Anal. Chem. 1983, 55, 135-138
135
CORRESPONDENCE
Ozone Sir: Gaseous reduced sulfur compounds are of potential importance in the global atmospheric sulfur budget. The major source of this reduced sulfur is thought to be bacterial production, the most prevalent biogenic sulfur gas being hydrogen sulfide (HzS),with dimethyl sulfide (DMS), carbonyl sulfide (COS), carbon disulfide (CS,), mercaptans, and disulfides commonly being present in smaller amounts (1,2). These sulfides are subsequently oxidized in the troposphere to SOz and sulfate, thus contributing to the naturally occurring background of the latter compounds. With the recognition of acidic precipitation us a global atmospheric problem, there is a continuing need for highly sensitive and selective detection methods for these important reduced sulfur gases. A number of analytical methods have been used to measure vapor phase organic sulfides (3-11). However, all of these methods suffer from interferences, analysis complexities, and/or lack of sufficient sensitivity to monitor reduced sulfur gases a t the sub-part-per-billion levels expected in ambient air (12, 13). A method for real-time measurement of H2S, based on the chemiluminescent reaction of H2S with chlorine dioxide (CIOz) has recently been reported (14). A detection limit of 3 ppb is achieved by using photon counting techniques with a cooled photomultiplier. A major drawback to this method is that the C102 reagent is not easily prepared or stored. Also, formation of elemental sulfur can occur in the chemiluminescent reaction chamber, coating the cell window and reducing sensitivity during continuous operation. With the aim of developing a simple, sensitive, selective real-time method for measurement of reduced sulfur gases, we have investigated the chemiluminescent detection of HzS, DMS, and other reduced sulfur compounds by their oxidation with ozone. The observation of chemiluminescence in the 300 to 400 nm wavelength region resulting from the gas-phase reaction of O3 with HzS, DMS, and methyl mercaptan (CH,SH) has been previously reported (15,16) and the emitting species has been identified as electronically excited SOz (16, 17). The homogeneous gas-phase reaction of ozone with hydrogen sulfide has been reported to be slow in studies at low pressures (18-21), and the oxidation mechanism is apparently complex. A reaction order for H2Sfrom 0.5 (18) to 2 (21)has been observed, and a heterogeneous reaction pathway has been proposed as a possible lsxplanation of the data (22). Reported here are results of a pireliminary investigation of the ozonereduced sulfur chemiluminescence,which also imply a complex reaction mechanism (possibly both homogeneous and heterogeneous) but indicate that ozone chemiluminescence may be an extremely useful tool for the detection of HzS, DMS, and other reduced sulfur species in the ambient atmosphere and in industrial applications (23). EXPERIMENTAL SECTION Figure 1 shows the experimental arrangement used in this investigation. A Monitor Labs Model 8410 ozone monitor was modified for use as a detector of the ozone-sulfide chemiluminescence. An electrical discharge ozone source, of the type used in Thermo-Electron oxides of nitrogen detectors, was connected to the former ethylene flow line of the 8410. A Metal Bellows Corp. Model 41 air pump was substituted for the rubber diaphragm pump in the 8410, since the rubber diaphragm would have
been destroyed by the excess ozone in the reaction mixture. A UV-transmitting, visible absorbing glass filter (Corning CS-7-60) was installed in front of the photomultiplier tube (Hamamatsu R-268) to restrict light detection to the 300-400 nm range. This filter has a peak transmittance of approximately 70% at 355 nm. No change was made in the detector electronics or in the stainless steel reaction chamber. The reaction chamber was used at room temperature and was not thermostated. The chamber and all connecting tubing were sealed against outside light. Exhaust gases from the pump were scrubbed by a large activated charcoal trap before venting into the air to destroy excess reagent ozone and to trap any toxic reaction products. Gas mixtures were prepared with a Dasibi Model 1005-C2 Gas Calibrator. Permeation tubes for HzS,DMS, CH3SH,thiophene, benzene, acetaldehyde, and propane were obtained from AID, Inc. Pressurized cylinders of ethylene and benzene in nitrogen, at approximately 1 ppm and 20 ppm, respectively, were prepared by successive dilutions starting from the pure compounds. Mixtures of 208 ppm NO in nitrogen and 99 ppm NOz in nitrogen were used, both in aluminum cylinders from Scott Specialty Gases. Optimization tests were run with H2Sand DMS, which showed that the maximum chemiluminescent emission from both compounds occurred at sample air and ozonizer flow rates of 100 cm3/min each and a reaction chamber pressure of just under 1 atm. These conditions were used for all sensitivity and interference tests discussed below. No separate optimization study was done for methyl mercaptan or thiophene.
