United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA-600/S7-81-041 July 1981 Project Summary Technique for Measuring Reduced -orms of Sulfur in Ambient Air Robert S. Braman and J A new technique for measuring low concentrations of volatile sulfur com- pounds in ambient air is The technique consists of tration of sulfur compoun isorption on gold metal co ited sand or gold foil surfaces followec mes M. Ammons discussed. ireconcen- s by chem- by thermal desorption, separation, and detection by flame photometry. Breakthrough capacities are on the order of 1 micro- gram total sulfur compounds. The unique aspect of this research is the derivatization of thiol type compounds principally H2S and CH3SH, by ethyl iodide for partial separation of H2S, DMS, SOz. COS, and CH3SH. Best suited for the 0.01 - 5 ppb range, the technique has been used to detect trace sulfur compound concentrations as low as 0.001 ppb by volume with 100L and larger sample volumes. Sample size detection limits depend upon the type of flame photometric detector used but are generally in the 0.1 ng range. Repeatability of mea- surements is ±5-8% relative standard deviation. Accuracy depends upon the compound. DMS and SO2 are detected as individual compounds. H2S, CS2 and COS are detected as a single compound on gold foil. Methylmer- captan and dimethyldisulf ide appear as a single compound. Field studies at two sites. Cedar Island, N.C. and Prairie View, Texas, have demonstrated that the technique is practical for field use and for determining vertical pro- files up to 10 meters above ground level. Hydrogen sulfide, dimethylsul- fide, sulfur dioxide and carbonylsulfide were the principal reduced forms of sulfur detected at these sites. This Project Summary was devel- oped by EPA's Environmental Sciences Research Laboratory, Research Tri- angle Park. NC, to announce key find- ings 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 most widely used commercial instrumentation for sulfur compounds in air employs the methods of Stevens, Mulik, O'Keeffe and Krost (1) and Stevens, O'Keeffe and Ortman (2) or modifications and improvements there- of. These methods are readily applicable for direct sulfur analyses at or above 1 ppb, and capable of separating several sulfur compounds in air in GC applica- tions. Ambient concentrations of H2S, (CHafeS and CH3SH compounds in non- polluted locations are in the 0.01 -1 ppb range. Consequently, these methods have only marginal detection capability and oxidation losses of H2S and CH3SH can be substantial. This study was undertaken to further explore the use of gold surface adsorp- tion as a preconcentration approach. In the improved methods reported here reduced sulfur compounds are adsorbed onto gold metal coated silica sand or gold foil. Chemisorbed and preconcen- trated sulfur compounds are derivatized to a Iky I sulf ides by ethyl iodide, desorbed ------- by heating, separated on a short column and then detected by means of a flame photometric detector. Preconcentration by means of gold coated silica sand was developed first, studied for interferences and used in the two field studies reported here. Nitrogen dioxide and ozone interfere if large sample volumes over 20-30 liters are used or if present in elevated con- centrations. These interferants are removed by use of a sodium carbonate filter. The method was used in field studies at Cedar Island, North Carolina and at Prairie View, Texas where H2S, DMS, SOz and COS were the major reduced compounds of sulfur detected. Diurnal variations and vertical profiles of sulfur compounds are reported. After the field studies, further methods development was carried out including investigation of gold foil preconcentra- tion and investigation of high molecular weight secondary amines for COS and CS2 preconcentration. Gold foil precon- centration was found to be less sensitive to oxidation than gold coated silica. The use of high molecular weight amines for COS and CS2 preconcentration was not successful. Preconcentration Tube Analysis Preconcentration tubes adsorb SOz, H2S, CH3SH (CH3)2S, COS and other sulfur compounds from air quantitatively at the flow rates used in sampling. These compounds are desorbed from the tubes in a two step analysis, trapped in a short liquid nitrogen cooled U-trap, separated during the heating of the U- trap and detected by means of a flame photometric detector. Figure 2 shows the