2084
Anal. Chem. 1990, 62, 2084-2087
fluorophore and quencher. Therefore, the sensitivity for static quenching cannot be predicted here. If both dynamic and static quenching processes are present, the sensitivity of the optrode will be increased and detection limits improved accordingly.
LITERATURE CITED Gerade, H. W. Toxicology and Biochemistry of Aromatic Hydrocarbons; Ekevier: Amsterdam, 1960. (2) Miller, E. C.; MHler, J. A. Chedcal Mutagens; Hollaender, A,, Ed.: P l a num Press: New York. 1971; Vol. 1. (1)
(3) Kelker, H. Ber. Bunsen-Ges. Phys. Chem. 1963, 67,698. Johnston, K.; Zlelinski, W. L., Jr. Anal. Chem. 1975, 47, 670. (5) Janini, G. M.; Muschik, G. M.; Zieiinski, W. L., Jr. Anal. Chern. 1976, 48, 809. (6) Radecki, H.; Lamparczyk, H.; Kaliszan, R. Chromatogmphia 1979, 12, 595. (7) Ziolek, A.; Witkiewicz, 2 . ; Dabrowski, R . J . Chromatogr. 1984, 299, 159.
(4) Janlnl, G. M.;
B. A.; Tarbet, B. J.: Brodshaw, J. S.; Lee, M. L. J . Chromatogr. 1986, 357,79. (9) Wise, S. A.; Bonnett. W. J.; Guenther, F. R.; May, W. E. J . Chromatogr. Sci. 1981, 19, 457. (10)Ferguson, J. L. Sci. Am. 1964. August, 77. (11) Aliev, D. F.; Gasanov, I. I.; Lisetskii, L. N. Zh. Fiz. Khim. 1969, 63, 558. (12)Novak, T. J.; Poziomek, E. J.; Mackay, R. A. Mol. Cryst. Liq. Cryst. 1973, 2 0 , 203. (13) Myi, J. Advances in LiquM Crystal Research and Applications: Bata, L., Ed.; Pergamon Press: Oxford, 1980. (14) David, D. J.; Hardy, E. E. Patent US 4040749, 1977. (15) Poziomek, E. J.; Novak, T. J.; Mackay, R. A. Mol. Cryst. Liq. Cryst. 1974, 2 7 , 175. (16) Weast, R. C.; Astle, M. J.; Beyer, W. H. CRC Handbook of Chemistry and Physics, 64th; CRC Press, Inc.: Boca Raton, FL, 1983. ( 8 ) Nishkke, M.; Jones,
RECEIVED for review April 10, 1990. Accepted July 6, 1990. Supported in part by the National Science Foundation through Grant CHE 87-22639 and by the Upjohn Co.
Chemiluminescent Method for Continuous Monitoring of Nitrous Acid in Ambient Air Yukio Kanda* and Masafumi Taira National Laboratory for High Energy Physics, Oho, Tsukuba, Ibaraki-ken 305, Japan
A continuous-flow method for measurlng atmospherlc HNO, concentratlon In real tlme has been developed that uses a chernlluminescent NO, monitor. A Na,CO, solutlon strips gaseous "0, from the atmosphere by means of pulllng an alr sample and the Wutlon through a glass coil and mlxlng continuously wlth ascorblc acid solution which reduces nitrite to NO. The mlxture Is led Into a gas-llquld separating coll conslstlng of mkroporow, PTFE tuMng. The NO evolved from the separatlng coll is swept out by a stream of clean air and detected with a chemllumlnescent NO, monitor. The technlque utlHzes a dual flow system and dual channel NO, monitor to correct poSnlve Interferences from NO, and peroxyacetyln#rate (PAN). The concentratlonof HNO, Is determkred by dmerence between the two measurements. SenSnMty of the method Is a functlon of the ratio of sampling flow rate to carrier gas flow rate, wMch permlts readlly a hlghly sensltlve measurement.
