Anal. Chem. 1990, 62, 994-996
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(5) Huggins, M. L. Ann. N . Y . Acad. Sci. 1942, 43, 1. (6) Orye. R. V.; Prausnltz, J. M. Ind. Eng. Chem. 1965, 5 7 , 18. (7) Hussey, C. L.; Parcher, J. F. Anal. Chem. 1973, 45, 926. (8) Laub, R. J.; Purnell, J. H.; Williams, P. S.;Harbison, M. W. P.: Martire, D. E. J . Chromatogr. 1978. 155, 233. (9) Conder, J. R.; Young, C. L. Physicochemical Measurement 6y Gas Chromatography; John Wiley & Sons: Chichester, 1979. (10) Deming. S. N.; Morgan, S. L. Anal. Chem. 1973, 45, 278A
( 11) Dohnal. V., Vesely. F., VinH. J., Collect. Czech. Chem. Commun. 1982, 47, 3188.
RECEIVED for review November 16, 1989. Accepted February
E,1990. The authors are indebted to the Research Fund of SR Serbia. Belgrade, for partial financial support.
Application of Hydrogen Storage Alloy for the Determination of Trace Impurities in High-Purity Hydrogen by Gas Chromatography Hiroshi Ogino,* Yoko Aomura, a n d Masashi Mizuno Technical Research Laboratory, Toyo Sanso Co., Ltd., 3-3, Mizue-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa 210, J a p a n
A hydrogen storage alloy was applied to determine the trace impurities, such as neon, argon, nitrogen, krypton, methane, and xenon, in hydrogen. This system consists of a gas chromatograph combined with a precolumn filled with the hydrogen storage alloy. The alloy efficiently retains hydrogen at room temperature and under the pressure of the carrier gas. By use of a photoionization detector, the detection HmKs achieved were as follows: 4.7, 0.02, 0.02, 0.01, 0.01, and 0.01 ppm for Ne, Ar, N,, Kr, CH,, and Xe, respectively.
INTRODUCTION For the determination of inherent gas impurities in hydrogen, gas chromatography (GC) has been widely used as the preferred method. The photoionization detector (PID), which is based on the emission from a direct current discharge in helium gas (1,2),is a universally sensitive detector ( 3 , 4 )and has especially high sensitivity for gases such as argon, nitrogen, krypton, methane, and xenon. We reported in a previous paper (5)that the PID is suitable for the determination of trace amounts of such gases. However, it has been hard to separate and accurately determine the trace amounts of impurities a t levels less than parts per million in hydrogen. The use of a high sensitivity detector for these gases results in a huge hydrogen peak overlapping other peaks that are later eluted. It is necessary to eliminate the hydrogen peak overlap. In general, hydrogen is eliminated prior to entering the analytical column of the GC with a hydrogen transfer system (palladium alloy tube) and/or copper oxide (CuO) catalyst. The former method utilizes the very high and selective permeability of the palladium alloy, heated between 500 and 625 "C, to allow transfer of the hydrogen ( I ) . The latter method requires temperatures over 800 "C. If the hydrogen sample contains oxygen, the system causes water formation. There are some problems with these methods relative to unwanted deactivation of the column from the resultant water. The resulting water should be adsorbed by a short precolumn filled with desiccant, such as a molecular sieve (e.g. MS-13 X) column. For the last two decades, hydrogen storage alloys such as LaNi, have provided purification and storage techniques for hydrogen gas ( 6 ) ,but there have been no published methods
* Author to whom correspondence should be addressed. 0003-2700/90/0362-0994$02 50/0
Table I. Hydrogen Storage Alloys" HSA-1 MmNi4SA10.5 HSA-2 LaNi,,,Al0,, HSA-3 LmNi4.7A10.3 HSA-4 Ti,.,Zro.,,Mno.,CrCuo,~ Mm and Lm are mischmetal and lanthanum rich mischmetal, respectively; mischmetal is a mixture of lanthanoid rare earth metals extracted from ores. (I
for applying hydrogen storage alloys to gas analysis techniques. This paper describes a new analytical method for determining trace impurities in hydrogen by the use of a photoionization detector (PID)and the precolumn separation using a hydrogen storage alloy (HSA).
