AUTOMATED ONLINE IDENTIFICATION AND MONITORING OF IMPURITIES IN GASES

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Theresa Moody
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1 JPACSM 127 AUTOMATED ONLINE IDENTIFICATION AND MONITORING OF IMPURITIES IN GASES Trace Analytical Inc. Menlo Park, CA ABSTRACT GC based gas analyzers with Reduction Gas Detector (RGD) and Flame Ionization Detector (FID) routinely measure single digit part-per-billion (ppb) level impurities in bulk gases, high purity process gases, air, lab, and field gas samples. Typical impurities detected include H 2, CO, CH 4, hydrocarbons, CO 2, oxygenates, arsine, phosphine, and carbonyl sulfide. These analyzers have been used for continuous online gas analysis in air separation plants and semiconductor fabrication facilities for more than a decade. Systems are configured for dedicated applications. Typically, a single analyzer is configured to analyze inert gases, air, oxygen, or other bulk gases such as H 2, CO 2, or NH 3 using dual gas chromatography channels and dual detectors. Gas blenders and stream selectors are routinely combined with the programmable analyzer to make a complete system for automated analysis of up to six gas streams, plus calibration gas and zero gas streams. of volatile organic compounds (VOCs) at single digit part-per-trillion (ppt) to mid-ppb concentrations. Typical applications include the use of adsorption traps to analyze C2-C11 VOCs in process gases by capillary column GC/FID. Sulfur compounds are analyzed by isothermal capillary columns and electrochemical detection. Gas chromatography utilizing packed column and isothermal conditions are of great utility in target compound analysis of process gases and in industrial hygiene applications in normal workplaces and cleanrooms. Different analytical configurations measure specific impurities in different sample gases. Impurities measured are analyzed with specific application packages. Sensitivity varies for different models, types of samples, and support equipment utilized. When more complex mixtures are present, capillary columns in isothermal or temperature program mode are needed to resolve the compounds. Ultimate sensitivity in ultra high purity environments can be achieved by prudent selection of materials and by maintaining high flow velocity. INTRODUCTION The Reduction Gas Detector (RGD) has been used for more than 2 years to analyze trace gases in gas mixtures. The RGD most often measures H 2 and CO impurities in N 2, Ar, He, O 2, air, CO 2, NH 3, and hydrocarbons. The analyzers made today often contain a Flame Ionization Detector (FID) to provide single digit ppb analysis of a broader range of carbon compound gaseous impurities. The RGD also measures other impurities at high ppt to ppb concentrations such as: Ethylene/alkenes Ketones/aldehydes Arsine/phosphine/hydrides Alkynes Alcohols Benzene 1,3-butadiene H 2 S, COS/sulfur gases The RGD is more sensitive than FID for the above classes of compounds, which are combustible in hot HgO, and is much less sensitive to alkanes and fixed gases, except for H 2 and CO. The RGD operating principal is based on mercury vapor detection by UV absorption. Reaction of reducing (combustible) gases with hot mercuric oxide (e.g. 265 C) produces mercury vapor: CO + HgO CO 2 + Hg The RGD output signal is based on the photometric determination of mercury vapor at 254 nm and exhibits a linearity up to 5 x 1 3. The mercury output of the detector is typically in the ppt concentration range and is scrubbed at the outlet of the detector. Figure 1 shows a schematic representation of the RGD. EXPERIMENTAL A micro-gas blender was employed to generate accurate calibration gas concentrations in the range of.1-1 ppb in a diluent gas. This was done by ratioing a low ppm standard with a zero gas at precise flow rates. The blender was used to verify LDLs and was also used to calibrate the gas chromatographs close to the expected sample concentrations of the impurities. Table I shows the precision obtained in the analysis of H 2, CO, CH 4, CO 2, and NMHC (ethane) utilizing the RGD for H 2 and CO and

