deviation decreases to 6 a t 16SYb2+ and to 7 millimass units a t 170Yb4+. Such oscillatory behavior in the mass dispersion is also observed by Venkataraghavan et al. (2) and may be attributed to the inhomogeneity of the magnetic field.
CONCLUSION We have developed a program for the complete semiquantitative evaluation of spark source spectra recorded on a photographic plate. Most of the arguments presently available to the analyst in the evaluation procedure are included in this program. Major aspects in our program are the high precision in mass assignments and the quantitative solution to interference problems. We have followed the philosophy that only those application programs
are really useful to the analyst which contain all evaluation criteria presently at his disposal. The considerable commitment of time and effort going into the development of such programs ultimately pays off in the speed, accuracy, and completeness gained by the automated evaluation.
ACKNOWLEDGMENT Robert Johnson's contribution, the furnishing of all the mass spectra needed in this study, is gratefully acknowledged. Received for review December 11, 1972. Accepted April 2 , 1973.
Gas-Phase Electron Paramagnetic Resonance Detection of Nitric Oxide and Nitrogen Dioxide in Polluted Air Hiromichi Uehara and Satoshi Arimitsu Sagamt Chemical Research Center, Nishionuma. Sagamihara-shi. Kanagawa, 229 Japan
Gas-phase electron paramagnetic resonance has been applied to the detection of NO and NO2 in polluted air. NO2 in polluted air is detected by Zeeman modulation at 1 atm of sample pressure, without interference from 0 2 and NO, and NO is detected by Stark modulation under the reduced sample pressure of less than 0.1 Torr without any interferences of coexisting substances. The sensitivity is better than 30 ppb for NO and NO2 with a sample volume of 1.5 I . when a novel low-temperature trapping technique is adopted. When the sample is observed without any enrichment, the minimum detectable limit is 10 ppm for N02.
Gas-phase electron paramagnetic resonance (gas-phase EPR) was applied to the detection of NO and NO2 in polluted air. Although NO and NO2 are highly responsible for the photochemical smog ( I ) , accurate determination is not always easy for parts-per-million (ppm) quantities of them. Conventional wet chemical methods seem to be still under discussion (2, 3 ) . Many instrumental detection methods ( 3 ) have been reported such as IR absorption, UV absorption. gas chromatography, chemiluminescent technique ( 3 ) ,IR absorption of tunable laser radiation (5), and a laser method based on the Zeeman modulation of absorption (6). Since any one of these monitors only one species, NO or NOz, complete analysis requires the chemical conversion of NO or NO2 to the other, which introduces inevitable uncertainties to the results. Recently, a carbon monoxide laser was used for a n independent detection of NO and NO2 ( 7). ( 1 ) P. A. Leighton. "Photochemistry of Air Pollution," Academic Press, New York. N. Y . . 1961. ( 2 ) For instance. areport in Chern. Week. 111(3).31 (1972). (3) W. Leithe. "The Analysis of Air Pollutants." Revised English ed, Ann Arbor Science Publishers, Mich., 1971. ( 4 ) A. Fontijn, A . J. Sabadell. and R. J. Ronco, Anal. Chern.. 42, 575 (1970). (5) L. B. Kreuzer and C. K. N. Patei, Science, 173,45 (1971). (6) A. Kaldor. Wm. 6. Olson, and A. G. Maki. Science, 176, 508 (1972). (7) L. 6. Kreuzer. N. D. Kenyon, and C. K. N. Patel, Science. 177, 347 (1972).
PJow, we report a new method using gas-phase EPR by which the separate determinations of NO and NO2 are easily and accurately accomplished without using any chemical reactions. The sensitivity is better than 30 partsper-billion (ppb) for NO and NO2 with a sample volume of 1.5 1. when a novel low-temperature trapping technique is adopted. When the sample is observed without any enrichment, the minimum detectable limit is 10 ppm for NO2.
