Anal. Chem. 1991, 63,2295-2300
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Determination of C,-C4 Alkanes by Ion Mobility Spectrometry Daniel R. Kojiro* NASA-Ames Research Center, Moffett Field, California 94035 Martin J. Cohen, Robert M. Stimac, and Roger F. Wernlund PCP, Inc., 2155 Indian Road, West Palm Beach, Florida 33409 Donald E. Humphry SETI Institute, 2035 Landings Drive, Mountain View, California 94043 Norishige Takeuchi TMAINorcal, 2030 Wright Avenue, Richmond, California 94804
FUght Instrumentation for the analyses of extraterrestrlal envlronments must often perform under severely restricted condltlonr. Often, the detection and Identlfkatlon of a multitude of chemlcal species ls required to fufflll the rclentlfk objectives of the mkrlon. It Ir therefore important that the analytical Indrumentam have universal rMponre. The gas ChromcrtograpMoCr moMllty spectrometer (GC-I-) has the potential to provkle the sampk separation, klentlflcatlon and sefndtMly necessary for a rucceseful analysis. However, the IMS has poor senrltlvity for fhe C1-C4 alkanes. The abundance of these molecules at varlour extraterrestrlal sHes lo often of great I”nce to exo#doglrk. Thk dudy foCU8e8 on opthnltlng IMS sample lonltatlon mechankms for the C1-C4 alkanes and other hydrocarbons of Interest to 8x0bWogY.
INTRODUCTION A detailed knowledge of the history and abundance5 of the biogenic elements and their compounds throughout the solar system can provide scientists with a basis for understanding the conditions necessary for chemical evolution and the origin of life. Flight experiments conducting in situ analyses have already produced a wealth of information on the environments of Mars and Venus. The Solar System Exploration Office at Ames Research Center has utilized gas chromatography as the analytical technique for two such exobiology flight experimenta. The Gas Exchange Experiment (GEX) (11, onboard the Viking mission to Mars, used a gas chromatograph to analyze changes in soil head space gases as part of a package of life detection experiments. The Pioneer Venus Large Probe Chromatograph (2) analyzed the atmosphere of Venus as the probe descended through the atmosphere. Future missions will require instrumentation capable of providing identification and quantitation of a multitude of molecular species over a wide range of concentrations. Analytical instrumentation that is both highly sensitive and precise, in terms of chemical identification, will be required to successfully carry out such analyses. For example, an analysis of the atmosphere of Titan, for the proposed Cassini mission to the Saturnian system, will involve the detection and quantitation of at least 15 atmospheric gas components (3). Concentrations of many of the compounds of interest are expected to be below 1 ppm (3). The Components of any aerosols, collected and pyrolyzed, can add many more organic molecules to be determined. The gas chromatograph-ion mobility spectrometer (GCIMS) has the potential to provide the sample separation,
identification and sensitivity necessary to do successful atmospheric analyses. The GC can be used to separate a very complex mixture into individual components or perhaps small groups of components. The IMS is then used to detect, quantify, and identify each component independent of its elution from the GC. When combined with a GC, two independent methods of sample identification result GC retention time and IMS spectra. This can significantly simplify the role of the GC column, since sample components may no longer have to be completely resolved. Coeluting components, molecules having the same or similar GC retention time, can be identified from their IMS spectra. Because the IMS operates at atmospheric pressure, interfacing with the GC is a very simple matter. However, a conventional IMS does not possess the universal response desired for flight GC detectors. The IMS response to saturated aliphatic hydrocarbons is particularly poor. The conventional ion mobility spectrometer (IMS), as shown in Figure 1,is an atmospheric pressure, chemical vapor detector capable of utilizing any one of a variety of host gases, including air, nitrogen, argon, helium, and others. The detection and identification of the sample Components begins with a source of energetic radiation, such as @ radiation, which ionizes the atmospheric pressure gas in the reaction region. This primary ionization initiates a sequence of ion-molecule reactions leading to the formation of sufficiently energetic reactant ions, which in turn ionize sample molecules and form product ions. The product ions and any remaining reactant ions, moving under the influence of an electric field, are admitted into a drift region by an ion gate. The electric field then draws the ions through the sample-free drift region where they are separated and subsequently collected according to their molecular weight, collisional cross section, and charge distribution. The pattern of product ions and ion drift times producd by this process becomes the ion mobility signature, observed at the ion collector of the sensor cell in a time frame of milliseconds. Concentrations down to parts per trillion (ppt) in the reaction region produce detectable electrical signals. A more detailed explanation of the ion mobility spectrometer can be found in ref 4. The normal laboratory and commercial operation of the IMS uses gases that contain trace species, in particular, water vapor at multiple parts per million. In a conventional IMS, the water concentration dominates the ion-molecule reaction sequence to produce, in the positive-ion mode, the water cluster reactant ion. With the normal development of this ion in the ion-molecule reaction scheme, saturated aliphatic hydrocarbons are only poorly detected at concentrations of 1ppm and less. In order to sensitively measure hydrocarbons then, some other method of IMS sample ionization must be
00032700/91/036~2295S02.50/0 @ 199 1 Amerlcan Chemical Sockty
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 20, OCTOBER 15, 1991
ELECTROMETER AMPLIFIER
I SNSOR CELL HWSINQ AND HEATER 'l\jfEATED DRIFTQU
"$P
Flgure 1. Conventional ion mobility spectrometer.