2 JPACSM 128 the FID with methanizer for CH 4, CO 2, and NMHC. The relative standard deviation of the dual GC channel analyzer ranged from 1.2 to 2.8% for the five gases in nitrogen at 5 ppb concentration. A chromatogram of 5 ppb H 2, CO, CH 4, CO 2, and NMHC in the universal nitrogen calibration gas is shown in Figure 2. Figure 1. Reduction gas detector (RGD) schematic. RESULTS Table II shows the GC/RGD response factors (RF) for unsaturates (ethylene, acetylene, propylene, and methyl acetylene, RF-1) compared to saturated hydrocarbons (ethane and propane, RF-2). The RGD was found to be 11 to 14 times less sensitive to saturated hydrocarbons than similar unsaturated hydrocarbons, as seen in the ratio of unsaturate/saturate hydrocarbon response factors (RF-1/RF-2). This allows isothermal, packed column analyses such as that shown in Figure 3 (1,3-butadiene and benzene in air at 13 ppb using a short 31 inch column in a fast three minutes). Table II data predicts low response to possible saturated hydrocarbon interferents in actual applications. In this work, the relative standard deviation of the 1,3-butadiene peak area was.6% in nine measurements and was 3.7% RSD for benzene. Figure 4 illustrates that the RGD responds in a linear manner to ethylene in an air/n 2 mixture from a low.54 ppb concentration to 288 ppb. Figure 5 is the GC/RGD chromatogram of ethylene at.54 ppb in air/n 2, ethylene at 1.1 ppb in air/n 2, and blank N 2. This chromatogram illustrates that the ethylene response at 1.1 ppb is twice that of.54 ppb. Challenging applications of the GC/RGD methods are illustrated in three chromatograms that exhibit low level determinations of the unstable, highly toxic arsine gas, (AsH 3 ). Figure 6 shows the GC/RGD analysis of 1 cc of a gas mixture in N 2 containing CO, phosphine (PH 3, 18 ppb), AsH 3 (27 ppb), and carbonyl sulfide (COS) in CO 2. Figure 7 shows a GC/RGD analysis of a 1 cc mixture of CO, PH 3, AsH 3, propylene, and 3 ppb ethlyene oxide (EtO) in CO 2. The semiconductor industry has a need to analyze arsine in air as related to process gas tool use and fugitive emissions for industrial hygiene purposes. Figure 8 shows the GC/RGD chromatogram of phosphine and arsine in air. Arsine was detected at 14 ppb in ambient air, which allowed for detection of this toxic gas well below the 5 ppb regulatory limit and is able to distinguish phosphine, another hydride, from arsine with a five minute total run time. of VOCs at mid part-per-trillion to part-per-billion concentrations. Typical applications include the use of adsorption traps to analyze C2-C11 VOCs in process gases by capillary column GC/FID. Sulfur compounds are analyzed by loop injection with isothermal capillary columns and electrochemical detection. Figure 9 is a chromatogram of benzene, toluene, ethyl benzene, m,p-xylene, o-xylene (BTEX) standard in air at 1 ppb. The air was trapped on an adsorption trap for three minutes with analysis by temperature programmed capillary GC/FID. Trapping is readily done for ten minutes to give a projected lower detection limit in single digit ppt range when peaks are stored with peak rejection set at 1 area counts. Figure 2. N 2 sample, 5 ppb of H 2, CO, CH 4, CO 2, and NMHC by dual channel packed column isothermal GC/RGD and GC/FID/M.

3 JPACSM 129 Table I Precision of Analysis of 5 ppb H 2, CO, CH 4, CO 2, and NMHC (Ethane) in Nitrogen by GC/RGD (1 cc), and GC/FID/Methanizer (5 cc). H 2 -RGD CO-RGD CH4-FID CO 2 -FID NMHC-FID Run # Peak Area Peak Area Peak Area Peak Area Peak Area 1 143,16 1,314,21 144,38 154, , ,161 1,31, ,71 154,5 241, ,711 1,37, , , , ,647 1,32,275 14, ,585 25, ,475 1,297, ,74 152,15 25, ,423 1,287, , ,744 25, ,674 1,275, , , ,954 Average 145,28 1,299, , ,42 246,74 Std Dev 3,113 13,8 2,75 4,244 5,972 % Std Dev Table II Hydrocarbons: RGD Unsaturates Versus RGD Alkanes Unsaturated Hydrocarbon RGD RF-1 Saturated Hydrocarbon RGD RF-2 RF-1/RF-2 Ratio C 2 H 4 2,7 C 2 H C 2 H 2 22,9 C 2 H C 3 H 6 42,1 C 3 H C 3 H 4 39,331 C 3 H RF = Response factor = area counts/ppb Note: Results may vary and are from a small data set. millivolt Peak Area Statistics: 14 1,3-BD RSD =.6%, n = 9 1,3-butadiene 12 Benzene RSD = 3.7%, n = 9 air 1 8 benzene retention time (seconds) Figure 3. Analysis of 1,3-butadiene and benzene in ambient air at 13 ppb using a 31 inch x 1/8 inch OD ss unibead 3S packed column. peak area counts 6,, 5,, 4,, 3,, 2,, 1,, conc. (ppb), [.54 ppb, 5.4ppb to 288 ppb] Figure 4. Ethylene GC/RGD response plot. millivolt RGD response 5.E+4 4.E+4 3.E+4 2.E+4 1.E+4 blank.e retention time (seconds) Figure 5. GC/RGD of ethylene at.54 ppb in N 2 /air, ethylene at 1.1 ppb in N 2 /air, and blank N ppb.54 ppb N2 blank