EXPERIMENTAL SECTION Apparatus and Conditions. A schematic diagram of the experimental apparatus is shown in Figure 1. A gas chromatograph with a PID (Hitachi, GC-263-30, Tokyo, Japan) was used in this experiment. A precolumn was installed between the gas sampler with a 1.5-mL loop and the analytical column, which was packed with a molecular sieve (MS-5A, 60/80 mesh) in 3 m X 3 mm i d . stainless steel tubing. The precolumn was 30 cm X 3/8 in. 0.d. stainless steel tubing and was packed with HSA (20/60 mesh) as shown in Figure 2. Silica fibers were packed intermittently in order to prevent any expansion problem of the precolumn, e.g., wall rupture, which might be caused by hydrogen absorption into the alloy. Operating conditions of the GC-PID and the precolumn were as follows: oven temperature, 80 "C; detector temperature, 100 "C; carrier gas, He, 50 mL/min; discharge gas, He, 44 mL/min; discharge potential, 750 V; precolumn temperature, a t room temperature (ca. 25 "C). Materials. The HSA used in the experiments are listed in Table I. They were purchased from Japan Metals and Chemicals Co., Ltd. (for HSA-1-3), and Daido Steel Co., Ltd. (for HSA-4), respectively. Reference gases were prepared by the gravimetric method and were supplied by Toyo Sanso Co., Ltd. Most of the experiments, which were done in order to establish the optimum operating conditions of the precolumn and gas chromatograph, were conducted with reference gases having the following compositions: [l] 9.0 ppm Ar and 9.0 ppm N, in H,; [2] 46.8, 9.4, 9.4, and 9.4 ppm for neon, argon, krypton, and xenon especically in H2. Activation of the HSA. Hydrogen absorption with HSA (e.g., LaNi,) is based on the following reaction: LaNi, + 3H2 .-+LaNi,HG + heat This is the reversible reaction of a solid alloy with hydrogen gas. C 1990 American Chemical Society
ANALYTICAL'CHEMISTRY. VOL. 62. NO. to, MAY 15, 1990 995 L
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Table 11. Activation and Regeneration Procedures of a Precolurnn Filled with HSA-3 activation steps
regeneration steps
precolumn turned to the activation position
precolumn turned to the activation position
1
1
evacuation (under vacuum at 140 "C for 30 min)
evacuation (under vacuum for 30 mi")
1
hvdroeen ahsorotion (at ca. 25
S
i
Figure 1. Schematic diagram of apparatus for determination of impurities in high-purity hydrogen: A, GC 6. interface: C, integrator: D, precolumn (HSA): E. heating block: F. heating controller: G. H, airactuated six-port rotary valves: I. sample loop: J. pressure transducer: K. L. mass flow controllers: M. hydrogen gas inlet for HSA activation: N. helium gas inlet for purge: 0. hydrogen and helium gases outlets: P. R. to rotary vacuum pump: 0.sample gas outlet: V,-V,. diaphragm stop vaives: S.T. sample gas inlet.
