Preconcentration and determination of hydrogen sulfide in air by flame

of sulfur In sea air sampled at this location, but major amounts of unidentified organosulfur compounds are also present in on-shore air. The concentr...
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992

ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978

Preconcentration and Determination of Hydrogen Sulfide in Air by Flame Photometric Detection Robert S. Braman," James M. Ammons, and Joseph L. Bricker Department of Chemistry, University of South Florida, Tampa, Florida 33620

Gold coated glass beads have been studied for use in preconcentratlng low concentrations of hydrogen sulfide ( H2S) and other sulfur compounds in air. Sulfur compounds and H2S are removed by heatlng, separated on a short U-trap column, and detected by a flame photometric detector with limits of detectlon of approximately 0.01 ng or 0.1 part per trillion v/v for 100-L samples. Application to field analyses at Cedar Island, N.C., Indicated that H2S was the major reduced form of sulfur in sea air sampled at thls locatlon, but major amounts of unldentifled organosulfur compounds are also present in on-shore alr. The concentration and distribution of sulfur compounds appeared to be influenced by the sampling height above the ground.

T h e most sensitive method for hydrogen sulfide (H2S)in air to date employed on a reasonably wide basis is the method of Stevens et al. (1,2) in which 10-mL air samples are analyzed by gas chromatography with a flame photometric type detector (FPD). This method is reasonably readily applied to analyses of samples containing several parts-per-billion and higher H2S. T h e chromatographic separation makes it readily applicable t o analyses of a mixture of sulfur compounds. Nevertheless, t h e concentrations of H,S and of organosulfur compounds in ambient air are usually so low t h a t ambient air cannot be analyzed directly without preconcentration. Natusch e t al. (3)have developed and applied an AgNO, impregnated filter type sampler for H2S in air. It had a detection limit for H2S near 6 parts per trillion (ppt) but lacked ability to detect organosulfur compounds. Ambient H2S concentrations in nonpolluting locations were found t o be below 0.1 ppb, thus providing a target analyte concentration for method development. Collection of sulfur compounds on gold coated glass beads followed by removal by heating in the presence of H2 has been studied and developed for the analysis of ambient air ( 4 ) . I t is here applied for t h e first time in a n environmental study. Details of t h e analytical procedure and its development are presented, together with the results of the environmental analyses. Recommendations for further development are given. Basis of the Method. A variety of metals coated onto glass beads were studied by Ammons ( 4 ) as preconcentrators of H2S and organosulfur compounds. Compounds were absorbed from air, desorbed by heating in hydrogen and detected using a d c discharge emission type detector. Gold was selected for further study because of its ease of regeneration for reuse and stability t o use in air. T h e F P D now used extensively in air analyses for sulfur compounds was selected as the detector for use in work here because of its superior sensitivity and selectivity. Absorption of H2Sand a variety of organosulfur compounds during sampling was found to be efficient as was their removal upon heating in a Hz-N2 mixture. Mixtures of H2S,CH,SH, and (CH&Sz were removed as the absorbed compounds with only small amounts of decomposition of the mercaptans and disulfides, if any. T h e apparatus design used in the method development is shown in Figure 1. T h e quartz burner was quite stable to 0003-2700/78/0350-0992501.OO/O

