Anal. Chem. 1998, 70, 1735-1742
Dimethyl Sulfide Measurement by Fluorine-Induced Chemiluminescence Alan J. Hills,*,† Donald H. Lenschow,† and John W. Birks‡
Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado 80307-3000, and Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309-0215
We have developed a high-speed sensor for dimethyl sulfide (DMS) based on its fast chemiluminescent reaction with molecular fluorine. Emission in the wavelength range 450-650 nm is monitored via photon counting. The instrument can continuously measure DMS with a response time of 0.1 s and is highly linear and sensitive. Limits of detection (S/N ) 1) are 39, 12, and 4 pptv DMS for 0.1-, 1-, and 10-s integration times, respectively. Sensitivity and response time allow the direct measurement of DMS fluxes in the marine atmospheric boundary layer by the eddy correlation technique. Selectivity has previously been measured and is sufficient for monitoring DMS in the marine boundary layer without significant interferences. The potential link between dimethyl sulfide (DMS) produced by phytoplankton in the ocean and cloud condensation nuclei in the marine troposphere has been postulated as a possible negative feedback mechanism having a significant effect on global climate. 1 This hypothesis has generated much interest in the measurement of DMS, which is the dominant sulfur species emitted by the ocean to the atmosphere (15-50 Tg yr-1, >97% of the total sulfur).2,3 This emission is large enough that DMS is the dominant biogenic sulfur species emitted to the atmosphere.4-6 To date, however, there exists no in situ, fast sensor for DMS that can be flown on research aircraft and used to directly measure oceanic DMS fluxes by eddy correlation techniques at atmospheric concentrations. In eddy correlation, fluxes are directly measured by measuring the covariance of a scalar quantity (like chemical concentration) with the small-scale vertical wind currents.7 For example, the eddy correlation flux of DMS at a given instant in †
National Center for Atmospheric Research. ‡ University of Colorado. (1) Charlson, R. J.; Lovelock, J. E.; Andreae, M. O.; Warren, S. G. Nature 1987, 326, 655-661. (2) Andreae, M. O. The Biogeochemical Cycling of Sulfur and Nitrogen in the Remote Atmosphere; Galloway, J. N., Charlson, R. J., Andreae, M. O., Rodhe, H., Eds.; D. Reidel Publishing Co.: Boston, 1985; pp 5-25. (3) Johnson, J., private communication. National Oceanic and Atmospheric Administration, October 5, 1994. (4) Andreae, M. O.; Andreae, T. W. J. Geophys. Res. 1988, 93, 1487-1497. (5) Guenther, A.; Lamb, B.; Westberg, H. (U.S. National Biogenic Sulfur Emissions Inventory). In Biogenic sulfur in the Environment; Saltzman, E. S., Cooper, W. J., Eds.; ACS Symposium Series 393; The American Chemical Society: Washington, DC, 1989. (6) Bates, T. S.; Cline, J. D.; Gammon, R. H.; Kelly-Hansen, S. R. J. Geophys. Res. 1987, 92, 2930-2938. S0003-2700(97)00963-3 CCC: $15.00 Published on Web 03/20/1998
© 1998 American Chemical Society
time is given by the product of the instantaneous vertical wind velocity, w′, and the fluctuation in concentration of DMS, c′DMS, at that instant. The integral of the covariance, w′c′DMS, over time is the flux, FDMS. The eddy correlation technique is a powerful tool that has been used for the measurement of H2O, CH4, CO, CO2, O3, NO, NOx, N2O, SO2, sensible heat, momentum, and aerosol fluxes. It is considered state-of-the-art since no assumptions or special conditions are required. Nevertheless, the technique is limited by the fact that fluctuations should be rapidly measured (e0.1-s time constant). No operational DMS sensor currently exists with sufficient speed and selectivity to perform this measurement. The low concentrations (10-300 pptv) and irreversible adsorption of DMS within analytical instruments have restricted the measurement of atmospheric DMS to grab sampling techniques.3,4,8-10 Typically, DMS is measured via cryogenic sampling or by sorbent trapping and then analyzed via gas chromatography with flame photometric, mass spectrometric, or electron capture detection.3,9,11-13 Loss of temporal resolution due to slow sampling (≈5-15 min) and analysis (≈15-30 min) times precludes the use of direct flux measurement by eddy correlation. To limit losses in precision and accuracy in the measurement of DMS at atmospheric concentrations, real-time measurement systems are preferred. Atmospheric pressure chemical ionization mass spectrometry has been used successfully to measure DMS in