RESULTS AND DISCUSSION As was observed in an earlier study (16) the chemiluminescence intensity decreased in the order CH3SH > CH3SCH, > H2S. Thiophene chemiluminescence was still less intense than that from HzS. Initially, tank oxygen was used to supply the discharge ozonizer in order to maximize the ozone concentration in the reaction chamber. Comparative tests were conducted, however, with both dry air and tank oxygen supplied to the ozonizer. Surprisingly, when air was used in the discharge ozonizer, the signal observed from DMS increased by a factor of 3 over that found when oxygen passed through the ozone source. This large enhancement of signal when air is used in the ozonizer is reproducible for DMS, but sensitivity for thiophene shows only small improvement and that for HzS and CH3SH decreases slightly, relative to the sensitivity obtained by using oxygen. Sensitivities for CH3SH, DMS, HzS, and thiophene are in the ratio 80:30:3:1 when dry air is used in the ozonizer. The improvement in sensitivity to DMS is not caused by trace species present in ambient air, since identical results are found with ultrapure air, ambient air that has been purified by means of charcoal scrubbers, or unpurified ambient air. Tests indicate that oxygen quenches the ozone/sulfide chemiluminescence more efficiently than nitrogen does, possibly by scavenging the radicals produced in the sulfide oxidation. However, this effect is not large enough to account for the 3-fold increase in chemiluminescence from DMS when air rather than oxygen is used in the ozonizer. The optimum sample and ozonizer gas flow rates have been found to be the same using air or oxygen for ozone generation. Studies have disclosed that the enhancement of chemiluminescence from DMS which occurs from reactions with ozonized air is caused by the presence of oxides of nitrogen
0003-2700/83/0355-0135$01.50/00 1982 American Chemical Society
136
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983 EXHAUST
t
OZONE MONITOR CALIBRATION --81R SOURCE
-
OR 02
-
OZONE SOURCE
Figure 1. Block diagram of apparatus used in the investlgation of ozone/suifide chemliuminescence.
(NO,) produced from Nz and Oz in the electrical discharge (24). Air passed through the ozonizer discharge was analyzed with a Thermo-Electron Model 14-B NO, detector and found to contain approximately 50 ppm of NO,. When a similar NO, concentration was produced by addition of standard mixtures of NO or NOz in nitrogen to ozonized oxygen downstream of the ozonizer, the sensitivity to DMS increased by more than a factor of 2, relative to that when only nitrogen was added. Improvement of the DMS signal could be achieved by the addition of NO or NOz to the ozone flow even when air was used in the ozonizer, provided the added NO, concentration was of similar magnitude to that of the NO, produced in the ozonizer. Addition of NO or NOz in nitrogen to the ozone flow produced no improvement in sensitivity to HzS, relative to that when only nitrogen was added. No signal was observed from mixtures of sulfides and NO or NO2 in the absence of ozone. In a second study, the composition of the gas passing through the discharge ozone source was varied by premixing oxygen and nitrogen. The chemiluminescent intensity from a constant concentration (165 ppb) of H2S was found to be nearly invariant with ozonizer gas composition, from pure oxygen to 10% Oz/90% Nz. In contrast, the signal from 44 ppb DMS was increased over its value with ozonized oxygen with mixtures varying from 95% Oz to 10% Oz. The maximum signal from DMS occurred at an ozonizer gas composition of 75% oz/25%Nz, being 3.5 times as great as the signal observed using pure oxygen. However, the maximum is broad and signal enhancement at an ozonizer gas composition of 20% Oz/80% Nz (i.e., air) was only 10% lower. In all cases the chemiluminescent emission dropped sharply to zero