PRINCIPLE Gaseous paramagnetic species in polluted air are only 0 2 , NO, and NO2. Although all of the substances are, in principle, observed by EPR, a detailed consideration shows that separate detection of NO and NO2 is possible. First, the fact is used that among 0 2 , NO, and NOz, those having an electric dipole moment are NO and NO2. The nonpolar oxygen molecule is not observed at all by Stark-modulating gas-phase EPR which was first proposed by Carrington et al. (8, 9). A second base for detecting NO and NO2 independently is that NO is a linear molecule, whereas NO2 is nonlinear. The principle of the gas-phase EPR of NO is shown in Figure 1. Since the NO molecule (211312) has axial symmetry and has also a large spin-orbit interaction, only the molecular-axis components A and Z of L and S, respectively, are quantized (Hund's case a ) ( I O ) . The rotational angular momentum K of the molecular frame is perpendicular to the molecular axis. The quantities A Z ( = 12) and N combine to give the total angular momentum J (the nuclear spin I is ignorable). The magnetic moment ( = A + 22') of NO originates from A and 2 . Since the total angular momentum J is conserved in free space, the applied external magnetic field H interacts with the J component of the
+
(8)A. Carrington. D. H. Levy. and T. A . Miller, Rev. Sci. Instrum.. 38. 1183 (1967). (9) A . Carrington. "Molecular Spectroscopy." Elsevier, London, 1968, p 157. (10) G. Herzberg. "Spectra of Diaiomic Molecules," 2nd ed, Van Nostrand, New York. N. y . , 1961.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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magnetic moment to cause the Zeeman effect. When the molecular rotation is excited, that is, the quantum number N (or J) becomes large, the J component of the magnetic moment and, accordingly, molecular g value (gJ)become small. Therefore, the EPR spectrum of NO for the higher rotational level is observed a t the higher resonance field. In fact, the resonance field is entirely different for each rotational level ( I I ) . Another essential factor for understanding the EPR spectrum of NO is that the electric dipole moment is fixed along the molecular axis on which the magnetic moment is also fixed. This enables us to observe the EPR spectra of the electric dipole transitions by use of the Stark modulation. In Figure 2 , the principle of the gas-phase EPR of NO2 is shown. Since NO2 is a nonlinear radical in the electronic ground state, the electronic orbital moment vanishes. There are, therefore, no large interactions which connect the electron spin S to the molecular frame rotation N (the small spin-rotation interaction is negligible). The magnetic moment originates only from S. Since the magnetic moment and the molecular frame rotation are independent with each other, the molecular g values gJ of NO2 for all rotational levels are identical (g 2). The electric dipole moment is fixed to the molecular frame as shown in Figure 2 . Thus, we cannot observe the EPR spectra of NO2 by Stark modulation, but we can obtain the magnetic dipole transitions by Zeeman modulation ( 1 2 ) . On the basis of the considerations described above, NO in polluted air is detected by Stark modulation under the reduced sample pressure of 0.1 Torr without any interferences of coexisting substances. To detect NO2 by Zeeman modulation, interferences of 0 2 are completely obviated as follows. 0 2 in the triplet ground state has a large spin-spin interaction and axial symmetry causing the molecular g value for each rotational level to be completely different. Moreover, as the forbidden transitions are “allowed” by the applied magnetic field ( I 3 ) , the gas-phase EPR spectra of 0 2 consist of many lines distributed in a wide range of the magnetic field. The spectra, therefore, smear out due to the pressure broadening of each rotational line when the sample pressure is increased up to about 1 atm. In fact, spectra of 0 2 a t 1 a t m of pressure were not observed at the g 2 resonance region. On the other hand, a single strong spectrum of NO2 is observed at 1 a t m of pressure. This result comes about by the piling-up of the EPR spectra for all rotational 2 resonance region as (and vibrational) levels a t the g stated above. In conclusion, NO2 in polluted air is detected by Zeeman modulation at 1 atm of sample pressure, without interference from 0 2 and NO.