real-time.8,9,14,15 Spicer et al.14 quote a detection limit for DMS of 2 pptv, but response time, ≈1 s, is somewhat slow for eddy covariance flux measurement. With modification, the instrument is potentially capable of DMS flux measurements from aircraft. However, the (7) Lenschow, D. H. In Biogenic Trace Gases: Measuring Emissions from Soil and Water; Matson, P. A., Harriss, R. C., Eds.; American Chemical Society: Washington, DC, Blackwell Science: Cambridge, MA, 1995; Chapter 5, pp 126-163. (8) Eisele, F. L.; Berresheim, H. Anal. Chem. 1992, 64, 283-288. (9) Berresheim, H.; Eisele, F. L.; Tanner, D. J.; McInnes, L. M.; Ramsey-Bell, D. C.; Covert, D. S. J. Geophys. Res. 1993, 98, 12701-12711. (10) Bates, T. S.; Johnson, J. E.; Quinn, P. K.; Goldan, P. D.; Kuster, W. C.; Covert, D. C.; Hahn, C. J. J. Atmos. Chem. 1990, 10, 59-81. (11) Bandy, A. R.; Scott, D. L.; Blomquist, B. W.; Chen, S. M.; Thornton, D. C. Geophys. Res. Lett. 1992, 19, 1125-1127. (12) Bandy, A. R.; Thornton, D. C.; Ridgeway, R. G., Jr.; Blomquist, B. W. In Isotope Effects in Gas-Phase Chemistry; Kaye, J. A., Ed.; ACS Symposium Series 502; American Chemical Society: Washington, DC, 1992; pp 409422. (13) Blomquist, B. W.; Bandy, A. R.; Thornton, D. C. J. Geophys. Res. 1996, 101 (D2), 4377-4392.
Analytical Chemistry, Vol. 70, No. 9, May 1, 1998 1735
sensor weighs 1200 lb and requires about 8 kW of uninterrupted electrical power, restricting use to a dedicated aircraft that has been modified for the payload.15 Also, both pressure (altitude) and water vapor interfere in the tandem mass spectrometry technique, with sensitivity decreasing 6-fold for a relative humidity increase of 10-92%. The use of chemiluminescence (the production of light from a chemical reaction) can result in pptv-level detection limits for certain gaseous species having high chemiluminescence quantum yields. This is partly because photomultiplier tubes provide single photon detection, and, similar to mass spectrometry, the chemiluminescence appears on a near-zero background. Detection is fast since the response time of chemiluminescence detectors is governed only by reaction cell residence time; subsecond response times are possible.16 Typically, the method is dependent on analyte concentration to the first power and is linear over at least 4 orders of magnitude. The nitric oxide detector invented by Fontijn et al., for example, can be used to measure either NO or O3 down to 5 pptv in real-time:17,18
NO + O3 f NO2* + O2
(1)
NO2* f NO2 + hν (λ ≈ 1200 nm)
(2)
Chemiluminescence has been used to sensitively and selectively quantify a host of gas-phase species, such as O3, H2O2, NO, NO2, ethene, and isoprene, in addition to an even greater variety of liquid-phase species.19-22 DMS, H2S, CH3SH, and other reduced sulfur compounds have been detected via ozone-induced chemiluminescence.23,24 Although much of the chemistry has not been resolved, the process is a several step oxidation, eventually leading to the formation of electronically excited SO2, which emits a near-UV photon upon return to the ground state,
reduced S-compd + O3 f f SO2*
(3)
SO2* f SO2 + hν (λ ≈ 340 nm)
(4)
The detector responds to most reduced sulfur species and exhibits sensitivities to DMS, H2S, and CH3SH of about 300, 100, and 4000 pptv, respectively. Unfortunately, the reaction is not selective, with (14) Spicer, C. W.; Kenny, D. V.; Chapman, E.; Busness, K. M.; Berkowitz, C. M. J. Geophys. Res. 1996, 101, 29137-29147. (15) Kenny, D. V.; Spicer, C. W.; Sverdrup, G. M.; Busness, K.; Hannigan, R. Proceedings of the Annual Meeting-Air Waste Management Association, 85th (Vol. 2A), 1992; Paper No. 92/67.01, 14 pp. (16) Guenther, A.; Hills, A. J. J. Geophys. Res., in press. (17) Fontijn, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970, 42, 575-579. (18) Ridley, B. A.; Howlett, L. C. Rev. Sci. Instrum. 1974, 45, 742-746. (19) Birks, J. W. Chemiluminescence and photochemical reaction detection in chromatography; VCH Publishers: New York, 1989; 291 pp.. (20) Hutte, R. S.; Sievers, R. E.; Birks, J. W. J. Chromatogr. Sci. 1986, 24, 499505. (21) Turnipseed, A. A.; Birks, J. W. J. Phys. Chem. 1991, 95, 6569-6574. (22) Hills, A. J.; Zimmerman, P. R. Anal. Chem. 1990, 62, 1055-1060. (23) Kelly, T. J.; Phillips, M. F.; Tanner, R. L.; Gaffney, J. S. Proceedings of the 75th Annual Meeting of the Air Pollution Control Association, New Orleans, Louisiana, June 20-25, 1982; 82-51.8. (24) Kelly, T. J.; Gaffney, J. S.; Phillips, M. F.; Tanner, R. L. Anal. Chem. 1983, 55, 135-138.