employed. The two alternate methods of IMS sample ionization studied here are a conventional IMS with very dry helium and a IMS using metastable helium ionization. In a very dry helium IMS, the water concentration is reduced to ppb levels in order to increase the lifetime of the hydrocarbon ions to the milliseconds required for measurement. Helium was chosen as the drift gas because it is the preferred carrier gas for flight GC system. At low water level concentrations, other species may become the reactant ion in the IMS. An intentionally added chemical, a reagent gas, can also be employed to form the reactant ion. An alternate approach to improving IMS sample ionization involves designing an IMS reaction region that utilizes metastable ionization. Lovelock (5) first described an argon ionization detector utilizing the metastable ionization process. Later detectors utilizing this mechanism employed helium. Helium, in the presence of a strong electric field, can be "excited" to a metastable state tbrough collisions with @ particles emitted by a radioactive source. These excited helium molecules, metastable helium, can then ionize molecules that have an ionization potential below 19.8 eV. This results in a nearly universal sample ionization mechanism with great sensitivity. The most widely used application of metastable ionization is the commercialy available helium ionization detector (HID), a universal, highly sensitive gas chromatographic detector. Several further descriptions of this detector and its ionization processes can be found in the literature including articles by Hartmann and Dimick (61,and Andrawes and Ramsey (7). The NASA version of this detector, the metastable ionization detector (MID) (81,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 modqlator (IO), the MID sample response range extends contiguously from 1 ppb to thousands of ppms. EXPERIMENTAL SECTION A very dry helium IMS test system, show in Figure 2, was built at PCP Inc. to investigate very dry IMS analyses. The helium drift gas was passed through cold traps containing 13X molecular sieves to remove water. The cold traps were cooled by using either dry ice or liquid nitrogen. This system was able to eventually reduce the water content in the IMS to below 30 ppb, as determined by the ratios of water cluster ions, as described by Kebarle et al. (11). According to Kebarle IH+(H@),1 PH20= -ZH+(H~~),, Kn-1.n PHB = partial pressure of water vapor I = intensity of subscripted ions measured in the mass spectrometer Kn-l,n= the equilibrium constant for the following reaction: H+(H20),1
+ H20
log Knqn=
H+(HzO),
lOOA T
I
1 $-$KToI
I
L--""*L%1o,I
I
Flgure 2. Very dry helium IMS flow diagram.
