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Anal. Chem. 1982, 5 4 , 826-828
Determination of Trace Amounts of Hydrogen and Its Isotopes in Methane Sir: The chromatographic separation of small amounts of substances in large quantities of solvent or substrate is a problem not only in environmental trace analysis but also in mechanistic investigations carried out in the fields of photochemistry and radiation chemistry, where the conversions are usually held very low. The products often have small differences in boiling point, polarity, and pK (I),to the substrate or its structural isomers (2). Additional complications may arise when separation of isotopes is needed and/or gaseous samples at reduced pressure must be analyzed. The present paper is addressed to the latter two aspects. We describe here the separation and analysis of trace (parts per million) quantities of H2/HD/D2 in y-irradiated CHI/ C5Dl2gaseous samples at 10 torr (total amount of hydrogen in the nanomole range). It turned out that the rather delicate separation of hydrogen isotopes on a column fiied with etched glass beads (3) could not be performed when a large excess of methane was present. We therefore tried to perform a preseparation of the sample in a device directly coupled to the gas chromatograph. A U-shaped tube (inner diameter 5 mm) equipped with two valves was filled with molecular sieve 5 A, attached to a vacuum line, and conditioned 2 h at 553 K under high vacuum. Then the gaseous sample in the irradiation vessel also attached to the vacuum line was allowed to expand into the loop and to condense onto the molecular sieve. The rest of the gases which were not trapped were pumped off with an automatically operated Toeppler pump, also connected to the vacuum line, into a calibrated sampling loop, and the pressure was measured with a “Barocell” absolute pressure gauge. The sampling loop served as an injection loop for the GC part of the system. The gases were subsequently separated on a column filled with the etched glass beads and detected by a thermal conductivity detector using helium as a carrier gas. Typically, samples of 0.5 L of irradiated CH4/C5DI2mixture at 10 torr contained a few nanomoles of the hydrogen isotopes
after absorption of a dose of 1 x 1020eV-g-* ( 4 ) . At a trap temperature of 96 K (liquid nitrogen) no hydrogen could be detected indicating that all gases were trapped. When the temperature was raised to 196 K (solid C02/2-propanol), the analysis was made impossible because of the high methane pressure building up. We ultimately achieved an optimal separation of hydrogen from methane when the trap was held at 123 K by means of a petroleum ether/liquid nitrogen slush. Even after 1h of automatic pumping only very small amounts of methane could escape the trap. For further analysis of the trapped gases the trap valves were closed, and the trap was removed and attached to a GC-MS system. After the trap which now served directly as an injection loop was‘ heated to 553 K, the rest of the radiolysis products were injected. However, it must be mentioned that hydrocarbons containing more than three C atoms were not quantitatively liberated from the molecular sieve under these conditions (no high vacuum applied).
ACKNOWLEDGMENT The author wishes to thank Ammanz Ruf and Monique Goel for technical assistance.
LITERATURE CITED (1) . . Neumann-SDallart. M.: Getoff, N. Int. J. Radlat. Phys. Chem. 1979, 13, 101. (2) LukBE, S. Chromatographla 1979, 12, 17. (3) Giumann, T.; Piringer, 0.;Weber, A. Chlmia 1970, 2 4 , 112. (4) Neumann-Spallart, M., unpublished results.
Michael Neumann-Spallart Institut de Chimie Physique Ecole Polytechnique FBdBrale de Lausanne CH-1015 Lausanne, Switzerland RECEIVED for review August 21,1981. Resubmitted December 21, 1981. Accepted January 25, 1982.
