Miniature triaxial metastable ionization detector for gas chromatographic trace analysis of extraterrestrial volatiles

Abstract

Gas chromatography has found highly successful application in NASA's flight programs. Gas chromatographs have been flown to both Mars and Venus where detailed compositional measurements were made. These instruments were quite small and relatively sensitive when compared to commercially available instruments; however, they do not appear adequate for future missions currently being planned. The earlier flight GC's had incorporated thermistor bead thermal conductivity cells as the detector. This detector requires very precise temperature control and only provides about 1 ppm sensitivity. Temperature stabilization causes the detector to be quite heavy, i.e., about 200 g. Greater sensitivity will be required for measurements of trace components in extraterrestrial environments. Review of other detector types revealed the metastable ionization detector as a likely candidate because of its superior thermal stability and high sensitivity. The metastable detector, first described by Lovelock as an argon ionization detector, has been studied and somewhat modified by others. The commercial design by Hartmann and Dimick was used for comparison purposes in our work. In the past, three features of the metastable detector are prominent: it has part-per-billion sensitivity, contamination must be carefully controlled, and anomalous response is common. Since it is an ionization detector, however, temperature instabilities do not cause the major perturbations experienced by the thermal conductivity detectors. This paper describes a miniature metastable ionization detector featuring an unconventional electrode configuration, whose performance characteristics parallel those of traditional design, while its weight is quite small. The prototype has been used in our laboratories routinely for 2 years, and the concept will be incorporated into a flight GC for use in the Space Shuttle.

Determination of C1-C4 alkanes by ion mobility spectrometry

“…Figure 3 shows a helium ionization IMS design with a specialized reaction region. The two small metastable ionization (MI) electrodes in the reaction region are similar to the electrodes in the triaxial metastable ionization detector (MID) (8) used in trace analyses at ARC. The electrometer and control system used were also custom designed and built at ARC but are similar to the controls for commercial IMS systems.…”

“…Several further descriptions of this detector and its ionization processes can be found in the literature including articles by Hartmann and Dimick (6), and Andrawes and Ramsey (7). The NASA version of this detector, the metastable ionization detector (MID) (8), is a component of the Cometary Ice and Dust Experiment (CIDEX) (9), currently part of the Comet Rendezvous Asteroid Flyby mission (CRAF). When combined with the NASA-developed voltage modulator (10), the MID sample response range extends contiguously from 1 ppb to thousands of ppms.…”