Sensitive Gas Chromatographic Detection of Nitrogen Dioxide Otto Grubner and Abraham S. Goldin Harvard School of Public Health, Department of Environmental Health Sciences, 66.5 Huntington Avenue, Boston, Mass. 021 15
Reaction between styrene and nitrogen dioxide yields products that can be separated by gas chromatography and detected by an electron capture detector. The number of products is reproducible and their concentration proportional to the concentration of nitrogen dioxide. Detection limits are such that two nanograms of nitrogen dioxide in a gas sample can be detected.
This paper reports a procedure for the analysis of nitrogen dioxide by gas chromatography, based on its reaction with styrene. Reaction products are separated by gas chromatography and detected by electron capture or flame ionization detectors. In preliminary experiments, as little as two nanograms of nitrogen dioxide have been detected. Up to now, gas chromatography of nitrogen dioxide has not been sensitive or reliable enough for use as an analytical method. Four general types of procedures have been used. The first of these (Greene and Pust) ( I ) employed molecular sieves pretreated with water as solid adsorbent for gas chromatographic columns. Oxides of nitrogen were separated on such a column and detected by a thermal conductivity detector. This method is relatively insensitive and may be subject to errors due to possible reaction of nitrogen dioxide with molecular sieves and/or oxidation of nitric oxide by the adsorbent. Direct sensitive gas chromatographic analysis of nitrogen dioxide with electron capture detection was reported by Morrison et al. (2,3). It was shown that nitrogen dioxide can be separated from oxygen on a column filled with fluoropak and coated with methyl silicone oil S.F. 96. The third method (4) uses porous polymer beads as solid adsorbent. The polymer is a cross-linked copolymer of ethylvinylbenzene and divinylbenzene (One form is sold by Waters Associates under the name Porapak). This method has been shown to give erroneous results because the polymer reacts with the nitrogen dioxide (5, 6). The fourth type of chromatographic method (7) is based on the catalytic reduction of nitrogen dioxide to nitrogen and subsequent gas chromatographic analysis of the nitrogen on a molecular sieve column. This method is probably reliable but is relatively tedious and limited by the sensitivity of the thermal conductivity detector. The strong tendency of nitrogen dioxide to react with organic compounds has been utilized analytically. For example, the most widely used method (Saltzman) for the determination of nitrogen dioxide in ambient air is based on the Griess reaction in which nitrogen dioxide forms a reddish purple azo dye with aqueous diazotized sulfanilic acid naphthylamine hydrochloride. Optical spectrophotom(1) S. A. Greeneand H. Pust, Anal. Chem., 20, 1039 (1958) (2) M . E. Morrison. R . G. Rinker, and W . H . Corcoran, Anal. Chem., 36, 2256 (1964). (3) M . E. Morrison and W. H . Corcoran, Anal. Chem., 39,225 (7967). (4) W. F. Willhite and 0. L. Hollis, J. Gas Chromafogr.,6, 84 (1968) ( 5 ) J. M . Trowell, J . Chromatogr. Sci., 9,253 (1971). (6) 0. Grubner, J. J. Lynch, J. W . Cares, and W. A . Burgess, Amer. Ind. Hyg. Assoc. J.. 33,201 (1972). (7) V. G. Berezkin and V. S. Gavrichev, Zavod. Lab., 37,901 (1971).
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etry of this dye has made it possible to measure about 40 nanograms of nitrogen dioxide. The investigations reported here used the reactive nature of NO2 to form products which could be separated by gas chromatography and detected by sensitive ionization detectors. The reaction between nitrogen dioxide and styrene was very suitable for this purpose. This reaction takes place at ambient temperatures at very low concentrations of both nitrogen dioxide and styrene. It is a fast reaction and yields several products that can be detected by electron capture and flame ionization detectors. The reaction is reasonably reproducible and the products formed are proportional to the amount of nitrogen dioxide in the sample.