1736 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
ozone chemiluminescing with a variety of other reactive compounds, especially alkenes. The total sulfur chemiluminescence detector, the sulfur chemiluminescence detector (SCD),25 is presumed to convert a small fraction (typically less than 1%) of the parent sulfur compound to SO in a hydrogen-rich flame or furnace. The SO produced is transported to a chemiluminescence reaction cell, where it reacts with ozone to form electronically excited SO2*. As for ozoneinduced chemiluminescence, the SO2* emission occurs in the near-UV.25-29 Sulfur compounds are thought to undergo the following sequence of reactions:
S-compd (H2, O2, heat) f SO
(5)
SO + O3 f SO2* + O2
(6)
SO2* f SO2 + hν (λ ≈ 340 nm)
(4)
Since the sulfur compound is first catalytically converted to SO, all sulfur compounds exhibit the same molar response, making the detector a universal sulfur detector. The reaction mechanism actually may be more complicated; at least at higher sulfur concentrations, an intermediate sulfur compound different from SO is formed, but it is converted to SO upon reaction with ozone.30 The SCD has found use as a chromatographic detector28,31,32 and as a total ambient sulfur monitor.26,33 It was reported to have a limit of detection of 80 pptv sulfur at S/N ) 2 in 1990,26 and subsequent refinements have further enhanced the sensitivity. The SCD is an excellent total sulfur monitor but cannot measure atmospheric DMS without interference from other sulfur species such as carbonyl sulfide, also present in the marine troposphere at a mixing ratio of ∼500 pptv. It is unlikely that a prefilter or catalyst could be devised which could selectively and rapidly remove sulfur compounds other than DMS while preserving the DMS temporal signature necessary for atmospheric flux measurements. The development of an eddy covariance fluorine-induced chemiluminescence detector is a logical extension of the reduced sulfur chemiluminescence detector first developed as a gas chromatography detector.34,35 This detector is based on the chemiluminescence that results from the reaction of fluorine with select reduced sulfur compounds. Intense chemiluminescence occurs upon reaction of fluorine with a sulfur-containing compound, provided that the S atom is bonded to a carbon that is, itself, bonded to at least one hydrogen atom (i.e., H-C-S functionality). DMS (CH3SCH3) is highly chemiluminescent with F2, producing HCF* (electronically excited HCF) and HF† (vibrationally excited HF, up to v ) 6), both of which emit visible light (25) Benner, R. L.; Stedman, D. H. Anal. Chem. 1989, 61, 1268. (26) Benner, R. L.; Stedman, D. H. Environ. Sci. Technol. 1990, 24, 1592-1596. (27) Benner, R. L.; Stedman, D. H. Appl. Spectrosc. 1994, 48, 848-851. (28) Ryerson, T. B.; Dunham, A. J.; Barkley, R. M.; Sievers, R. E. Anal. Chem. 1994, 66, 2841-2851. (29) Schorran, D. E.; Fought, C.; Miller, D. F.; Coulombe, W. G.; Kelslar, R. E.; Benner, R. L.; Stedman, D. H. Environ. Sci. Technol. 1994, 28, 1307-1311. (30) Burrow, P. L.; Birks, J. W. Anal. Chem. 1997, 69, 1299-1306. (31) Shearer, R. L. Anal. Chem. 1992, 64, 2192-2196. (32) Johansen, N. G.; Birks, J. W. Am. Lab. 1991, 23, 112-119. (33) Jodwalis, C. M; Benner, R. L. J. Geophys. Res. 1996, 101, 4393-4401. (34) Nelson, J. K.; Getty, R. H.; Birks, J. W. Anal. Chem. 1983, 55, 1767-1770. (35) Mishalanie, E. A.; Birks, J. W. Anal. Chem. 1986, 58, 918-923.
