CO2 laser-based pulsed photoacoustic ammonia detection

ARTURO VALLESPI; VERÓNICA SLEZAK; ALEJANDRO PEURIOT; GUILLERMO SANTIAGO

INTERNATIONAL JOURNAL OF THERMOPHYSICS

SPRINGER/PLENUM PUBLISHERS

Lugar: New York; Año: 2013 p. 1661 - 1665

0195-928X

Detecting ammonia traces is relevant in health, manufacturing and security areas, among others. Since ammonia presents a strong absorption band (the ν2 mode) around 10 m, we study some of the physical properties which may influence its detection by means of pulsed photoacoustic (PA) spectroscopy with a TEA CO2 laser. The characteristics of the ammonia molecule and the laser intensity may result in a non-linear dependence of the PA signal amplitude on the laser fluence. We found that ammonia absorption can be described as a simple two-level system with power broadening. As NH3 is a polar molecule it strongly undergoes adsorption phenomena in contact with different surfaces. Therefore, physical adsorption-desorption at the cell´s wall is studied. A theoretical model, based on Langmuir?s assumptions, fits well to the experimental results with stainless steel. Related to these studies, measurements led to the conclusion that, at the used fluence values, dissociation by multiphotonic absorption at the 10P(32) laser line may be discarded. A calibration of the system was performed and a detection limit around 190 ppb (at 224 mJ/cm2) was achieved.