INTRODUCTION Measurement of H N 0 2 in the atmosphere was first made by Perner and co-workers (I, 2), employing a differential optical absorption spectrometer. They reported that the HNOz concentrations at several sites in Western Europe and in the United States ranged from 0.02 to 8 ppb. The technique is a specific, absolute method using absorption by HNOz at 354.1 and 368.1 nm, along with very long path lengths (0.75-2.26 km), permitting direct measurement of ambient H N 0 2 concentrations. Other methods for measuring HNOz require a certain preconcentration technique, because the concentration in the atmosphere is too low for the determination by a direct analysis. Use of a diffusion denuder is one of the preconcentration techniques widely used in air pollution studies. Ferm and Sjodin (3) have reported on the use of a sodium carbonate coated denuder for collecting HN02. The HNOz 0003-2700/90/0362-2084$02.50/0
was extracted in water and determined spectrophotometrically by the Greise-Saltzman method. Braman et al. ( 4 ) have developed a sequential, selective hollow tube preconcentration and chemiluminescent analysis system for nitrogen oxide compounds, including HN02. The H N 0 2 was absorbed on a diffusion denuder coated with potassium oxide-iron oxide and was thermally desorbed as NO and subsequently detected via the chemiluminescent reaction of NO with 03. These measurement systems combined with denuder technique provide an integrated concentration or an average concentration. Concentration-time profiles observed by Perner et al. have shown a characteristic time dependence of HN02; the gradual build-up of HN02 during the night and its rapid decay after sunrise. The results have provided several significant suggestions on the atmospheric chemistry of HNOz through computer simulations (5). This indicates the importance of continuous measurement in elucidating basic atmospheric chemistry of HN02. This paper describes a simple method for the continuous monitoring of ambient HN02. The technique utilizes a coil for stripping HN02 from air and a chemiluminescent monitor for detecting NO produced by the subsequent reduction of NO, in the scrubbing solution. Presented here are the results of method development performed by using a high-purity HNOz source and the results of preliminary measurements of ambient HN02.
EXPERIMENTAL SECTION Reagents. All chemicals used were of reagent grade from Wako Chemical Industries, Ltd. All aqueous solutions were prepared with high-purity water from a Millipore Milli-Q purification system. A standard stock solution of 100 c(g of N02-/mL was M Na2C03from sodium nitrite dried at 110 prepared in 5 X "C. Working standard solutions of nitrite were prepared by diluting the stock solution to the desired concentration with 5 x M Na2C03solution. Standard HN02gas mixtures were generated by a continuous generation system described previously (6). A standard cylinder 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990 2085
Figure 1. Schematic diagram of dual channel flow system: A,
stripping coil; B, peristaltic pump: C, mass flow controller; D, air pump; E, debubbler; F, clean air input; G, gas-liquid separating coil; H, NO, monitor; I, recorder.
a
1.6rnm .^ . ID i u - ~ u r nCOIL . .. . , a
I I i
5cm* 'Om' ID
I
Scrubbing solution
b
I r II I I
waste
Stripping coil (a)and gas-liquid separating coil (b): A, inlet of mixture of scrubbing solution and ascorbic acid solution; B, silicone tube: C. microborous PTFE tube. Figure 2.
gas mixture containing of 3.02 ppm NO in N2, purchased from Nihon Sanso Co., was diluted to the desired concentration with high-purity N2by means of Kofloc gas blending system (Kojima Seisakusho). A calibrated permeation tube for NO2 with an emission rate of 15.5 bg/min at 30 "C was obtained from Gastec Co. A diffusion tube for HN03 was made in the laboratory from an empty glass diffusion tube (3-mm i.d., 10-cm length) and a 1:l mixture of concentrated HN03 and H2S04. The mass loss rate was measured gravimetrically and found to be 5.35 pg/min at 30 "C. These tubes were kept at 30 OC in a Gastec PD-1B permeater and was flushed with a flow of high-purity Nz. Peroxyacetyl nitrate (PAN) was synthesized by nitration of peracetic acid in n-tridecane, according to the procedure of Gaffney et al. (7). A diluted PAN vapor was prepared by injecting an appropriate microliter amount of PAN in n-tridecane solution into a Tedlar bag containing known volume (ca. 50 L) of highpurity Nz.The concentration was determined by ion chromatography following alkaline hydrolysis of PAN to acetate (8). Apparatus. A schematic diagram of dual channel flow system is shown in Figure 1. The configuration of a stripping coil with separator, shown in Figure 2a, is similar to that used by Lazrus et al. (9). A 10-turn coil was made of a 50 cm length of glass tubing of 1.6-mm i.d. Figure 2b shows a eight-turn gas-liquid separating
0
0.1
0.2
0.3 0.4 Pumping rate, mL/min
Effect of pumping rate of scrubbing solution on HNOp collection efficiency: sample flow rate, 2.0 L/min: (0)1 X lo-' M Na,C03, (0) 1 X lo3 M Na,CO,, (0)1 X lo4 M Na,CO,, (A)water. Figure 3.