Figure 1. Principle of the gas-phase EPR of NO
-
-
Figure 2. Principle of the gas-phase EPR of NO2
-
EXPERIMENTAL
7 0
1 mm Figure 3. Dimensions of C-band cylindrical TEo, 1 mode cavity 1898
NO was detected with Stark modulation using a C-band E P R apparatus (14). Figure 3 shows the C-band airtight EPR cavity (4.47 GHz) which has an internal volume of 670 cm3. T h e cavity itself is a sample cell for the detection of NO. A pair of the cavity endplates constitutes the Stark electrodes. T h e optimum sensitivity was obtained a t the sample pressure of 0.05-0.1 Torr. Gasphase E P R spectra of NO (2113;2 J = 3 / 2 ) were observed a t a resonance field of about 4 kG. The centered line of M J , M I = %, O+?/2, 0 was used for monitoring N O pollution. The Stark ac (100 kHz) voltage of 53 V/cm (peak-to-peak value) and the dc bias of 26 V / c m were fixed for all the measurements of NO. NO2 was detected by Zeeman modulation at 1 a t m of sample pressure, using an X-hand spectrometer. The sample cell was a (11) R . L. Brown and H. E. Radford. Phys. R e v , 147, 6 (1966). (12) T. J. Schaafsrna. Chern. Phys. Lett., 1, 16 (1967). (13) M. Tinkham and M . W. P. Strandberg, Phys. Rev.. 97, 951 (1955) (14) H. Uehara, Mol. Phys., 21, 407 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, S E P T E M B E R 1973
10 m m i.d. quartz tube which had a Teflon (Du Pont) stopcock a t one end. T h e sample volume is 10 cm3. T h i s sample tube was inserted into a n X - b a n d cavity (9.2 GHz). A single line of NO2 was observed a t 3.2 kG with a peak-to-peak line width of 260 G. All t h e measurements of NOz were made with a Zeeman modulation width of 20 G which corresponded to t h e maximum power supplied by our X-band spectrometer. During t h e process of taking the sample tube into and out of t h e cavity, much care was taken in keeping various spectrometer adjustments unchanged. T h e power saturation effects d u e t o t h e incident microwaves were carefully eliminated. A Takachiho P.G. 30-1. cylinder of NO (>98.5%) and a hiatheson lecture bottle of NO2 (99.iYo) were used a s the standard samples. T h e pressure was measured by a Texas Instruments Model 145 precision pressure gauge with micron gearing (0,00086 Torr/count). D i r e c t M e a s u r e m e n t s . A gas-phase E P R spectrum of 500 ppm NO2 in polluted air is shown in Figure 4A. T h e sample volume was 10 cm3. As is seen in Figure 4A. NO2 in polluted air was detected without any interferences from 0 2 and other coexisting species. N 2 0 4 has no influence on t h e measureT h e equilibrium 2 N 0 2 ments of NO2 a t t h e ppm range. To examine the detection limit, E P R spectrum of 40 ppm NO2 in room air was measured, a s shown in Figure 4B. From this signal-to-noise ratio, a detectable limit ( i i ' NO2 was obtained to be 10 ppm for t h e direct measurement of the polluted air sample. Since NO is detected under the reduced sample pressure of less t h a n 0.1 Torr, the detectable limit for the direct measurements is rather high and is a few hundred p p m . T h e linearity of responses of the EPR spectrometer was confirmed. T h e EPR signal intensity varied linearly