Anal. Chem. 1883, 65, 84-86
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TECHNICAL NOTES
Real-Time Measurement of Dimethyl Sulfoxide in Ambient Air H. Berresheim,'l+ D. J. Tanner? and F. L. Eiselet Georgia Institute of Technology, Atlanta, Georgia 30332
INTRODUCTION Dimethyl sulfoxide (DMSO) is formed in the atmosphere as an intermediate product of the OH addition reaction with dimethyl sulfide (DMS).' In turn, OH also attacks DMSO to form dimethyl sulfone (DMSO2),one of the stable end products of atmospheric DMS oxidation. Recently, it has been hypothesized2 that the emission of dimethyl sulfide (DMS) from the world's oceans and its oxidation in the atmosphere may be responsible for controlling cloud formation over remote oceanic regions. Therefore, it has been further suggested that atmospheric DMS chemistry plays a significant role in global climate change. To verify this hypothesis instrumental techniques must be used or developed which are capable of measuring DMS, its major oxidants, and its individual oxidation products, including DMSO, in the real atmosphere, preferably with a high time resolution. Previously, we have already reported the successful development of different mass spectrometric techniques for realtime measurement of DMS, SO2,CH3S(02)0H, HzS04, and OH in the atmo~phere.3-~In this paper we describe a new highly sensitive method for real-time measurement of DMSO in ambient air. The first measurements of gaseous DMSO and DMSO2 in the atmosphere were reported by Harvey and Lange6 Typical levels for both compounds were in the low parts-per-trillion by volume (pptv) range. Further measurements over the North Atlantic showed a large range of concentrations observed for both compounds covering a t least 3 orders of m a g n i t ~ d e .The ~ method used by these investigators has recently been described in more detail by Lang and Brown.8 It involves preconcentration sampling of both compounds from air by adsorption on Tenax GC contained in stainless steel tubes. The loaded tubes are extracted with methanol, and the extracts are further concentrated by solvent evaporation and then analyzed on a gas chromatograph equipped with a Hall electrolytic conductivity detector. Sample extraction and analysis are performed off-line in the laboratory usually within hours to weeks after the samples have been collected and stored in a refrigerator. This method has two major disadvantages which limit its usefulness in studying the chemical mechanisms involved in atmospheric DMS oxidation and may even produce biased results. First, sampling integration times are typically on the order of 8 h or longer for determination of low pptv levels School of Earth and Atmospheric Sciences. Georgia Tech Research Institute, Physical Sciences Laboratory. (1)Yin,F.;Grosjean,D.; Seinfeld, J. H.J.Atmos. Chem. 1990,11,390. (2)Charlson, R. J.; Lovelock, J. E.; Andreae, M. 0.;Warren, S. G. Nature 1987,326,655. (3)Eisele, F. L.; Berresheim, H. Anal. Chem. 1992,64,283. (4)Eisele, F.L.; Tanner, D. J. J. Geophys. Res. 1991,96,9295. (5)Tanner, D.J.; Eisele, F. L. J. Geophys. Res. 1991,96,1023. (6)Harvey, G. R.; Lang, R. F. Geophys. Res. Lett. 1986,13,49. (7)Pszenny, A. P.; Harvey, G. R.; Brown, C. J.; Lang, R. F.; Keene, W. C.; Galloway,J. N.; Merrill, J. T. Global Biogeochem. Cycles 1990,4, 367. +
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0003-2700/93/0365-0084$04.00/0
of DMSO and DMSO2. Therefore, only daily-based average concentrations of both compounds can be determined with this method. Since real-time measurements are not possible, the kinetics of DMS oxidation and production of DMSO and DMSOzcannot be studied under real atmospheric conditions. Second, the method may be biased by possible artifact formation of DMSO and DMSO2 occurring in the Tenax traps during sampling due to air oxidation of DMS which is also trapped on Tenax GC. Lang and Brown* reported tests in which no artifact formation of DMSO or DMSO2 was observed using mixtures of DMS and ozone-enriched air. The use of an oxidant scrubber was considered to be unnecessary. However, other investigators have observed significant losses of DMS occurring in air-sampling traps and have emphasized the use of a scrubber to prevent this problem.9 To this date, neither the oxidant nor the product species involved in the observed loss of DMS during preconcentration sampling have been identified. Therefore, results obtained from measurements of DMSO and DMSO2 using preconcentration sampling without an efficient scrubber in the sampling line may be ambiguous and must be interpreted with caution. In this paper we describe a new technique for measuring DMSO in ambient air at sub-pptv levels which does not require sample preconcentration. This technique is based on atmospheric pressure chemical ionization mass spectrometry (APCI/MS) and involves the use of a flow reactor which has been described in detail in an earlier paper.3 The high sensitivity of APCI/MS for measuring DMSO in air was first recognized by Karasek and co-workers.lOJ1They used DMSO as a prototype gas to demonstrate the sensitivity of APCI in combination with ion mobility spectrometry, a technique which they termed plasma chromatography (PC). This system could be further coupled with a quadrupole mass spectrometer. For a combined PC/MS system a detection limit of 1 pptv DMSO in dry air was estimated based on 10-min signal integration.12 Similar to this previous application the present APCI/MS technique makes use of the relatively high proton affinity of DMSO (211 kcal/mol) compared to HzO(166 kcal/mol) and most other atmospheric trace gases.l3 The present system can detect DMSO mixing ratios in ambient air down to approximately 0.5 pptv at 60-s signal integration.