coil. It consisted of two tubes. The inner tube was made of microporous PTFE tubing (1-mm i.d., 2-mm o.d., 50-cm length, 1-Nrn pore size, 60% porosity, Sumitomo Denko, Inc., Poreflon tube TB-21), and the outer tube was made of a 3-mm4.d. silicone tubing. A carrier gas flows in the annulus between the two tubes. The debubbler was made of a 2-cm length of microporous PTFE tubing. Flows of sample air and carrier gas were controlled by SEC-410 mass flow controllers (Stec Corp.). Two-channel peristaltic pumps (LKB 2132 Microperpex, Bromma, Sweden; Masterfelx PA-21A, Cole-Parmer, Chicago, IL) were used for pumping the reagent solutions. Measurements of HN02 and NO were made with two chemiluminescent monitors. A Dylec DY-8400 NO, monitor (Dylec Corp.) was used mainly in the experiments on parameter optimization. A Monitor Labs 8840 NO, analyzer (Monitor Labs, Inc., San Diego, CA) was used in the measurements of ambient HNOP This instrument is a dual-channel monitor with two reaction chambers and two photomultiplier tubes. To simultaneously measure the NO from the two flow systems, the NO, flow line in the instrument was modified to lead directly to the reaction chamber by bypassing the converter. Procedure. Gaseous HN02 is stripped from the atmosphere by means of concurrently pulling air sample and scrubbing solution M through a stripping coil. A scrubbing solution of 5 X Na2C03is pumped continuously into the inlet of the stripping coil at a rate of 0.20 mL/min. Air is pulled through the coil after passage through a Teflon filter at a flow rate of 2.0 L/min. The scrubbing solution is isolated from the scrubbed air in a vertical separator column by gravity and is continuously pumped from the bottom of the separator together with about 0.05 mL/min of the scrubbed air. After passing through a tublar debubbler, the scrubbing solution is added to a flow of 0.20 mL/min of 0.1 M ascorbic acid in 0.05 M H$04 and the mixture is then fed through a gas-liquid separating coil. The evolved NO is swept into a NO, monitor by room air freed of NO by passage through a cobalt(II1) oxide coated denuder ( 4 ) . On the other hand, to measure the interference effects from NO2and PAN, the scrubbed air is led to the stripping coil in the second channel flow system that is identical with the first.
RESULTS AND DISCUSSION HN02Collection. Our previous work (6) showed that the flow-type generation system for HN02, based on the reaction of aqueous solution of NaN02 with H2S04,can continuously provide pure H N 0 2 of known concentrations. In addition, HNOz vapor was found to be entirely reduced to NO by the converter of the chemiluminescent NO, monitor and fully detected as NO, compound. Accordingly, measurement of the collection efficiency of the stripping coil for H N 0 2 was carried out by monitoring the HNOz generator effluent downstream of the coil with a NO, monitor. Measurements were made for the scrubbing solutions of water and Na2C03solutions of 1 X lo4, 1 X and 1 X 10" M at the pumping rates ranging from 0.05 to 0.4 mL/min and at a fixed sampling rate of 2.0 L/min. As shown in Figure 3, the quantitative collection of HNOz (>98%) in the stripping
2086
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19,OCTOBER 1, 1990
b oL\
0.2 0.1 Ascorbic acid. M Flguro 4. Effect of ascorbic acid concentration on reduction of nlrite to NO: (0) NO, (0)"0,. 0
coil was obtained at NaZCO3concentrations above 1 X M and at pumping rates higher than 0.2 mL/min. Effect of the air sampling rate on the HNOZcollection was also examined at a 0.2 mL/min of 5 X M NaZCO3solution. The collection efficiency was found to diminish at sampling rates above 2.0 L/min. This result indicates that above 2.0 mL/min, the residence time of air in the coil is insufficient for establishing partition equilibrium of HNOz between the sample air and the scrubbing solution. Reduction of Nitrite to NO. In selection of the reducing agent for the conversion of nitrite in a solution to NO followed by chemiluminescent or ultraviolet absorption detection of NO, Cox (10) and Nagashima et al. (11) used the following criteria: (a) the reaction must be rapid and quantitative at low nitrite levels and (b) the reagent should be of low volatility. They have reported the employment of 0.2 M NaI in 13 M acetic acid and 0.13 M NaI in 13 M phosphoric acid, respectively. The reducing agent chosen for this study was ascorbic acid solution in dilute HzS04. This reagent is easy to handle and was found to meet the above criteria. The effect of HzSO4 concentration in ascorbic acid solution on the reduction of nitrite was examined by using nitrite standard solution. A 2.0 ppm NO; solution and 0.1 M ascorbic acid solutions prepared in different concentrations of HzS04were pumped separately into the gas-liquid separating coil at the same rate of 0.20 mL/min. The NO evolved from the coil was purged by a clean air at a rate of 2.0 mL/min and determined with a NO, monitor. The maximum reduction efficiency was obtained when the HzS04 concentration was between 0.02 and 0.1 M. The H2S04 concentration of 0.05 M was chosen for further experiments. Figure 4 shows the effect of ascorbic acid