Sampling small volumes of ambient ammonia using a miniaturized gas sampler

The development of a gas sampler for a miniaturized ambient ammonia detector is described. A micromachined channel system is realized in glass and silicon using powder blasting and anodic bonding. The analyte gas is directly mixed with purified water, dissolving the ammonia that will dissociate into ammonium ions. Carrier gas bubbles are subsequently removed from the liquid stream through a venting hole sealed with a microporous water repellent PTFE membrane. A flow restrictor is placed at the outlet of the sampler to create a small overpressure underneath the membrane, enabling the gas to leave through the membrane. Experiments with a gas flow of 1 ml min–1, containing ammonia concentrations ranging from 9.4 ppm to 0.6 ppm in a nitrogen carrier flow have been carried out, at a water flow of 20 µl min–1. The ammonium concentration in the sample solution is measured with an electrolyte conductivity detector. The measured values correspond with the concentration calculated from the initial ammonia concentration in the analyte gas, the fifty times concentration enhancement due to the gas–liquid volume difference and the theoretical dissociation equilibrium as a function of the resulting pH

Ammonia sensors and their applications - a review

Many scientific papers have been written concerning gas sensors for different sensor applications using several sensing principles. This review focuses on sensors and sensor systems for gaseous ammonia. Apart from its natural origin, there are many sources of ammonia, like the chemical industry or intensive life-stock. The survey that we present here treats different application areas for ammonia sensors or measurement systems and different techniques available for making selective ammonia sensing devices. When very low concentrations are to be measured, e.g. less than 2 ppb for environmental monitoring and 50 ppb for diagnostic breath analysis, solid-state ammonia sensors are not sensitive enough. In addition, they lack the required selectivity to other gasses that are often available in much higher concentrations. Optical methods that make use of lasers are often expensive and large. Indirect measurement principles have been described in literature that seems very suited as ammonia sensing devices. Such systems are suited for miniaturization and integration to make them suitable for measuring in the small gas volumes that are normally available in medical applications like diagnostic breath analysis equipment

Micro evaporation electrolyte concentrator

The sensitivity of miniaturized chemical analysis systems depends most of the time on the obtainable detection limit. Concentrating the analyte prior to the detection system can enhance the detection limit. In this writing an analyte concentrator is presented that makes use of evaporation to increase the ion concentration of an electrolyte. The evaporation rate can be enhanced using forced convection. In order to control the evaporation rate a nitrogen flow is fed over a liquid channel covered with a hydrophobic vapor permeable membrane.Water vapor can pass through this membrane in contrast to water itself because of the hydrophobic nature of the membrane surface. An electrolyte conductivity detector is used to measure directly the concentration effect as a function of the nitrogen flow velocity. The influence of the convective nitrogen flow and the residence time of the analyte inside the concentrator are investigated in this paper. It is shown that the evaporation rate is enlarged with an increase in convective flow. The concentration effect is also enhanced when the residence time of the analyte inside the concentrator is increased. The higher concentration enhancement due to the longer residence time, however, results in an increase in water vapor present in the nitrogen flow. This results in a lower normalized evaporation rate when the available evaporation time is enlarged

Miniaturized measurement system for ammonia in air

The development of a miniaturized ammonia sensor made using microsystem technology is described. Gas is sampled in a sampler comprising two opposite channels separated by a gas permeable, water repellent polypropylene membrane. Subsequently, the acid sample solution is pumped into a selector where an alkaline solution is added to ionize all sampled ambient acid gasses, resulting in an enhanced selectivity. In the selector, the ammonia can diffuse through a second membrane into a purified water stream where an electrolyte conductivity sensor quantifies the resulting ammonium concentration. The realized system is shown to be selective enough not to be influenced by normal ambient carbon dioxide concentrations. Experiments with a gas flow of 3 ml/min, containing ammonia concentrations ranging from 9.8 to 0.3 ppm in a nitrogen carrier flow, into a 15 μl/min sample solution flow and finally into a 5 μl/min purified water stream have been carried out and show that the system is sensitive to ammonia concentration below 1 ppm. \u