Tracing Atmospheric Pollutants by Gas Chromatographic Determination of 'Sulfur Hexafluoride Russell N. Diet21 and Edgar A. Cote Department of Applied Science, Brookhaven National Laboratory, Upton, N . Y . 11973 Based on a nitric oxide treated molecular sieve column which quantitatively elutes SF6 prior to the components of air, laboratory and portable electron capture gas chromatographic procedures have been developed to determine SF6 down to a sensitivity of less than 4 X 10-13 cm3/cm3 without preconcentration. The laboratory chromatograph can measure the SF6 concentration in collected air samples at a rate of 1 every 5 min. Application to plume studies by simultaneous determination of SOz, sulfate, and SF6 showed that the measured decrease of pollutant concentration was primarily attributable to dilution. The portable real-time instrument, which has been used in aircraft and land vehicles, can continuously measure the SF6 tracer gas down to a sensitivity of 4 X 10-13 cm3/cm3 for a duration of 40-60 sec with a response time of.about 3 sec for a tenfold concentration change. The capability of airborne power plant plume crosswind concentration gradient determinations using SF6 is demonstrated. Tracing air masses for 100 or more kilometers during unstable meteorological conditions appears feasible. W
Monitoring of our environment is becoming increasingly important both to the concerned public and to pollution control researchers. Numerous instruments and methods are available for measuring gaseous and particulate pollutant concentrations (Hochheiser et al., 1971) in the atmosphere but tracing and determining the source of these pollutants, especially a t distances greater than 15 km, have not been as extensively and successfully studied. Sulfur hexafluoride has proved to be a useful tracer for such dispersion studies because of its ultrasensitive detection by electron capture chromatography, chemical stability in the presence of other atmospheric pollutants and sunlight (Saltzman et al., 1966), and normally low concentration in the atmosphere-less than 2 parts in 1013 by volume (Lovelock, 1971). Early chromatographic methods of analysis employed alumina or silica gel and charcoal columns in series (Clemons et al., 1968; Turk et al., 1968; Hawkins et al., 1971), but the SF6 peak always eluted on the tail of the large oxygen peak preventing the determicm3/cm3) nation of low concentrations of SF6 without preconcentration. Concurrently with Simmonds et al. (1972), an improved laboratory electron capture gas chromatograph method for the direct determination of collected air samples containing as little as a few parts of SF6 in loi3 parts of air has been developed (Dietz and Cote, 1971); the technique and its application to plume tracing and chemistry studies are described in this paper. Additionally, a portable gas chromatograph, which has been developed for real-time determination of SF6 in meteorological tracer studies using aircraft and land vehicles (Dietz et al., 1972), is also described including examples of the determination of SF6 concentration gradients in power plant plume crosswind measurements. The application to plume chemistry studies for determining chemical conversion by measuring changes in the pollutant-to-SF6 concentration ratio is also indicated. ITo whom correspondence should be addressed. 338
Environmental Science & Technology
Experimental Laboratory Chromatograph. The laboratory instrument used in these determinations was a Varian Aerograph Model 1532-2B with a concentric tube type 250-mCi tritium foil electron capture detector. The 1-mV recorder was equipped with a Disc Integrator and a low pass noise filter consisting of two 3000-ohm resistors and a 100-bf capacitor a t the input terminals. The filter reduced the amplified detector noise by about a factor of five without affecting the SF6 signal. Ultrapure Nz from Baker or Matheson was satisfactory as the carrier. gas after passing through an anhydrone dryer; the use of a BTS catalyst gas purification train (Maak and Sellars, 1965) did not produce any significant improvement in detector sensitivity. The 17-ft by yg-in. gc column (a11 tubing was stainless steel) and the 3-ft by l/g-in. precut column contained Molecular Sieve 5A treated with nitric oxide (Dietz, 1968) to reduce tailing of the SF6 peak; following treatment the columns were purged with carrier gas overnight a t 325°C. The purpose of the precut column was to prevent components of the atmospheric sample other than SF6 from reaching the detector. Optimum sensitivity was achieved with the detector a t 100°C and 65 V dc a t a carrier gas flow rate of 14 cm3/min. When accumulated contaminants began to affect the detector standing current (about 4-6 weeks), the column was cleaned by heating overnight a t 300°C with the carrier gas flowing. The precut column was normally outgassed a t the same temperature every two or three days. Details of the plumbing configuration are presented elsewhere (Dietz and Cote, 1971). Portable Chromatograph. The portable gas chromatograph used in this study was an Anayltical Instrument Development Model 510 equipped with a 200-mCi tritium foil parallel plate type electron capture detector designed to operate under pulsed mode with 5% methane in argon as carrier gas. The only major modifications to the original instrument consisted of replacement of the Teflon cylinder within the detector with one of glass since Teflon absorbs SF6, removal of the needle injection port, and