Figure 1. Schematic diagram of the fast DMS sensor test configuration (not to scale).
upon return to their ground states:36
CH3SCH3 + F2 f f HF† and HCF*
(7)
HCF* f HCF + hν (λ ) 500-700 nm)
(8)
HF† f HF + hν (λ ) 660-750 nm)
(9)
The kinetics of reaction 7 have been investigated at room temperature.21,37 The reaction is one of the fastest known chemical reactions between two closed-shell, neutral molecules, with k7(298) ) 1.6 × 10-11 cm3 molecule-1 s-1. The rapid rate of the reaction offers significant advantages as a chemiluminescence sensor since completion can be attained at room temperature, with low reactant concentration, in a small reaction cell, and in a short time. The speed of reaction 7 is also believed to be largely responsible for the experimentally verified, large selectivity for DMS compared to other species.34,35 EXPERIMENTAL SECTION Chemiluminescence Detector. The DMS-F2 detector assembled to test the concept of ambient DMS monitoring is shown in Figure 1. A converted fast isoprene sensor16 served as the basis for the instrument. The reaction cell is constructed of stainless steel with a glass window on one end and has a volume of 39 mL. Sample air at about 1.2 standard liters per minute (slpm) containing DMS and a dilute fluorine mixture in He of 0.10 slpm are introduced at opposite sides of the reaction cell. Reaction 7 occurs adjacent to a photomultiplier tube (PMT) photocathode. The PMT detector (Hamamatsu, HC-135) counts photons exiting the reaction cell. A colored glass band-pass filter (400-600 nm) or Pyrex (BK-7) window transmitted light between the reaction cell and the PMT. Product gases were evacuated from the reaction cell and then scrubbed of F2 and HF via a two-stage chemical trap, which first converts excess F2 to CF4 on activated carbon, and then HF to H2O on Ascarite II: (36) Glinski, R. J.; Mishalanie, E. A.; Birks, J. W. J. Photochem. 1987, 37, 217231. (37) Turnipseed, A. A. Doctoral dissertation, NCAR/CT-127, 1990; 139 pp.
2F2(g) + C(s) f CF4(g)
(10)
HF(g) + NaOH(s) f NaF(s) + H2O(g)
(11)
Molecular fluorine should be handled with extreme caution since a fluorine release can be fatal. Even though reactions 10 and 11 were successful in destroying F2 and HF, all pump exhaust should be vented to a suitable fume hood, and fluorine warning and monitoring devices should be employed if possible. Preliminary studies of the DMS-F2 sensor concept, including those involving H2 doping, used a converted liquid chromatography detector (Sievers Research, Inc., model 207) with a redsensitive PMT. A large pump (Sargent-Welch 1375) provides system vacuum. Mean residence time in the reaction cell is 0.012 s at 5 Torr. To avoid the restriction of a flow meter in the high-conductance plumbing, a flow rate vs pressure calibration was established by placing a flow controller at the inlet of the reaction cell and recording total cell pressure as a function of flow rate. The system flow rate then could be accurately assessed from the pressure. This calibration exhibited no discernible drift over the course of the study. All surfaces in the flow system were stainless steel or Teflon in order to resist the reactive chemicals and reduce memory effects. All experiments were performed at room temperature. Gas Mixtures. Flow rates of fluorine in helium (5-8%, Matheson) of 0.010-0.200 slpm were monitored via a mass flow meter constructed of Monel and verified via the total flow/ pressure calibration. DMS-in-air mixtures were generated by diluting either a DMS cylinder standard (1.04 ppmv DMS in N2, Scott-Marrin) or the output of a permeation system (Kintek). Monthly mass measurements on the permeation tube gave a mass loss of DMS of 2.0 × 10-10 g s-1 (4.2 × 10-9 slpm). Effluent from the DMS cylinder standard or the permeation oven was diluted with zero air in a glass mixing tube (5 cm i.d. × 1.2 m length) (see Figure 1). Zero-air flow rates of 2-20 slpm when mixed with the cylinder standard or permeation effluent resulted in DMS mixing ratios of 150 pptv-5.0 ppbv. Since the output of the dilution tube was variable and as high as 20 slpm while the maximum sampling rate into the DMS-F2 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
