Rare Gases of the Atmosphere Gas Chromatography Using a Thermal Conductivity Detector and a PaI I adium Trans moduI ator J. E. Lovelock,l P. G . Simmonds, and G . R . Shoemake Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, Calu. This paper reports on the application of the palladium transmodulator combined with a small volume thermal conductivity detector to the determination of the rare gases in air. The analysis was performed directly on a 10-ml sample of air using a single column operated at room temperature. A gain in sensitivity of l o 3 is demonstrated. The system described was developed for planetary atmospheric analysis but is of general use wherever gas analysis at high sensitivity i s required.
A SINGLE GAS CHROMATOGRAPHIC system which can determine permanent gases with accuracy, and which is sensitive enough t o detect traces at the parts per by volume level has so far eluded discovery. Accurate analysis t o a concentration of is possible by the use of the well established catholic detectors such as the thermal conductivity o r ionization cross section detectors. Analysis at high sensitivity for concentrations less than is also possible using the helium ionimtion detector but with doubtful accuracy and a very limited dynamic range. The limitation of sensitivity with the well established catholic detectors first mentioned above can in principle be overcome by using a palladium transmodulator (1) between the column and the detector. The advantages conferred by this combination increase as the volume occupied by carrier gas within the detector is decreased. There follows a description of the use of a small volume thermal conductivity detector and a palladium transmodulator in the difficult analytical problem of the direct determination of the rare gases present in air. NATURE OF THE PROBLEM
A component eluted from a chromatographic column is diluted by the carrier gas in proportion to the carrier gas volumetric flow rate and the time interval of its elution defined as peak base width. The least attenuation is obviously with Present address, Bowerchalke, Nr. Salisbury, Wilts, England. ( I ) J. E. Lovelock, K. W. Charlton, and P. G. Simmonds, ANAL. CHEM., 41, 1048 (1969). 1958
peaks of narrow base width such as with high resolution columns and soon after elution of the air peak. Where a mixture of compounds needs to be analyzed, the requirements of their separation preclude the choice of these optimum conditions for any but the first component t o be eluted, at least if the analysis is to be carried out as a single step. If one or more components of a mixture follow closely a major component, then even more severe restraints are placed upon the analysis. The tail of the major component even under ideal conditions may give a much larger and more varying signal than does the trace which follows. To slow the analysis so that the separation is greater or t o use a smaller sample, both carry the penalty of further reductions in detectivity. The direct analysis of air for its content of rare gases is a practical exercise which serves well t o illustrate this problem. Air contains the rare gases in the following concentrations, expressed as parts per million by volume: helium 5.2, neon 18, krypton 1.14, xenon 0.087, and argon 9.3 X lo3. A well designed thermal conductivity detector might readily detect the presence of any of these gases except xenon at the stated concentrations were they dispersed in the proper carrier gas ; for example, nitrogen for helium and neon or hydrogen for argon and krypton. Yet the direct determination of rare gases in air by gas chromatography using a thermal conductivity detector has never been reported and would in practice be most difficult t o achieve. This problem would be formidable even with a sensitive detector such as the helium ionization detector. Helium itself and neon would not be detected and the detection of krypton in air would present a difficult if not impossible problem, because there is no known column which can sufficiently separate krypton from atmospheric nitrogen t o prevent the tail of the major nitrogen peak over-riding the small trace peak of krypton. Mixtures of krypton and nitrogen can be contrived in the laboratory to demonstrate their separation and determination by gas chromatography, but this is a very different matter from their determination in air where their concentrations differ by nearly six orders of magnitude. Some measure of the degree of dilution by carrier gas can be gathered from the analysis illustrated in Figure 4 later
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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Figure 1. Gas flow circuit diagram of the chromatograph showing hydrogen purifier A, sample valve B, oxygen remover C, column D,mixing value E, second carrier flow restrictors F, transmodulator G, detector H,and oven Z
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Figure 2. The carrier gas mixing valve Primary carrier hydrogen enters at A. The second carrier is fed into the hydrogen at C by a coaxially disposed 0.075-mm i.d. capillary tube. The mixed carrier passes along B to the transmodulator