conversion of the detector to dc operation by installing an adjustable potentiometer on an internally available +15 V dc terminal. The chromatograph contained its own rechargeable batteries 'for electrometer and detector operation (sufficient for >10 hr). Additionally, 350 W a t 120 V ac was required for operation of a modified Varian 7-port solenoid-operated sampling value, a Honeywell Model 19 recorder, and a positive-displacement sampling valve. The mode of operation of this real-time instrument was frontal chromatography since a column short enough to elute the SF6 in real-time (2-4 sec) by conventional chromatography was too short to effect a separation of the SF6 from the air of the sample. During sampling, the argon backflush gas in the column was displaced with the air being analyzed with the recorder output in the form of a step (frontal) in place of the usual SF6 peak. A constant height for the SF6 frontal was indicative of a constant concentration in the air being sampled; when the SF6 concentration varied, as when crosswind traversing a plume spiked with SF6, the SF6 frontal height also varied in a similar fashion. A schematic diagram of the flow system and sampling
valve is shown in Figure 1. In the backflush position pure argon was sent to the detector (ports 4-5) while the column (6-ft by lb-in. packed with Molecular Sieve 5A, 60-80 mesh) was backflushed with argon (ports 7-6, 2-3). Port 3 was connected to the upstream side of the back-pressure regulator so that the column could be brought to sampling pressure prior to switching. The argon was regulated to 20 psig (gauge 2) producing a 70 to 80 cm3/min backflush and the needle valve was adjusted for a detector purge of 40 cm3/min; the detector voltage was approximately 7 V dc. The molecular sieve column was treated with nitric oxide as previously described as was the 3-in. by l/B-in. molecular sieve dryer ahead of the detector. This latter short column was necessary to produce a stable baseline since trace impurities from the sampling valve (lubricant vapors, etc.) would have otherwise adversely affected the detector. About 20 sec before sampling, the air-sampling pump was activated and the back-pressure regulator was set to 11 psig (gauge 1) to purge the pump and sampling valve between ports I b and l a (the latter was an added modification to the original valve). After energizing the solenoid switching valve, approximately 20 cm3/min of the air to be analyzed was passed through the column (ports lb-2). Both the columns and the detector were a t 25°C. About 39 sec after sampling commenced, the SF6 frontal reached the detector followed by the 0 2 frontal 46 sec later. This gave an effective continuous determination of SF6 for a duration of three quarters of a minute-sufficient, for example, to traverse a plume crosswind section. At this point, the solenoid switching valve was de-energized to backflush the remaining components of the atmospheric sample through the main vent (the air-sampling pump was turned off and the back-pressure regulator was reduced to 0 psig). Depending on the concentration range of the SFs, the column was backflushed for 1-4 min before the next sampling commenced in order to remove the 0 2 from the previous sample. Both the laboratory and real-time portable chromatographs were calibrated with gas mixtures ranging from 2 X to part of SF6 per part of air. The gas mixtures were precisely prepared by pressure-dilution measurements from a working standard of 85 ppm SF6 in N2 mixture prepared by the same technique. This latter mixture was checked for accuracy using an F&M Model 810 chromatograph (Hewlett-Packard) with a thermal conductivity detector and molecular sieve column (Dietz, 1968) calibrated with pure SF6. Atmospheric Sampling Cannisters. For determination of SF6 with the laboratory instrument, 900-cm3 steel bottles, each equipped with a toggle valve, critical orifice, filter, and quick-connect fitting, were used in an aircraft to obtain samples from power plant plumes as described elsewhere (Manowitz and Tucker, 1969). Analyses were performed on a routine basis by either pressurizing the collection bottles to about 2 atm absolute with ultrapure N2, allowing ample time for mixing, and expanding through the chromatograph sample loop or by evacuating the sample loop up to the collection bottle and allowing the undiluted sample to expand into the loop. When suitable pressure corrections were applied, the results agreed within 3% by either method.
Res u 1t s Calibration. The concentration of the SF6 working standard as determined by the TC chromatograph was 85 f 2 ppm-in excellent agreement with that calculated from the pressure-dilution measurements of 84 1 ppm. When the SFGin air mixtures prepared from the working
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standard was used, the calibration curves for both electron capture chromatographs were nearly linear up to 10-9 cm3/cm3 with a limit of detection a t three times noise of about 4 X 10-13 cm3/cm3, Laboratory Chromatograph. Backflushing the precut column a t the appropriate time after sample injection prevented the major fraction of the oxygen from the air sample from reaching the gc column and detector, thus considerably shortening the time for a complete analysis. In the upper chromatogram of Figure 2, the precut column was backflushed 1.5 min after sample injection; this caused most of the oxygen of the laboratory air sample to be vented and allowed only a small amount (
