How to Calibrate an Ammonia Analyzer

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 N_t (N_total).

The software subtracts NO from NOx and NOx from N_t and provides outputs of NO₂ and NH₃ respectively. Some models can output NH₃ along with NO, NO₂, NOx, and N_t 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 Tips

Calibration factors are used to correct the NO, NO₂, NOx, NH₃, and N_t 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 N_t 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/N_t. 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.