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in this paper. The peak widths at the base of helium, neon, argon, and krypton were 27, 27, 120, and 300 seconds, respectively. At a carrier gas flow rate of 110 ml min-’ and using a 10-cc sample, this corresponds t o peak concentrations in the carrier gas of helium 2, neon 7.2, argon 1000, and krypton 0.04 parts per million by volume or dilutions of 2.5-,
2.5-, 11-, and 27-fold, respectively. The combination of a small volume thermal conductivity detector and a palladium transmodulator has the capacity t o solve this practical problem, partly because of the great enhancement of sensitivity made possible by the transmodulator, in practice nearly 1000-fold; but also because the transmodulator enI
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Figure 3. Analysis of air containing 1 of helium and 2.3 % methane Column 4.5 m long X 6 mm id., 5-A molecular sieve. Hydrogen Row 67 ml min-l. Sample size 10-ml thermal conductivity detector, recorder F.S.D. 1mV. Chart speed 5 minutes per inch ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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ables the choice of any convenient carrier gas to be used t o convey the sample through the column (1). With the krypton nitrogen separation, a singularly convenient artifice is the use of nitrogen as the second carrier gas in the transmodulator, in which event the detector is no longer embarrassed by the close proximity of a huge signal from nitrogen itself. In the experiments reported below, a thermal conductivity detector was chosen t o illustrate the advantages of the combination of a transmodulator and a catholic concentration sensing detector, because it is the most commonly used of this detector class. The same arguments apply, however, to any concentration sensing detector provided only that the sensing volume is small enough t o take advantage of very low second carrier gas flow rates. The detector property which determines the limit of improvement possible with a transmodulator is the volumetric time constant: VP/UT,where Vis the sensing volume, U the carrier gas flow rate and P and T the gas pressure and temperature, respectively. For the resolution of early peaks, a time constant not exceeding 2 seconds is desirable, thus with a detector l ml in volume the minimum second carrier flow rate would be 0.5 ml per second. With such a detector, it would be difficult to achieve a sensitivity improvement by transmodulation greater than 5-fold under practical conditions; by contrast the detector used in these experiments had a sensing volume of 2.5 pl which enabled a sensitivity gain of 1000 t o be achieved. A similar improvement in response time comes from operation at reduced pressures and makes possible the use of the transmodulator with a n ionization cross section detector or a mass spectrometer.
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Figure 1 shows the arrangement of the components used in the chromatographic system, each of them is described separately as follows: HYDROGEN PURIFIER. The hydrogen supply was purified by passing it through a tube 30 cm long by 3 mm i.d. of palladium 25% silver alloy, heated t o 400 "C and enclosed within a stainless steel jacket. The impure hydrogen was supplied t o the jacket and a small proportion of it vented t o air t o carry away impurities with it. The pure hydrogen flowed from the interior of the thimble. SAMPLEVALVE. A Pye Unicam sample valve was used which is a 90" turn Teflon (Du Pont) construction and was equipped with a 10-ml sample loop. OXYGENREMOVER.Pa!ladized molecular sieve was made and activated according t o the method described by Krejci et al. (2). A 20-cm length of 6-mm i.d. glass tubing was filled with this reagent and inserted between the output of the sample valve and the inlet t o the column. COLUMN.The column was a 5-meter length of 6-mm i.d. stainless steel tubing filled with a mixture of 5-A molecular sieve containing 10% of activated carbon, both 80 t o 100 mesh. The mixed adsorbent was activated by heating t o 300 "C for 14 hours with a stream of nitrogen flowing. TRANSMODULATOR. The column outlet was taken t o a T-connection made as illustrated in Figure 2 into which nitrogen was introduced a t a rate of 0.1 ml min-l. This rate of flow was maintained by the application of nitrogen at 10 psi t o a 300-cm length of 0.075-mm i.d. silver capillary tubing. The mixed carrier gases then passed into the transmodulator which was constructed from a 60-cm length of 1.5-mm i.d. palladium 25% silver alloy tubing, wound as a helix 1 cm in diameter and 5 cm long. The tubing was (2) M. Krejci, K. Tesarik, and J. Janak, in "Proceedings of the 2nd
Biannual International Gas Chromatography Symposium, 1959," H. J. Noebels and N. Brenner, Ed., Instrument Society of America, Pittsburgh, Pa., 1959. 1960