concentration on the reduction of nitrite to NO. The yield of NO was increased with the increase in ascorbic acid concentration and the quantitative reduction of nitrite was obtained at concentrations above 0.05 M. At lower ascorbic acid concentrations, a monitor in the NO, mode gave obvious response, indicating the generation of some nitrogenous species besides NO. It was ascertained to be HNOz by ion chromatographic analysis following sampling of the gas mixture with a NazC03-impregnated filter. The results are also shown in Figure 4. No attempt was made to examine the effects of temperature and length of a gas-liquid separating coil, because the quantitative reduction was obtained by using a 50-cm-long coil at room temperature. Interferences. Of common air pollutants coexisting with HNOZ,PAN and NOz yield nitrite ions in alkaline solution and, hence, would produce positive interferences CH&(O)OONOZ 20HCH3COO- + NOz- + 02 + HzO (1) 2NOZ + 20HNOz- + NO3- + H 2 0 (2)
+
A
-
Interferences from PAN and NOz were investigated by passing each standard gas mixture through the stripping coil at a rate of 2.0 L/min. Absorption of the gases into a 0.20 mL/min of 5 X M NaZCO3solution in the coil was checked by monitoring the concentration downstream of the coil with the NO, analyzer and was quantitated by determining the concentration of nitrite ion in the NaZCO3solution by the spectrophotometric method (12). The results showed that the passage of each of 240 ppb PAN and 225 ppb NOz gas mixtures yielded the amount of nitrite ions corresponding to absorption of 4.6 f 0.2 ppb and 1.6 f 0.3 ppb ( N = 5), respectively. In other words, the positive interference was only 1.9% for PAN and 0.4% for NOz. However, in urban areas where PAN levels sometimes exceed 40 ppb (13,14) and the NOz concentration typically ranges a factor of 10-20 more than the HNOZconcentration (Z), these could present problems and should be considered. The correction for these interferences was made by sampling with two stripping coils connected in series; the effects were measured with the second channel flow system. Since PAN and NOz have oxidizing properties, they might partially oxidize the scrubbed HNOZ before it reacts with ascorbic acid and produce negative as well as positive interferences. To test the negative interferences from PAN, NOz, and the more abundant oxidant, 03,a 0.2 ppm NOz- standard solution was pumped into the stripping coil at a rate of 0.20 mL/min and each of 135 ppb PAN, 118 ppb NOz, and 1.6 ppm O3gas mixtures was passed through the coil at 2.0 L/min. The NO evolved by the reduction of NO, with ascorbic acid was then monitored with the NO, analyzer. The results showed positive interferences from PAN and NOz and no interference from 03.The observed effects of PAN and NOz were equal to those estimated from their concentrations and the positive interference effects described above. Hence PAN and NOz produce no negative interferences and O3has no effect on the method. Kaiser and Wu (15)have reported that reaction 3 proceeds relatively rapidly on the surface of glass vessel.
NO
+ NO2 + H20
2HNOz
(3)
Ferm and Sjbdin (3)have observed the HNOz formation due to the heterogeneous reaction during sampling with a NazC03-coated denuder. An additional experiment was performed to check the possibility of production of artifical HNOz in the stripping coil during sampling. The gas mixture containing 130 ppb NO and 220 ppb NOz was passed through the stripping coil at a rate of 2.0 L/min, while pumping a solution of 5 X M NaZCO3into the coil at a rate of 0.20 mL/min. Analysis of the NaZCO3scrubbing solution showed no formation of HNOz in the coil under these conditions. Gaseous HNO, was collected quantitatively in a scrubbing solution of 5 X M NaZCO3,but nitrate ion was found not to be reduced to NO with ascorbic acid; the HN03 produces no interference in the present method. Sensitivity, Calibration Graph, and Limit of Detection. The analytical sensitivity of the present method, expressed as a slope of a straight line calibration graph, would be expected to be directly proportional to the ratio of sampling flow rate to carrier gas flow rate; for a constant sampling flow rate, the concentration of NO produced by chemical reduction would increase with a decrease in the carrier gas flow rate. This is demonstrated in Figure 5 where the NO concentrations measured with the NO, monitor are given for different carrier gas flow rates. In these experiments, a gas mixture of 8.5 ppb HNOz was sampled with 0.20 mL/min of 5 X M NaZCO3 at a fixed flow rate of 2.0 L/min and the subsequent reduction was carried out with 0.20 mL/min of 0.1 M ascorbic acid in 0.05 M H2S04. A high sensitivity can be obtained when sampling flow rate is high and carrier gas flow rate is low.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
2087
Bo-
40
a n
d
z 20
d 0.4
0- ' 0
Anal. Chem. 1990, 62, 2084-2087
fluorophore and quencher. Therefore, the sensitivity for static quenching cannot be predicted here. If both dynamic and static quenching processes are present, the sensitivity of the optrode will be increased and detection limits improved accordingly.