EXPERIMENTAL Styrene used in our experiments was the product of the Eastman Kodak Co., bp 33-35 “C/8 mm. This compound gave a single gas chromatographic peak (styrene) with a small trace impurity having a slightly shorter retention time than styrene. No other peaks were observed. Various concentrations of styrene in air were prepared by injection of a known amount of saturated styrene vapor into a 500-ml sampling flask containing ambient air. Concentrations of styrene for calibration purposes were prepared by introducing known amounts of liquid styrene into the sampling flask. Test concentrations of NO:! in air were produced with “Dynacal” permeation tubes produced by Metronics Associates, Inc., Palo Alto, Calif. The permeation rate of NO:! from the tubes (10 cm long) a t 20 “C was determined to be 8200 ng/min gravimetrically. Some comparative samples were taken from nitrogen dioxide lecture bottles as sold by Matheson Co. Various concentrations of nitrogen dioxide in air ranging from 1 to 150 ppm were prepared by inserting a 10-cm nitrogen dioxide permeation tube suspended on a Teflon (Du Pont) tape into a 500-ml sampling flask for a known period of time a t 20 “C. After the insertion of the tube, the flask was closed with a stopper wrapped with paraffin film. The stopper was taken out momentarily for removal of the tube and immediately replaced. Insertion of the tube into the sampling flask for one minute would give a n approximate concentration of 4 ppm (v/v) of NO2 in the 500-ml sampling flask. It was expected the concentration of nitrogen dioxide in the flask would be directly proportional to the time during which the permeation tube was inserted. This expectation is based on the assumption that the concentration of NO2 in the flask is so low compared to t h a t in the tube that it does not affect the permeation rate--i e , that the permeation rate is diffusion controlled (8). The predictability of NO:! concentration as a function of time was checked experimentally. Eight different concentrations of NO:! in air ranging from 4 to 89 ppm were prepared as described. A known amount of the liquid Griess reagent was then added to each sampling flask and let stand for 20 minutes (with occasional shaking) in order to absorb all the NO2 and develop the color. The liquid was subsequently withdrawn from the sampling flasks and analyzed by standard colorimetric procedure. The results were interpreted by the method of least squares and the obtained regression line could be described by: c = 3.9 t + 0.066 where c is the concentration of NO2 in the flask in ppm; t, time of insertion in minutes. This value was used for all our experiments as the permeation rate of the tube for the technique described earlier. There is good agreement with the rate of 4.05 ppm/min estimated from weight loss of the tube with time. The standard error of estimate of c on t was k0.6 ppm. (8) 0 . Grubner, C . Martin, and W. A. in Amer. lnd. Hyg. Assoc. J.
Burgess, submitted for publication
Table I. Characteristic Data of Products of Reaction between Nitrogen Dioxide and Styrene Column temperature 60 " C
Peak
1
blto
1.57 662
Kovats indices
3
5
2 1.87 69 7
2.18 721
4 2.56 746
2.87 764
2 1.57 753
3 1.71 772
4 2.28 836
2.78 880
6 3.62 800
7 6.37 887
a
Styrene
7.37 910
1000
13.2
Column temperature 100 " C
Peak
tr / to
Kovats indices
1 1.36 720
Styrene
5
3.92 956
Figure 1. Gas chromatogram of products of reaction between nitrogen dioxide and styrene, column 60 "C
Figure 3. Gas chromatogram products of reaction between nitrogen dioxide and styrene detected by EC and FID sirnultaneously
RESULTS AND DISCUSSION
Figure 2. Dependence of the peak-height on concentration of
nitrogen dioxide, column 6 0 "C
Mixtures of nitrogen dioxide with styrene in a wide range of concentrations were prepared, both by adding nitrogen dioxide to styrene and by adding styrene to nitrogen dioxide. In one case, the NO2 permeation tube was immersed in the sampling flask containing air with a known amount of styrene. In the other, the NO2 permeation tube was placed in the sampling flask with air alone and the styrene added after removal of the tube. Both ways of mixing produced qualitatively and quantitatively identical mixtures, provided the concentrations of the reagents were the same. A gas chromatograph, Varian Associates Series 200 was used for the separations of styrene and styrene-NO2 reaction products from air. The column was Ys in. by 12 ft, the packing was Chromosorb with 5% SE30 as the liquid phase. The number of theoretical plates of the column was approximately 2000. The outlet of the column was provided with a 1:l splitter so that the eluted compounds were detected simultaneously by flame ionization and electron capture detectors and recorded by a dual channel strip chart recorder. Samples of the gaseous mixtures were introduced on the column by a 1-ml glass syringe through a septum into a block heated to 170 "C. The column temperature was kept a t 60 "C in one series of experiments and a t 100 "C in the other. Detectors were heated to 150 "C. Nitrogen was used as a carrier gas in all experiments with a linear velocity of 7 cm/sec.