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Figure 4. Analysis of air, 10 ml but transmodulator on. Hydrogen flow 110 ml rnin-'. NS flow 0.1 rnl min-l. Recorder F.S.D. 2 mV. Chart speed 5 minutes per inch flattened during winding t o form the helix, so that the dead volume of its interior was minimized. It was heated either by the passage of a current of 10 amperes directly through it, or by mounting in a n oven a t 350 "C. A commercially available version supplied by Trienco (Raleigh, N.C.) was also tried and functioned as well as the one described. The nitrogen outlet from the transmodulator was conveyed along a short, IO-cm length of 0.25-mm i d . silver capillary tubing directly t o the sensing volume of the detector; the silver tube was brazed into the end of the palladium alloy tubing. DETECTOR. The detector used was a Servomex MK158 (Servomex Controls, Ltd., Crowborough, Sussex, England) thermal conductivity detector, which has a chamber volume of 2.5 pl. The detector was operated at a temperature of 110 "C t o prevent water condensation within it and a t a filament current of 100 mA. The entire apparatus was mounted in and on a Pye Model 104 gas chromatograph (Pye Instruments, Cambridge, England) and the detector power supply and signal processing electronics were those supplied with the chromatograph. RESULTS AND DISCUSSION
Figure 3 is a chromatogram of a 10-ml air sample with 1 of helium and 2 . 3 z of methane added. The transmodulator was disconnected. The recorder full scale deflection was 1 mV and the hydrogen flow rate 67 ml per minute. In spite of the close t o ideal conditions for analysis, no trace of the neon peak can be seen on this chromatogram. Figure 4 shows the
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same analysis with the same column but with the transmodulator in operation, with the recorder full scale deflection set at 2 mV to accommodate the helium and neon peaks, and nitrogen flowing as the second carrier gas at 0.1 ml min-l. Switching on the transmodulator caused the hydrogen flow to increase to 110 ml min-l and accounts for the faster elution of the gases. The helium and neon are almost resolved and with a peak signal of 1.6 MV it is clear that either of these gases at a concentration of lo-’ could have been detected. The argon signal was offscale to over 100 mV and is followed by a trace peak of water, the product of the small quantity of oxygen escaping removal in the palladized sieve. A small peak for nitrogen is seen and is attributable to the great increase in nitrogen flow through the detector as the 8 ml of nitrogen emerge from the column. Finally krypton, as a small but distinct and reproducible peak emerges 8 minutes after the nitrogen. The gain in sensitivity achieved with this combination is close to 1000-fold which raises the sensitivity of the thermal conductivity detector to low concentrations of gases to the same level of that of the sensitive ionization detectors. This achievement was made possible through the outstandingly good design of the detector with its sensing volume of only 2.5 p1. It permitted a gas volumetric time constant of 1.5 sec when the gas flow was only 1.6 p1 per sec. This short time constant is necessary for the resolution of the helium and neon peaks. A palladium tube is qualitatively different from a diffusion concentrator in that all of the hydrogen is removed in one pass. If “all” seems too strong a term, it might be better to say that the inlet and outlet hydrogen pressure ratio of a palladium tube heated in air has been observed to exceed loll. It follows that given a large enough sample, a column able to cope with it, and a small enough detector, the ultimate sensitivity gain could be exceedingly large. Practical limita-
tions with present day gas chromatography equipment suggest 1 pl as the smallest practical detector volume and 1 liter per minute as the highest practical carrier gas flow rate. With these conditions and assuming a sample volume of 100 ml, the gain in sensitivity possible by transmodulation would be 105. The gain in sensitivity of 1000 times in the experiments here reported is therefore not necessarily the most that can be achieved. Further refinements in the analysis described could also have been achieved by such techniques as temperature programming. The aim of this research was the development of a gas chromatographic system for planetary atmospheric analysis and which would be able to determine atmospheric constituents under realistic conditions down to parts per 106 by volume. In a planetary atmospheric analysis, it is likely that a mass spectrometer would be used rather than a thermal conductivity detector. It is clear however from these experiments that such a substitution would have made possible the direct analysis of air for its content of rare gases and also their isotopic distribution. The analysis of planetary atmospheres for other trace constituents such as those of biological interest for example, methane and carbon monoxide, etc., is also possible but was not attempted in these experiments. The atmospheric methane peak was, however, occasionally seen as a small negative deflection beyond the krypton peak on the chromatogram shown in Figure 4.
RECEIVED for review July 8, 1971. Accepted August 10, 1971. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.
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