a need for reliable traceable ammonia gas standards as well as in situ analytical procedures for the monitoring of ammonia in ambient air.
The permeation method is an effective tool for dynamically generating precise gas standards with a low uncertainty in the concentration range of a few nmol/mol to several µmol/mol in an inert carrier gas, e. g. pure nitrogen or purified ambient air. Here, we present our ammonia gas standard generator as well as results of the characterisation of its individual components supporting the uncertainty assessment according to GUM for stable gas concentrations in this range.
In order to detect ammonia in the nmol/mol-range, a suitable sensor has to be developed. In this contribution, we therefore additionally present first approaches on the development of such a sensor using optical fluorescence as transduction mechanism due to its intrinsically high sensitivity and high spatial resolution. Incorporation of a fluorescent dye, which shows fluorescence enhancement in the presence of ammonia, into a polymer matrix allows to reversibly recognize low amounts of ammonia. It can be concluded that fluorescence sensor is a robust tool for measurements of ammonia; however it needs calibration for the planed use.