The Haber-Bosch process, which produces industrial ammonia (NH3), contributes 1.2% of the world's CO2 emissions, making it a major climate change contributor. Therefore, it has become crucial for the scientific community to create new processes for ammonia production. In the last ten years, electrochemical nitrogen (N2) conversion to ammonia has gained popularity as a study area because it offers the potential to replace the energy- and environmentally-damaging Haber-Bosch process.
However, since this field is still in its infancy, the ammonia production rates are often quite low, making it easy for the observed levels to come from many sources of contamination (unstable N-containing compounds, N2 gas source, human breath, air, etc.).
In recent studies, rigid isotopic labeling measures were included in investigations on converting nitrogen to ammonia. Surprisingly, it was discovered that the most active aqueous solution metallic catalysts did not produce any NH3. This finding shows that, when 14NH3 and N2 gases are present, NH3 detection alone is insufficient and that, in addition, 15NH3 detection is required. A technique like this may guard against false-positive findings while also revealing the existence of pollutants.
We need analytical techniques that can be linked in-line and offer (quasi-) real-time information on product generation to conduct mechanistic investigations with the requisite time resolution.
The isotopologues of NH3 (14NH3 and 15NH3) are often utilized in physiology's metabolic tracing investigations, which aid in identifying the cell's primary biosynthetic routes. Environmental monitoring, including industrial process control and exhaust gas measurement, is yet another essential use (e.g., propulsion, pharmaceutical, and chemical).
PA Detection Method
Photoacoustic detection enables the in situ and nondestructive measurement of isotopes. A potent method to assess concentrations at low levels is photoacoustic spectroscopy. In a photoacoustic detector, molecular absorption of modulated optical light in gases, liquids, and solids results in the generation of acoustic pressure waves recorded by a microphone. In gases, the relationship between the produced sound's amplitude and the concentration of the absorbed gas component is direct.
How the Study was Conducted
By using a near-infrared photoacoustic (NIR-PA) system, this study intended to establish a relatively simple but reliable, completely autonomous, and a durable system for the selective, quick, and sensitive detection of ammonia isotopes.
Previously, there was just one study on measuring 14NH3 and 15NH3 selectively, and the photoacoustic technique effectively estimates NH3 concentration in general. This work outlines a newly designed photoacoustic approach for the quick and simple simultaneous detection of 14NH3 and 15NH3 isotopologues.
Components of Gas Production System
The gas production system was used to carry out spectral measurements, calibrations, cross-sensitivity analyses, and response time studies. The two primary components of the gas production system include the NIR-PA system, run either by an external cavity diode laser (ECDL) or a distributed feedback (DFB) diode laser, and the ammonia gas generating unit, which produces different combinations of 14NH3 and 15NH3.
Modes of Gas Producing Unit
The gas-producing unit was operated in either a mass-flow controller mixing mode or a mode based on chemical reactions. The system's chemical reaction component is skipped in the first mode of operation, and the gases from two cylinders are combined using precise mass-flow controls.
For the chemical reaction-based producing mode, a nitrogen cylinder was used to purify the gas mixture created in the system's chemical reaction part through the photoacoustic cell.
The photoacoustic spectra of the 14NH3 and 15NH3 isotopologues were obtained using gas samples from gas cylinders and the chemical reaction, respectively. Measurement wavelength optimization started by utilizing an ECDL to record the photoacoustic spectra of the two isotopologues and water vapor.
The operating software of the NIR-PA system separates the collected photoacoustic signals into two values, PA14 and PA15, with a high sensitivity to the isotopes 14NH3 and 15NH3 and a low cross-sensitivity to water vapor after measuring at each of the selected wavelengths.
The researchers developed a photoacoustic system based on a DFB diode laser for the sensitive, precise and rapid detection of isotopically tagged NH3.
The measurement wavelengths of the isotopologues that are among the strongest in the intended wavelength range and lie sufficiently near one another were selected to construct the system operating software so that a concentration measurement cycle could be completed in less than a second. This made it possible to fully use the photoacoustic detection method's innately rapid response even in challenging circumstances.
Due to its short response time, robustness, and high sensitivity, the system is projected in various real-world applications, from electrocatalytic N2 production to biological research.
Emily Awuor Ouma, Helga Huszár, László Horváth, Gábor Szabó, Csaba Janáky, and Zoltán Bozóki (2022) Development of a Near-Infrared Photoacoustic System for Selective, Fast, and Fully Automatized Detection of Isotopically Labeled Ammonia. Analytical Chemistry. https://pubs.acs.org/doi/10.1021/acs.analchem.2c01191
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