Halocarbon Interferences in Chemiluminescent Measurements of NO, Surendra B. Joshi" Northrop Services, Inc., Environmental Sciences Group, P.O.
Box 12313, Research Triangle Park, N.C. 27709
Joseph J. Bufalini U.S. Environmental Protection Agency, Research Triangle Park, N.C. 277 1 1
w Anomalous NO, responses were observed when halocarbons were irradiated in the presence of oxides of nitrogen. Interferences to a chemiluminescent NO, monitor using a heated carbon converter were studied for phosgene, trichloroacetyl chloride, chloroform, chlorine, and hydrochloric acid. Data show positive interference in the NO, monitor due to the test compounds, either individually or as mixtures. Substituting a FeS04 converter for the heated carbon converter may solve the problem. This information may be of particular interest to smog chamber researchers and to those making NO2 measurements in areas where halocarbon levels are relatively high. It is speculated that excited 0 2 molecules may be the interfering compound, but additional work is needed to substantiate this hypothesis.
Researchers ( I , 2) have reported that certain commercial chemiluminescent monitors respond to other oxides of nitrogen in addition to NO and NO2. In this work, we discuss some unusual responses observed by a chemiluminescent NO, monitor when halocarbons were passed through a heated carbon converter. Figure 1shows the data obtained when tetrachloroethylene
was irradiated in the presence of NO,. In Figure 2, similar data are shown for a chloroform/NO, system. Both experiments were performed in a 440-Lborosilicate glass and Teflon smog chamber equipped with fluorescent light banks of blacklights and sunlights. The first order dissociation specific rate for NO2 for this chamber is 0.5 min-l. Figures 1 and 2 show anomalous NO2 responses after the halocarbon begins to react. Wet colorimetric analyses for NO2 (3) indicated that both of the experiments showed negligible concentration of NO2 in the systems. An earlier publication ( 4 ) on the photooxidation of chlorinated ethylenes reported that HC1, phosgene, and trichloroacetyl chloride are some of the halogen-containing products from the tetrachloroethylene/NO, system. These compounds were examined for possible interferences to the NO, mode of the chemiluminescent detectors, i.e., a test gas containing HC1, phosgene, or trichloroacetyl chloride was passed through the heated carbon converter of the NO, analyzer. In these experiments Tedlar bags were filled with tank air and spiked with NO/N02. In some instances the bags were not spiked with NO/N02. The NO, instrument was then attached to the bag, and the initial NO or NO2 measurements were obtained. The bags were then spiked with the test halogencontaining compounds, either individually or in mixtures. The
0 NO2 CHEMILUMINESCENT (CARBON CONVERTOR] TEMP. = W0F
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Figure 1. Irradiation of 4 ppm (vlv) tetrachloroethylene and 0.2 ppm NO, 0013-936X/78/0912-0597$01.0010 0 1978 American Chemical Society
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Flgure 2. Chloroform (4 ppm) and 0.2 ppm NO, irradiation Volume 12, Number 5 , May 1978
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NO or NO2 was again measured. The results of the interference studies are shown in Table I. The NO2 response changes by as much as 330% when phosgene and HCl are added to the NO/N02 mixture. Trichloroacetyl chloride also has a similar interference effect. With 5 ppm (v/v) of trichloroacetyl chloride, an interference of approximately 250% was observed when NO2 was at 0.125 ppm concentration. Chlorine gas also shows an interference for NO2 measurements. Positive responses were caused by Clz and trichloroacetyl chloride also in the absence of NO,. Finally, this Clz and halocarbon interference appears to increase with increasing carbon converter temperature. Chemiluminescent monitors using high-temperature stainless steel converters (such as for monitoring "3) may be subject to greater interference for the chlorinated hydrocarbons. Figure 3 shows the NO2 response of the chemiluminescent analyzer while monitoring NO, in an air mixture with increasing concentrations of trichloroacetyl chloride. The interference is practically linear up to 1.1ppm (v/v) of trichloroacetyl chloride. The interference problem appears to be eliminated when
an FeS04 converter is used in place of the carbon converter. These data are also shown in Figure 1.However, reported data on an FeS04 converter operated a t room temperature show decreasing efficiency of conversion (NO2 NO) at high humidities ( 5 ) .This problem with the FeS04 converter can be eliminated if the converter is heated to 32 "C (6). At this time it is not known what the interfering compound could be since the chemiluminescent instrument used in this study has an optical filter on the photomultiplier tube that cuts off at approximately 6000 A. Therefore, any excited species emitting at wavelengths longer than 6000 A would be detected. The sensitivity of the photomultiplier tube, however, drops off at or near 8500 A. Therefore, the interfering excited intermediate would have to emit between these wavelength limits (6000-8500 A). Ozone, when irradiated in the presence of chlorine, reacts very quickly (7), presumably as a result of the fast reaction, C1+ O3 C10 0 2 . By analogy, it is postulated that similar reactions occur in the thermocatalytic degradation of chlorine-containing compounds. Thus, a compound like trichloroacetyl chloride is first catalytically degraded by the hot carbon converter to C10 or C102 and COz. The C10 or ClOz
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Table 1. Interferences of Halogen-Containing Compounds on NO, Measurements lnltlal response, ppm
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Final response, ppm
% Change in response NOx NO
Number
NO
NOx
NO
NOx
1