I
---..------
5 - -> Figure 3. Metastable ionization electrode for IMS: (1) electric field rings; (2) Ion gates: (3)1" source holder: (4) end repeller; (5) electrodes; (6) sample gas Inlet; (7) exhaust. I
7
where Tis the absolute temperature and A and B are constants that Kebarle has determined for each normally occurring n - 1,n pair listed in his article. Thus if the above reaction is in equilibrium, the water concentration can be calculated from the water cluster ion ratios observed. Water concentration in parts per million is simply ppm HzO PH&X 106/760 A commercial IMS-MS (PCP Model MMS-160) was used to provide the IMS spectra and mass spectra identification of ions. This instrument system has been described in detail elsewhere (4). The normal IMS housing was modified so that the carrier gas supply fed directly into the source end of the cell. In addition, a port was added for a purge gas supply that swept the outside parts of the cell stack. Both the drift and purge gas supplies were connected to a common line from a single flow controller. All gas linea exposed to sample were heated to 100-200 "C. Samplea were injected by using a six-portCarle microvolume purged-sample valve equipped with sample loops of either 40-pL or 1.0-mL volume. Data readout and acquisition were via several devices. Mass spectra were viewed with a Nicolet 1072 digital averager. Ion mobility spectra were taken with an EC&G Princeton Applied Research Model 4203 signal averager or with an IBM compatible computer with an ASPB-1 advanced signal processing board and associated rapid data acquisition program. Chromatogram were recorded on a Linseis Model LM-24 strip chart recorder. Other pertinent instrumentation and equipment parameters are as follows: MMS-160: gate, 10-cm drift; gate width, 0.2 ms; PAR 4203 mode, 28 exponential average; dwell, 20 ps; full scale expand value, 22; ASPB-1:number of waveforms summed, 20; number of samples per waveform, 640; gate pulse repetition period, 20.832 ms; gate width, 200 ps; sampling period or dwell, 31.25 ~ s GC ; column type, '/le in. X 5 ft. stainless steel tubing, 2,6-dichlorophenyl isocyanate on 1OO/150 mesh Porasil C (12),flow rate 7.5 mL/min helium at 34 psig, temperature ambient (25-30 OC). To investigate helium ionization ion mobility spectrometry, several custom built IMS driitubes were built at Ames -arch Center (ARC).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 eptems. The only significant modification is the capability to provide a very high
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12.04 msec
J
Sample vent
2.6 ppm
Sample In
Figure 4. Metastable ionization GC-IMS. voltage to the MI electrodes for production of metastable helium. A Bertan 205A-05R high-voltage power supply provided high voltage for the IMS and MI electrodes. The IMS spectra were monitored on a Kikusui COS 5100 oscilloscope. Further data reduction as well as spectra monitoring was provided by a Nicolet LAS 1275 computer with a NEC PC-8023A-C printer. Typical parameters for the IMS were as follows: number of waveforms summed, 50; number of samples per waveform, 1024; gate pulse repetition period, 20 ms; gate width, 200 1 s ; sampling period or dwell, 20 ps; temperature, 200 O C ; drift gas flow rate, 100 mL/min. Sample was introduced to the IMS by using a Carle gas sample valve. Figure 4 shows the GC-MID-IMS system. Gas samples could be injected directly into the IMS or could f i t be separated in the GC column. Gas eluting from the column first passed through a Carle 100 thermal conductivity microdetector (TCD) before entering the IMS. The column used was similar to the one used in the dry helium work and was operated at temperatures ranging from 20 to 60 OC. Carrier gas (flow rate, 8 mL/min) and the IMS drift gas was 99.9999% helium (Scientific Gas Products). A Matheson Hydrox purifier was used to further purify the helium. Helium flow was controlled with a Brooks 58503 mass flow controller. RESULTS AND DISCUSSION A conventional IMS employs a Nia3ionization source in an electric field drift tube operated at atmospheric pressure. The typical clean host gas is air with a water vapor content of approximately 10 ppm. In the absence of added sample molecules, the high-energy electrons emitted by the source generate a cascade of ion-molecule reactions in the air, terminating, in the positive mode, with the water cluster ion. The number of water molecules in the cluster is dependent upon temperature and water vapor concentration. The water cluster ion will generally enable the ionization of introduced sample chemicals via the proton-transfer reaction
RH++ S
---*
SH+ + R
where R and S are reactant (water) and sample species, respectively. The heat of reaction is simply AHR PAR - PAS where PA is the proton affinity of R and S as noted. A negative heat of reaction indicates that the reaction is exoenthalpic and thermodynamically favored. For most ionmolecule reactions, the reaction efficiency is quite high for moderately exoenthalpic reactions (at least 10 kcal mol-' exoenthalpic) and drops precipitously for thermoneutral and endoenthalpic reactions (13). However, even endoenthalpic reactions can be observed a t relatively high sample concentrations, which favor the forward reaction due to the law of
Figure 5. Background positive IMS spectrum in air. 7.60 mm
-
0.30 ppm Hfl
NH,', NO* 5.76 m m
I
d
b
Flgurr 6. Background IMS spectrum In very dry helium.