.T,'b
atm for.30 mi")
.,
It evacuation (under vacuum at 140 "C for 30 min)
1
nrecolumn turned back to the analytical position (heat up to 180 "C for 30 min) I precolumn cooled down to ea. 25 "C
4
.I
precolumn turned back to the analytical position (heat up to 180 "C for 30 mi") 1 precolumn cooled down to ea. 25 "C
GC analysis
I
A
Flgure 2. Cross section of prewlumn packed with H S A A, ' I , - ll, in. reducer: 6, ' I s in. X 30 cm stainless steel tubing: C. silica fibers (ca. 5 mm height): D. 20/60 mesh HSA. The ahsorption is exothermic and the desorption is endothermic. The hydrogen absorbed at the lower temperature is chemically bound within the hydride crystal lattice and it is desorbed from the hydride crystal lattice in the HSA-hydride a t higher temperatures if the pressure of the gas containing hydrogen is maintained nearly constant: these conditions for absorption and desorption of hydrogen are dependent on the metal composition of the alloy used. The HSAs require activation. Initially, the alloy is pulverized and then packed into a precolumn tubing. The precolumn is evacuated to remove the remaining air with a rotary vacuum pump, and is then filled and pressurized with high-purity hydrogen (Toyo, Sanso, A grade, 99.9999% (v/v)) up to 5-6 atm at room temperature after valves K, and K, were closed. This condition is held for a minimum of 30 min. Next, valve K, is closed and valve K, is opened to evacuate the precolumn using the vacuum pump. During its evacuation, the precolumn is heated to 140 O C and allowed to stand for 30 min. The heating unit is removed from the heating block of the precolumn so that the precolumn temperature can return to ambient. Valve K, is then clmed and valve K, is opened. The precolumn is then filled with hydrogen to 5-6 atm of pressure. This condition is maintained for 30 min in order to completely activate the precolumn (HSA). To assure the complete activation of the HSA, repeat the evacuation and repressurizing with hydrogen two or three times. Finally, valve F switchs back to the analytical position, and the precolumn is then further heated at 180 "C for 30 min in order to release hydrogen from the HSA-hydride. If the precolumn is activated with the previously described procedure, regeneration, if required, of the precolumn (HSA) can be accomplished with only evacuating it at 140 OC under vacuum. The activation and regeneration procedures are summarized in Table 11. The activated HSAs should he stored in precolumn, which was sealed after filling with inert gas such as helium and argon in order
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0
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TIME (MINI
Figure 3. Typical chromatograms of Ar ( t o ppm) and N, (to ppm) in H, obtained with different HSAs: A, HSA-1: 6, HSA-2 C. HSAJ: D, HSA-4. The GC-PID conditions are given in the text. to protect HSA that may be oxidized or burned with oxygen. The HSA can be disposed of after deactivation, e.g. by dipping it in water or passing carbon monoxide through the precolumn.
RESULTS AND DISCUSSION Selection of the HSA. Four hydrogen storage alloys were evaluated as to whether hydrogen can be completely absorbed by them and whether the resulting residual gases can pass through the precolumn without any losses or contamination. All of the HSAs listed in Table I showed hydrogen absorption at the conditions examined under the pressure of the carrier gas (He) and a t room temperature. Typical chromatograms obtained from these HSAs are shown in Figure 3. The HSA-1 showed that nitrogen was partially absorbed in the precolumn and that the nitrogen peak area increased with increasing number of sample injections. The HSA-4 showed not only the largest capacity of hydrogen ahsorption among the alloys examined in this study hut also the perfect absorption of nitrogen as shown in Figure 3. This alloy is unsuitable for the determination of nitrogen. On the other hand, HSA-2 and -3 had improved performance where only hydrogen was absorbed without any loss of other impurities such as argon, nitrogen, krypton, methane, and xenon. HSA-3 was used in the following experiments. The precolumn could be regen-
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
B
lAr
L- 4 Y H . ,
TIME
Figure 4. Chromatographic profiles obtained by ordinary (A) and the present (B) methods: attenuation, X64 for (A) and X8 for (B); GC-PID