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sensor was only 1.2 slpm, a method was needed to sample a fraction of the dilution tube output. The solution employed was to bore a 0.051-cm hole in a VCO blind nut (Cajon, Macedonia, OH) and attach the nut to a length of 1/4 in. black Teflon tubing: 0.051 cm is near the optimum orifice diameter for this particular flow system, balancing high sample air flow, and hence photon yield, against pressure quenching and reduced chemiluminescence signal. One end of the tubing was placed near the outlet of the flow tube, and the other end was plumbed directly to the reaction cell inlet. Since the inlet flow rate into the reaction zone was fixed only by total cell pressure and inlet orifice diameter, care was needed to ensure that the dilution tube outlet flow rate was greater than the sample inlet flow rate. If this relationship were not maintained, room air would be sampled into the sensor, invalidating a calibration. The minimum dilution plus standard gas flow rate in this work was 2.1 slpm, nearly twice the sample inlet flow rate of 1.2 slpm. An additional advantage of the orifice inlet was that low pressure (2-6 Torr) existed from the orifice to the reaction cell, minimizing surface adsorption. To evaluate the effect of water vapor on sensitivity, rapid step functions in humidity were generated by creating two flow paths for dilution air. One path used zero cylinder air directly. A second path bubbled the air through 1 L of purified water. A glass frit (∼10 cm2) was used to increase the dilution air/water contact area. Toggle valves rapidly switched between the two flow paths. Air streams exiting the mixing tube had relative humidities of 0% or 74%, as measured with a capacitive thin-film hygrometer (Vaisala, Humitter). Intermediate humidities were generated by substituting metering valves for the toggle valves. As has been done previously with other chemiluminescence sensors,38,39 the DMS-F2 sensor was fitted with a prehumidifier, which could add H2O vapor from liquid water into the sensor sample inlet line just prior to the reaction cell. At the H2O vapor flow rate of 0.3 slpm, 25% of the sample inlet air was water vapor. Verification of DMS Mixing Ratio. DMS concentrations were assessed by comparing the diluted DMS cylinder flow with that generated by the permeation system and by independent gas chromatographic analysis. The analysis was performed by adjusting the output of the permeation/dilution system to produce a 2.2 ppbv DMS sample air stream. Three-liter aliquots of permeation/dilution air were then sampled into Tedlar bags. A volume of 1.2 L of air was passed through Carbotrap 200 multistage solid adsorbent cartridges (Supelco). The samples were desorbed onto a DB-Petro column (0.25 mm × 100 m, 0.5 mm film, J&W Scientific) in a Hewlett-Packard (HP) 5890 series II gas chromatograph. Quantification was made using an HP 5921A atomic emission detector tuned for carbon (496 nm). Replicate samples gave 1.6 and 1.7 ppbv DMS, 33% lower than the calculated permeation/dilution value. As DMS is susceptible to scavenging on surfaces and may not have been quantitatively transferred from the Tedlar bags and/or adsorbent trap, we conclude that the chromatographic analysis is in reasonably good agreement with the mixing ratio calculated from mass loss of the permeation tube. The cylinder standard was not verified using the chromatographic technique but compared favorably with the permeation method. (38) Carroll, M. A.; McFarland, M.; Ridley, B. A.; Albritton, D. L. J. Geophys. Res. 1985, 90, 12853-12860. (39) Ridley, B. A.; Grahek, F. E. J. Atmos. Ocean. Technol. 1990, 7, 307-311.
1738 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
Figure 2. (A) Raw sensor signal as function of [DMS]. Sample inlet flow rate, 1.2 slpm; pressure, 6.4 Torr; F2/He flow rate, 0.075 slpm; inlet orifice diameter, 0.051 cm; data rate, 1 Hz; S/N ) 1 at 12 pptv (for noise ) 1σ). (B) Calibration data for the fast DMS sensor. Data correspond to the trace shown in (A). Slope, 3.6 counts s-1/pptv DMS; intercept, 1830 counts s-1; R ) 0.999.