EXPERIMENTAL SECTION Apparatus and Method. The apparatus used in the present work has been described in detail in an earlier paper.3 Therefore, (8)Lang, R. F.; Brown, C. J. Anal. Chem. 1991,63, 186. (9)Saltzman, E. S.;Cooper, D. J. Biogenic Sulfur in the Enuironment; ACS Symposium Series; American Chemical Society: Washington, DC, 1989,Vol. 393,p 330. (10)Karasek, F. W. Res.lDeu. 1970,21,25. (11)Cohen, M. J.; Karasek, F. W. J. Chrornatogr. Sci. 1970,8,330. (12)Franklin GNO Corp. AlphalZZ plasma chromatograph-mass spectrometer PCP, Inc.: West Palm Beach, FL, 1970. (13)Lias, S.G.; Liebam, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984,13,695. 0 1993 American Chemical Society
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only a brief summary of the experimental setup is given here. One major section of the apparatus consists of a selected-ion, chemical ionization flow reactor. A tailor-made carrier gas mixture is continuously passing through the reactor in a laminar flow. After entering the reactor, the carrier gas is ionized by (Y 1 bombardment from a radioactive 241Amsource, forming specific reactant ions. On the other hand, the sample molecules enter the reactor through a central axis tube downstream from the Y 241Amsource and are subsequently ionized by proton-transfer reactionswith the carrier gas reactant ions. The spatial separation a between the production of reactant ions and subsequent chemical 0 ionization of the sample is crucial to the high sensitivity and specificity achievable with the present apparatus. This spatial separation significantly reduces the number of radical or metastable species interferingin the production of sample ions, making the ion chemistry in the reactor straightforward, and results in the formation of well-known product ions. Wall losses are virtually eliminated due to the laminar flow conditions in the DMSO (PPW reactor. In the present application the targeted sample species is Figure 1. Typical calibration curve for DMSO at 5-s slgnal integrathm DMSO. As mentioned earlier,DMSO has a relativelyhigh proton Indbidual points represent means from 10 repetitbe measurements. affinity compared to most other atmospheric gases. This allows Standard error bars are smaller than symbols shown. for an efficient production of (DMSO)H+product ions by proton exchange with initial NH4+ions in the flowreactor. Subsequently, the use of Nz carrier gas with (HZO),H+ as the primaryreactant DMSO concentrations are determined from the (DMSO)H+/ ion species is probably sufficient. However, during a preNH4+ ion ratio. Because of the high proton affinity of DMSO, liminary attempt t o measure DMSO in the field on Sapelo the sample air can be introduced directly into the flow reactor Island, GA, in April 1990 very ambiguous results were without requiring prior gas chromatographic (GC) separation of obtained. Direct chemical ionization of the sample air was DMSO from other atmospheric compounds. This major advanused; i.e., initial reactant ions and sample product ions were tage makes it possible for the first time to measure DMSO in air formed in the same reaction region. A large number of varying on a continuous basis with a high time resolution (typically 60-5 and unknown reactant ion species were formed in the flow sample integration in ambient air). The carrier gas mixture used in the present application consists of NZcontaining a few parts reactor which either did not react with DMSO or in some per billion by volume (ppbv) of NH3 and HzO. The NZgas is cases may have reacted but led t o dissociative products. The supplied from a liquid nitrogen dewar and cleaned from impurities results were thus ambiguous, and calibration was not possible. in a high-pressure (20 psi) liquid nitrogen trap. Downstream In addition to the large number and diversity of initial reactant from the trap and prior to entering the reactor the carrier gas is ion species occurring in the flow reactor the problems mixed with a continuous flow of NH3 (12 ng/min at 30 "C) encountered in these earlier field experiments were likely originating from an in-line NH3 permeation device (VICI related to large differences between the proton affinities of Metronics). Using a typical N2 flow rate of 1L/min, this produces DMSO and species such as HzO involving relatively higha NH3 mixing ratio of approximately 15 ppbv in the carrier gas. energy transfer reactions. Therefore, a more specific gentler Radioactive bombardment of the carrier gas mixture prior to method of ionizing DMSO was chosen, using NH3 in the carrier sample ionization in the reactor produces (HzO),H+ ions which undergo charge-transfer reactions with NH3 to form (HzO),,,NH4+ gas to make NH4+the dominant reactant ion species. Since due to the higher proton affinity of NH3 (204kcal/mol) compared the proton affinity of NH3 (204kcal/mol) is also relatively to Hz0. Both n and m depend on the HzO and NH3 content of high but lower than that of DMSO, the production of (DMSO)the carrier gas used and on temperature and pressure. When H+ ions in this mode is not affected by species other than encountering the sample air, the ammonium ions react with NH4+ and no dissociation of the product ions is observed DMSO molecules to produce (DMSO)H+ ions which are then because the energy transfer during the reaction is relatively guided by electrical fields into the Nz buffer gas and through small (0.3 eV). small sample apertures into a low-pressurecollisional dissociation Figure 1shows a typical DMSO calibration curve (blank chamber and from there into a modified Extrel quadrupole mass subtracted) based on a signal integration time of 5 s (rz = spectrometer. In the present application sample air was continuously flowing into the reactor at a rate of 0.1 L/min through 0.9994, slope = 33.4, intercept = 26.1 counts). Stable a 1/3z-in.