flow fluctuations during the cold trapping of desorbed sulfur compounds and separation on t h e glass bead U-trap. A GCA/McPherson Instrument Co. monochromator was first used for wavelength selectivity but was replaced by a 394-nm interference filter to simplify t h e detector and improve t h e limits of detection. With the apparatus shown, the limit of detection was found to be approximately 0.01 ng sulfur per sample. This limit of detection combined with an assumed sampling time of 30 min, sampling rate of 2 L/min and concentrations of H2S, 50 ppt; dimethyl sulfide (DMS), 8 ppt; and mixed organosulfur compounds (R-S), 10 p p t permits calculation of the effectiveness of preconcentration. Under t h e above assumed conditions (close to real conditions), the H2S,DMS, and R-S signals observed would be respectively 300,48, and 60 times their limit of detection. It was with the above method design in mind that some 12 sampling stacks were prepared for use in field sampling a t Cedar Island, N.C. T h e single major apparatus substitution for the field analyses was the use of a Meloy Instrument Co. 185A F P D detector instead of the quartz burner assembly shown in Figure 1. EXPERIMENTAL Apparatus. Laboratory studies in development of the method were carried out with the apparatus shown in Figure 1. Analyses at the field site were done with the Meloy 185A FPD with data recorded on a Linear Instruments Corp. integrating strip chart recorder. The field analysis apparatus consisted of two parts, a H2Sdesorption apparatus and the analysis train. The integrating recorder had 4 times off scale integrating capacity which permitted a reasonably wide range of sample sizes. The quartz tube detector shown in Figure 1 was constructed of 10-mm 0.d. quartz tubing with an interior tubing of 6-mm 0.d. Typical gas flow rates were H2, 420 mL/min; air, 125 mL/min; and carrier gas mix, H2 70 mL/min; He, 72 mL/min. An Elhygen hydrogen generator (Varian Aerograph Model 9652) and tank Hz were both used as carrier gas hydrogen on occasion. No detectable background of sulfur compounds was found in any of the carrier gases employed. Sulfur Dioxide Scrubbers. Sulfur dioxide is the greatest potential interference and must be removed because it is adsorbed onto gold and is partially converted to H2Sduring analysis. An acid base reaction with Na2C03treated glass beads was used to remove SO2. Sulfur dioxide was found to be absorbed by glass beads treated with small amounts of Na2C03while H2S,DMS, CH3SH, and dimethyl disulfide (DMDS) compounds were not, under the air sampling conditions used. The arrangement of the SO2 absorption tube with the rest of the preconcentration stack is shown in Figure 2. Scrubbing tubes were prepared from 8G-120 mesh glass beads cleaned by acid washing. Some 5 g of cleaned, dry glass beads are treated with 45 fig of Na2C03,dried, and packed into glass tubes. These tubes are then treated by passing 10 ppb H2S through them for 30 min to condition the tubes. The conditioning process avoids the loss of a fraction of the H2S in the first several analyses. The SO2 absorbing tubes are also tested for slow bleeding off of SO2 by addition of 10-100 ng of SO2 to the scrubbers and analyzing for passage of SO2 during a 15-min pumping period. No bleeding off of SO2 was noted. Suitably prepared SOz scrubbing tubes pass less than 1% of the SO2,do not absorb more than 1-2% of H2Sor organosulfur compounds passed through C 1978 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 H2.Air Burner

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Flgure 3. Comparison of thioacetamide and permeation tube standards 1-1

Figure 1. Apparatus arrangement for analysis of preconcentration tubes Filter Holder with

Glass Fiber Filter

i:

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=SO2

PTFE Connectors

i

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2/ \

-=-/ Scrubber

Gold Samplins Tube

Figure 2. Arrangement of preconcentration sampling stack

them, and have an estimated sample capacity of 500 L. A substantial amount of testing went into the development of the SOz scrubbing tubes. Solid granular Na2C03was too alkaline and could not be properly conditioned. A commercial SO2 scrubber had a very low capacity for SO2. The scrubbing tube described above had a much greater capacity for SO2than the commercial model, likely because of its larger size and resultant larger amount of NaZCO3present on the surface. It is recommended that the SOzscrubbing tubes in commercial use by others be tested periodically for SO2 retention efficiency, since they do not have an infinite lifetime of useful service. P a r t i c u l a t e Filters. Filters tested for particulate removal were: Gelman Instrument Go., glass fiber filters Type A-E and Nuclepore membrane filters, 5, 2, 1, 0.2, and 0.1 pm pore size. Filters were tested for passage of a HzSO4 aerosol using a venturi type aerosol generator and dilute aqueous H2SO4 The glass fiber filters and the meter pore size membrane filters were over 90% effective in stopping the aerosols generated while the other pore sizes permitted passage of substantial amounts of the H2S04 aerosol. The glass fiber filters were selected for use since they were readily available and effective. All volatile sulfur compounds tested, SO2,HzS, CH3SH, and DMS were found to pass the glass fiber filters. The glass fiber filters should be tested for retention of small particulate sizes before use in urban areas where the very fine automobile exhaust sulfur particulate from catalytic converters may be a problem. Preconcentration Tube Preparation. Preconcentration tubes were prepared using 60/80 mesh glass beads (Varian Instruments, Inc.) and gold chloride prepared from gold foil. The beads were washed with concentrated HCl, dried, and treated with enough AuCl, to give approximately a 10% (wt) coating of gold metal after H2 reduction. Sampling tubes were approximately 8 inches long including connectors and were packed with approximately 3 cm of coated beads. Preconcentration tubes are rendered blank prior to use simply by putting them through the analysis procedure. Several analyses are occasionally needed to reduce the blank H a value to less than 0.1-1.0 ng. If tubes did not readily give small blank H2S responses upon repeated analysis, they were cleaned by passing a few mL of concentrated HC1 through them followed by distilled water Sulfate sulfur is incompletely removed from tubes by the analysis procedure. Preconcentration tubes were also tested for efficiency in absorbing H2S and DMS from air prior to use. This was done with

