Detection of Gas-Phase Ammonia Using Photothermal Interferometry.

Ammonia in the atmosphere plays an important chemical role in diverse ecological and physical systems. It is challenging to measure because it adheres to many commonly used sampling materials, is soluble in water, and forms particles. A new instrument has been built to detect gas -phase ammonia in ambient air by photothermal interferometry. Ammonia molecules in sampled air are excited to upper vibrational rotational states of the nu_2 band of ammonia by absorption of 9.22 mu m light from the 9R30 line of a carbon dioxide laser. Excited ammonia molecules transfer this energy as heat to the air by collisions. The temperature and consequent refractive index change is measured as a phase shift in one arm of a sensitive homodyne interferometer. The CO_2 laser output is modulated at 1.2 kHz, and the AC signal from the interferometer is measured with a lock -in amplifier. The +/-2sigma detection limit of the interferometer at this frequency is 1.2 times 10^{ -7} radians, only a factor of four above the quantum noise limit. The ammonia detector is calibrated by dynamic dilution of two permeation tube outputs and by standard addition. Permeation rates are verified by NH _3 conversion to nitric oxide on hot platinum foil followed by chemiluminescent NO detection, and are monitored by mass loss history. The sample cell, inlet and calibration plumbing is glass to minimize NH _3 sample loss by wall adsorption. Sample flow is 2 l/min, with residence time less than one second. The detector is zeroed by sampling through tubing filled with H_3PO_4-coated Teflon. A Stark signal shift of 7.9 mV was measured for 507 ppb of NH_3 with 925 V/mm of applied field density as an alternative zeroing method. The linear dynamic range of the detector is broad, (greater than 5 orders of magnitude), and the detector is sensitive, with a 31 ppt 4sigma lower detection limit measured in 100 s. Signal gain is insensitive to water vapor and CO_2. The instrument response is fast, with a 1 s e-folding response time to a large step change in ammonia concentration, and corresponding 1 s 4sigma detection limit of 250 ppt.