LITERATURE CITED Gerade, H. W. Toxicology and Biochemistry of Aromatic Hydrocarbons; Ekevier: Amsterdam, 1960. (2) Miller, E. C.; MHler, J. A. Chedcal Mutagens; Hollaender, A,, Ed.: P l a num Press: New York. 1971; Vol. 1. (1)
(3) Kelker, H. Ber. Bunsen-Ges. Phys. Chem. 1963, 67,698. Johnston, K.; Zlelinski, W. L., Jr. Anal. Chem. 1975, 47, 670. (5) Janini, G. M.; Muschik, G. M.; Zieiinski, W. L., Jr. Anal. Chern. 1976, 48, 809. (6) Radecki, H.; Lamparczyk, H.; Kaliszan, R. Chromatogmphia 1979, 12, 595. (7) Ziolek, A.; Witkiewicz, 2 . ; Dabrowski, R . J . Chromatogr. 1984, 299, 159.
(4) Janlnl, G. M.;
B. A.; Tarbet, B. J.: Brodshaw, J. S.; Lee, M. L. J . Chromatogr. 1986, 357,79. (9) Wise, S. A.; Bonnett. W. J.; Guenther, F. R.; May, W. E. J . Chromatogr. Sci. 1981, 19, 457. (10)Ferguson, J. L. Sci. Am. 1964. August, 77. (11) Aliev, D. F.; Gasanov, I. I.; Lisetskii, L. N. Zh. Fiz. Khim. 1969, 63, 558. (12)Novak, T. J.; Poziomek, E. J.; Mackay, R. A. Mol. Cryst. Liq. Cryst. 1973, 2 0 , 203. (13) Myi, J. Advances in LiquM Crystal Research and Applications: Bata, L., Ed.; Pergamon Press: Oxford, 1980. (14) David, D. J.; Hardy, E. E. Patent US 4040749, 1977. (15) Poziomek, E. J.; Novak, T. J.; Mackay, R. A. Mol. Cryst. Liq. Cryst. 1974, 2 7 , 175. (16) Weast, R. C.; Astle, M. J.; Beyer, W. H. CRC Handbook of Chemistry and Physics, 64th; CRC Press, Inc.: Boca Raton, FL, 1983. ( 8 ) Nishkke, M.; Jones,
RECEIVED for review April 10, 1990. Accepted July 6, 1990. Supported in part by the National Science Foundation through Grant CHE 87-22639 and by the Upjohn Co.
Chemiluminescent Method for Continuous Monitoring of Nitrous Acid in Ambient Air Yukio Kanda* and Masafumi Taira National Laboratory for High Energy Physics, Oho, Tsukuba, Ibaraki-ken 305, Japan
A continuous-flow method for measurlng atmospherlc HNO, concentratlon In real tlme has been developed that uses a chernlluminescent NO, monitor. A Na,CO, solutlon strips gaseous "0, from the atmosphere by means of pulllng an alr sample and the Wutlon through a glass coil and mlxlng continuously wlth ascorblc acid solution which reduces nitrite to NO. The mlxture Is led Into a gas-llquld separating coll conslstlng of mkroporow, PTFE tuMng. The NO evolved from the separatlng coll is swept out by a stream of clean air and detected with a chemllumlnescent NO, monitor. The technlque utlHzes a dual flow system and dual channel NO, monitor to correct poSnlve Interferences from NO, and peroxyacetyln#rate (PAN). The concentratlonof HNO, Is determkred by dmerence between the two measurements. SenSnMty of the method Is a functlon of the ratio of sampling flow rate to carrier gas flow rate, wMch permlts readlly a hlghly sensltlve measurement.