permeation tubes and the sampling pumps. Gold coated tubes may be used repeatedly. No specific test was made of this but some gold tubes have been used over 50 times in laboratory experiments. Acid washing reduces the useful tube life by loosening the gold coating on beads. Small pieces of organic matter on the tubes turn to carbon during analysis and can absorb sulfur compounds if used again. Tubes can be cleaned of carbon contamination by heating the tubes to analysis temperature in the presence of oxygen. Organic compounds in air samples apparently do not cause this type of interference. In later work the glass beads were found to contain small amounts of sulfur compounds which slowly reduced and produced the small blanks encountered. The beads were also found to contain substantial amounts of iron and zinc. Compounds of these elements are generally removed in the bead cleaning process with acid or by oxidation with oxygen while heating the tubes to 700-800 "C. Iron is a particularly troublesome interference and should be removed from all parts of the analysis system which encounters the sample. S t a n d a r d s a n d Calibration. Two types of standards have been studied for calibration of the FPD detectors used in sulfur in air analyses and for determining the operational characteristics of the analysis system. These are the diffusion tube standards of the type developed by O'Keefe and Ortman ( 5 ) and aqueous solutions of thioacetamide. The testing of preconcentration systems is rendered difficult because of the need to test them at very low air concentrations. A manifolded mixing system was used to dilute H2S,DMS, and CH3SH from gas diffusion tubes constructed of 6-mm 0.d. glass tubing connected with 2-cm lengths of Teflon tubing. Tubes were prepared using H2S,SO2and CH3SH gases or liquids in the case of other sulfur compounds. Low diffusion rates were obtained reasonably constant over several days to several weeks. Since this type of diffusion tube was not calibrated by weighing it was necessary to use a different approach for standardization. Thioacetamide in aqueous solutions was found to be a reproducible calibration standard for H2S. Stock solutions approximately 2000 ppm were diluted to 4 ppm and microliter sized samples of the latter were injected onto gold coated bead columns. Analysis of the gold tubes gave H2S peaks. The reproducibility of thioacetamide standards was found to be =k4.9% relative for 21 samples of 16 ng H2S on five different gold tubes. A comparison of two gold tubes indicated no significant difference in response between the tubes at the 80% confidence interval. Similar reproducibility was later experienced using permeation tubes as standards for H2S,DMS, CH3SH,and SO2on different gold tubes. Ammonium sulfate and H2S04were also tested as standards. Both gave H2S peaks on analysis. The thioacetamide was more stable on standing. The thioacetamide standard was compared to weighed diffusion tube standards using the Meloy SA 185 detector in work at Cedar Island. Figure 3 shows that both give approximately the same calibration curve. Calibration curves data for H2S, DMS, and CH3SH were obtained using U-trapped standards and the Meloy 185A FPD. Despite the linearizer on the detector instrumentation, some curvilinear character is observed in these calibration curves. This is not surprising since fluorescence self absorption effects are

