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(24) McKay, J. F.; Cogswell, T. E.; Weber, J. H.; Latham. D. R. Fuel 1975, 5 4 , 50. (25) Bunger, J. W.; Thomas, K. P.; Dorrence. S. M. Fuel 1979,5 8 , 183. (26) Liotta. R.; Brons, G.; Isaacs, J. Fuel 1983,62, 781. (27) Liotta. R. Fuel 1979,56,724. (28) Kawahara, F. W. Anal. Chem. 1968,40. 2073. (29) Smith, R . V.; Tsai, S. L. J . Chromatogr. 1971,61. 29. (30) Brooks, J. B.; Alley. C. C.; Liddle, J. A. Anal. Chem. 1974,4 6 , 1930. (31) Alley, C. C.; Brooks, J. B.; Choudhary. G. Anal. Chem. 1976,48, 387. (32) Parish, R . C.; Stock, L. M. J. Org. Chem. 1965,30, 927. (33) Tedder. J. M. Chem. Rev. 1955,5 5 , 787. (34) Darbre, A. I n Handbook of Derivatives for Chromatography; Blau, K., King, G. S., Eds.; Heyden Press: Philadelphia, PA, 1978; Chapter 2. (35) Blau, K.; King, G. S. I n Handbook of Derivatives for Chromatography; Blau, K., King, G. S..Eds.; Heyden Press: Philadelphia, PA, 1978; Chapter 3. (36) Bellamy, L. J.; Williams, R. L. J. Chem. SOC. 1957,4294. (37) Hewltt, C. D.; Silvester, M. J. AMrichimica Acta 1988,21(1), 3. (38) Grizzle. P. L.; Green, J. 8.; Sanchez, V.; Murgia, E.; Lubkowitz, J. Prep.-Am. Chem. SOC., DivPet. Chem. 1981, 26. 839. (39) Green, J. 8.; Grizzle, P. L.; Thomson, J. S.; Hoff, R. J.; Green, J. A. Fuel 1985,6 4 . 1581. (40) Green, J. B.; Hoff, R. J.; Woodward, P. W.; Stevens, L, L. Fuel 1984, 63, 1290. (41) Green, J. B.; Stierwalt, B. K.; Green, J. A,; Grizzle, P. L. Fuel 1985, 6 4 , 1571. (42) Green, J. 8. J. Chromatogr. 1986,3 5 8 , 53. (43) Green, J. 8.; Thomson, J. S.; Yu, S. K.-T.; Treese, C. A,; Stierwalt, B. K.; Renaudo. C. P. P r e p . Pap .-Am. Chem. SOC.,Div. Fuel Chem. 1986,31(1), 198. (44) Green, J. B.; Grizzle, P. L.;Thomson, J. S.; Shay, J. Y.; Diehl, B. H.; Hornung, K. W.; Sanchez, V. "Analysis of Heavy Oils: Method Development and Application to Cerro Negro Heavy Petroleum. Distillation and Determination of Routine ChemicalPhysical Properties"; US. DOE Contract No. DE-FC22-83FE60149, NIPER Topical Report No. 160; February 1988. (45) Yu, S. K.-T.; Green, J. B. "Infrared-Chemical Derivatization Method for Determination of Total Hydroxyls and Carboxyls in Petroleum and Syncrudes"; U.S. DOE Contract No. DE-FC22-83FE60149, NIPER Topical Report No. 363; September 1988. 146) Dzidic, I . ; Somervllle. A. C.: Raia. J. C.: Hart. H. V. Anal. Chem 1988,60, 1318
(47) Cross, A. D.; Jones, R. A. Practical Infrared Spectroscopy, 3rd 4.; Butterworths, Ltd.: London, 1969; pp 38-42. (48) Cross, L. H.; Rolfe, A. C. Trans. Faraday Soc.1951,47, 354. (49) Allen, D. T.; Grandy, D. W.; Jeong. K.-M.; Petrakis, L. Ind. f n g . Chem. Process Des. Dev. 1985,24, 737. (50) Sutterfield, D.; Lanning. W. C.; Royer, R. E. Upgrading Coal Liquids; Sullivan, R. F., Ed.; Advances in Chemistry 156; American Chemical Society: Washington, DC, 1981; Chapter 5. (51) Seifert, W. K. Progress in the Chemistry of Organic Natural Products; Herz, W., Grisebach, H.. Kirby, G. W., Eds.; Springer-Verlag: New York, 1975; Vol. 32, pp 1-49. (52) Ross, A. M.; Whalen, D. L.; Eldin, S.; Pollack, R. M. J. Am. Chem. SOC. I98eq 110, 1981. (53) Keeffe. J. R.; Kresge, A. J.; Yin, Y. J. Am. Chem. SOC. 1968, 110. 1982.
