United States Environmental Protection Agency Atmospheric Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S3-85/016 Apr. 1985 &EPA Project Summary Chemical Transformations in Acid Rain: Volume I. New Methodologies for Sampling and Analysis of Gas-Phase Peroxide Roger L. Tanner New methodologies for sampling and analysis of gas-phase peroxides (H2O2 and organic peroxides) using (a) diffusion denuder tubes and (b) gas- to-liquid transfer with prior removal of ozone have been investigated. The purpose was the development of an interference-free method for deter- mining H2O2(g) in ambient air. A denuder approach using ferrous (1, 10-phenanthroline)-coated tubes was unsuccessful for, although H2O2 was removed, the capacity was low and ozone was also removed, possibly through surface decomposition to H2O2 and its radical precursors. Gaseous peroxide in compressed airstreams could be collected in im- pingers without artifact formation from surface ozone decomposition if O3 was first removed by gas-phase titration with nitric oxide. This Project Summary was developed by EPA's Atmospheric Sciences Research Laboratory, Research Triangle Park, NC, to an- nounce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The purpose of this research task was to develop fundamentally new methods for sampling and analysis of gas-phase hydrogen peroxide (H202) and organic peroxides, if possible, through the use of diffusion-denuder tubes. In addition, sampling methods for H202 using gas-to- liquid transfer and capable of avoiding in situ production of H202 from ozone (03) decomposition and other processes were studied. The goal of the research was an interference-free method for gas-phase peroxides with 0.1 ppb limit of detection and 15 min time resolution. Much analytical effort has been expend- ed in the past few years in measuring gaseous and aqueous H202 following the recognition that H202 could oxidize dissolved S(IV) rapidly throughout the pH range of rain, cloud and fog waters. Fur- thermore, the high solubility of H202 in water led to significant H202 concentra- tions in cloudwater. Methods for deter- mining liquid-phase H202 have been developed using several approaches: luminol chemiluminescence, p-hydroxy- phenylacetic acid dimer fluorescence, scopoletin fluorescence quenching and peroxyoxalate chemiluminescence. At- tempts to measure gas-phase hydrogen peroxide by collection in impingers or by other dissolution techniques have been shown to be generally unreliable due to the in situ formation of hydrogen peroxide from low-solubility constituents of am- bient air during collection by impingers. It is suspected that surface-initiated ozone decomposition via H02~ and 02~ in- termediates is the likely mechanism of "artifact" H202 formation. ------- Two possible research approaches for artifact-free sampling of H202(g) were in- vestigated in this project: selective, reac- tive sampling onto a coated denuder tube, employing H202 redox chemistry and sup- pression of in situ H202 formation by selective removal of ozone. This study describes a denuder ap- proach using ferrous (1,10-phenanthroline) coated tubes that was successful: the capacity for H202 was low and ozone was also removed, possibly through surface decomposition for H202 and its radical precursors. However, the study also documents that gaseous peroxide in com- pressed airstreams can be collected in im- pingers without artifact formation from surface ozone decomposition if 03 is first removed by gas-phase titration with nitric oxide. Experimental Experiments in this task were con- ducted by using a system in which gas- phase hydrogen peroxide was reproduci- bly generated in the 1-500 ppbv range by multiple dilutions with compressed air, with facilities to subsequently humidify the air and add ozone and nitric oxide to the diluted airstream (see Figure 1). Parallel airstreams were then formed with appropriate mixing chambers, and perox- ide was collected in parallel series of 2 or 3 midget impingers that were followed by flow meters, charcoal traps for ozone and H202, and a common, selectable inlet line to the ozone analyzer. Diffusion denuder tubes containing coatings of Fe(ll)-1, 10-phenanthroline were tested in ex- periments by placing them in the flow streams after division at points A and B, just prior to impingers 1-1 and 2-1, respectively. Aqueous H202 in each of the six impingers was analyzed after each ex- periment using the POHPAA fluorescence method by sample injection into a flow in- jection system. For the most of the experiments con- ducted in this task, an alternate, manually operated "stopped-flow" approach to aqueous H202 analysis was used. In this