860
Anal. Chem. 1984, 56,860-862
Miniature Triaxial Metastable Ionization Detector for Gas Chromatographic Trace Analysis of Extraterrestrial Volatiles Fritz H. Woeller,* Daniel R. Kojiro, and Glenn C. Carle Solar System Exploration Office, NASA-Ames
Research Center, Moffett Field, California 94035
Gas chromatography has found highly successful application in NASA's flight programs. Gas chromatographs have been flown t o both Mars ( I , 2 ) and Venus (3, 4) 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 t o 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 (5),has been studied and somewhat modified by others. The commercial design by Hartmann and Dimick (6) 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.
EXPERIMENTAL SECTION The detectors were assembled into breadboard GC instruments fitted with commercial gas sampling valves (Carle, Valco), using sample loop volumes of 100 pL and 1 mL. The detector was mounted on top of the input head of a Keithley 642 electrometer (Keithley Instruments, Cleveland, OH). The detector was electrically shielded but special temperature control was not used. One detector electrode was connected directly to the electrometer input with a short spring. The other electrode was connected through a 1.5-MQ resistor to a Keithley 240 A power supply (0-1200 V dc). The resistor is required to limit current, thus supressing arcing between electrodes as a result of inadvertent overvoltage. The output of the electrometer was connected to a Hewlett-Packard 3388 integrator. For comparison studies, He ionization detectors from the Varian Model 2732 trace gas analyzer were substituted directly for the miniature detector in the breadboard instrument. The breadboard was fitted with 1 mm i.d. porous polymer columns of the type Porapak N, Porapak Q, or custom-made poly(diviny1benzene)beads (7). Carrier gases were 99.999% He (Matheson) and 99.996% Ne (Linde). Cylinders were fitted with high-purity type, welded-diaphragm regulators (Tescom). Additional purification was effected by Matheson 8302 Hydrox Purifiers to remove traces of 0, and H20. The purifiers were preconditioned by attaching an old column at the exit as flow resistance and flowing gas while the purifier was turned on for at least 72 h prior to connecting it into the analytical GC. During operation in the carrier gas system, the purifiers were operated
at half the rated voltage. This satisfactorily removed trace contaminants while suppressing troublesome outgassing experienced with new, unused purifiers. The miniature metastable ionization detector is shown in Figure 1. It has an internal volume of 80 KLand weighs about 1g. The body of the detector is formed by a tube 17 mm long with a bore of 3.2 mm. Brass or steel has been used in construction interchangeably. A 0.44 mm i.d. vent tube is provided in the side of the body 13.75 mm from the upper end of the body. The body supports a cylindrical 0.2 mm thick Ti foil coated with Ti3H2(250 mCi nominal) on the surface facing the electrodes. The cylinder is formed by carefully rolling a flat 12.5 mm square foil over a smooth 3.5 mm rod. The cylinder is then press-fitted into the detector body. The two electrodes are fabricated from 0.55 mm diameter Nichrome wire which was stretched to 0.43 mm diameter to straighten and stiffen the wire. The electrodes are held in place by a 3.2 mm 0.d. insulator made from AD-99 alumina (Coors Porcelain, Golden, CO) with two parallel longitudinal holes. Each electrode is fitted with a 0.74 mm 0.d. by 0.44 mm i.d. stainless steel sleeve slightly longer than the procelain insulator which provides the proper electrode spacing and alignment. The electrodes and sleeves are then fitted into the insulator so that the sleeve is extended slightly beyond either end of the insulator. The electrodes are measured to be extended 14.5 mm from the end of the insulator into the detector interior, while on the external end, any convenient length of electrode wire may be left for electrical connections. The resulting gap between electrodes is 0.8 mm and the closest electrode-to-wall gap is 0.45 mm. The insulator containing the electrodes is inserted into the detector so that the electrodes are 1.5 mm back from the top of the body. The detector body is fitted with a brass or stainless steel cap of 4.2 mm i.d. to which a carrier inlet tube of 0.44 mm i.d. is attached. The cap shown here extends 7 mm down over the body. The detector components, i.e., body, insulator, sleeves, and electrodes, are bonded to their respective neighbors with cyanoacrylate adhesive. The cap is bonded last after the foil has been inserted. The adhesive, after curing by mild heat, has excellent dielectric properties, can be removed by prolonged soaking in warm acetone, and does not contaminate the detector since application is external only. Detector senstivity was verified by repeated 1 : l O dilution of several certified commercial standards near 50 ppm, independent preparation of our own mixtures, and several cross-checks with the aid of exponential dilution.