RECEIVEDfor review October 19, 1989. Accepted March 3, 1989. This work was sponsored by the U S . Department of Energy under Cooperative Agreement DE-FC22-83FE60149. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Universal Sulfur Detection by Chemiluminescence Richard L. Benner and Donald H. Stedman* Department of Chemistry, University of Denver, Denver, Colorado 80208
A universal sulfur detector (USD) capable of measuring both reduced and oxidized sulfur compounds in the low picogram range is descrlbed. Products of a hydrogen flame are drawn through a critical orlflce into a low-pressure flow system and then mixed with ozone. Sulfur compounds entering the flame produce sulfur monoxide which undergoes a chemiiuminescent reaction with ozone. The excited state of sulfur dloxide produced emits light with a peak wavelength of 350 nm. The USD, which has been tested in the real-time mode, has a linear response from sub part per billion by volume (ppbv) to sub part per million by volume S, and has a detection limit of about 0.13 ppbv (==l pg of Ses-') with a time constant of 2 s. The molar response to sulfur was found to be identlcal for the five low molecular weight compounds tested. The USD has no interferences from water vapor or CO,. Hydrocarbon interference Is mlnhlred by adjustment of the residence time of the sample in the flame. The base line signal drift is typically undetectable over 24 h of continuous use.
INTRODUCTION Sensitive and selective detection of sulfur compounds is of considerable interest in a number of widely diverse fields (1-3). Nearly as diverse are the methodologies and detectors reported 0003-2700/89/036 1-1268$0 1.50/0
(4-9). By far the most widely used sulfur-selective detector is the flame photometric detector (FPD), which has been reviewed thoroughly by Farwell and Barinaga (10). Chemiluminescence detection schemes have also been reported for reduced sulfur species with O3 (9),CIOz ( I I ) , and Fz(7). These have not gained wide acceptance because of interference problems, difficulties in handling reagents such as CIOz, drastic differences in sensitivity to different species, or lack of detection for oxidized species. The FPD is based on the production of sulfur atoms in a Hz/Oz flame, which combine to form electronically excited Sz*.The Sz*has chemiluminescence emission bands at 384 and 394 nm (10). The intensity of the emission is fundamentally nonlinear in sulfur concentration and has significant interferences from other species, which can be either in the positive or negative direction (10). The addition of sulfur compounds at a constant background concentration has been used to linearize the FPD output and decrease the detection limit but decrease the dynamic range. When used as a real-time detector for atmospheric monitoring, the FPD suffers from HzO and COz interference (12). Hydrocarbons are the major source of interference in gas chromatographic uses (10). Theoretical and experimental studies have shown that a large portion of sulfur entering a flame produces SO (2, 13, 14). In fact, the SO is present at concentrations about 10 times higher than that of atomic sulfur. Even though SO is a free 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
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Quartz Sample Tip
Ozone
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Schematic diagram of the USD operated in the real-time
mode. radical, it is sufficiently stable to be sampled in a flow system (15). Halstead and Thrush (16) observed a strong blue chemiluminescence of SOz* from the reaction of O3 with SO. On the basis of these observations, we hypothesized that the SO produced in a flame would be easy to detect and linear in the sample sulfur concentration and provide lower detection limits than those obtained for the FPD. Akimoto et al. (17) have shown that the SOz* emission is identical with that observed from reactions of SO, CH3SH, CH3SCH3, and HzS with ozone and conclude that SO is an intermediate in the reactions of each of these species. Since HzS is also formed in a hydrogen-rich flame, the possibility that a portion of the observed chemiluminescence is due to the more complex HZS/O3 reaction (18,19) cannot be ruled out. Introduction of ozone directly into a flame is not a feasible means of exploiting the SO + O3 reaction because of the thermal instability of O3at flame temperatures. An uncooled sampling probe based on the design described by Fristrom and Westenberg (20) has been used to draw the entire flame to low pressure and room temperature where reaction with O3 is possible. The pressure of the gases drops to approximately 10 Torr; thus all chemical reactions are quenched (20), there is no possibility of condensation of the water produced in the flame, and the gas mixture is rapidly transported to a light-tight reactor for the chemiluminescent reaction.
EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the present version of the instrument is shown in Figure 1 (21).In the first version, a home-built photometer was utilized. For most of the data discussed herein, a chemiluminescence detector (RCD, Model 270, Sievers Research, Boulder, CO) was used. Modifications to the detector included the following: replacement of the standard photomultiplier tube (PMT) with a blue-sensitive PMT (Model R268, Hamamatsu, Japan), replacement of the glass window with fused quartz, and addition of an optical filter (7-54, Corning Glass Works, Corning, NY). The optical filter transmits between 240 and 410 nm with a peak transmittance of 82% at 320 nm. In addition, the reaction cell was modified with larger plumbing to accommodate larger sample flow rates. Larger flow rates were achieved by replacement of the standard 37 Lamin-' vacuum pump with a 300 Lsmin-' pump (Model 1012, Alcatel, France). A high-capacity ozone generator (Model RF, Triple-0-International Systems, Nirim, IN) was used because it could produce nearly 10 times more ozone at 80 standard cm3.min-' than the ozone generator built into the RCD. The sample tip and burner assembly are shown in Figure 2. This quartz assembly was built in-house and contains a diffusion flame. Dilution air used in the dynamic dilution calibration system (Figure 1)was metered by a rotameter. Hydrogen and calibration standards of sulfur gases were metered with mass flowmeters (Model FM360, Tylan, Carson, CA).
+
To Reaction Cell Figure 2. Custom-built quartz burnerlsample tip assembly. The sample tip contains an orifice of 0.1 mm diameter. The sample tip can be moved to adjust the residence time of the sample in the flame from 1 to 40 ms. The burner body is 12 mm o.d., and all other tubing is 6 mm 0.d.
Reagents. Dilution air was obtained from the laboratory bench but first passed through an activated charcoal adsorbent bed. All tubing between the sample orifice and the RCD reaction cell was coated with halocarbon wax (Series 1200, Halocarbon Products, Hackensack, NJ) to minimize loss of the SO to wall reactions. Oxygen supplied to the ozone generator and the hydrogen were standard grade (US Welding, Denver, CO), and no provisions were made to remove contaminants from the gases. The SO2calibration standard (99 ppmv in air) was obtained from Matheson (East and HzS (217 ppbv Rutherford, NJ), and the SF6(51 ppmv in Hz) in helium) standards were obtained from Scott Specialty Gases (Longmont, CO). RESULTS AND DISCUSSION Design Considerations. Preliminary testing of the USD for sulfur has been performed with the detector in the realtime mode. It is important that the postflame pneumatic system be maintained at the lowest possible pressure for the following reasons: (1)Halstead and Thrush (16) have determined that the intensity of the chemiluminescence reaction is inversely proportional to pressure with a half-quenching pressure of about 0.02 Torr. (2) The sample stream is about 25% water vapor. The pneumatic system pressure must be maintained below about 50 Torr so that the partial pressure of water vapor is below the vapor pressure of water at room temperature. This ensures that condensation does not occur in the pneumatic system. (3) The sample probe design used has been shown to quench all postflame reactions if the pressure across the sample orifice drops by a factor of 10 or more (20). I t was found that an orifice with a diameter of about 0.1 mm provided the necessary pressure in the pneumatic system. All data presented here were collected under the following conditions: sample air flow of 400 standard cm3.min-', a cell pressure of 10 Torr, and 5 cm3.min-' O3 in 80 standard cm3.min-' of Oz. Addition of the UV filter decreased a high base-line signal equivalent to 40-50 ppbv to less than 2 ppbv. The burner/sample tip assembly was designed to allow the sample tip to be moved relative to the hydrogen inlet, which acts as the flame origin. At the flow rates used, the residence time of the sample in the burner is 4 ms-cm-'. The burner is designed to allow the flame residence time to be adjusted from 1 to 40 ms. System Performance. Base-Line Stability. It was observed after nearly 2 months of working with the instrument that the base-line signal would start at a very low value but
1270
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drift upward to off-scale within a few hours after flame ignition. This background chemiluminescence increased when the ozone generator was turned off and only O2 was reacting with the sample gases. The signal disappeared completely when the Oz flow was stopped. The absolute intensity of the background signal was not sufficiently intense to allow spectral analysis. It was found that addition of a small amount of a halogenated compound (e.g. any chloroflurocarbon, CCl,, etc.) to the flame would eliminate the base-line drift without affecting the sensitivity to sulfur compounds. Apparently, the reactive species responsible for the background luminescence is scavenged by the halogens. The present instrument introduces CFzC12into the dilution air. The CF2C12has been added at two concentrations-40 and 180 ppmv; both produce an equally low and stable base line. With the addition of CF2C12,the base-line drift during 24 h of continuous operation is undetectable and is approximately 0.2 ppbv during 75 h of continuous operation. E f f e c t of Hydrogen Flow. The sensitivity of the USD to SO2 as a function of equivalence ratio is shown in Figure 3. The equivalence ratio is defined as the ratio of the actual hydrogen flow rate to the hydrogen flow rate needed for stoichiometric combustion. It can be seen that the optimal equivalence ratio is between 1.6 and 1.8. This optimum equivalence ratio is independent of the sample residence time in the flame (i.e. sample tip position). We believe that the reason for the sharp optimum is related to the equilibrium