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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978

involved and are known to be a function of sample sizes. The limit of detection was calculated from the response of small sample sizes, 1-5 ng of HzS and recorded noise. For HzS it is approximately 0.01 ng. Other organosulfur compounds are not so sensitively detected. In any case environmental concentrations of H2Sencountered were sufficiently large so as t o give well more than 1 ng per sample. Procedure for Sampling a n d Analysis. Sulfur compounds absorbed onto the gold preconcentration tubes are removed by heating the tubes to approximately 5 W O O "C for 5 min by means of a resistance heating wire coil. The carrier gas carries the removed sulfur compounds to the liquid N2 cooled U-trap. Sulfur compounds in the U-trap are then removed by heating the U-trap to 100-200 "C after removal of the liquid Nzcontainer. In the field analyses procedure, the cold U-trap was removed from the desorption apparatus and attached to the analysis train in which it was degassed for 20 s with Nzprior to heating and analysis. The HzS desorption apparatus could not be attached directly to the Meloy 185A analyzer because the latter was too sensitive to pressure and gas flow changes which occur upon removal of the liquid Nz. Gold sampling tubes were reblanked prior to reuse and assembled into sampling stacks with the filters and S O z scrubber tubes. These were sealed with laboratory paraffin film and stored until use. Sampling was timed by stopwatch. Sampling stacks were calibrated for flow rates while attached to the same pump used in sampling. Sample flow rates were found to be reproducible within &5?&relative over the several days of field sampling. Gold tubes were analyzed as soon as possible after sampling. Tubes were also reblanked directly after sampling. Analyses were carried out at the field site using the 0-1 and 0-10 mV ranges of the recorder to provide the desired response sensitivity. Nearly all analyses were done at the lower sensitivity range. Peak area response data were obtained using an integrating recorder and compared to the calibration curves for analyses. Hydrogen sulfide, CH3SH, and DMS are reported as ng/L and ppb (vol). The unidentified organosulfur compounds were analyzed using the H2S calibration curve because a suitable calibration curve was not available.

RESULTS AND DISCUSSION Efficiency Tests of the Sampling System. All gold tubes tested for H2Sretention from air at air flow rates up to at least 3 L/min show better than 99% efficiency under a variety of humidity conditions between 50% and 95% relative humidity. Gold tubes do exhibit a finite capacity for sulfur compounds. This is approximately 500 to 1000 ng depending upon the gold tube tested but is related to the amount of gold present. Since t h e gold tube preconcentrators are intended for use with sample sizes well below 100 ng, the sulfur capacity is not approached. A similar capacity effect and efficiency was observed for DMS. Several comparisons were made of H2S absorbed by and removed from t h e gold tubes vs. t h e HzS evolved from t h e diffusion tube dilution system and collected directly in the U-trap. No difference was noted in the several instances of this type of test. T h e agreement of t h e thioacetamide standards t o diffusion tube standards also supports the good HzS efficiency observed. No losses of H2S,DMS, or CH3SH were noted on either t h e glass fiber filters or in the SOz scrubbing tubes. E f f e c t of N i t r o g e n Oxides a n d Ozone. A sometimes ignored aspect of trace H2S analysis is the effect of O3 and nitrogen oxides (NO,) on the collection system used. From the known oxidizing nature of O3and NOz, one would predict that either should oxidize H2S and perhaps also some of the other reduced forms of sulfur. Moreover, the amounts of 0, and NO, present in air are generally much larger than the HzS or other reduced forms of sulfur present. Non-urban concentrations of NOz and O3 are in the 5-20 and 5-50 ppb concentration range according to recent reviews (6, 7). Since H2Smay be expected to be from 0.05 to 1.0 ppb, it is apparent

Table I. Effect of NO, on H,S Recovery Tubes not protected H,S

Present, ng 11.5 11.5 11.5 11.5 11.5 11.5 11.5 11.5

NO2

Recovered, Concentra- Amount, ng tion, ppb ng 1 1 . 5 (100%) 20 120 6.9 (10%) 28 430 5.6 (49%) 43 1060 1.4 (12%) 140 1710 With SO, scrubber 11.6 (101%) 25 305 10.4 (90%) 12 824 12.0 (104%) 105 2600 5.2 (45%) 138 3420