analysis apparatus arrangement. Separation of sulfur compounds is accomplished in the U-trap part of the analysis system during heating. Filter The U-trap is constructed of 6 mm o.d. borosilicate glass and is 15 inches long packed with OV-3, 15% on Anakrom C- 22, 40/50 mesh, Analabs, Inc., New Haven, Conn. The FPD was constructed using an interference filter, quartz burner and MacPhearson Co. monochrometer. A logarithmic amplifier was used to lin- earize detector response to sulfur on one of the two analyzers assembled. The first desorption step involves derivatization of sulfur compounds on the preconcentration tube. Ethyl iodide vapor is injected into the nitrogen car- rier gas stream with the liquid nitrogen p f* — ^ 10 mm O.D. PTFE Teflon Connector Quartz Wool i-:':-::::9i.v;;::':8 I • ' A ' r3f l"' •/ ** KB v i Au Coated AJ03 %&>- 6 mm O.D. Quartz \ Polystyrene Analytical Procedures Sampling Air sampling was accomplished using quartz absorption tubes packed with gold metal coated white beach sand. Tubes were 10 mm o.d. and 20 cm long having a 3 cm packed section as shown in Figure 1. The sand was gold coated by first mixing the sand with auric chloride, and then taking it to dryness with stirring on a hot plate. This was followed by hydrogen reduction at elevated temperature. Enough gold chloride was used to give a 5%-10% gold coating by weight on the sand. The packed section was in 2 1cm lengths separated by quartz wool to prevent cracking of the quartz because of thermal expansion during heating in the analysis procedure. Flow rates through a filter and absorp- tion tubes were generally in the 1-3 L/mn range and were determined for each set of sampling pump, tube and filter used in air sampling. Air sampling was generally done at 2 meters above the ground. In certain instances sampling was done with towers at several distances above the ground at generally, 10, 5, 2, 1,0.5 and 0.2 meters. Figure 1. Preconcentration tube design. Hi-Air Burner Sampling Tube "" —I PTFE Connector T° autotransformefi Adaptor / To autotransformer Cold-trap PM Module Filter To Amplifier and Strip Chart Recorder Dewar Figure 2. Analysis apparatus arrangement. ------- Dewar in place and passes through the preconcentration tube which is then heated to 300-400°C for 2-4 minutes. Ethyl iodide, far in excess of the sulfur present, is adsorbed on the gold surface and the excess is collected in the cold U- trap. Upon heating, some of the ethyl iodide converts the RSH and H2S present to the ethyl derivatives, likely by two steps. C2HSI + Au - Aul" + CaH6+ 2 C2H5+ + H2S - (C2Hs)zS + 2H+ Aul~ + H+ - HI • Au Alkylthiols are converted to the ethyl derivative by the same reaction. C2H5I + C4H9SH - C4H»SC2H5 + HI Sulfur dioxide and dimethylsulfide do not react. Carbonyl sulfide and carbon bisulfide, if present and adsorbed on the gold, are also converted to diethyl sul- fide (DES). In natural samples COS is observed to be adsorbed onto the quartz wool part of the preconcentration tube where it is not converted in the alkylation process. It is removed from the quartz wool during the first heating step and is recovered as COS in the analysis of the U-trap. The second step in analysis deter- mines S02. The carrier gas is switched to hydrogen and the U-trap is cooled. The preconcentration tubes are then heated to 600-700°C. Sulfur dioxide still adsorbed onto the gold is converted to H2S which is desorbed and trapped by the U-trap. The U-trap is allowed to warm up, and H2S is detected. Response calibration is done using permeation tubes (for SOa,"H2S, DMS) and an air mixing manifold. Permeation rates determined gravimetrically are corrected for temperature effects. Re- sponse curves are prepared by absorbing known amounts of the low concentra- tions of diffusion tube standards onto blanked preconcentration tubes followed by their analysis. The typical reproducibility of standards on absorption tubes has been deter- mined from several sets of standardiza- tion experiments and was found to be ± 4.5 - 5% for DMS, ± 8 -10% for SO* all in units of relative standard deviation. Gold Foil Preconcentration Because of low recoveries of H2S occasionally noted when using Au coated sand, work was initiated using gold foil packed tubes. Gold foil tubes gave 50% higher response than the Au coated sand indicating a lower loss of H*S. Because of the better response of the gold foil tubes several were prepared and