INTRODUCTION Measurement of H N 0 2 in the atmosphere was first made by Perner and co-workers (I, 2), employing a differential optical absorption spectrometer. They reported that the HNOz concentrations at several sites in Western Europe and in the United States ranged from 0.02 to 8 ppb. The technique is a specific, absolute method using absorption by HNOz at 354.1 and 368.1 nm, along with very long path lengths (0.75-2.26 km), permitting direct measurement of ambient H N 0 2 concentrations. Other methods for measuring HNOz require a certain preconcentration technique, because the concentration in the atmosphere is too low for the determination by a direct analysis. Use of a diffusion denuder is one of the preconcentration techniques widely used in air pollution studies. Ferm and Sjodin (3) have reported on the use of a sodium carbonate coated denuder for collecting HN02. The HNOz 0003-2700/90/0362-2084$02.50/0
was extracted in water and determined spectrophotometrically by the Greise-Saltzman method. Braman et al. ( 4 ) have developed a sequential, selective hollow tube preconcentration and chemiluminescent analysis system for nitrogen oxide compounds, including HN02. The H N 0 2 was absorbed on a diffusion denuder coated with potassium oxide-iron oxide and was thermally desorbed as NO and subsequently detected via the chemiluminescent reaction of NO with 03. These measurement systems combined with denuder technique provide an integrated concentration or an average concentration. Concentration-time profiles observed by Perner et al. have shown a characteristic time dependence of HN02; the gradual build-up of HN02 during the night and its rapid decay after sunrise. The results have provided several significant suggestions on the atmospheric chemistry of HNOz through computer simulations (5). This indicates the importance of continuous measurement in elucidating basic atmospheric chemistry of HN02. This paper describes a simple method for the continuous monitoring of ambient HN02. The technique utilizes a coil for stripping HN02 from air and a chemiluminescent monitor for detecting NO produced by the subsequent reduction of NO, in the scrubbing solution. Presented here are the results of method development performed by using a high-purity HNOz source and the results of preliminary measurements of ambient HN02.
EXPERIMENTAL SECTION Reagents. All chemicals used were of reagent grade from Wako Chemical Industries, Ltd. All aqueous solutions were prepared with high-purity water from a Millipore Milli-Q purification system. A standard stock solution of 100 c(g of N02-/mL was M Na2C03from sodium nitrite dried at 110 prepared in 5 X "C. Working standard solutions of nitrite were prepared by diluting the stock solution to the desired concentration with 5 x M Na2C03solution. Standard HN02gas mixtures were generated by a continuous generation system described previously (6). A standard cylinder 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990 2085
Figure 1. Schematic diagram of dual channel flow system: A,
stripping coil; B, peristaltic pump: C, mass flow controller; D, air pump; E, debubbler; F, clean air input; G, gas-liquid separating coil; H, NO, monitor; I, recorder.
a
1.6rnm .^ . ID i u - ~ u r nCOIL . .. . , a
I I i
5cm* 'Om' ID
I
Scrubbing solution
b
I r II I I
waste
Stripping coil (a)and gas-liquid separating coil (b): A, inlet of mixture of scrubbing solution and ascorbic acid solution; B, silicone tube: C. microborous PTFE tube. Figure 2.
gas mixture containing of 3.02 ppm NO in N2, purchased from Nihon Sanso Co., was diluted to the desired concentration with high-purity N2by means of Kofloc gas blending system (Kojima Seisakusho). A calibrated permeation tube for NO2 with an emission rate of 15.5 bg/min at 30 "C was obtained from Gastec Co. A diffusion tube for HN03 was made in the laboratory from an empty glass diffusion tube (3-mm i.d., 10-cm length) and a 1:l mixture of concentrated HN03 and H2S04. The mass loss rate was measured gravimetrically and found to be 5.35 pg/min at 30 "C. These tubes were kept at 30 OC in a Gastec PD-1B permeater and was flushed with a flow of high-purity Nz. Peroxyacetyl nitrate (PAN) was synthesized by nitration of peracetic acid in n-tridecane, according to the procedure of Gaffney et al. (7). A diluted PAN vapor was prepared by injecting an appropriate microliter amount of PAN in n-tridecane solution into a Tedlar bag containing known volume (ca. 50 L) of highpurity Nz.The concentration was determined by ion chromatography following alkaline hydrolysis of PAN to acetate (8). Apparatus. A schematic diagram of dual channel flow system is shown in Figure 1. The configuration of a stripping coil with separator, shown in Figure 2a, is similar to that used by Lazrus et al. (9). A 10-turn coil was made of a 50 cm length of glass tubing of 1.6-mm i.d. Figure 2b shows a eight-turn gas-liquid separating
0
0.1
0.2
0.3 0.4 Pumping rate, mL/min
Effect of pumping rate of scrubbing solution on HNOp collection efficiency: sample flow rate, 2.0 L/min: (0)1 X lo-' M Na,C03, (0) 1 X lo3 M Na,CO,, (0)1 X lo4 M Na,CO,, (A)water. Figure 3.