4 JPACSM 13 RGD millivolt response CO PH3 18 ppb AsH3 27 ppb COS retention time (sec) Figure 6. CO, PH 3, AsH 3, and COS in CO 2 by GC/RGD. RGD millivolt response CO PH3 AsH3 C3H6 EtO 3 ppb retention time (sec) Figure 7. CO, PH 3, AsH 3, C 3 H 6, EtO (3 ppb) in CO 2 by GC/RGD. 7/31/ 3:42 conc (ppb) ret. t.(sec) peak area benzene toluene ethyl benzene m,p-xylene Figure 9. BTEX in air at 1 ppb using three minute adsorption trapping with capillary temp. programmed GC/FID; estimated lower detection limit is 5 ppt for a ten minute trapping time. Figure 1 is a chromatogram of compressed air spiked with benzene analyzed by temperature programmed capillary GC/FID. The air was trapped on an adsorption trap for ten minutes and the smallest peaks at 11 area counts at retention time of 672 seconds represents a lower detection limit of 5 ppt (.2 ug/m 3 ) for hexane. Figure 11 is a chromatogram of 131 ppb dimethyl sulfide in N 2 by capillary isothermal GC/ELCD. The estimated lower detection limit single digit ppb is based on the observation of a sulfur compound peak at 3.7 ppb. Figure 8. Phosphine and arsine (14 ppb) in ambient air by GC/RGD.

JPACSM 131 1/1/21 9: ug/m3 RT(sec) peak area methane 84.1 324,35 97.1 36,849 112.9 12,828 ethane 4.1 149.6 2,62.7 26. 3,422 4. 211.2 2,196 propane 32.4 265.9 162,19 butane 12.8 394.

5 JPACSM 131 1/1/21 9: ug/m3 RT(sec) peak area methane , , ,828 ethane , , ,196 propane ,19 butane ,184 pentane ,423 C6 hydrocarbon benzene ,733 Figure 1. Compressed air spiked with benzene using ten minute adsorption trapping and capillary temperature programmed GC/FID; estimated lower detection limit is 5 ppt (.2 µg/m 3 ). SUMMARY GC based gas analyzers with Reduction Gas Detector (RGD) and Flame Ionization Detector (FID) routinely measure single digit part-per-billion (ppb) level gaseous impurities in bulk gases, high purity process gases, air, lab, and field gas samples. Typical impurities detected include H 2, CO, CH 4, hydrocarbons, CO 2, oxygenates, arsine, phosphine, and carbonyl sulfide. These analyzers have been used for continuous online gas analysis in air separation plants and semi-conductor fabrication facilities for more than a decade. Gas blenders and stream selectors are routinely combined with the programmable analyzer to make a complete system for automated analysis of up to six gas streams, plus calibration gas and zero gas streams. of VOCs at single digit part-per-trillion (ppt) to ppb concentrations. Typical applications include the use of adsorption traps to analyze C2-C11 VOCs in process gases, N 2, noble gases, CO 2, H 2, O 2, CO, and air by temperature programmed capillary column GC/FID. Sulfur compounds are analyzed by isothermal capillary columns and electrochemical detection. In the broader scope of industrial gas monitoring, where bench analyzers (IR, CLD, FID, paper tape, etc.) are fine for particular tasks, chromatography offers several benefits, particularly as one reaches the sensitivity limits of the bench instruments. Chromatography is superior in cases where interfering compounds/species may be present. In addition, a single GC can measure multiple components in multiple samples, something that might only be accomplished with a huge array of dedicated analytical instruments. 9/7/ 15:23 ppb RT (sec) Peak Area Dimethyl sulfide ,322 Sulfur compound ,99 Figure ppb dimethyl sulfide in N 2 by capillary isothermal GC/ELCD; estimated lower detection limit = low ppb.

PROCESS AND ENVIRONMENTAL APPLICATIONS OF GAS CHROMATOGRAPHY WITH THE REDUCTION GAS DETECTOR

JPACSM 18 PROCESS AND ENVIRONMENTAL APPLICATIONS OF GAS CHROMATOGRAPHY WITH THE REDUCTION GAS DETECTOR Marco Succi SAES Getters S.p.A., Milano, Italy James G. Moncur Independent Consultant KEYWORDS: REDUCTION