The U.S. National Library of Medicine defines ammonia as a “colorless inorganic compound of nitrogen and hydrogen with the formula NH3, usually in gaseous form with a characteristic pungent odor. Ammonia is irritating to the skin, eyes, nose, throat, and lungs. It is essential for many biological processes and has various industrial applications.”
There are ammonia analyzers that enable professionals to measure ammonia (NH3) concentrations in ambient air. These analyzers utilize chemiluminescence technology for a reaction that produces a characteristic luminescence with intensity proportional to the amount of Nitric Oxide (NO).
State-of-the-art ammonia analyzers utilize several methods of operation. Some examples:
- Mode 1: The basic operating principle in all modes for the measurement of NO, the sample is mixed with ozone in the reaction chamber. This reaction produces a characteristic luminescence with intensity proportional to the concentration of NO.
- Mode 2: In the NOx mode, the sample is passed through a molybdenum convertor which reduces any NO2 in the sample to NO. This is then transported to the reaction chamber where the sample is measured as NO + NO2 converted to NO, and reported as NOx.
- Mode 3: In the third mode, the sample is passed through a high temperature convertor before going to the analyzer where the NH3 is converted to NO for analysis. This is then measured as NO + converted NO2 + converted NH3, and reported as Nt (Ntotal).
The software subtracts NO from NOx and NOx from Nt and provides outputs of NO2 and NH3 respectively. Some models can output NH3 along with NO, NO2, NOx, and Nt to the display or electronic outputs. (See Ammonia Chemiluminescent Gas Analyzer product specification sheet.)
These analyzers are highly reliable, but should be calibrated to ensure results are consistently accurate. Below are tips for calibrating Thermo Scientific Model 17i Ammonia Analyzer, but one should always follow the specific instrument’s operating instructions supplied by the manufacturer.
Calibration factors are used to correct the NO, NO2, NOx, NH3, and Nt concentrations readings that the instrument generates using its own internal calibration data. Normally, the instrument is calibrated automatically; however, the instrument can also be calibrated manually using the Calibration Factors menu on the instrument.
The NO, NOx, and Nt background corrections are determined during zero Calibration. 100% N2 or zero air is acceptable to perform zero calibrations.
For span calibrations, typically NO balance N2 is used for NO/NOx/Nt. For NH3 calibrations, NH3 balance N2 is typically used. For NH3, however, you should not run the calibration gas for long periods of time as it can strip the iron oxide layer from the NH3 hi temp converter. If NO2 is to be calibrated, a GPT (Gas Phase Titration) test should be run same as for NO/NOx analyzer to determine converter efficiency.
You cannot obtain a NO2 calibration gas bottle at a low enough concentration for this test. Using a multi-gas calibrator to dilute NO2 gas is also NOT recommended as when diluting the NO2 with air the NO2 can react and turn into NO skewing the results low. A GPT test is how to perform calibrations on the NO2 channel.
For the Model 17i, O2 is not necessary for calibrations but is for reading sample gas. Calibrations are typically run for only a short amount of time and will not deplete the iron oxide layer in the high temp NH3 converter. The sample must contain the O2 level to keep the self-regenerative iron oxide layer intact over longer periods of time. If O2 is used in the calibration zero or span gas, that is also acceptable.
For additional details, consult the user manual or contact your manufacturer’s representative.