Analysis of the Products of Reaction between Styrene and Nitrogen Dioxide. Column at 60 "C. The reaction products of the styrene-nitrogen dioxide interaction have been analyzed by gas chromatography at a temperature of 60 "C. Eight reaction product peaks have been detected by the electron capture detector, all of them having retention times shorter than styrene. A typical gas chromatographic spectrum of this mixture is shown in Figure 1. Characteristic data and Kovats's indices of particular peaks are listed in Table I. Concentration Dependence. The relationship between NO2 concentration and the heights of peaks 3 and 5 was measured and evaluated. Three flasks were prepared containing the test concentration of NO2 and different concentrations of styrene (16, 48, and 160 ppm). Samples, 0.5 ml, were withdrawn for gas chromatography. The results are summarized in Figure 2. The peak heights were found to be linear functions of NO2 concentration with slopes of 0.91 mm (ppm)-I for peak 3 and 1.25 mm (ppm)-1 for peak 5 . The spectrum under given conditions is surprisingly reproducible with an error well within the limits of the gas chromatographic method when a gas syringe is used for sample introduction. The peaks No. 7 and 8 represent probably the largest fraction of the reaction products. They may not, however, be the best suited for the analysis of nitrogen dioxide at this temperature because of their relatively large retention times. In this respect peaks No. 3 and 5 call for attention. These are clearly discernible from the base line of the spectrum even at very low concentration of nitrogen dioxide in the mixture. ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
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:
-t
i
I1
cm IO
5
I
0
20 40 Concentration of
NO^ 6o pprn
80
Figure 5. Dependence of the peak height on concentration of nitrogen dioxide, column 100 "C
Figure 4. Gas chromatogram of products of reaction between nitrogen dioxide and styrene, column 100 "C
The reaction products and their gas chromatographic peaks were independent of styrene concentration, even when this concentration was less than the NO2 concentration. The reason for this is not known, and the possibility that styrene or a polymer is retained in the system cannot be ruled out. This does not affect the utility of the procedure for analysis. N o attempt has yet been made to identify the components of the reaction between styrene and nitrogen dioxide at this low concentration level. Since the reaction products are less strongly retained than styrene, they are probably nitrated or oxidized fragments of the styrene molecule rather than nitro or oxo derivates of styrene. It is difficult to make quantitative estimates about the products of the reaction since the chemical composition of the compounds and the response of the electron capture detector to them remain unknown. To gain some insight on this matter, several tests were run in which the flame ionization detector was used at its highest sensitivity. A typical spectrum of such a test is shown in Figure 3. Each product detectable by ECD can also be detected by FID, but with considerable difficulty. On the assumption that the signal of the flame ionization detector for a particular product is not much different from that for styrene, one could say that the concentration of most single products of the reaction is between 10 to 500 ppb, under the conditions described in Figure 3. With our present equipment, 2-3 nanograms of NO2 can be detected after reaction with styrene. We believe that this limit may be improved by at least one order of magnitude by the use of modern, highly sensitive EC detectors, which were not available for this study. Column at 100 "C. A similar series of experiments was performed and the reaction products were analyzed by gas chromatography with a column temperature of 100 "C. In these analyses, five characteristic peaks were observed, as shown in Figure 4. The relation between NO2 concentration and the heights of peaks 1, 4, and 5 was determined as previously described. The linear relationship is shown in Figure 5 . The slopes of the lines were found to be 0.77 mm (pprn) for peak 1, 1.45 for peak 4, and 0.69 for peak 3. The products of the nitrogen dioxide interaction with styrene give a larger signal a t higher temperature at the expense of resolution. Once again the signal is proportion946
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Figure 6. Gas chromatogram from ambient concentration of nitrogen dioxide