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a large excess of H2 causes the flame temperature to decrease, which shifts the equilibrium to the left. Effect of Flame Residence Time. Figure 4 shows how sensitivity and signal to noise ratio (S/N) change as a function of flame residence time. A very sharp peak in the S / N is observed with a flame residence time of approximately 2.5 ms. The signal and S/N decrease very rapidly at shorter flame residence times, probably because the combustion is incomplete and SO is not formed. At longer flame residence times the signal reaches a constant value at the SO equilibrium concentration, but the noise increases due to a less stable flame which "flickers". Effect of O3 Flow and Concentration. The RCD ozone generator was used during initial testing. The O3 concentration produced with O2 is twice that with dry air, and a corresponding difference in sensitivity is observed. Switching to the larger ozone generator (increasing O3 concentration by 10) increased sensitivity by a factor of 2. The ozone flow rate at which the sensitivity is optimum corresponds to 5 cm3 O3 per minute in 80 standard cm3-min-' of 02.The flame produces a large amount of NO, which also reacts with the 03, and may be the reason why such a large amount of O3 is needed to optimize the sulfur signal. Sensitivity, Detection Limits, and Dynamic Range. Dynamic dilution calibration with a 99 ppmv SOz standard has shown a linear response between 0.4 ppbv and 1.5 ppmv (=3 to 13000 pg of S s s - ' ) . Figure 5 shows a calibration curve obtained for SO2 with a flame residence time of 2.5 ms. The slope of the line is 12.1 f 0.2 mV.ppbv-' with an intercept of 2.2 f 9.0 mV. The calibration system could not produce accurate concentrations below 0.4 ppbv and therefore detection limits are determined by extrapolation of the sensitivity to base-line noise. The extrapolated detection limit for SO2 is 0.13 ppbv with a signal/peak-to-peak noise of 2. Table I shows the relative response of five sulfur compounds which have been normalized to SO2 response. Each of the slopes for the calibration lines have slope standard deviations of less than 5% of the slope, demonstrating good precision in the measurements. The SO2,SF6,and H2S standards were certified to an accuracy of -+lo% by the manufacturer. The CH3SCH3and C2H,SH sources were made in-house by more approximate volumetric dilution methods. The USD has equal response to each species within the error of accuracy of the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
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Figure 6. Effect of flame residence time on the degree to which hexane interferes with the sulfur signal. There is no interference with a flame residence time of approximately 7.5 ms. calibration sources. With ozone flowing into the reaction cell (with the flame off) the output current is about twice the dark current. The output increases by a factor of 2 when the flame is burning and ozone is flowing. The base-line noise with the flame off (Le. electronic noise only) is 50% of that with the flame on (electronic plus chemical noise). The SO production efficiency may be increased by operating the flame at lower than ambient pressure. This possibility has not yet been investigated. The possibility that the chemiluminescent precursor is H2S,rather than SO, will also be investigated in future studies. Interferences. Two common interfering species for the FPD operated in the real-time mode are C02and water vapor. For the USD there is no effect on either the base-line signal or the response to a given concentration of SO2 for water vapor between 0.4% and 3% (~12-83%relative humidity at 23 "C). The same H2O concentration variation would be expected to reduce a FPD response by 5 % (21). Changing the COPconcentration from 350 ppmv to 1700 ppmv produced no detectable change in response to SO2 in the USD. The same COPconcentration variation would be expected to reduce a FPD response by 80% (12). In chromatographic analysis, two compounds that commonly coelute are methyl ethyl sulfide (MES) and hexane. The hexane enhances the sulfur signal at low sulfur concentration and quenches the sulfur signal at high sulfur concentration in the FPD (10). A 90-L Tedlar bag was filled with air containing approximately 25 ppbv MES and analyzed with the USD while successively increasing the concentration of hexane in the bag. The results of these tests are shown in Figure 6 for various flame residence times. Figure 6 demonstrates that it is possible to eliminate the interference from hydrocarbons by adjusting the flame residence time. To test this hypothesis the response to SO2 as a function of heptene concentration at a fixed flame residence time of 7.5 ms was measured and is shown in Figure 7 . When the heptene concentration was lo5 times greater than the sulfur concentration, only a 15% enhancement of the MES signal was observed. The USD did not have any detectable response to sulfur-free air with either hexane or heptene concentrations up to 4000 ppmv. Apparently the flame chemistry is perturbed by the hydrocarbon in such a way that SO production is affected to a minor extent. It should be pointed out that the effect on the USD response from the hydrocarbons reported here is 102-104times less than that reported for the FPD (10, 22).