approach, three volumes of sample were mixed, with one volume of peroxidase/ POHPAA/EDTA solution in pH 8.5 TRIS buffer. The premixed sample was allowed to standard for 2-3 min, and then an ali- quot was injected directly into the fluorimeter flow cell for analysis. The flow cell was rinsed out thoroughly with TRIS buffer between injections. Extensive washing of the sample lines in contact with catalase solution was required before reuse. Results and Discussion Considerable effort was expended to improve the POHPAA fluorescence technique for aqueous-phase peroxide. These efforts were required because low levels of peroxide would have to be analyzed for passive denuder-collection Mixing Chamber NO/ A/2 H202 samples or for artifact-free impinger collections (if such could be achieved). Without the improved analytical sensitivity attained by these efforts, the stated goals of this task (0.1 ppbv, 15 min time resolu- tion) would not have been possible. The final limit of detection achieved during this effort was 0.3 ppb aqueous (~ 0.01 /Jvl), This sensitivity was just barely ade- quate to attain the task goal for the artifact-suppression or denuder ap- proaches. Denuder Tube Sampling Pyrex denuder tubes, 0.6 cm OD by 30 cm long, were coated with ferrous-! 1, 10-phenanthroline)-sulfate solution in methanol. They were then placed in the parallel airstreams downstream from the mixing chamber and, in the case of line 2, after the addition of NO in N2 to the airstream. Various admixtures of com- pressed air and ozone and/or H202 were passed through the denuders. Nitric oxide (6.2 ppm after dilution) was added to line 2 and the apparent H202 in each of im- pingers 2-1 through -3 and 1-1 through -3 (see Figure 1) was determined using the POHPAA fluorescence technique. Four- teen experiments were conducted to test the denuder sampling technique: The results obtained suggested the following: (1) Ferroin-coated denuder tubes of the type tested have limited capacity for H202 removal, but they also remove F/M 2 2-1 2-2 Impingers 2-3 Vent Excess Thru Charcoal 8410 03 Monitor F/M 1 1-1 1-2 Impingers 1-3 I 1 Optional Humidifier Figure 1. Modified apparatus for W2O2 generation and impinger collection. 2 ------- 03 at continuously decreasing efficien- cy. (2) Ferroin-coated denuders do not pre- vent artifact H202 formation in im- pingers downstream of the denuder. (3) It appears that ozone reacting on the denuder surface may generate artifact H202 on partially exhausted denuder tubes. We do not wish to assert that all iron(ll) complexes would exhibit the same behavior when used as denuder coatings as did ferroin, but only that the denuder approach as defined was heretofore un- successful in removing H202 quantitative- ly. In addition, complications are intro- duced because of co-removal of ozone and the likely co-reaction with the ferroin. Gas-to-Liquid Sampling After O3 Removal As noted above, previous observations have suggested that artifact H202 formed in gas-to-liquid sampling using impingers or other approaches seems to be related to levels of ozone and one or more other air constituents. As a result, sampling ap- proaches in which ozone is selectively removed from the sampled air may be successful in eliminating the artifact formation of H202 while simultaneously transmitting H202 at high, reproducible ef- ficiency to an aqueous solution for POHPAA analysis of H202. Unreported data suggest that the amount of H202 formed in bubblers is non-linearly related to 03 concentration in sampled air, but since 03 reaction on the bubbler surfaces appears to be the initial and limiting step in artifact H202 formation, removal of 03 prior to sampling should effectively eliminate the process. The evidence that titration with excess NO removes artifact H202 formation in im- pinger collection of H202 is shown in Table 1. Hydrogen peroxide (calculated to be 28 + 2 ppb in stream 1 and 22 ± 4 ppb in stream 2) was admitted without O3 or NO in Expts. 1 and 2; peroxide was found in roughly equal amounts in bub- blers 1-1 and 2-1. No peroxide was found in subsequent bubblers. Ozone at 327 ± 13 ppb was admitted to the system in Expts. 3 through 9, with NO (6.2 ppm) present in stream 2 only for Expts. 4 through 9. In Expt. 3, addition of ozone alone to both streams produced H202 in all impingers with most being found in im- pinger 1 of each stream. Addition of ozone + H2O2 mixtures to stream 1 and 03/H202/NO to stream 2 (Expts. 4, 5, and 9) produced additional peroxide in all stream 1 bubblers consistent with the results of Expt. 3, but no peroxide was formed in stream 2 bubblers sampling NO- containing air. The amount of artifact H202 sampled from 03-containing airstreams was variable, and appears to be reduced in Expts. 