mass action. Since reaction efficiencies can easily vary by 6 orders of magnitude or more between exoenthalpic and even mildly endoenthalpic reactions, IMS detection limits may vary over the same range, depending upon the chemical being measured. If the reaction rate constant for a given chemical sample approaches the collision rate constant, it is possible to detect ppt concentrations, depending upon the capabilities of the data acquisition system. Alkanes as a class have relatively low proton affinities and, accordingly, poor detectabilities in the conventional IMS. The proton affinities of the series methane through butane range from 5.73 to 6.73 eV (14). The proton affinity of water is 7.16 eV (14). These chemicals are thus virtually nondetectable in a conventional IMS. Even a compound such as hexane, with a somewhat higher proton affinity, is detectable only in the ppm region. This effect was observed early in the development of the IMS and, therefore, virtually no work has been done up to this point on the detection of the C1 to C, alkanes. Karasek et al. (15) used the IMS to analyze the C6 to,C16 alkanes in their study of the relationship between ion mobility spectrometry and chemical ionization mass spectrometry (CIMS). They injected 0.1-pL amounts of sample vapor directly into the carrier gas stream to produce the spectra for comparison, and no quantitative results were given. One solution to the problem of alkane detection was to shift the relative reaction energetics such that the formation of
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nl
7.93 mwc (290 mV)
1.45 ppm H20
2.14 ppm H20
Flguro 7. (a) Propane IMS spectrum in very dry helium. (H,O),.H+ is at 7.92 ms. Product ions are at 9.60, 11.36, 12.98, and 14.36 ms. (b) Propane IMS spectrum in air. (HzO),-H+ Is at 12.04 ms. Product ions are at 12.62, 13.02, and 14.8 ms. I 7.94 msec (220 mV)
-> E
Y,
PE
2
n
7.71 mrsc (353 mV)
II
n
7.66 mwc (316 mV) 4.0 ppm H20
->
l k E
5 3 E n % ui
Flguo 8. (a-c) Butane IMS s m a in very
heiium with increasing water concentration. The response to butane decreases as water concentration increases.
n
12.04 mwc (BBO mV)
Flgure 8. (a) Butane IMS spectrum In very dry heiium. (H20),.H+ is at 7.94 ms. Product ions are at 9.68 and 1136 ms. (b) Butane IMS spectrum in air. (&O),*H+ is at 12.04 ms. Product ions are at 12.84, 13.06, 13.34, and 14.04 ms.
alkane ions becomes more favorable. The reactions leading to the formation of the water cluster ion are all strongly exoenthalpic and may proceed with rate constants approaching the collision rate constant. Calculations based on these as-
sumptions can then be made to determine the rate at which these reaction sequences are completed. At the typical 10 ppm water vapor concentration in the conventional IMS host gas, the reaction sequence leading to the formation of the water cluster ion is effectively completed in less than 0.1 ms. This means that the higher energy ions, which precede the formation of the water cluster ions, are consumed on the same time scale. However, as the water vapor concentration is reduced, the lifetimes of these higher energy intermediate ions are increased. Using a similar approach for 100 ppb water vapor levels, it takes 5 ms or longer for the water cluster ion formation reaction sequence to be substantially completed. This approaches typical drift times in the IMS and allows more alkane ions to be formed from the higher energy intermediate ions and to survive drifting to the collector to be observed. Figure 5 shows a typical IMS background spectrum with air drift gas. The water cluster ion, at 12.04 ms is the dominant ion. Figure 6 shows the IMS background with dry helium as the drift gas. Since the drift gas is now helium, which requires different electric field settings and is of lower density than air, the respective drift times will be different. Again the water ion, here at 7.69 ms, is the dominant ion present, even though the water concentration is much lower.