RESULTS AND DISCUSSION The new configuration permits the physical dimensions of the detector to be small. I t allows a symmetric, two-electrode array within a cylindrical radioactive source that can best be described as a triaxial configuration. The advantage in compactness for coaxial designs was previously pointed out by Lovelock et al. (8) for cross section detectors and by Poole and Zlatkis (9) for electron capture detectors. One significant advantage of the triaxial geometry is that the source is not an electrode so that the source, along with the detector body, and the inlet and vent tubes are all grounded. Consequently, the connecting tubing needs no isolation thus simplifying the electrical and mechanical hookup. Further, in the event of accidental arcing, the source is not affected. Flight applications of a GC detector imply other constraints beyond simplicity of connection and small pysical size. The major constraints are that it must be able to detect a t least 10 ppb concentrations, it must not require large allocations of resources such as carrier gas and power, and it must be
This article not subject to US. Copyright. Published 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 56. NO. 4. APRIL 1984
.c
CARRIER
w2
881
= 2 x 10-1
L
0
Flguo 2. Chromatogram Obtained with miniature detector (example. C2H, = 2 X 10.' = 200 ppb): column. Porapak N. 200-325 mesh. 8 m length. 1 mm i.d.. 20 'C: delenor. 20 OC. 300 Vl1.74 X 10 'A: STEELSLEEVE NICHROME WIRE
Flgure 1. Cutaway view of miniature trlaxial ionization detector. Lengths are in mm. Refer lo the text lor diameters. compatible with other miniaturized components of a flight GC. The miniature triaxial detector was found to easily meet such constraints. In comparing the triaxial detector geometry with the flat-plate style, we found the standing currents to he comparable in magnitude. In the triaxial design, the electrodeto-electrode distance is 0.8 mm vs. 1.0 mm in customary flabplate detectors. On the other hand, the triaxial detector has a much smaller effective electrode surface. T h e closest anode-to-source distance (of significance in noise generation by i3 particles) is 0.45 mm, however, the mean distance is near 1 mm which is of course true in the flat-plate array also. Figure 2 shows a chromatogram obtained with the miniature triaxial detector using He carrier gas at 12.2 scm3/min. T h e minimum detectable concentration was shown to he 10 ppb or less for all components. The measured sensitivity for the triaxial detector was 8 X lo-" mol/s for methane and 9.5 x W5mol/s for carbon dioxide. At this extreme sensitivity, the linear dynamic range was estimated as 6 x lo2 for both gases. The suitability of sensitive ionization detectors for operation at elevated temperature8 has already been reported by others (10). We have found that the thermal drift stability is also excellent. During a temperature change of 30 "C in 15 min, the base line drift at full sensitivity (300 V/ 3 x 10-9 A) for our detector was as small as l(rloA, which corresponds to ten times the width of the base line noise. Clearly this detector does not need significant thermal control and it approaches the sensitivity requirements for solar system exploration. Here, however, the use of He carrier may occasionallyinterfere with the strategy of analysis, calling for a n alternate carrier. Upon substituting Ne (99.996%) as carrier gas, purged by a Matheson purifier a t half-voltage, we obtained almost the same standing current/voltage curve and mass flow sensitivities for CH, and COP as with He carrier. Both the triaxial and the flat-plate design (Varian) were used interchangeably on identical breadboard assemblies, and in the several electrode voltage regimes referred to in the literature as the cross section ( l l ) ,saturation, and multipli-
Row rate. 12.2 scm3/mln: Sample size. 1 cm'(verlica1 scale height = 7 X 10.'' A; range 2.97 lo 3.67 nA).