at ozonizer gas mixtures approaching 100% N2. If the O3 chemiluminescent method is to be useful for ambient sulfide detection, it is important to determine the effect of NO, in the sample air on the measurement of sulfide compounds in that air. Mixtures of NO and NOz with both DMS and HzS have been prepared by adding standard mixtures of NO or NOz in nitrogen to the diluent air downstream of the calibrator. No change in signal from HzS was found with the addition of NO or NOz, as expected from the discussion above. Enhancement of the signal from DMS was observed a t parts per million levels of added NO,, when oxygen was used in the ozonizer, but a decrease in signal with added NO, was observed when air was used in the ozonizer. For the present report the key finding is that NO or NOz concentrations of 100 ppb or less in the sample air have
negligible effects on the intensity of DMS chemiluminescence, regardless of the composition of the ozonizer gas; hence it is clear that no interference in ambient DMS detection will result from the presence of ambient NO,. Although the detailed mechanism of enhancement of DMS chemiluminescence by NO, is still unknown, a few comments can be made. Hales et al. (18)discuss reports of NO, interactions with H2S but support a photolytic pathway for oxidation by HzS by NO,. Photolytic reactions are ruled out in the present study by the construction of the reaction cell and plumbing system. In the study by Becker et al. (19),under reaction conditions widely different from those in the present work, NO2was found to quench the chemiluminescence from HzS, but not from CH3SH or DMS, probably by reaction with chain-propagating HS radicals formed in HzS oxidation. Such an effect was not observed in this study. The final steps in the mechanism of ozone/sulfide chemiluminescence are
so + 0 3
- soz*+ +
0 2
(1)
S02* ---* SO2 hv (2) Although NOz is known to react with SO more than 100 times faster than ozone does, it is clear that enhancement of DMS chemiluminescence caused by NO, does not involve an increased rate of SO oxidation, since in that case similar effects would be expected for all the sulfides studied. In the ozone stream any nitric oxide (NO) produced in the discharge would be rapidly converted to higher oxides, including NO3,which has been shown to be a strong oxidizing agent for reactive hydrocarbons (25,26). It may be that the effects of NO, on the chemiluminescent oxidation of DMS are caused by the presence of NO3, in equilibrium with NzO6,in the ozonized air. Whereas the oxidation of HzS or CH3SH by ozone is thought to be initiated by the breaking of a hydrogen-sulfur bond (19, 20), e.g. H2S 0 3 HSO HOP (3) this cannot occur with dimethyl sulfide. We propose that the enhanced chemiluminescence from DMS in the presence of NO, is caused by some species, probably NO3,which attacks the sulfur-carbon bond more effectively than ozone does. Further studies using other organic sulfides will be performed to investigate this hypothesis. Calibration curves have been obtained for HzS and DMS, using air in the ozone source, since this gives optimum sensitivity for DMS and nearly optimum sensitivity for HzS. These calibration curves are shown in Figure 2. As stated
+
-+
+
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
70c
Table I. Compounds Tested for Interference in the Detection of Gaseous Sulfides by Ozone Chemiluminescence
/
compound ethylene benzene propane nitric oxide nitrogen dioxide acetaldehyde sulfur dioxide
: U.
40.
0
cL 40
1_u
32
137
40
Figure 2. Calibration curves for H2Sand DMS: (a) H,S, 9/1/81 (e), 12/22/81 (0); (b) DMS, 12/16/81 (O),single points on 1/14/82 (A), and 2/9/82 (0). In both (a) and (b), curves are drawn through the earliest calibration points only. previously, the sensitivity for DMS is about an order of magnitude higher than that for HzS. At low sulfide concentrations (e.g.,