RESULTS AND DISCUSSION Sensitivity and Drift. Figure 2A shows the DMS-F2 detector signal as a function of time for a series of different DMS concentrations generated with the dilution system. “Instrument zero”, shown on Figure 2A, was measured by disconnecting the permeation oven apparatus at the dilution tube, sampling only zero air. The resulting calibration plot for the data trace of Figure 2A is shown in Figure 2B. The data are highly correlated, with a correlation coefficient of 0.999. The range of [DMS] spanned only a factor of 30, but the detector has previously demonstrated a range of linearity of greater than 1000.34 The calibration data show a sensitivity (slope of Figure 2B) of 3.6 photons s-1 pptv-1 DMS. Assuming that 50% of the light produced in the reaction cell reaches the PMT and that it has a quantum efficiency of 2% at 600 nm (product literature), then 3.6 photons s-1 pptv-1 equates to 1 photon emitted per 106 DMS molecules reacting with F2. This is comparable to other gas-phase chemiluminescence detection systems. The only experimentally controlled parameter affecting the photon yield is pressure. Photon yield decreases with pressure, presumably due to collisional quenching of emitting species. Like all chemiluminescence systems, the reagent, F2, produces a background signal due to its interaction with surfaces or contaminants in the reaction cell, resulting in low-level light generation. For this setup, the reaction
F2 + reaction cell f products + hν
(12)
generated a background B ≈ 1800 photon counts s-1. The chemiluminescence background decreased as a function of time and stabilized after about 1 h. Variations in reactant (F2) concentration can cause the zero level to shift. Here, the concentration of fluorine in the reaction cell exhibited small variations because of F2/He regulator “creep”, dominating the total measured drift of 7 pptv DMS h-1. The limit of detection is determined primarily by the photon counting shot noise. Sensitivity of the DMS-F2 sensor was estimated by taking the standard deviation of the photon counts for the 234 pptv trace of Figure 2. The mean of the signal plus background is 2639 counts s-1, and the standard deviation of the trace is 53 counts s-1. The noise level agrees within statistical error with the expected photon shot noise of σn ) 51 counts s-1 (square root of signal plus background). Thus, the signal-to-noise ratio, S/N, is given by
S/N ) S/σn
(13)
For a background level of 1830 counts s-1 and a signal of 3.6 counts s-1 pptv-1, DMS mixing ratios giving S/N ) 1 are calculated to be 39, 12, and 4 pptv for integration times of 0.1, 1.0, and 10.0 s, respectively. Error in Measuring Covariance. A primary objective of the DMS sensor is to measure the eddy flux (i.e., the covariance of fluctuations in scalar concentration and vertical air velocity, as discussed, for example, by Lenschow7) of DMS from an aircraft. We use the above background and sensitivity numbers to estimate the error variance contributed by the system noise and compare this with the error variance contributed by measuring over a horizontal flight path of limited duration or length in the marine planetary boundary layer (PBL). From Ritter et al.,40 the total error variance for measurement of a flux F over a period T is
σw2 σ (F;T) ) (4σs2τ + σn2∆t) T 2
(14)
where σw2 and σs2 are the vertical velocity and DMS variances in the PBL, respectively, τ is the integral scale of the vertical velocity, and ∆t is the sampling interval. The basic units of σn are counts/ (time interval),which is then converted into DMS concentration in pptv by multiplying by the instrument scale factor. The first term on the right side of eq 14 is the contribution to the error variance resulting from a sampling time of finite duration.41 To illustrate the relative contribution of system noise to the covariance estimate, we establish as a criterion that the error due to limited sampling time should be greater than or equal to the error due to system noise. That is, (40) Ritter, J. A.; Lenschow, D. H.; Barrick, J. D. W.; Gregory, G. L.; Sachse, G. W.; Hill, G. F.; Woerner, M. A. J. Geophys. Res. 1990, 95 (D10), 1687516886. (41) Lenschow, D. H.; Mann, J.; Kristensen, L. J. Atmos. Ocean. Technol. 1994, 11, 661-673.