-o.d.Teflon line guided inside the stainless steel central concentrations lower than approximately 4pptv were difficult axis tube. The sample flow into the reactor was maintained by to produce due to limitations of the gas flow dilution system. keeping the reactor slightly (1Torr) below ambient pressure. During ambient air measurements standard addition caliCalibration. Gaseous DMSO standards in the low pptvrange brations were performed using a 60-sintegration cycle for were produced using a DMSO permeation device (VICI Metronics) in combination with a multistage flow dilution ~ y s t e m . ~ better precision. The estimated 2u detection limit of the described method in ambient air is 0.5 pptv DMSO a t 60-5 The permeation rate of the DMSO source (7.4 0.4 ng/min at integration. Figure 2 shows results obtained from field 35 "C) was determined by repeatedly measuring the weight loss measurements of DMSO with the present system. T o our of the permeation cell over a period of 3 months using a Mettler knowledge these data represent the first real-time measuremicrobalance. Except for the gravimetric measurements, the DMSO cell was permanently stored in a permeation oven at 35 ments of DMSO in ambient air. The measurements were OC under a constant flow of ultraclean Nz gas. In the laboratory conducted on April 24,1991, at a Pacific coastal site (Cheeka DMSO standards were produced using N2 as the dilution gas. Peak) in northwestern Washington. The site is located on Measurements in the field were calibrated by standard addition the Olympic peninsula near Cape Flattery a t an elevation of using dynamic mixtures of the DMSO/N2 effluent from the approximately 500 m above sea level. The measurements permeation system with ambient air. were made with a time resolution of 60 s. The results clearly show a correlation between daylight intensity and production RESULTS AND DISCUSSION of DMSO due to photochemical oxidation of DMS. Peak DMSO levels up to 3.2pptv were observed in phase with solar The choice of carrier gas in the present selected-ion APCI/ radiation peaks in the late morning hours and a t noontime. MS technique is crucial for obtaining optimal results with DMSO levels were below the detection limit at night. As respect to a specific sample species. For many applications
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Local Time (hr) Figure 2. DMSO concentrations measured in marine air at a Pacific coastal site In northwestern Washington on April 24, 1991. Slgnal integration was 60 8. A discussion of the negative values is given In the text.
shown in Figure 2 the data scattering around the detection limit also include some negative values which resulted from a background signal variation equivalent to k0.2 pptv DMSO at 60-8 integration. Standard addition calibrationsin ambient air were typically made once or twice a day (here between 18 and 21 h) and were also subject to the observed background signal variation. The overall accuracy of the method including (14)Busfield, W.K.;Ivin, K.J. Trans. Faraday SOC. 1961,57,1044.
calibration is estimated to be approximately 20%. In the future isotopically labeled DM%O will be used in standard addition calibrations to eliminate the present calibration problem and further improve the accuracy of the method. As mentioned earlier, a systematic study of atmospheric DMS oxidation requires measurements of both DMSO and DMSO2 in air. Dimethyl sulfone is a relatively stable compound with a much weaker basicity compared to DMSO. Therefore, direct measurements of atmospheric DMSO2 in the low pptv range are not possible with the present technique. Certain modifications are required to convert DMSO2 into a more reactive product. We are currently developing a selected-ion APCI/MS technique specifically for real-time atmospheric DMSO2 measurements. The sample analysis involves on-line GC separation of DMSO2 from other sulfur compounds followed by postcolumn pyrolysis to S0214in the presence of a gold catalyst. The SO2 produced by decomposition of DMSO2 is then analyzed as described earlier? Preliminary experiments using DMSO2 standards in the pptv range have shown very promising results.
ACKNOWLEDGMENT We thank T. S.Bates of NOAA/PMEL and R. J. Charbon of the University of Washington for the opportunity to conduct field measurements a t Cheeka Peak research station. This work was supported by the National Science Foundation, GrantsATM-8813594and ATM-9021522, and by the Georgia Tech Research Institute, Grant E8904-046. RECEIVEDfor review April 10, 1992. Accepted October 5, 1992.