used in subsequent experiments. The detec- tion system was recalibrated using a gold foil tube as the preconcentration standard tube. The resulting calibration curves had less point scatter than previous curves using the gold coated sand tubes, especially for small sample sizes in the 0.5-10 ng range. Dimethyl sulfide seems to be quite stable towards oxidation loss of sample. Both the gold foil and gold sand tubes exhibited good scrubbing and recovery efficiencies. No DMS was detected on the quartz wool and silica tubes and essentially all of the DMS was observed to pass. Gold foil tubes under ambient sam- pling conditions have given reproducible results. Standards checked on the foil tubes following field use have consist- ently been on the calibration curve showing outside use dots not degrade the tubes. After about eight months and approximately 500 analyses one original Au foil tube showed a decrease in re- covery of about 15%. The foil was removed and placed in a new quartz holding tube. The response returned to normal. Microscopic examination of the now somewhat cloudy original quartz holding tube showed that the surface had become etched. It is believed that the vastly increased surface area due to the etching caused partial loss of the sample in a mechanism similar to that observed for the silica tube. A set of three gold foil tubes were used for 15 samples of H2S of 4.0 ng each. The repeatability of analyses was 4.5% r.s.d. This rose to approximately 8% near the detection limit. The detec- tion limit as 2 x RMS noise for H2S using the foil tube was 0.115 ng H2S as H2S. For DMS it was 0.33 ng as DMS. Conclusions and Recommendations The preconcentration of reduced compounds of sulfur on gold metal surfaces has been studied, improved and applied in field analyses. The major improvement made was the addition of a derivatization step in the procedure. The derivatization of thiol compounds by ethyl iodide eliminates sulfur dioxide and sulfate interferences in preconcen- tration prior to desorption and analysis. Sample volumes up to 50 liters can be preconcentrated with minimal inter- ference from ozone or nitrogen oxides copresent in air so long as sodium carbonate scrubbers are employed. Carbonyl sulfide is collected and identi- fied separately on gold coated sand but carbon disulfide is derivatized and not distinguished from hydrogen sulfide. On gold foil H2S, CS2 and COS all appear as the same derivative. Within these limitations the method is useful for preconcentrating H2S, (CHa)2S, COS, CHaSH and CH3-S-S-CH3 at concentra- tions as low as 1 ppt. Gold foil preconcentrators appear to have the advantage of easier adaptability to automated use than the gold coated sand tubes. Polymeric secondary amines of several types preferentially absorb COS and CS2 while not absorbing HaS. Unfortunately, thermal desorption of the COS and CS2 is incomplete and the approach was not successful. Field use of the method at Cedar Island, N.C. and Prairie View, Texas indicated that COS, H2S, S02 and DMS exhibit a diurnal variation with increased production in salt marshes during the day. These sulfur compounds are pro- duced in the ground over land and dif- fuse into ambient air. Although suitable for qualitative ex- amination of vertical profiles, the sam- pling time required and precision of the method are probably not sufficient for accurate calculation of dry deposition rates. Further study of the analytical ap- proach should include modification to selectively separate and determine COS and CS2. Gas to particle conversion at the ground level in air and further verifica- tion of the diurnal variation at locations other than over salt marshes are also needed. References 1. R.K. Stevens, J.D. Mulik, A.E. O'Keeffe, and K.J. Krost, Anal. Chem., 43, 827(1971). 2. R.K. Stevens, A.E. O'Keeffe, and G.C. Ortman, Environ. Sci. Technol., 3, 6.52(1969). « UAOOVCRNtKNT mOTM OFFICC: 1»1 .757-012/7215 ------- Robert S. Braman and James M. Ammons are with the University of South Florida, Tampa. FL 33620. William McClenny is the EPA Project Officer (see below). The complete report, entitled "Technique for Measuring Reduced Forms of Sulfur in Ambient Air," (Order No. PB 81-179 772; Cost: $8.00, 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: Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United'States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 . PS 0000329 ------- |