A. Yamato

H. Tsuji

N. Tuboi

T. Kawano

T. Sugiyama

T. Uda

National Institute for Fusion Science (NIFS-Repository)

§ 4. Infinitesimal Concentration HydrogenAnalyzer Using TRDKawano, T., Sugiyama, T., Yamato, A., Uda, T.,Tuboi, N., Tsuji, H. (GTR Tee)By combining the functions of a gas chromatographand an atomic absorption spectrophotometer, we havedeveloped an infinitesimal concentration hydrogen analyzercapable of analyzing hydrogen concentrations of from ahundred ppb down to a few ppb. The analyzer comprises agas chromatograph and atomic absorption parts as shown inFig.l. The gas chromatograph part consists of a carrier gaspurifier, a valve box and a separation column (Hydroisopack), which is immersed in liquid nitrogen in a vacuumbottle vessel. The atomic absorption part contains a tracerreduction detector (TRD, Round Science Inc.) and a gettertube. The TRD is composed of a mercuric-oxide bed and amercury vapor lamp.Using the gas chromatograph functions, a sample gasis drawn into the analyzer, where it mixes with purifiedcarrier gas and goes through a separation column (its role isname "Gate Keeper (Inert Gas Purifier)" (Aeronex Inc.).Purified carrier gas is sent into a valve box, which injects thesample gas into the carrier gas stream.Besides carrier gas, we have to deal with impuritiescontained in the sample gas because the impurities likeoxygen and carbon dioxide react with mercuric oxides in thebed and so disturb the expected reaction of mercuric oxidewith hydrogen. Thus, we employed the following liquidnitrogen cooling method. The sample gas is drawn in alow-temperature separation column, immersed in liquidnitrogen, and cooled to the liquid nitrogen's temperature (77K). Then all impurities with boiling points higher than liquidnitrogen temperature will be deposited and removed fromthe streaming carrier/sample gas, in particular, oxygen(boiling point 90 K) and carbon dioxide (194.5 K) which arethe main components of air and strong reducing agents formercuric oxides.Thus, while carrier and sample gases pass through thegas chromatograph part, impurities hindering a highsensitivity measurement are removed, and only hydrogengas remains in the carrier gas to be transported to the atomicabsorption part.In the atomic absorption part, an atomic absorptionspectrophotometer with a high sensitivity to mercury vaporis used to detect hydrogen reacting in the TRD. The TRDdetects hydrogen with the following reductionreaction:Infinitesimal Concentration Hydrogen Analyzer1 Configuration of analyzer developed for measurement ofmfinitesimal concentrations of hydrogen gas.shown later) before being sent to the atomic absorption part.The atomic absorption spectrophotometer analyzes thehydrogen concentration through a reduction reaction ofmercuric-oxide with hydrogen.Regarding the measurement of a very low hydrogenconcentration (ppb level), we have to consider severalproblems. The first is the impurities contained in the carriergas, because one of the impurities is often hydrogen. In ourcase, neon gas was used as the carrier gas and acommercially available "pure" neon gas may contain up to 2ppm of hydrogen, which is far lager than the hydrogenconcentration we intend to measure. Therefore, we had toensure that such impurities had been removed from thecarrier gas before the measurement. For this purpose, weemployed a gas purifier, which is marketed under the brandThat is, hydrogen is drawn through a bed ofmercuric oxides and reacts with the mercury oxidesto produce free mercury vapor. The mercury vaporamount is measured by using its spectral absorption,the method usually employed in an atomicabsorption spectrophotometer, and the mercuryconcentration corresponds exactly to that of thehydrogen contained in the sample.At this stage, we should outline the safetymeasures regarding use of mercury vapor. Themercury vapor resulting from the mercuric oxidereducing reaction with hydrogen is harmful tohumans. Therefore, a getter tube is attached at theoutlet of the mercury oxide bed of the TRD. Thepresent getter tube is stuffed with a sufficientamount of activated carbon grains. These getter materialscompletely absorb the mercury vapor discharged from thebed.Next, we· carried out a corroborative experiment toestablish the detection limit for hydrogen. Test sample gaseswith low concentrations of hydrogen in nitrogen gas (5, 10,20, 50, and 100 ppb) were prepared. We repeated thehydrogen analyses ten times for each of the test samplegases by using the present analyzer. As a result weconcluded that the analyzer combining the functions of a gaschromatograph and atomic absorption spectrophotometer issufficiently promising for measurement of infinitesimalhydrogen concentrations below 100 ppb.OutletGellerTRDAtomic absorptionLiq. N2 vesselHydro isopackValveboxGas chromatographCarrier gaspurifierCarriergas--1--+[, •••Sample •gas-+----.1.257

Infinitesimal Concentration Hydrogen Analyzer Using TRD

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