with NO2 concentration in t h e air sample over the range of 30 to 500 ppm. T h e average deviation Idxlav of t h e measured values ( x , ) from the linear line ( + o l ) was 4.8 pprn ( / d s l a v = Z r = 1" Ixl - x o r l / n ) . Detections of Low Concentrations of NO a n d NO2. In order t o detect NO2 whosr concentration was less t h a n 10 p p m , a lowtemperature trapping technique was used. Since t h e vapor pressures of NO2 are 0.0024 and 0.00033 Torr a t -105.5 and -120"C, respectively ( 1 5 ) . NO2 at ppb range can be cold trapped a t temperatures below -150°C. NO2 was not detected, however, in t h e revaporized gas when trace NO2 in air was trapped at low temperatures. This is attributed to the fact t h a t NO2 reacts with water condensed on t h e wall by t h e scheme (16)
A
I
2 .o
+
+
-+
HNOz + H N 0 3 2 N 0 + HzO
4.0kG
t
3.0
2.5
3.5 kG
Figure 4. Gas-phase X-band EPR spectra of NO2 in air
Samples. were observed directly under 1 atm of pressure: ( A ) 500 ppm NOz, ( 8 ) 40 ppm NOn
t PUMP
+
2 N 0 2 HzO 3HNOz "03
3.0
II
A
PUMP
CL
J
(1)
B
(2)
It was impossible t o remove t h e water vapor sufficiently by desiccating t h e sample gas with a desiccant such as P205. T h e difficulty was settled by trapping SO2 on a low-temperature desiccant (P205).T h e sample gas must be kept out of contact with lowtemperature sites except t h e desiccant, because water condensed on the wall quenches NO2 by Equation 1. After noncondensables were pumped out, NO2 even in the p p b range was quantitatively recovered in the revaporized gases. T h e low-temperature trapping apparatus is schematically shown in Figure 5. T h e apparatus was made of Pyrex glass (Corning) except for Teflon stopcocks C 0 - c ~ .Greased stopcocks and greased joints are unusable because of absorption of NO2 on grease. In Figure 5, A is a 1.5-1. sample tube. P is a Pyrex glass circulation pump. a n d D is a preliminary desiccating t u b e which contained P205. T is a U-shaped tube, t h e inside wall of which is entirely coated with a Silm of P205 T h e 10-cm3 quartz sample cell B described above was attached by a greaseless joint J. T h e polluted air sample is introduced into A through D with a flow rate o f 300 cm3/min. In typical experiments, T was cooled to -150°C by a low-temperature bath LB. After circulation of t h e sample gas for 15 min, noncondensables are pumped out while condensables are trapped at T. Then removing LB and warming 'r up to 50°C. the revaporized gases are introduced into the sample cell B . Measurements were made after cell B was filled u p t o 1 a t m with additional nitrogen or clean air. T h e gas-phase E P R spectra of 8 p p m a n d 0.38 p p m S O 2 i n air are shown in Figure 6A a n d 6B, respectively. T h e sample used for cold trapping was 1.5 I. Sor each. T h e quantitative nature of this trapping technique was confirmed using standard samples of NO2 in air. A plot of the E P R signal intensity o f concentrated NO2 to original NO2 concentra(15) A . Charlesand G . Egerton. J. Chem. SOC.,105, 647 (1914) (16) F. A. Cotton and G . Wilkinson, "Advanced Inorganic Chemistry," Interscience. New York. N. Y . , 1967.