5 to 9 in comparison to Expts. 1 to 4. Indeed, no artifact H202 was formed in impingers 2-2 and 2-3 during Expts. 6 to 8. In contrast, only in Expt. 6 was a small amount of H202 found in the 03/NO/air- stream. This could be due to inadequate mixing of NO with the ozone/airstream, but is more likely the result of desorption of H202 retained in the mixing chamber from the previous experiment. Never- theless, the preponderance of evidence suggests that ozone is removed sufficient- ly fast that no measurable artifact H202 is formed. Artifact H202 is formed in variable amounts when an airstream containing about 300 ppb 03 is sampled. Collected amounts correspond to about 3-15 ppb of gaseous H202 in the first bubbler and roughly an order of magnitude lower in subsequent bubblers. This differs somewhat from results reported by others in which roughly equal amounts of perox- ide were formed in subsequent bubblers, and indeed, it is different from our own early results, suggesting that the extent of the process in which artifact H202 is form- ed is quite dependent on the presence of other atmospheric constituents in addition to 03. That these constituents may differ widely in their aqueous solubility is sug- gested by the low production rate of ar- tifact H202 in impingers 2 and 3 compared to impinger 1 for the experiments sum- marized in Table 1. Conclusions The research approaches investigated in this task for artifact-free sampling of H202(g) included selective, reactive sampling onto a coated denuder tube, employing H202 redox chemistry and sup- pression of in situ H202 formation by selective removal of ozone. A denuder approach was attempted employing Fe(ll)-1,10-phenanthroline- coated glass tubes. Hydrogen peroxide was removed by such tubes but collection efficiencies less than calculated values were observed even with relatively fresh tubes. This suggested that surface deple- tion of sorption sites was reducing the capacity of dry coating on the diffusion tube. In addition, ozone was removed to a significant extent by the Fe(ll)-phenan- throline denuder tubes, which indicated a lack of specificity for H202 and raised the spectre of surface decomposition of 03, possibly to gas-phase H02 and/or H202. Table 1. Collection of H2O2 from Ozone-Containing and Ozone-Free Air Streams Experiment No. ; 2 3 4 5 6 7 8 9 H202 (fJ\ Composition H202/Air " O3/Air 03/H2O2/Air " 03/A/r " " 03/H202/Air VI) in Sampled Bubbler 1 0.58 0.55 0.36 0.96 0.63 0.15 0.044 0.047 0.69 Gas Stream 1 Ave, Bubbler 2 + 3 ND* •• 0.022 0.018 0.016 ND ND ND 0.060 H202 (fuM Composition H2O2/Air " O3/Air 03/H2O2/NO/Air " 03/NO/Air " " 03/H202/NO/Air ') in Sampled Gas Bubbler 1 0.56 0.39 0.31 0.52 0.45 0.026 ND ND 0.50 Stream 2 Ave, Bubbler 2 + 3 ND ND 0.014 ND ND ND ND ND ND *ND = none detected {•< Blank) Gas Phase Concentrations: [Ozone] = 327 ± 13 ppb (Expts. 3-9) [H2O2] = 28.0 ± 2.0 ppb I Stream 1); 22.2 ± 3.6 ppb IStnvn 2) [NO) = 6.2 ppm Sampling Conditions: Air sampled for 30 min 0.50 L/min in each One. ------- Thus, removal of H202 onto denuders by nominally H202-specific chemisorption does not appear to offer significant ad- vantages over gas-to-liquid sampling for gaseous H202 analysis. Suppression of in situ production of H202 in gas-to-liquid sampling (bubblers, impingers) by upstream titration of the ozone in the sampled airstream has been successfully demonstrated for cases in which compressed air is used. Gaseous hydrogen peroxide was collected com- pletely (>99%) in the first bubbler whereas confluent ozone produced measureable peroxides in the second and third bubblers; no peroxide was observed in the second and third bubblers for those experiments in which 03 (100-300 ppb) was titrated by 6 ppm NO prior to bubbler collection. The addition of 6 ppm NO does not significantly interfere with POHPAA analysis of collected aqueous H202. Roger L Tanner is with Brookhaven National Laboratory, Upton, NY 11973. Marc/a C. Dodge is the EPA Project Officer (see below). The complete report, entitled "Chemical Transformations in A cid Rain: Volume I. New Methodologies for Sampling and Analysis of Gas-Phase Peroxide, "(Order No. PB 85-174 425/AS; Cost: $8.50, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at' Atmospheric Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 U S GOVERNMENT PRINTING OFFICE 1985-559-016/27031 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 POSTAGE & FEE'S : ~ fl PEŁA- i* I ~ PERMIT No"(i< Official Business Penalty for Private Use $300 OOOC329 PS U S ENVIR PROTECTION AGENCY REGION 5 LIBRARY 230 S DEARBORN STREET CHICAGO IL 6 060 A ------- |