4σs2τ g σn2∆t
(15)
Using the relations obtained by Lenschow7 for typical marine PBL conditions and a mean DMS concentration of 100 pptv, eq 15 leads to the condition that the surface flux should be g2.3 pptv m s-1. Based on typical oceanic DMS flux estimates,42 the S/N is adequate for operational field measurements, although the averaging length would be somewhat longer than for a noise-free instrument. Since the total system noise is a combination of background noise and noise contributed by the sensitivity of the instrument, the noise can be reduced by either reducing the background noise or increasing the sensitivity. For example, if the background noise could be eliminated completely, this would reduce the noise contribution to the flux error by a factor of 2.5. If, instead, the sensitivity were increased by a factor of 10, this would decrease the noise contribution by a factor of 6.4. Response Time. Figure 3 shows a plot of photon counts per 0.1 s as a function of time as the DMS source was modulated at the orifice inlet as described above. The detector time constant is about 0.1 s. Since the mean residence time in the 39-mL reaction cell at a flow of 1.2 slpm and 5 Torr is 0.012 s, adsorptive memory effects in the inlet tube and reaction cell most likely dominate the response time. The sensor has a sufficiently rapid response to be used both as a high-resolution vertical profiler and for eddy covariance flux measurements from aircraft. Extent of Reaction. It is desirable to drive the chemiluminescence reaction to >99% completion so that maximum photon yield is attained and to ensure that a small perturbation to reagent F2 concentration or other conditions such as cell residence time do not have the potential to generate a large photon yield fluctuation. Extent of reaction can be calculated using pseudofirst-order kinetics. This assumption is valid since [F2] . [DMS], thereby making the term k7[F2] invariant. Extent of reaction can be calculated by the equation
[DMS]t/[DMS]o ) exp(-k7[F2]t)
(16)
Typical values for parameters related to these experiments are T ) 298 K, P ) 5 Torr, total flow rate ) 1.2 slpm, flow rate of 8% F2/He ) 0.075 slpm, and reaction cell volume ) 39 mL. Assuming piston flow, we calculate a residence time in the reaction cell of 0.012 s, [F2] ) 8.1 × 1013 molecules cm-3, and extent of reaction ≈ 100% ([DMS]0.012s/[DMS]o ) 1.8 × 10-7). Nevertheless, as the F2 concentration is increased, some gain in signal is realized. Thus, the initial reaction of F2 and DMS is not the only important reaction leading to chemiluminescence production. This observation is consistent with the previous finding that HCF*, formed in secondary reactions, is the principal emitter when F2 is in large excess over DMS.21 Chemical Amplification by Doping the Reaction Cell with H2. Although the DMS sensor described here measures light primarily from HCF*, additional light is generated by HF† emission. Since future work may involve detection of reduced sulfur species which do not produce significant HCF*, experi(42) Erickson, D. J.; Ghan, S. J.; Penner, J. E. J. Geophys. Res. 1990, 95, 75437552.
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Figure 3. Response-time characteristics for the fast DMS sensor. DMS sample modulated on/off via proximity of DMS delivery line to inlet orifice. Conditions: data acquisition rate, 0.1 s; reaction cell pressure, 4.1 Torr (total flow rate, 0.98 slpm); F2/He flow rate, 0.057 slpm; H2 flow rate, 0 slpm; sample air flow rate, 0.92 slpm; sheath air flow, >30 slpm; permeation air flow rate, 0.50 slpm; DMS permeation rate, 4.0 × 10-9 slpm; sample inlet orifice diameter, 0.037 cm; [DMS] ) 1.5 ppbv; calculated cell residence time, 0.011 s.
ments were performed in an attempt to enhance the HF† channel. Hydrogen doping was investigated as a means of enhancing the sensitivity of detection from this channel. Previous work showed that trace F atoms are produced in the reaction of F2 with DMS.21 In the presence of H2, trace F atoms catalyze the production of highly vibrationally excited HF as follows:
F + H2 f HF + H
(17)
H + F2 f HF† + F
(18)
HF† f HF + hν
(19)
net: H2 + F2 f 2HF + hν
(20)
The reaction of H with F2 produces vibrationally excited HF† with v e 9.43 The catalytic cycle can be repeated several times within the residence time of the reaction cell, allowing for chemical amplification of the signal. With the visible-light-sensitive bialkali PMT, no enhancement in the signal was observed due to H2 doping, as would be expected since the dominant emission is from HCF*. However, when a red-enhanced, multialkali PMT (Hamamatsu, R1925) was used, about a 6× enhancement in sensitivity to 1.1 ppbv DMS was observed for [F2] ) 3.9 × 1014 molecules cm-3 and an added [H2] of 7.6 × 1015 molecules cm-3, consistent with production of vibrationally excited HF† (Figure 4.) The signal amplification occurred at much lower concentrations of H2 and leveled off at hydrogen concentrations well below 1015 molecules cm-3. This behavior is consistent with the fact that reaction 18 is the rate-determining step of the cycle of reactions 17-19. The time constant for the reaction of F with H2 is calculated to be only 5 µs for [H2] ) 7.6 × 1015 molecules cm-3 (43) Kompa, K. L.; Pimentel, G. C. J. Chem. Phys. 1967, 47, 857.
