A method of in situ chemical analysis by use of small, simple, robust sensors has been proposed. The basis for the proposal is the discovery that chemical reactions emit acoustic waves, and that characteristic frequency spectra are associated with specific reactions. The acoustic frequencies of interest range from about 0.1 to several megahertz. If a newly recorded acoustic spectrum were to match a previously recorded spectrum of a known chemical reaction, then the newly recorded spectrum would be deemed to indicate the presence of that reaction. The amplitude of a spectrum would be taken as an indication of the rate of the corresponding reaction.

A chemical-analysis instrument based on this concept would include an array of miniature piezoelectric acoustic sensors, each with a typical area of no more than a fraction of a square millimeter. The sensors would be able to withstand accelerations up to about 105 times that of normal Earth gravitation (about 106 m/s2).

The instrument would include circuitry for flash analog-to-digital conversion of the sensor outputs, plus a moderately powerful digital data processor that would Fourier-analyze the digitized sensor outputs to obtain acoustic spectra. The instrument would also include electronic circuitry for (1) matching the newly acquired spectra with previously recorded spectra of known reactions and/or (2) recording and/or transmitting the newly acquired spectra for further processing by a computer to find matches to known spectra. The durations of typical chemical reactions of interest range from tens of seconds to hours; on this time scale, there would be no difficulty in time-multiplexing readouts from multiple sensors.

In the original proposed application, such an instrument would be used to find the chemical compositions of gases and dust in the Martian atmosphere, soil, and/or rocks. A similar terrestrial use would be monitoring gaseous, liquid, and solid pollutants in the field. For this purpose, the piezoelectric transducers would be coated with materials that engage in known chemical reactions with the pollutants or other substances of interest. Coatings could include adhesives to assist in collection of dust (and/or magnets to assist in collection of magnetic dust). Alternatively or in addition, reactive liquids could be injected into reaction sites at analysis time. Because the sensors would have low thermal masses, heating the sensors to reaction temperatures (if necessary) would consume little power -- no more than milliwatts at the highest temperatures likely to be needed.

This work was done by Frank Hartley of Caltech forNASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category.


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In Situ Chemical Analysis via Acoustic-Emission Spectra

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This article first appeared in the January, 1999 issue of NASA Tech Briefs Magazine.

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