Table 11. Effect of 0 , on H,S and DMS Recovery (unprotected tubes)' H2S

Present, Recovered, ng ng % 0 3 , Pg 11.5 10.5 91 6.64 87 13.3 11.5 10.0 11.5 8.6 75 19.9 11.5 4.5 39 33 DMS H,S Present, Recovered, found, ng ng ng 0 3 , IJg 29.6 1 3 (46%) 4.9 0.66 28.6 6.7 (23%) 6.4 3.3 28.6 12.3 (39%) 9.2 6.6 28.6 1.8 (6%) 6.0 33 Two experiments with SO, scrubbers gave 100% recoverv of DMS. that the oxidizers are in substantial excess. Preliminary experiments indicated that NO2 and O3oxidize sulfur compounds trapped on gold surfaces while NzO had no effect. Nitric oxide appears not to be an interference but it oxidizes t o NOz which is an interference. The effect of NOz and O3 on HzS and DMS collected on gold tubes was studied in a laboratory apparatus designed to produce low concentrations of these gases. Nitrogen dioxide was delivered from a syringe drive apparatus and diluted by air. T h e NOz concentration of the resulting air sample was analyzed by the Saltzman (8) method. Ozone a t low concentrations was produced by a Welsbach Instrument Co. O3 generator and determined by iodometric titration. Samples of H2Sand DMS were pumped onto blanked gold tubes from a dynamic gas sampling system employing permeation tube standards. They were then exposed to various amounts of 0, or NOz and analyzed for sulfur compounds. This type of experiment was carried out both with and without the use of the SO2 scrubbing tubes to determine if the later afforded protection against the oxidizing compounds. Tables I and I1 give t h e results of the two studies. Unprotected tubes lose HzS, probably as SO2displaced from the tubes by the oxidizing agents which were present in amounts sufficient to entirely saturate t h e absorption sites on the gold. Dimethylsulfide is oxidized to H2S. In both cases the use of a SOz t r a p reduces the interference effects to a n acceptable extent. Similar experiments were carried out using outside air sampling. Ozone was 30-50 ppb during the time of the experiments; NO2 was not determined at the time of experiment. Outside air may be considered to be an urban type air as the experiment was carried out in the city of Tampa, Fla. Results,

ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978

Table 111. Effect of Outdoor Air on DMS Recovery Tubes Not Protected DMS Air Present, Recovered, Volume, Time, ng ng L min 26.5 (93%) 24 10 28.5 19.0 (67%) 48 30 28.5 5.0 (18%) 143 60 28.5 Protected 30.0 (105%) 71 30 28.5 28.5 (100%) 71 30 28.5 28.5 (100%) 71 30 28.5 32.5 (86%) 79 33 38 shown in Table 111, indicate that unprotected tubes having H2S or DMS on them exhibit losses if air samples exceed 24 L in volume. This is in accord with laboratory experiments which showed that exposure of tubes to more than a few micrograms of O3 caused losses of sulfur compounds or demethylation. Again, use of C 0 2 absorption tubes afforded protection. APPLICATION T O F I E L D ANALYSIS E n v i r o n m e n t a l S a m p l i n g a n d Analyses. A major objective of the field sampling and analysis work was to test operation of the gold tube preconcentration and analysis system and to determine the identity and range of concentrations of reduced forms of sulfur present in air in a remote salt marsh area location. Several types of sampling studies were planned including: samplings of on-shore open ocean air, sampling as a function of height above the ground, sampling and analyses as a function of time (diurnal variations), and sampling of vegetation emission. Some of each of these were done although the time variation study could not be carried out over more than a few hours because of difficulties in maintaining the sampling tubes in suitably blank condition. Characteristics of t h e Location. Cedar Island is located on the east coast of North Carolina protected from the open Atlantic only by a narrow, low barrier island strip of the Cape Hattaras chain. The island is largely salt marsh area with a partly pine forested sandy high ground. This location was chosen for study as it is reasonably remote with little vehicular traffic within 2 miles of the sampling and analysis location. Houses are located no closer than 1 mile from the sampling location. A pine grove, grass, low shrubs, and salt marsh grass were present at the location. The vegetation, wind direction, and fence appeared to influence results of sulfur analyses. Ozone concentrations were 10 ppb; NO2 was not detected a t this location (less than 2 ppb). These interferences were therefore much lower in concentration than in urban locations. S a m p l i n g Towers. Air samples were obtained using a 10-m high mast which could be raised and lowered to attach sampling tubes and fittings. Tygon tubing attached to the mast connected the small vacuum pumps (Neptune DynaPumps, Fisher Scientific Co.) with sampling stacks. Samples were taken a t 10, 5, 2, 1, and 0.1 m above the ground. Also available a t the location was an abandoned radar antenna tower from which 20-m high samples could be taken. Five sample sets were taken in the course of the field work. Data from two typical sets of samples are given in Table IV. Sample set 1 was taken at a location as close as possible to the edge of Pamlico Sound. There was no mud flat region a t this location. Notable is the high percentage of H2S and low percentage of other reduced forms of sulfur. Set 2 was taken in an open field location 50 m inland from the edge of Pamilico Sound with wind directions from a nearby salt marsh.