coil. It consisted of two tubes. The inner tube was made of microporous PTFE tubing (1-mm i.d., 2-mm o.d., 50-cm length, 1-Nrn pore size, 60% porosity, Sumitomo Denko, Inc., Poreflon tube TB-21), and the outer tube was made of a 3-mm4.d. silicone tubing. A carrier gas flows in the annulus between the two tubes. The debubbler was made of a 2-cm length of microporous PTFE tubing. Flows of sample air and carrier gas were controlled by SEC-410 mass flow controllers (Stec Corp.). Two-channel peristaltic pumps (LKB 2132 Microperpex, Bromma, Sweden; Masterfelx PA-21A, Cole-Parmer, Chicago, IL) were used for pumping the reagent solutions. Measurements of HN02 and NO were made with two chemiluminescent monitors. A Dylec DY-8400 NO, monitor (Dylec Corp.) was used mainly in the experiments on parameter optimization. A Monitor Labs 8840 NO, analyzer (Monitor Labs, Inc., San Diego, CA) was used in the measurements of ambient HNOP This instrument is a dual-channel monitor with two reaction chambers and two photomultiplier tubes. To simultaneously measure the NO from the two flow systems, the NO, flow line in the instrument was modified to lead directly to the reaction chamber by bypassing the converter. Procedure. Gaseous HN02 is stripped from the atmosphere by means of concurrently pulling air sample and scrubbing solution M through a stripping coil. A scrubbing solution of 5 X Na2C03is pumped continuously into the inlet of the stripping coil at a rate of 0.20 mL/min. Air is pulled through the coil after passage through a Teflon filter at a flow rate of 2.0 L/min. The scrubbing solution is isolated from the scrubbed air in a vertical separator column by gravity and is continuously pumped from the bottom of the separator together with about 0.05 mL/min of the scrubbed air. After passing through a tublar debubbler, the scrubbing solution is added to a flow of 0.20 mL/min of 0.1 M ascorbic acid in 0.05 M H$04 and the mixture is then fed through a gas-liquid separating coil. The evolved NO is swept into a NO, monitor by room air freed of NO by passage through a cobalt(II1) oxide coated denuder ( 4 ) . On the other hand, to measure the interference effects from NO2and PAN, the scrubbed air is led to the stripping coil in the second channel flow system that is identical with the first.
RESULTS AND DISCUSSION HN02Collection. Our previous work (6) showed that the flow-type generation system for HN02, based on the reaction of aqueous solution of NaN02 with H2S04,can continuously provide pure H N 0 2 of known concentrations. In addition, HNOz vapor was found to be entirely reduced to NO by the converter of the chemiluminescent NO, monitor and fully detected as NO, compound. Accordingly, measurement of the collection efficiency of the stripping coil for H N 0 2 was carried out by monitoring the HNOz generator effluent downstream of the coil with a NO, monitor. Measurements were made for the scrubbing solutions of water and Na2C03solutions of 1 X lo4, 1 X and 1 X 10" M at the pumping rates ranging from 0.05 to 0.4 mL/min and at a fixed sampling rate of 2.0 L/min. As shown in Figure 3, the quantitative collection of HNOz (>98%) in the stripping
2086
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19,OCTOBER 1, 1990
b oL\
0.2 0.1 Ascorbic acid. M Flguro 4. Effect of ascorbic acid concentration on reduction of nlrite to NO: (0) NO, (0)"0,. 0
coil was obtained at NaZCO3concentrations above 1 X M and at pumping rates higher than 0.2 mL/min. Effect of the air sampling rate on the HNOZcollection was also examined at a 0.2 mL/min of 5 X M NaZCO3solution. The collection efficiency was found to diminish at sampling rates above 2.0 L/min. This result indicates that above 2.0 mL/min, the residence time of air in the coil is insufficient for establishing partition equilibrium of HNOz between the sample air and the scrubbing solution. Reduction of Nitrite to NO. In selection of the reducing agent for the conversion of nitrite in a solution to NO followed by chemiluminescent or ultraviolet absorption detection of NO, Cox (10) and Nagashima et al. (11) used the following criteria: (a) the reaction must be rapid and quantitative at low nitrite levels and (b) the reagent should be of low volatility. They have reported the employment of 0.2 M NaI in 13 M acetic acid and 0.13 M NaI in 13 M phosphoric acid, respectively. The reducing agent chosen for this study was ascorbic acid solution in dilute HzS04. This reagent is easy to handle and was found to meet the above criteria. The effect of HzSO4 concentration in ascorbic acid solution on the reduction of nitrite was examined by using nitrite standard solution. A 2.0 ppm NO; solution and 0.1 M ascorbic acid solutions prepared in different concentrations of HzS04were pumped separately into the gas-liquid separating coil at the same rate of 0.20 mL/min. The NO evolved from the coil was purged by a clean air at a rate of 2.0 mL/min and determined with a NO, monitor. The maximum reduction efficiency was obtained when the HzS04 concentration was between 0.02 and 0.1 M. The H2S04 concentration of 0.05 M was