1740 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
and k17 ) 2.6 × 10-11 cm3 molecule-1 s-1,44 while the time constant for the reaction of H with F2 is calculated to be 1.6 ms for [F2] ) 3.8 × 1014 molecules cm-3 and k18 ) 1.6 × 10-12.45 Thus, within the 12-ms residence time of the detection cell, the reaction can cycle about seven times, consistent with the observed signal enhancement factor of about 6. Optimization. DMS-F2 sensor optimization is straightforward, with several variables dominating the chemiluminescence response. Like many chemiluminescence systems, the sensor is mass flow dependent; i.e., the more sample air passed through the cell per unit time, the more light is produced. This was validated by throttling the pumping speed with a valve and by operating with two different pumps. Since light levels are low and photon-counting statistics determine the noise in the S/N ratio, one wishes to maximize the rate of light production. A competing effect is the quenching of emission by collisional deactivation at higher pressures. Thus, detection is optimized by (i) using the largest pump practical, (ii) maximizing effective pumping speed via the use of large-diameter, short-length reaction cell exhaust tubing, and (iii) selecting an optimum inlet orifice diameter that provides large sample flow while not overly quenching the chemiluminescent reactions. Note that, for an atmospheric monitor, there are no constraints on sample flow rate. Increased reaction cell volume beyond that needed for complete reaction and mixing is a minor enhancer of sensitivity. For this particular combination of vacuum pump, reaction cell, and inlet line, maximum sensitivity occurred for an inlet orifice diameter of 0.051 cm, which resulted in a reaction cell pressure of 6.4 Torr (44) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; Evaluation No. 12, National Aeronautics and Space Administration: Washington, DC, 1997. (45) Homann, K. H.; Schweinfarth, H.; Warnatz, J. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 724.
Figure 4. Effect of H2 doping on chemiluminescence signal. Sensor, modified Sievers Research, Inc., model 207; [DMS] ) 1.1 ppbv; H2 flow, 0 (300 s); Ptotal ) 3.9 Torr; F2/He flow, 0.056 slpm; permeation flow, 0.26 slpm; sample flow, 1.0 slpm; dilution flow, 3.06 slpm.
and an inlet flow rate of 1.2 slpm. As discussed above, because the reaction of DMS and F2 is rapid, fluorine concentration had only a secondary effect on sensitivity. An optimum flow rate for the 8% F2/He mixture was 0.075 slpm. Perhaps the greatest improvement in S/N could be achieved by reducing the background signal; this is discussed below in the Future Improvements section. Selectivity. The DMS-F2 system is unique in that large amounts of HCF* are formed when reaction 7 is fuel lean (normal sensor operation). Emission from HCF* occurs at 500-700 nm, which is much bluer than the HF† chemiluminescence at 670900 nm, which can result from the reaction of fluorine with other reduced sulfur compounds, or the weak chemiluminescence resulting from mixing alkenes and fluorine.36 Selectivity of the fluorine-induced chemiluminescence detector has previously been characterized for a wide range of compounds.34,35,46 Selectivity over hydrocarbons is in the range 103-107, with selectivity being the lowest for alkenes. These studies have shown that the detector is largely insensitive to alkanes, alkynes, aldehydes, carboxylic acids, amines, aromatic hydrocarbons, halogenated organics, and the inorganic species O2, N2, CO, CO2, NO, and O3. From the reported selectivities, we estimate that none of these species will provide a significant interference in measuring DMS over the oceans. As stated earlier, fluorine-induced chemiluminescence with sulfur compounds occurs only for those sulfur compounds having H-C-S bonding. This eliminates interferences from several atmospheric sulfur species often present at significant concentrations, i.e., SO2, CS2, OCS, and H2S. The only known interfering compound which could produce a significant signal in the marine troposphere is DMSO. Fortunately, DMSO fluxes and concentrations are much lower than those of DMS, (46) Fitzwater, D. A.; Glinski, R. J. J. Photochem. Photobiol. A: Chem. 1993, 74, 91-97.
resulting in calculated errors of