CONCLUSIONS The USD for sulfur has been shown to be a very sensitive, selective, and linear response detector when operated in the
I
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H e p t e n e C o n c e n t r a t i o n ['IOv / v ] Flgure 7. Relative response of methyl ethyl sulfide with various amounts of background heptene. Flame residence time is 7.5 ms. real-time mode. It provides detection limits at levels similar to those reported for flame photometric detectors but at response times that are at least 30 times faster. The USD does not suffer from the interference problems experienced by the FPD from water vapor or carbon dioxide. The small interference experienced from hydrocarbons can be minimized by adjustment of the residence time of the sample in the flame. The USD also provides uniform response to different sulfur compounds which should greatly enhance its utility as a gas chromatographic detector.
ACKNOWLEDGMENT The authors thank Sievers Research for the use of their RCD and, in particular, Richard Godec for many helpful suggestions.
LITERATURE CITED (1) Georgii, H. W. Atmos. Environ. 1878, 12, 681-690. (2) Kramlich, J. C. "The Fate and Behavior of Fuel Sulfur in Combustion Systems"; Ph.D. Thesis, Washington State University, Pullman, WA, 1980. (3) Hanson, M.; Ingvorsen, K.; Jorgensen, B., Limnol. Oceanogr. 1878, 2 3 , 68-76. (4) Brody, S. S.;Cheney, J. E. J. Gas Chromatogr. 1888, 4, 42-46. (5) Gage, D. R.; Farwell, S.0. Anal. Chem. 1980, 52, 2422-2425. (6) Hall, R. C. J. Chromatogr. Sci. 1874, 72, 152-160. (7) Nelson, J. K.; Getty, R. H.; Birks, J. W. Anal. Chem. 1883, 5 5 , 1767- 1770. (8) Worrell, W. L.; Liu, 0.G. J. Electroanal. Chem. 1884, 166, 355-362. (9) Kelly, T. J.; Gaffney, J. S.; Phillips, M. F.; Tanner, R. L. Anal. Chem. 1883. 55. 138-140. (10) Farwell, S. 0.; Barinaga, C. J. J. Chromatogr. Sci. 1888, 2 4 , 483-494. (11) Spurlin, S. R.; Yeung, E. S. Anal. Chem. 1882, 5 4 , 318-320. (12) Weber, D.; Olsen, K. 8.; Ludwick, J. D. Talanta 1880, 27, 665-668. (13) Zachariah. M. R.; Smith, 0. I. Combust. Flames 1887, 69, 125-139. (14) Muller, C. H.; Schofield, K.; Steinberg, M.; Brolda, H. P. Int. Symp. Combust. 1878, 17, 867-879. (15) Halstead, C. J.; Thrush, B. A. Proc. R . Soc. London 1888, 295, 363-379. (16) Halstead, C. J.; Thrush, B. A. Proc. R . Soc. London 1888, 295, 380-398. (17) Akimoto, H.; Finlayson, B. J.; Pitts, J. N., Jr. Chem. Phys. Left. 1871, 12, 199-202. (18) Cadle, R. D.; Ledford, M. Int. J. Air Wafer Pollut. 1888, 10, 25-30. (19) Hales, J. M.; Wilkes, J. 0.; York, J. L. Atmos. Environ. 1888, 3 , 657-667. (20) Fristrom, R. M.; Westenberg, A. A. Flame Structure; McGraw-Hill: New York, 1965. (21) Stedman, D. S.;Benner, R. L. United States Patent Pending, 1988. (22) Sugiyama, T.; Suzuki, Y.; Takeuchi. T. J. Chromatogr. 1873, 8 0 , 61-67.
RECEIVED for review December 12,1988. Accepted March 13, 1989. This work was supported in part by The National Aeronautics and Space Administration under Grant NGT50318, Sievers Research, Inc., and the United States Department of Energy under Grant DOE-AC05-88ER80864, and the National Science Foundation under Grant ATM-8620365.