Figure 5. System for low temperature trapping of NOz ( A ) 1.5 I. sample tube; (8)10-cm3 quartz sample cell; (c0-C~)Teflon stopcock; ( D ) preliminary desiccating tube; ( P ) Pyrex glass circulation p u m p ; (T) U-shaped tube, inside wall of which is coated with a film of P205; (LB) low temperature bath
B
Figure 6. Gas-phase EPR spectra of NO2 in air detected by the use of enrichment technique of low-temperature trapping (A) 8
ppm NO?. (B) 0.38 ppm NO2
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G
3963
(19) reported t h a t a n absorber using triethanolamine on firebrick removed NO2 completely, without any intluence on t h e N O concentration in the gas for any NO/NOz ratio normally occurring in ambient air. Figure 7 shows t h e gas-phase EPR spectra of 1 ppm NO in room air. An air sample of 500 cm3 was cold adsorbed. T h e pressure of the desorbed gas in the cavity was 0.05 Torr. From the signal-to-noise ratio of 30 in Figure 7 , the detectable limit of N O was found to be 30 ppb. This cold-adsorption technique using 80-cm3 standard samples of N O in air gave the average deviation 16xjav of 2.1 ppm of the measured values from the linear line over the range of 10 t o 90 ppm. The pressure of the 80-cm3 standard samples supplied for the cold adsorption was l/3 a t m in order t o eliminate the errors due to the oxidation of NO. Since the oxidation reaction
2N0
Figure 7. Gas-phase C-band EPR spectrum of 1 ppm NO in air Cold-adsorption technique was used
A
B
Figure 8. Examples of the detections of NO and NO2 in exhaust gases (A) 45.5 ppm NO in automobile exhaust at idling. (B) 90 ppm NO? detected directly in a sample of smoke-duct exhaust
tion disclosed a completely linear relation between them over the range investigated (0.3 to 30 pprn). The average deviation Ibxlav defined above was, for example, 0.014 ppm (for n = 6) in the range of 0.3 to 1.6 ppm. From Figure 6B, the detectable limit of NO2 was 30 ppb. N O was detected using the conventional cold-adsorption technique ( 1 7 ) , when t h e concentration was less t h a n a few hundred ppm. T h e pollutants in air were adsorbed on cold adsorbent a t -78°C. Nonadsorptive Nz and 0 2 were pumped out, and then, by heating the adsorbent u p to about 200°C. t h e pollutants desorbed were introduced into a n evacuated C-band cavity. The pressure of the sample containing N O was 0.05 Torr in the cavity when 500cm3 room air sample was cold adsorbed. T h e adsorbent used was firebrick (60-80 mesh Chamelite F K ) of 1 g. When NO2 coexisting has a possibility of producing N O on the cold adsorbent (18), t h e errors for NO measurements can be eliminated by t h e removal of NO2 in advance. Levaggi e t al Young and A . D. Crowell, "Physical Adsorption of Gases." Butterworths, London, 1962. (18) S.A.Greeneand H.Pust,Ana/. Chem., 30,1039 (1958). ( 1 7 ) D. M.
1900
+0 2
---*
2N02
(3)
has a negative activation energy (ZO), errors for the KO2 and NO measurements may arise from the low-temperature treatment. In such cases, the errors were eliminated by reducing the sample gas pressure for the low-temperature treatment as described above, because reaction 3 is a termolecular reaction.
APPLICATION Figure 8A shows the EPR spectra of NO in a sample of automobile exhaust. The sample used for the detection was 80 cm3. The sample pressure in the cavity was 0.06 Torr. The peak height indicates the NO concentration to be 45.5 ppm. The EPR spectrum of NO2 detected directly in a sample of smoke-duct exhaust of an air conditioning boiler is shown in Figure 8B. This indicates the presence of 90 ppm NOz. The sample volume was 10 cm3. The present gas-phase EPR method for the NO and NO2 detection is applicable to wide varieties of the polluted air samples. Moreover, the detection limit of NO2 can be improved by an order of magnitude if the Zeeman modulation width is increased to more than 100 G and if the sample is made to fill the entire X-band cavity space. In this report, we have proposed a novel principle for the detection of NO and NO2. The results of the quantitative analysis were very satisfactory, as were briefly described above. A detailed quantitative discussion will be published in the near future. ACKNOWLEDGMENTS The authors wish to thank Yonezo Morino for attracting their attention to air pollution problems. They are also indebted to Y. Ijuin for his helpful assistance in technical aspects. Received for review October 30, 1972. Accepted February 9, 1973. (19) D. A. Levaggi, W. Siu, M. Feldstein, and E. L. Kothny, Environ. SCi. Techno/.,6, 250 (1972) (20) I . C . Hisatsune and L. Zafonte. J. Phys. Chem.. 73, 2980 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
