A small, lightweight, low-power instrument, denoted a proton-transferreaction/ ion-mobility spectrometer (PTR-IMS) has been developed for detecting airborne organic compounds at concentrations in the sub-parts-per-billion range. Instruments like this one could be used on distant planets (such as Mars) to search for trace organic compounds indicative of life as well as numerous potential terrestrial uses: A few examples include medical applications (e.g., analyzing human breath to detect compounds associated with certain deadly diseases such as lung cancer and cirrhosis of the liver), lawenforcement applications (detecting airborne traces of explosives and drugs), environmental monitoring (detecting airborne pollutants and toxins), and military applications (detecting chemical warfare agents).

The Hollow-Cathode Ionizer generates reactant ions which then ionize target organic species.The target ions are then analyzed in the ion-mobility spectrometer.

Portable gas-chromatography /massspectrometry (GC-MS) instruments that have commonly been used heretofore for detecting airborne organic compounds are characterized by three major shortcomings: (1) insufficient sensitivity for detecting sub-parts-perbillion concentrations, (2) susceptibility to undesired fragmentation of large organic molecules, and (3) the need for high vacuum and thus for high-vacuum equipment, which contributes greatly to size, weight, and mechanical complexity. In contrast, the PTR-IMS offers sensitivity adequate for detecting concentrations at the parts-per-billion level; operates in such a manner as not to fragment large organic molecules; and requires only a partial vacuum that can be generated by equipment smaller, lighter, and less complex than that needed for GC/MS.

The PTR-IMS includes a hollowcathode ionizer, HCI (see figure), that is designed to generate reactant ions (RH+, where R is a reactant molecule) and to exploit a proton-transfer reaction. The HCI operates at a pressure in the approximate range of 1 to 5 torr (≈ 0.1 to 0.7 kPa). H2O is introduced into the discharge region inside the HCI, giving rise to the proton- transfer reaction

RH+ + M → MH+ + R,

where M is the target molecule and MH+ is the desired product ion. R is chosen to be a molecule that has a proton affinity slightly less than that of M, in which case the probability for fragmentation channels of the proton- transfer reaction is low and the process of “soft” ionization is dominant. Moreover, the reaction is highly selective: Molecules that have proton affinities lower than that of R do not enter the reaction. H3O+ is the most suitable proton-donor reactant ion for investigating trace chemical species in either Martian or Earth air because H3O+ does not react with CO2, CO, H2O, O2, N2, He, Ne, Ar, or Xe.

The ions produced by the HCI are introduced along with a sample of air into a reaction chamber. Product ions generated in the reaction chamber are detected and analyzed in the IMS, which was chosen over conventional mass spectrometers and other instruments because it offers the sensitivity needed for detection in the sub-partper- billion range, can handle a wider range of molecules, and does not require a high vacuum (it can even operate at normal terrestrial atmospheric pressure). In the IMS, a bias voltage produces an electric field that causes the reactant and product ions to drift in a desired direction. At the downstream end of the drift region, ions are detected by use of a high-gain electrometer; typically, the detected ion current can be on the order of a picoampere. A microprocessor controls the operation of the instrument and the acquisition, processing, and display of data.

In order to enable the determination of drift velocities, the ions are introduced into the drift region in pulses at time intervals typically between 20 and 40 ms. Within the drift region, the ions undergo spatial separation based on both mass and shape. For a given electric-field strength, the drift velocity of a given ion species is directly proportional to its specific mobility. Smaller ions tend to travel faster. By measuring the ion-drift times under a particular set of conditions, one can construct ion-mobility tables that can be used for identification of unidentified target ion species.

This work was done by Isik Kanik and Luther Beegle of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
Mail Stop 202-233
4800 Oak Grove Drive Pasadena
CA 91109
(818) 354-2240

Refer to NPO-21187, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Portable Instrument Detects Very Dilute Airborne Organics

(reference NPO-21187) is currently available for download from the TSP library.

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This article first appeared in the March, 2002 issue of NASA Tech Briefs Magazine.

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