and precolumn operating conditions are given in the text. erated by evacuating the hydrogen absorbed for only 30 min a t 140 "c in vacuo. In an attempt to confirm whether the retained hydrogen was released with the elapse of time, the following experiments were conducted with the use of a short precolumn, which was packed with the HSA-3 into 30 cm X 1/4 in. 0.d. stainless steel tubing. After activation of the short precolumn, 1.6 mL of hydrogen gas, containing 10 ppm each of argon and nitrogen, was injected into this GC system. The gas chromatograph was run continuously for 240 min in order to confirm the elution of hydrogen. It was found that the hydrogen could be strongly absorbed with the HSA since the hydrogen peak did not appear a t the experimental conditions of room temperature and a carrier gas pressure of 2-3 atm. Next, the same sample gas was automatically and repeatedly injected at 10min intervals in order to determine the capacity of hydrogen absorption. The result showed that this HSA could absorb ca. 15-17 mL of hydrogen under the proposed conditions and then showed the elution of hydrogen. However, this capacity is not large enough to analyze a number of samples. In the following experiments, a larger sized precolumn (30 cm X 3/8 in. 0.d. stainless steel tubing) was used. This size of precolumn, which contained ca. 32 g of HSA-3, could absorb 32 mL of hydrogen under the present condtions even after several regenerations. For the 1.5-mL sample size, analyses of more than 20 samples can be analyzed before regeneration is required. Recoveries of Trace Impurities. In order to confirm the recovery of the trace impurities, the following experiments were carried out. The peak area of each component that passed through the precolumn was compared with that obtained without the precolumn. Sample gases used in both experiments were prepared by diluting the standard gas, containing Ne (46.8 ppm), Ar (9.4 ppm), Kr (9.4 ppm), and Xe (9.4 ppm) in He and N2 (9.0 ppm) and CH, (11 ppm) in He, with pure hydrogen or helium. Mass flow controllers (STEC, MS-400, Kyoto, Japan) with *2% deviations of precision were used. If a large amount of hydrogen was analyzed by the GC with a highly sensitive detector such as the PID and without the precolumn, i t required reducing the amount of hydrogen in the sample gas. The results showed that neon, argon, nitrogen, krypton, methane, and xenon were not absorbed in the precolumn. However, carbon monoxide and carbon dioxide were irreversibly adsorbed with the precolumn. Both carbon monoxide and carbon dioxide tended to poison the HSA. These experiments were conducted sep-
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Figure 5. Typical chromatograms of mixture gases: (A) Ne (23.4 pprn), Ar (4.7 ppm), Kr (4.7 ppm), Xe (4.7 ppm) in H,; (B) N, (3.2 ppm), CH, (5.0 ppm) in H;, attenuation, X8; other conditions are the same
as in Figure 3. arately with a standard gas containing 10 ppm CO and 10 ppm C 0 2 in H2 and an activated charcoal (60/80 mesh, 3 m X 3 mm i.d. stainless steel tubing) column. Typical Gas Chromatogram. Typical gas chromatograms of the standard mixture of Ar and N2 in hydrogen obtained by the present method (with the precolumn) and an ordinary method (without the precolumn) are shown in Figure 4. The chromatograms show that if the sample containing a large quantity of hydrogen was directly injected into the gas chromatograph, it is impossible to separate and detect trace impurities such as argon, nitrogen, and methane since their component peaks overlap with the huge hydrogen peak. On the other hand, the proposed method provides suitable separation and successful detection of their impurities without any difficulties because hydrogen is completely removed from a sample gas as shown in Figures 4 and 5. Reproducibility and Detection Limits. The reproducibilities (relative standard deviation, n = 7) were 2.1% ,0.60%, 2.2%,0.42%, 0.47%, and 0.89% for Ne (46.8 ppm), and Ar (0.94 ppm), N2 (0.90 ppm), Kr (0.94 pprn), CH4 (1.1ppm), and Xe (0.94 ppm), respectively, under the present system. The detection limits, which were calculated from the minimum peak area of the integrator detection, were 4.7,0.02,0.02,0.01, 0.01, and 0.01 ppm for Ne, Ar, N2, Kr, CHI, and Xe, repectively. The calibration curves were almost similar to the results described in our previous paper (5). In conclusion, hydrogen storage alloy can be practically applied for the gas chromatographic determination of trace impurities in hydrogen. This method will offer detection limits of less than parts per million for impurities in hydrogen if a gas chromatograph is equipped with a high sensitivity detector such as the photoionization detector (PID) and a helium ionization detector (HID) (7).
LITERATURE CITED (1) Cowper, C. J.: DeRose A. J. The AnaWsk of Gases by Chromafography; Pergamon Press, Ltd.: Oxford, 1985. (2) Xiuqi, L.: Huannan, H.; Jianying, 2.; Bohua, Y.; Pingtian, M. Fresenius' 2.Anal. Chem. 1988, 337,520-524. (3) Yamane, M. J . Chromatcgr. 1964, 74. 355-367. (4) Kawazoe, K.; Kamo, T.: Takata, Y.: Shikama. N. Buneski Kagaku 1985, 3 4 , 309-313. (51 Oaino. H.:Aomura. Y.: Komwo. M.; Kobayashi. T. Anal. Chem. lS89, 67,2237-2240. (6) Sandrock, G. D.; Huston, E. L. CH€MT€CH 1981, Dec, 754-762. (7) Andrawes, F ; Greenhouse, S. J . Chromatogr. S d . 1988. 26, 153-159.
RECEIVED for review December 26,1989. Accepted February 26, 1990.
