Three Methods of Detection of Hydrazines

Concentrations could be measured more accurately than in prior methods.

Three proposed methods for measuring trace quantities of hydrazines involve ionization and detection of hydrazine derivatives. These methods are intended to overcome the limitations of prior hydrazine-detection methods.

Hydrazine (Hz), monomethylhydrazine (MMH), and unsymmetrical dimethylhydrazine (UDMH) are hypergolic fuels and are highly reactive, toxic, and corrosive. A capability to measure concentrations of hydrazines is desirable for detecting leaks and ensuring safety in aerospace settings and in some industrial settings in which these compounds are used. One of the properties (high reactivity) that make it desirable to detect trace amounts of hydrazines also makes it difficult to detect hydrazines and measure their concentrations accurately using prior methods: significant amounts are lost to thermal and catalytic decomposition prior to detection. Further complications arise from the “sticky” nature of hydrazines: Sample hydrazine molecules tend to become irreversibly adsorbed onto solid surfaces with which they come into contact during transport to detectors, giving rise to drift in detector responses.

altIn each proposed method, the reactive, sticky nature of hydrazines would be turned to advantage by providing a suitably doped substrate surface with which the hydrazines would react. The resulting hydrazine derivatives would be sufficiently less sticky and sufficiently more stable so that fewer molecules would be lost to decomposition or adsorption during transport. Consequently, it would be possible to measure concentration with more sensitivity and less error than in prior techniques.

The first proposed method calls for the use of a recently developed technique known as desorption electrospray ionization (DESI), in which a pneumatically assisted micro-electrospray at ambient pressure is directed at a surface of interest. In this case, the surface of interest would be that of a substrate described above. The impingement of the electrically charged micro-droplets in the spray upon the substrate would dislodge and ionize the hydrazine derivative molecules, giving rise to stable ejected hydrazine derivative ions, which would then be detected and quantitated by use of a mass spectrometer, ion-mobility spectrometer, or other suitable instrument (see figure).

The second proposed method calls for the use of another recently developed technique known as desorption atmospheric-pressure chemical ionization (DAPCI), in which an atmospheric-pressure corona discharge in the vapor of toluene or another suitable compound is used to generate projectile ions. In this case, the ions would be made to impinge on the substrate, with consequent ejection and ionization of stable hydrazine derivative ions as described above. Again, as described above, the hydrazine derivative ions would be detected and quantitated by use of a mass spectrometer, ion-mobility spectrometer, or other suitable instrument.

In the third proposed method, one would use yet another recently developed desorption-and-ionization technique known as direct analysis in real time (DART). In this technique, a plasma containing excited-state atoms and ions is formed in a gas (e.g., helium or nitrogen) that has a high ionization potential. The excited-state atoms and ions impinge on the surface of a solid sample, causing desorpion of low-molecular weight molecules from the sample. In the proposed method, the sample would be the substrate, from which the hydrazine derivative molecules would be desorbed. The desorbed hydrazine derivative molecules would then be ionized and detected as described above.

This work was done by Timothy Griffin of Kennedy Space Center and Cristina Berger of ASRC Aerospace Corp. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. KSC-13121/2/3