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lsobutane
n-Butane
Ethane
Propyne
IMS CHROMATOQRAM OF HYDROCARBON MIXTURE ~
~
_
_
_
_
_
Flguro 10. Dry helium IMS analysis of
model Titan atmospheric gas mixture.
The mass identification of the peaks in the ion mobility spectra was accomplished by using the coupled mass spectrometer. The technique involved here has been previously described in detail (4). Using the mass spectrometer, the ion mass or masses comprising any substantial peak in the ion mobility spectrum can be unambiguously determined. Thus the nature of the specific ions detected can be elucidated (i.e. molecular ion, fragment ion, adduct ion, etc.) and the underlying reactions forming the observed ions may be worked out. The ion mobility peaks themselves are unique to the specific background or sample molecules present in the IMS and serve to empirically identify them. However, with the mass spectrometer, it can be shown, for example, that the single 'water ion" peak in the ion mobility spectrum is, in fact, an equilibrium mixture of various cluster and adduct species that drift in the atmospheric host gas as one peak. Ion mobility peaks resulting from the introduction of sample eluting from the coupled GC are identified experimentally from the GC retention times. In unknown mixtures, both ion mobilities and GC retention times can be used in a complementary fashion to confirm chemical identity with more certainty than either technique alone. The ion mass identifications provide useful information as the analytical technique is developed, but these are not crucial to the ultimate usefulness of the analysis for the identification and measurement of alkanes. Figures 7 and 8 show comparisons of the IMS spectra obtained from propane and butane with air and dry helium drift gas. The water concentration in the helium system is less than 1 ppm. Although there is 25 times more sample in the air analyses, the dry helium spectra produce much larger sample/product ion peaks. The effect of lower water concentrations is shown in Figure 9, where butane is analyzed with different amounta of water present in the IMS helium drift gas. As the water concentration increases from parta a through c, the response to butane decreases. Other ionization processes appear to take over as the dominant water ion is removed. No other reagent gas w a ~introduced to the IMS during these experiments. Early results, using a dry air system, with water levels of 2.6-1.5 ppm, indicate that no such ionization of
I 4
1
I
I
50
100
I
I
I
I
I
I
200 300 350 400 450 APPLIED volts
Flgwe 11. Current vs voltage curve for metastable bnizatlon detector. 150
-
a 7 100 L X
50-t 0
Figwe 12.
electrodes.
100
Volts 300
*'
400
500
Current vs voltage curve for IMS wlth metastable Ionization
saturated aliphatic light hydrocarbons takes place in dry air. A t present it is not clear whether the air inhibits the ionization of the hydrocarbons or helium is required for such ionization to occur, or both. A new set of experiments, using a dry
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ms
1
2
.
3
11.8677~
3
8
9
v
b
cdrc:
. .
1
11.3965~ 0 ns 5.1000 ms 10.220 ms 15.340 ms
1
2
3 min Collecting electrode current
10.9254~ 0 ns
5.1000 ms 10.220 ms 15.340 ms
Fbure 13. GC metastable ionization-IMS analysis of hydrocarbon mixture.