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Table IV. Tower Sample Data Sample Set l ( 8 - 2 4 - 7 7 , 1650-1720 h ) DMS H2S -___ R-Sng/L Height, m ppb ng/L ppb ng/L =H,S 1.46 -d 10 0.94 5 0.59 0.90 0.02 0.06 0.001 0.40 2 0.26 0.93 0.004 1 0.60 0.1 0.24 0.37 0.03 0.08 Sample Set 2 (8-28-77, 2000-2100 h, 40 L)

loa

0.078 0.046 0.12 0.58 0.90 0.80 0.084 0.22 5 0.69 1.06 0.055 0.058 0.09 0.021 2b 0.86 0.038 0.10 0.48 lC 1.33 0.11 0.1 0.16 0.036 0.095 0.04 DMDS, 0.11 ng/L as H,S. a DMDS, 0.15 ng/L as H,S. - not DMDS, 0.09 ng/L as H,S CH,SH, 0.21 ng/L. detected.

’

Table V. Samples Taken at 20 Meters nglL 0.29 0.45 0.10 0.16 0.23 0.36 0.19 0.29 0.20 0.30 0.94 1.45 0.53 0.81 a DMS, 0.03 ng/L; 0.01 ppb DMDS, 0.02 ng/L as H,S. DMS, 0.22 ng/L; 0.08 ppb CH,SH, 0.02 ng/L. ppb

8-27-7 7 8-27-77 8-27-77 2-27-77 8-27-77a 8-28-77 8-28-77’

1200-1300 1305-1425 1425-1550 1550-1715 2030-2130 1610-1710 1715-1816

h h h h h h h

A substantially higher organosulfur compound content is evident. A pattern of sulfur concentration was noted in all of the tower studies with a maximum at 1m and total reduced sulfur concentrations increasing in the height above the ground. Although this was likely influenced by the micrometeorology of the area, the variation indicates a possible problem of picking a representative height for air sampling. Sampling at 5 or 10 m was likely more indicative of sulfur in the general location. Samples were taken from the radar tower located near the other two sample locations a t a height above ground of approximately 20 m. Results of analyses are given in Table V. Hydrogen sulfide was the major reduced sulfur component in nearly all cases. Very little contribution was noted from organosulfur compounds. Some of the 20-m samples were taken at the same time as the 10-m tower studies. Comparison of the 20-m and 10-m tower samples, where available, showed substantial agreement in composition and, generally, in concentration of reduced sulfur compounds found. A number of miscellaneous samples were taken with data given in Table VI. Discussion. A comparison of ocean air samples to on-shore samples was made and is shown in Table VII. Ocean air was defined as air samples taken near the water’s edge with wind from the Ocean direction. On-shore is defined as samples taken further inland with wind generally from the land direction. On-shore samples and the pine grove samples all had much higher amounts of organosulfur compounds present. This is an agreement with Rasmussen (9) who found a number of different organosulfur compounds associated with biological sources. During the development of the analytical method, the major objective was to provide a method principally for H2S, CH,SH, and DMS and these were satisfactorily separated in the U-trap. However, a much more complex mixture of sulfur

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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978

Table VI. Miscellaneous Samples H2S ppb nglL 0.28 0.43

Location Seaside edge (8-22-77, 1240 h ) Seaside edge (8-22-77, 1050 h ) Seaside edge (8-26-77, 1630 h ) Above water, marsh Above ground, marsh Pine grove