a1 to the concentration of NO2 and can be used for analytical purposes. In order to test the sensitivity of the method, air containing 0.1 to 0.2 ppm of NO2 was reacted with styrene and 5 ml were injected on the column. Even though such a large sample overloaded the column with severe degradation of the resolution, the spectrum presented in Figure 6 clearly indicates the presence of NOz-styrene reaction products. Change of the Spectrum with Time. Various mixtures of styrene and nitrogen dioxide were repeatedly analyzed at intervals of fifteen and thirty minutes in order to find out if there was any change of the spectrum with time. In most cases no change was observed within the first forty minutes, followed by a slow and small decrease of total heights of all the peaks, including that of styrene. The ratio of the heights of peaks, however, did not change within the limits of our error of observation. It is difficult to properly interpret this phenomenon which was, in our opinion, due to slow leakage of the mixture out of the sampling flask or due to sorption of some products by the flask or stopper, rather than due to decomposition of the products. Influence of Water, Ozone, Nitric Oxide, Chlorine, and Sulfur Dioxide. Water, nitric oxide, ozone, and chlorine might be expected to either react with styrene or interfere with the reaction between styrene and nitrogen dioxide. Therefore, five sampling flasks containing 500 ppm of styrene in air plus 1) saturated water vapor, 2) 1% chlorine, 3) 170nitric oxide, 4) 0.5% ozone, and 5) 1% sul-
fur dioxide were prepared. The content of each sampling flask was analyzed by gas chromatography. No additional peak except that of styrene was observed in any of these analyses. It may be, therefore, concluded that styrene does not react with any of the test gases to produce gas chromatographically detectable compounds. Different amounts of nitrogen dioxide were then gradually added to each of the sampling flasks and the resulting mixtures analyzed. The gas chromatographic spectrum of the reaction products between styrene and nitrogen dioxide corresponded to that recorded in the absence of any possibly interfering gas. Thus, we may assume that water, ozone, chlorine, nitric oxide, and sulfur dioxide, even in concentrations exceeding those of nitrogen dioxide, do not interfere with the reaction between nitrogen dioxide and styrene. Comparison of the Reactions of Nitrogen Dioxide with Griess Reagent and with Styrene. The Saltzman reaction for nitrogen dioxide is based on the adsorption of this gas in liquid, Griess type reagent, as mentioned earlier. This reaction is among the most sensitive in colorimetric analysis. One can assume that the affinity of nitrogen dioxide to the diazotized sulfanilic acid is high. To check whether the affinity of nitrogen dioxide to styrene is of comparable order, four sampling flasks containing 40 ppm of nitrogen dioxide were prepared. Styrene vapor, 1600 ppm, was added to two of the flasks, and then the contents of all four flasks were analyzed by the standard (Saltzman) colorimetric procedure. The results in samples containing styrene were 40% lower than in samples containing no styrene. It can be, therefore, assumed that the affinity of nitrogen dioxide to styrene is of the same order as that to Griess reagent. Other Reactions of Nitrogen Dioxide with Organic Compounds. For the purpose of possible gas chromatographic analysis of nitrogen dioxide we have also studied reactions between nitrogen dioxide and benzene, toluene, xylene, pentene, 1,l-dimethylhydrazine and triethanolamine. Only the reaction with xylene produced significant
quantities of compounds that could be detected by electron capture detector. The reaction with styrene, however, was far superior to any other reaction studied so far.
CONCLUSION The interesting reaction between styrene and nitrogen dioxide yields products that can be detected by sensitive ionization detectors used in gas chromatography. Particularly, the response of electron capture detector is strong enough to make possible the detection of reaction products in the concentration range well below 1 ppm. Furthermore, the products obtained by the reaction under the same experimental conditions are reproducible and their amount is proportional to the concentration of nitrogen dioxide. This supports our thesis that the reaction between styrene and nitrogen dioxide can serve as a basis for rapid and simple detection and determinations of nitrogen dioxide in gaseous mixtures. The detection limits of the reaction products are, very low and permit the detection of 2 X l o e 9 gram of NO2 in a sample. As far as we know, no other existing analytical technique can be used to analyze such a small quantity of nitrogen dioxide. The reaction between styrene and nitrogen dioxide has not yet been described in the literature. Its analytical application would be definitely enhanced if the reaction products were identified and the mechanism of the reaction was known. Studies of kinetics and equilibria between particular components is also needed. We intend to continue our work in this direction. ACKNOWLEDGMENT We are indebted to W. A. Burgess and J. J. Lynch for their useful discussions and valuable comments. Received for review November 30, 1972. Accepted January 24, 1973. Supported in part by USPHS National Institute of Environmental Health Science Grants ES00503, ES33332, and Center Grant No. ES0002.
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