nitrogen system, is now being developed to further test these results. As a further test of the dry helium IMS,a mixture of several hydrocarbons expected to be components of Titan's atmosphere was analyzed. The mixture contained ethane, ethylene, acetylene, propane, n-butane, isobutane, and propyne in a range of concentrations from 20-200 ppm. The major product ion peak was monitored to produce a chromatogram and IMS spectra were taken continuously. Figure 10 shows the resulting chromatogram and the IMS spectra taken as each component eluted from the column. The concentration of water present in the IMS during this analysis was 30 ppb. Further improvements in IMS sensitivity to the various hydrocarbon components is expected as the water concentration is decreased. These results demonstrate the feasibility of the dry helium approach. As mentioned earlier, metastable helium ionization was also investigated as a possible IMS sample ionization mechanism for hydrocarbon analysis. When voltage is applied to the electrodes in a MID,a current is produced from ionization of impurities in the carrier stream and @ particle emission from the radioactive source. This background or standing current increases as the voltage applied to the electrodes is increased. Figure 11 shows the typical current vs voltage curve obtained for an MID. The MID is at its most sensitive in region 3 of the curve, often referred to as the multiplication region. An early test of metastable ionization-IMS (MI-IMS) feasibility is shown in Figure 12. Here, a high-voltage potential was applied between the repeller end plate and ring number 1in the IMS reaction region. No other changes to the conventional IMS design were made in this experiment. As the voltage was increased, an increase in the current produced was recorded that was very similar to that shown in Figure 11. However, no significant new ions appeared in the IMS spectra under these conditions. A specially designed IMS reaction region for metastable ionization was then designed and built. Several versions of a helium ionization reaction region for the IMS have been designed and tested, one of which is shown in Figure 3. The IMS, using the metastable ionization reaction region, was used as a detector for a GC and produced both a conventional chromatogram and IMS spectra from a GC analysis of a hydrocarbon mixture. The output of the MI-IMS collecting electrode was monitored by a strip chart recorder
to produce a chromatogram of the mixture comparable to the total ion current chromatogram produced by a gas chromatograph-mass spectrometer. Ion mobility spectra were taken of each GC peak and signal-averaged to produce an IMS spectrum for sample identification. Figure 13 shows both the chromatogram produced from the collecting electrode and the IMS spectra taken from each peak. The spectra produced, however, were not as clear as conventional IMS spectra or even the spectra produced in the dry helium IMS. The strong electric field introduced in the reaction region, to generate the metastable helium, affects the gating, resolution, and collection of the ions. Methods to define the extent of this effect and to control it are now underway. Clearly, though, there is a different manner of sample ionization taking place in the reaction region. Registry No. H20, 7732-18-5;ethane, 74-84-0;ethylene, 7485-1;acetylene, 74-86-2;propane, 74-98-6;n-butane, 106-97-8; isobutane, 75-28-5;propyne, 74-99-7;helium, 7440-59-7.
LITERATURE CITED Oyama, V. I.; Berdahl, E. J. J . oeophVs.Res. 1977, 82, 4669-4676. Oyama, V. I.; Carle, (3. C.; Woelbr, F.; Poltack, J. 8.;Reynolds, R. T.; Crab, R. A. J . Qsophys. Res. 1960, 85, 7891-7902. Hunten, D. M.; Tomesko, M. 0.; Flasars, F. M.; Samwlson, R. E.; Strobel, D. F.; Stevenson, D. J. In Sam;Gehrels, T., Mattlmws, M. S., Eds.; University of Arlzona Press: Tucson, AZ, 1984, pp 671-759. Spangler, 0.E.; Cohen, M. J. In Pyasma ChroM&mphy; Can, T. W., Ed.; Plenum Press: New Ywk, 1984; Chapter 1. Lovelock, J. E. J . Ckmatogv. 1956, 1 , 35-46. Hartmann, C. H.; Dimlck, K. P. J . Ges Chnrmetogr. 1966, 4 , 163- 167. Andrawes. F.; Ramsey, R. J . Chromatogr. Scl. 1986, 24, 513-518. Woeiler, F. H.; Kojiro, D. R.; Carle, 0.C. Anel. C M . 1964, 56, 860-862. Carle, 0.C.; O'Hara. E. J.; Clark, E. C. Pub/. Astron. Soc.Pac. 1965, 9 7 , 895-896. Carle, 0.C.; Kojlro, D. R.; Humphry, D. E. US. Pat No. 4,538,066, 1985. Kebarie, P.; Searles, S. K.; Zolla, A.; Scarborough, J.; Arshadl. M. J . Am. Chem. Soc. 1967, 89, 6393. Pollock, 0.E.; Kojko, D. R.; Woeller, F. H. J . Chrometogr. Scl. 1962, 20. 176-181. Harrison, A. 0.Chemhxl Ionlzatbn Mess S p e c t r ~ t r y CRC ; Press, Bow Raton. FL, 1983 p 27. Llas, S. G.; Lkbman, J. F.; Levln, R. D. J . Phys. chsm. Ref. Deta 1964, 73,NO. 3, 695-808. Karasek, F. W.; Denny, D. W.; DeDecker, E. H. A M I . chsm. 1974, 4 6 , 970-973.
RECEIVEDfor review February 8,1991.Accepted July 9,1991. This research was supported by the NASA Exobiology Flight Experiments Program.