AIDS FOR ANALYTICAL CHEMISTS Determination of Trace Impurity Gases by Metastable Transfer Emission Spectrometry Thomas H. Ramsey’ and Mike11 D. Nelson Ouallty and Rellability Laboratories, Chemical Materials Division, Texas Instruments, Incorporated, Dallas, Texas 75265
A relatively new analytical technique, metastable transfer emission spectrometry (MTES), for quantitative detection of elemental or molecular species in a gaseous matrix at very low concentrations ( 104/cm3)has been described by Sutton et al. (1, 2). Briefly, active nitrogen, N2*,is mixed with a gas flow containing various impurity species to be detected. The mechanism, described in detail elsewhere (3) essentially consists of monitoring the light emission from atoms in the sample gas as molecular species are broken up by the active nitrogen. Quantitative determinations can be obtained by measurement of the +aracteristic line intensity resulting from the photon emission. The spectra result from the energy N
OO03-2700/82/0354-0826$0 1.2510
transfer between the active nitrogen and the impurity atoms which decay and emit radiation. The intensity of a given line is proportional to the concentration of that impurity in the sample gas. The power of the method derives from the fact that a single impurity atom will be excited and decay several times while in front of the detection optics. Once excited, decay times are on the order of lo-* s. The fluorescence, most of which occurs in the ultraviolet- region, is monitored by conventional techniques. Of particular interest in the manufacture of polysilicon are any carbon or oxygen containing molecular species ( C H I , CO, COz)in inert gases that could contribute to carbon impurities 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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Table I. Relative Intensity Observed by MTES for Impurity Gases in Argon re1 intens hydrocarbon re1 intens ( CN content, (NO p emission) ppb emission)
MONITOR McCARROLL C A V I T Y
n
1/2"UUARTZ TUBE
Cfi MICROWAVE GENERATOR
G A S M I X I N G CHAMBER OR SAMPLE SOURCE
100.6 74.3 51.1 30.2 20.0 11.7 '7.0 1.1
592 421 296 169 112 61 32 18
98.4 68.0 47.1 28.3 14.6 6.9
CO, content, ppm
15.2 10.2 7.0 3.9 1.9 1.0
n
b
Figure 1. Metastable transfer emission spectrometry apparatus.
in the productioln of semiconductor grade silicon. In this work, argon iaamples containing known impurity gases were monitored by observing the 388-nm band for CN emission (4). This paper, describes experimental work in which argon containing various amounts of impurity gases representative of plant gas impurities was analyzed by MTES. The apparatus developled in this laboratory is shown schematically in Figure 1. Argon is frequently used in certain parts of silicon refining processes.
EXPERIMENTAL SECTION The active nitrogen was produced with an Opthos Instruments microwave power supply, operating at 2450 MHz, coupled to a McCarroll-type standing wave cavity. Light was collected, without benefit of special optics, by a 0.2-m grating monochromator and detected by an RCA IP28 photomultiplier tube. Through the use of a photometer, a strip-chart recorder was the final data display. The reaction chamber was fabricated from in. quartz tubing to allow UV radiation to pass. This tube is much smaller than those discussed in the litteratmebut affords a very compact instrument. The reaction chamber pressure is adjusted during emission detection to operate in a range of 7-20 torr. The argon used was Matheson UHP (0.1ppm total hydrocarbon). The hydrocarbon standard gmi was 2250 ppm isobutane and 2250 ppm methane with the balance argon. Gas mixtures or dilutions were made in a 23-L glass bell jar. A Wallace and Tiernan '720 degree sweep, differential vacuum guage was used to measure the vacuum in the jar. All dilutions were made by partial pressures and no compressibility factors were taken into account as the pressures used were below 1atm. Repeated evacuations, pressurizations, and additions were made to obtain the desired mixtures for analysis and to produce calibration curves. The jar could be evacuated, gases bled into the desired partial pressure, and the contents magnetically stirred. The contents could then be introduced into the MTES reaction chamber. Carbon dioxide standards were made by the same method. The source of C02was dry ice placed in a stainless steel sample cylinder in such a manner as to avoid H20 contamination. The COz was then mixed with argon in the bell jar as described above. Oxygen was also investigated. The O2source was room-air bled into the bell jar at known pressures, RESULT8 AND DISCUSSION Typical results for argon-containing hydrocarbons are shown in Table I. In order to verify the results, we sent duplicate samples for four different concentrations (0.5-10 ppm) to an independent laboratory for GC analysis. Agreement within 1 ppm was obtained for all samples; however, the GC was unable to detect any hydrocarbon below 1 ppm. The good linearity of the MTES data below 1 ppm indicates excellent
I
400
nm
380
Figure 2. Spectrum of 388-nm CN emission (A) without HCI and (B) wlth HCI additions to Matheson UHP N, (