chosen for further experiments. Figure 4 shows the effect of ascorbic acid concentration on the reduction of nitrite to NO. The yield of NO was increased with the increase in ascorbic acid concentration and the quantitative reduction of nitrite was obtained at concentrations above 0.05 M. At lower ascorbic acid concentrations, a monitor in the NO, mode gave obvious response, indicating the generation of some nitrogenous species besides NO. It was ascertained to be HNOz by ion chromatographic analysis following sampling of the gas mixture with a NazC03-impregnated filter. The results are also shown in Figure 4. No attempt was made to examine the effects of temperature and length of a gas-liquid separating coil, because the quantitative reduction was obtained by using a 50-cm-long coil at room temperature. Interferences. Of common air pollutants coexisting with HNOZ,PAN and NOz yield nitrite ions in alkaline solution and, hence, would produce positive interferences CH&(O)OONOZ 20HCH3COO- + NOz- + 02 + HzO (1) 2NOZ + 20HNOz- + NO3- + H 2 0 (2)
+
A
-
Interferences from PAN and NOz were investigated by passing each standard gas mixture through the stripping coil at a rate of 2.0 L/min. Absorption of the gases into a 0.20 mL/min of 5 X M NaZCO3solution in the coil was checked by monitoring the concentration downstream of the coil with the NO, analyzer and was quantitated by determining the concentration of nitrite ion in the NaZCO3solution by the spectrophotometric method (12). The results showed that the passage of each of 240 ppb PAN and 225 ppb NOz gas mixtures yielded the amount of nitrite ions corresponding to absorption of 4.6 f 0.2 ppb and 1.6 f 0.3 ppb ( N = 5), respectively. In other words, the positive interference was only 1.9% for PAN and 0.4% for NOz. However, in urban areas where PAN levels sometimes exceed 40 ppb (13,14) and the NOz concentration typically ranges a factor of 10-20 more than the HNOZconcentration (Z), these could present problems and should be considered. The correction for these interferences was made by sampling with two stripping coils connected in series; the effects were measured with the second channel flow system. Since PAN and NOz have oxidizing properties, they might partially oxidize the scrubbed HNOZ before it reacts with ascorbic acid and produce negative as well as positive interferences. To test the negative interferences from PAN, NOz, and the more abundant oxidant, 03,a 0.2 ppm NOz- standard solution was pumped into the stripping coil at a rate of 0.20 mL/min and each of 135 ppb PAN, 118 ppb NOz, and 1.6 ppm O3gas mixtures was passed through the coil at 2.0 L/min. The NO evolved by the reduction of NO, with ascorbic acid was then monitored with the NO, analyzer. The results showed positive interferences from PAN and NOz and no interference from 03.The observed effects of PAN and NOz were equal to those estimated from their concentrations and the positive interference effects described above. Hence PAN and NOz produce no negative interferences and O3has no effect on the method. Kaiser and Wu (15)have reported that reaction 3 proceeds relatively rapidly on the surface of glass vessel.
NO
+ NO2 + H20
2HNOz
(3)
Ferm and Sjbdin (3)have observed the HNOz formation due to the heterogeneous reaction during sampling with a NazC03-coated denuder. An additional experiment was performed to check the possibility of production of artifical HNOz in the stripping coil during sampling. The gas mixture containing 130 ppb NO and 220 ppb NOz was passed through the stripping coil at a rate of 2.0 L/min, while pumping a solution of 5 X M NaZCO3into the coil at a rate of 0.20 mL/min. Analysis of the NaZCO3scrubbing solution showed no formation of HNOz in the coil under these conditions. Gaseous HNO, was collected quantitatively in a scrubbing solution of 5 X M NaZCO3,but nitrate ion was found not to be reduced to NO with ascorbic acid; the HN03 produces no interference in the present method. Sensitivity, Calibration Graph, and Limit of Detection. The analytical sensitivity of the present method, expressed as a slope of a straight line calibration graph, would be expected to be directly proportional to the ratio of sampling flow rate to carrier gas flow rate; for a constant sampling flow rate, the concentration of NO produced by chemical reduction would increase with a decrease in the carrier gas flow rate. This is demonstrated in Figure 5 where the NO concentrations measured with the NO, monitor are given for different carrier gas flow rates. In these experiments, a gas mixture of 8.5 ppb HNOz was sampled with 0.20 mL/min of 5 X M NaZCO3 at a fixed flow rate of 2.0 L/min and the subsequent reduction was carried out with 0.20 mL/min of 0.1 M ascorbic acid in 0.05 M H2S04. A high sensitivity can be obtained when sampling flow rate is high and carrier gas flow rate is low.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
2087
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40
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d
z 20
d 0.4
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