0.59

0.90 -

0.15 0.23 DMS, 0.038 ppb, 0.10 ng/L 3.57 5.5 1.69 2.6 1.57 2.42 DMS, 0.15 ppb, 0.40 ng/L; CH,SH, 0.005 ng/L; R-S 2.7 ng/L as H,S 0.20 0.30 DMS, 0.012 ppb, 0.03 ng/L; R-S 0.71 ng/L as H,S

Pine grove

HzS

Others DMS, 0.015 ppb, 0.038 ng/L

2.42 ng/L

Table VII. Percentage Composition of Reduced Forms of Sulfur Ocean air samples On-shore air samples No. of Location, samCorn%of height ples % H,Sa pound t o t a l s b Seaside, 5 96.9 i. 4.7 H,S 53.2 0.1-10 m CH,SH 1.2 Seaside, 5 98.8 i 2.2 0.1-10 m Seaside, 5 99.5 i. 3.6 DMS 12.1 l m DMDS 7.9 Seaside, 4 96.1 t 6.2 R-S 25.4 20 m a Other sulfur compounds were only occasionally detected in small amounts. Average values of 1 7 samples. ~

~~~~

0.1 ppb. Consequently, the SO2 capacity of the scrubber tubes was not approached during sampling. The method has certain limitations. The sulfur compounds CS2 and COS are an interference since both appear as H2S after absorption on the gold tubes and H2 reduction. Although some of these are removed by the SOz scrubbing traps, an improved separation is needed. In addition, some decomposition of mercaptans to H2S was noted during hydrogen reduction and generally was a function of the total amount present. No decomposition of DMS was observed. Finally, the method does have the advantage of providing analytical information on HzS and DMS in air a t ambient concentrations in nonpolluted locations. It should substantially aid studies of the environmental chemistry of sulfur.

ACKNOWLEDGMENT The authors thank Robert K. Stevens, William McClenny, and Charles Bennett of the Development Branch, Environmental Protection Agency, Research, Triangle Park, N.C., for their aid in arranging for field work reported here.

Ingii

0.71 na/L as HIS

R-S

t ,

I

1

1

I

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0

1

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5

time. min

Figure 4. Analysis of preconcentrated compounds from pine grove

sample compounds was found as shown in Figure 4 and the unseparated, less volatile organosulfur compounds had to be reported as a group. An improved separation method is now under study. A gas chromatographic type of FPD air analyzer was in use during sampling of ambient air. Only HzS was detected in ambient air and a t concentrations in agreement with the preconcentration method described here. No SOz was detected at the limit of detection of the method which is approximately

LITERATURE CITED (1) R. K. Stevens, J. D. Mulik, A. E. O'Keefe,and K. J. Krost, Anal. Chem., 43, 627 (1971). (2) R. K. Stevens, A. E. O'Keefe, and G. C. Ortman, fnviron. Sci. Techno/., 3, 652 (1969). (3) D. F. Natusch, H. B. Kionis, H. D. Axelrod, R. J. Teck, and J. P. Lodge, Jr., Anal Chem., 44, 2067 (1972). (4) J. M. Ammons, "Selective Metal Surfaces for the Analysis of Ambient Concentrations of H,S", M.S. Thesis, University of South Florida, Tampa, Fia., June 1976. (5) A. E. O'Keefe and G. C. Ortman, Anal. Chem., 38. 760 (1966). (6) "M?dical and Bokgiic Effects of Environmental Pollutants. Nbogen Oxides", National Academy of Sciences, Washington, D.C., 1977, pp 56-98. (7) "Medical and Biologic Effects of Environmental Pollutants, Ozone and Other Photochemical Oxidants", National Academy of Sciences, Washington, D.C., 1977, pp 126-194. (6) 8. E. Saltzman. Anal. Chem., 26 1949 (1954). (9) R. A. Rasmussen, Tellus, 26, 254-260 (1974).

RECEIVED for review January 27, 1978. Accepted March 7, 1978. This work was supported by the National Science Foundation through RANN program grants ENV 76-09585 and ENV 76-09585-02.