In a proposed method for rapidly detecting hydrazine in air at concentrations at or above 10 parts per billion by volume (ppbv), tunable diode lasers (TDLs) and photodetectors would be used to measure infrared absorption spectra of both ammonia and hydrazine simultaneously. In this method, one would take advantage of the fact that (1) ammonia is formed in the decomposition of hydrazine and is always present when hydrazine is present and (2) the spectral features attributable to ammonia are much stronger than those attributable to hydrazine, and thus ammonia can be detected more easily. In a typical situation in which hydrazine is suspected of leaking and in which one could rule out an alternative source of ammonia (e.g., an open bottle of household ammonia solution or window cleaner), an ammonia spectrum could thus be taken as an indication of hydrazine.

Laser Beams of a TDL Spectrometer are reflected several times across a sample space. The proposed instrument could have a similar optical configuration and would utilize two laser beams: one for detecting hydrazine and one for detecting ammonia.

This method could be implemented by a variety of TDL spectrometers. A typical instrument would include a sample cell, one end of which would be occupied by a sensor head containing two near-infrared TDL/photodetector pairs (one pair for hydrazine and one for ammonia) capable of operating at room temperature. A mirror or a retroreflector would also be included on the sensor head. A retroreflector would be mounted on the other end of the sample cell. The various optical components would be positioned and oriented to make each laser beam traverse the cell multiple times to obtain a total optical path length of 40 m. The instrument might resemble the one shown in the figure, which is a previously developed TDL spectrometer. On the basis of previous experience with TDL spectrometers, the proposed instrument could be expected to weigh 5 to 10 lb (≈2.3 to 4.5 kg).

The atmosphere to be sampled would be drawn through the sample cell, the interior of which would be maintained at a pressure of 0.1 atm (≈10 kPa). Extrapolation from experiments on similar instruments indicates that the proposed instrument could detect hydrazine in a concentration as low as 20 ppbv and ammonia in a concentration as low as 2 ppbv. Thus, although the instrument could not measure hydrazine concentrations below 20 ppbv, it could nevertheless give a warning that hydrazine is likely to be present.

This work was done by John Houseman, Chris Webster, Randy May, and Mark Anderson of Caltech for NASA's Jet Propulsion Laboratory.

NPO-20680

Photonics Tech Briefs Magazine

This article first appeared in the September, 2000 issue of Photonics Tech Briefs Magazine.

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