cation regions (12). Both detector types could he operated at typically 300 V with a standing current of 3 X 10-9A and noise widths of 10-"A. The specific responses were similar for both, after taking all the recommended precautions to ensure system purity (13),and allowing extensive conditioning periods of up to several weeks in some instances when a major changeout of cornponenu had been conducted. Some peak deformities and inversions were ohsewed during conditioning. and we plan to document such phenomena for a forthcoming i n c e peak anomalies only appear t n affect the simplest paper. S analyte species (that is, those with high ionization potentials), we agree with Andrawes et al. (12) that the universal usefulness of the ionization detector as a sensitive, reliable device has been much underrated. The conditions of system purity essential to successful operation can, with some expertise, he brought under control. CONCLUSION The principal goal of developing a miniature, highly sensitive ionization detector for flight applications was nchieved. Improved fabrication techniques will primarily he aimed at replacing the ceramic insulator and adhesive honding by glass-mmetal seals and brazing procedures. I t is anticipated that the first step of a reduction in overall detector mass can be followed by a reduction in the active volume, which would eliminate the need for makeup gas when capillary columns are to be used. ACKNOWLEDGMENT T h e assistance of M.P. Hughes in the acquisition of the radioactive sources and their handling and monitoring has been a great contribution to this work. LITERATURE C I T E D 111 (2;
Ovama. V.
1 : %dahl. 0 J J Geon% Res 1977. 82. 4669-4676. &Own.~F. S.:Adelson. H. E.; Chap&. M C ; CbusBn. Cole. A J ; Crag n. J. 1..Day. R. J.. Debenham. C H.. FoRney. R E ; G1.e. R 1 ; HBNBY. D W : KCODP. J. L.. LOB,. S J , Lawn. J , .r.. POner. W . 0 , Rosiak. 0. T. Rev..Sd. Inshum. 1978. 43. 139-182. Dyama, V. I.: Carle. 0. C.; Wosller. F.; Polbck. J. 0.: Reynolds. R. T.: Craig. R. A. J . Geaphys. Res. 1980. 85 [No. A 13). 7691-7902. Oyama. V. 1.: Carle. 0. C.: WoeIIer. F.; Rocklin. S.;Vagrin. J.; Pone,. W.; Rwbk. G.: Reichwein. C. IEEE Trans. Geosd. Remote Sens;ng 1980. GE-18.85-93. Lovelock. J. E. J . Chmmslogr. 1958. 1 . 35-46. Hartman". C. H.; Dimick. K. P. J . wls Chromatogr. 1966. 4 .
-
(3) (4) (5) (6)
163-167.
(7) WoeIIer. F. H.; Pollock, G. E. J . Chromalogr. 1978. 137-140. ( 8 ) Lovelock. J. E.; Shosmake. 0. R.: ZblkiS. A. Anal. Chem. 1964. 36.
1410-1415.
862
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
(9) Poole, C. F., Zlatkis, A., Eds. "Electron Capture"; Elsevier: Amsterdam, 1981;p 14. (IO) Andrawes, F. F.; Gibson, E. K. HRC CC,J. H@hResolut. Chromatogr. Chromatogr. Commun . 1982, 265-266. (11) Lovelock, J. E. Anal. Chem. 1961, 33, 162-178. (12) Andrawes, F. F.; Brarell, R. S.; Gibson, E. K. Anal. Chem. 1080, 52, 891-896.
(13) Andrawes, F. F.; Byers, T. 8.;Gibson, E. K. Anal. Chem. 1981, 53, 1544- 1545.
for review June 20, lg83. Resubmitted December 2,1983. Accepted January 14, 1984.
CORRECTION Determination of Fluoride at Low Concentrations with the Ion-Selective Electrode Erik Kissa (Anal. Chem. 1983,55, 1445-1448). There is an unfortunate error in the Apparatus section, appearing on page 1445. The fluoride ion selective electrode was used in combination with a single junction electrode, Orion Model 90-01, instead of a double junction electrode. However, a double junction electrode, Orion Model 90-02, can be used when needed.
